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COLLEGE  OF  PHYSICIANS 
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A  TEXT-BOOK 

OF 

HUMAN  PHYSIOLOGY 


BRUBAKER 


A  TEXT-BOOK 


OF 


HUMAN   PHYSIOLOGY 


INCLUDING  A  SECTION  ON 


PHYSIOLOGIC  APPARATUS 


BY 

ALBERT  P.  BRUBAKER,  A.M.,  M.  D. 

PROFESSOR   OF    PHYSIOLOGY    AND    MEDICAL  JURISPRUDENCE  IN  THE   JEFFERSON  MEDICAL   COL- 
LEGE;   FORMERLY  PROFESSOR  OF  PHYSIOLOGY  IN    THE  PENNSY^LVANIA   COLLEGE   OF 
DENTAL  SURGERY;  FORMERLY  LECTURER  ON  PHYSIOLOGY  AND  HYGIENE 
IN  THE  DREXEL  INSTITUTE  OF  ART,  SCIENCE,  AND  INDUSTRY 


FIFTH  EDITION.     REVISED  AND  ENLARGED 
WITH  1  COLORED  PLATE  AND  359  ILLUSTRATIONS 


PHILADELPHIA 

P.   BLAKISTON'S   SON   &   CO. 

1012  WALNUT  STREET 


Copyright,  1916,  By  P.  Blakiston's  Son  &  Co. 


THE.MAPLE'PBESa.TORK.PA 


TO 

KENNETH  M.  BLAKISTON 

LOYAI.   FRIEND   COURTEOUS   GENTLEMAN 

GENEROUS   PUBLISHER 

THE  PRESENT  EDITION  OE  THIS   WORK 

IS 

AFFECTIONATELY   DEDICATED 


PREFACE  TO  FIFTH  EDITION 


In  the  preparation  of  the  fifth  edition  of  the  Text-book  of  Physiology 
the  attempt  has  been  made  once  again  to  increase  its  value  to  those  for 
whom  it  is  primarily  intended,  viz.,  students  of  medicine  and  practitioners 
who  desire  to  keep  in  touch  with  the  progress  of  physiology.  With  this 
view  the  text  has  been  subjected  to  careful  revision  and  in  many  sections 
entirel}'  rewritten.  Some  of  the  anatomic  diagrams  have  been  eliminated 
and  physiologic  diagrams  inserted.  The  suggestions  and  criticisms  con- 
tained in  reviews  and  book  notices,  in  letters  from  teachers  and  practi- 
tioners have  been  considered  and  in  many  instances  adopted,  with  benefit 
to  the  text,  for  all  of  which  the  writer  expresses  his  appreciation. 

Among  the  subjects  which  have  been  added,  enlarged  or  revised 
may  be  mentioned  the  metabolism  of  the  food  principles;  carbohydrate 
metabolism;  the  electric  currents  of  the  heart  and  their  graphic  registra- 
tion, the  electro-cardiogram;  animal  heat;  internal  secretion;  the  localiza- 
tion of  functions  in  the  cerebral  cortex;  the  autonomic  nerve  system,  etc. 
These  changes  have  necessitated  the  elimination  of  less  important  mate- 
rial, but,  nevertheless,  some  thirty-seven  additional  pages  have  been 
added  to  the  general  body  of  the  text. 

To  those  teachers  and  students  who  have  recommended  and  used  this 
work  and  to  whom  I  am  indebted  for  generous  praise,  kind  criticisms,  and 
helpful  suggestions,  I  wish  to  express  my  sincere  thanks  and  trust  that  in 
its  improved  form  it  will  continue  to  meet  their  approval. 

Once  again  I  desire  to  express  my  appreciation  of  the  unwearied  and 
invaluable  assistance  of  Mr.  I.  A.  Hagy  in  preparing  the  manuscript  for 
the  press. 

To  Messrs.  P.  Blakiston's  Son  &  Co.  I  am  greatly  indebted  for  their 
encouragement  and  generosity  in  the  promotion  of  everything  that  per- 
tains to  the  material  value  of  this  work. 

A.  P.  B. 


PREFACE  TO  FIRST  EDITION. 


The  object  in  view  in  the  preparation  of  this  volume  was  the  selection  and 
presentation  of  the  more  important  facts  of  physiology,  in  a  form  which  it  is 
believed  will  be  helpful  to  students  and  to  practitioners  of  medicine.  Inas- 
much as  the  majority  of  students  in  a  medical  college  are  preparing  for  the 
practical  duties  of  professional  life,  such  facts  have  been  selected  as  will  not 
only  elucidate  the  normal  functions  of  the  tissues  and  organs  of  the  body,  but 
which  will  be  of  assistance  in  understanding  their  abnormal  manifestations 
as  they  present  themselves  in  hospital  and  private  work.  Both  in  the  selec- 
tion of  facts  and  in  the  method  of  presentation  the  author  has  been  guided  by 
an  experience  gained  during  twenty  years  of  active  teaching. 

The  description  of  physiologic  apparatus  and  the  methods  of  investiga- 
tion, other  than  those  having  a  clinical  interest,  have  been  largely  excluded 
from  the  text,  for  the  reason  that  both  are  more  appropriately  considered  in 
works  devoted  to  laboratory  methods  and  laboratory  instruction,  and  for  the 
further  reason  that  the  student  receives  this  information  while  engaged  in  the 
practical  study  of  physiology  in  the  laboratory,  now  an  established  feature 
in  the  curriculum  of  the  majority  of  medical  colleges.  For  those  who  have 
not  had  laboratory  opportunities  a  brief  account  of  some  essential  forms  of 
apparatus  and  the  purposes  for  which  they  are  intended  will  be  found  in  an 
appendix. 

I  wish  to  acknowledge  my  indebtedness  to  Professor  Colin  C.  Stewart  for 
many  valuable  suggestions  in  the  preparation  of  different  sections  of  the 
volume;  to  Dr.  Carl  Weiland  for  assistance  in  the  chapter  on  vision;  to  Dr. 
Joseph  P.  Bolton  for  excellent  suggestions  on  questions  relating  to  physiologic 
chemistry. 


IX 


TABLE  OF  CONTENTS. 


Page 
CHAPTER  I. 
Introduction i 

CHAPTER  II. 
Chemic  Composition  of  the  Hum.vj  Body      6 

CHAPTER  III. 
Physiology  of  the  Cell 24 

CHAPTER  IV. 
Histology  of  the  Epithelul  and  Connective  Tissues 31 

CHAPTER  V. 
The  Physiology  of  Mov'ement • 39 

CHAPTER  VI. 
The  Physiology  of  the  Skeleton 47 

CHAPTER  VII. 
General  Physiology  of  Muscle- tissue 51 

CHAPTER  VIII. 
The  General  Physiology  of  Nerve-tissue 90 

CHAPTER  IX. 
Foods 116 

CHAPTER  X. 
Digestion 136 

CHAPTER  XI. 
Absorption 210 

CHAPTER  XII. 
The  Blood 230 

CHAPTER  XIII. 
The  Circulation  of  the  Blood 264 

CHAPTER  XIV. 

The  Circulation  of  the  Blood  (Continued) 328 

xi 


xii  CONTENTS. 

Pagb 


CHAPTER  XV. 


Respiration 


377 


CHAPTER  XVI. 
Animal  Heat     438 

CHAPTER  XVn. 
Excretion 453 

CHAPTER  XVIII. 
External  Secretions 476 

CHAPTER  XIX. 
Internal  Secretion 490 

CHAPTER  XX 

Metabolism 509 

CHAPTER  XXI. 
The  Central  and  Peripheral  Organs  of  the  Nerve  System 523 

CHAPTER  XXII. 
The  Medulla  Oblongata;  the  Isthmus  of  the  Encephalon;  the  Basal  Ganglia    .    .  551 

CHAPTER  XXIII. 
The  Cerebrum 570 

CHAPTER  XXIV. 
The  Cerebellum 603 

CHAPTER  XXV. 
The  Cranial  Nerves 610 

CHAPTER  XXVI. 
The  Autonomic    Nerve  System 640 

CHAPTER  XXVII. 
Phonation;  Articulate  Speech 658 

CHAPTER  XXVIII. 
The  Senses  of  Touch,  Taste  and  Smell 669 

CHAPTER  XXIX. 
The  Sense  of  Sight 679 

CHAPTER  XXX. 
The  Sense  of  Hearing 713 

CHAPTER  XXXI. 
Reproduction 726 

APPENDIX. 
Physiologic  Apparatus 741 

Index 767 


TEXT-BOOK  OF  PHYSIOLOGY 


CHAPTER  I 
INTRODUCTION 


An  animal  organism  in  the  living  condition  exhibits  a  series  of  phe- 
nomena which  relate  to  growth,  movement,  mentality,  and  reproduction. 
During  the  period  preceding  birth,  as  well  as  during  the  period  included 
between  birth  and  adult  life,  the  individual  grows  in  size  and  complexity 
from  the  introduction  and  assimilation  of  material  from  without.  Through- 
out its  life  the  animal  exhibits  a  series  of  movements,  in  virtue  of  which  it 
not  only  changes  the  relation  of  one  part  of  its  body  to  another,  but  also 
changes  its  position  relatively  to  its  environment.  If,  in  the  execution  of 
these  movements,  the  parts  are  directed  to  the  overcoming  of  opposing 
forces,  such  as  gravity,  friction,  cohesion,  elasticity,  etc.,  the  animal  may 
be  said  to  be  doing  work.  The  result  of  normal  growth  is  the  attainment 
of  a  physical  development  that  will  enable  the  animal,  and,  more  especially, 
man,  to  perform  the  work  necessitated  by  the  nature  of  its  environment  and 
the  character  of  its  organization.  In  man,  and  probably  in  lower  animals 
as  well,  mentality  manifests  itself  as  intellect,  feeling,  and  volition.  At  a 
definite  period  in  the  life  of  the  animal  it  reproduces  itself,  in  consequence 
of  which  the  species  to  which  it  belongs  is  perpetuated. 

The  study  of  the  phenomena  of  growth,  movement,  mentality,  and 
reproduction  constitutes  the  science  of  animal  physiology.  But  as  these 
general  activities  are  the  resultant  of  and  dependent  on  the  special  activities 
of  the  individual  structures  of  which  an  animal  body  is  composed,  physi- 
ology in  its  more  restricted  and  generally  accepted  sense  is  the  science  which 
investigates  the  actions  or  functions  of  the  individual  organs  and  tissues  of 
the  body  and  the  physical  and  chemic  conditions  which  underlie  and  deter- 
mine them. 

This  may  naturally  be  divided  into: 

1.  Individual  physiology,  the  object  of  which  is  a  study  of  the  vital  phenomena 

or  functions  exhibited  by  the  organs  of  any  individual  animal. 

2.  Comparative  physiology,  the  object  of  which  is  a  comparison  of  the 
vital  phenomena  or  functions  exhibited  by  the  organs  of  two  or  more 
animals  of  different  species,  with  a  view  to  unfolding  their  points  of 
resemblance  or  dissimilarity. 

Hmnan  physiology  is  that  department  of  physiologic  science  which 
has  for  its  object  the  study  of  the  functions  of  the  organs  and  tissues  of 
the  human  body  in  a  state  of  health. 

Inasmuch  as  the  study  of  function,  or  physiology,  is  associated  with 
and  dependent  on  a  knowledge  of  structure,  or  anatomy,  it  is  essential  that 


2  TEXT-BOOK  OF  PHYSIOLOGY 

the  student  should  have  a  general  acquaintance  not  only  with  the  structure 
of  man,  but  with  that  of  typical  forms  of  lower  animal  life  as  well. 

If  the  body  of  any  animal  be  dissected,  it  will  be  found  to  be  composed 
of  a  number  of  well-defined  structures,  such  as  heart,  lungs,  stomach,  brain, 
eye,  etc.,  to  which  the  term  organ  was  originally  applied,  for  the  reason 
that  they  were  supposed  to  be  instruments  capable  of  performing  some 
important  act  or  function  in  the  general  activities  of  the  body.  Though  the 
term  organ  is  usually  employed  to  designate  the  larger  and  more  familiar 
structures  just  mentioned,  it  is  equally  applicable  to  a  large  number  of 
other  structures  which,  though  possibly  less  obvious,  are  equally  important 
in  maintaining  the  life  of  the  individual — e.g.,  bones,  muscles,  nerves,  skin, 
teeth,  glands,  blood-vessels,  etc.  Indeed,  any  complexly  organized  struc- 
ture capable  of  performing  a  given  function  may  be  described  as  an  organ. 
A  description  of  the  various  organs  which  make  up  the  body  of  an  animal, 
their  external  form,  their  internal  arrangement,  their  relations  to  one  another, 
constitutes  the  science  of  animal  anatomy. 

This  may  naturally  be  divided  into : 

1.  Individual  anatomy,  the  object  of  which  is  the  investigation  of  the  construc- 
tion, form,  and  arrangement  of  the  organs  of  any  individual  animal. 

2.  Comparative  anatomy,  the  object  of  which  is  a  comparison  of  the  organs 
of  two  or  more  animals  of  different  species,  with  a  view  to  determining 
their  points  of  resemblance  or  dissimilarity. 

If  the  organs,  however,  are  subjected  to  a  further  analysis,  they  can  be 
resolved  into  simple  structures,  apparently  homogeneous,  to  which  the 
name  tissue  has  been  given — e.g.,  epithelial,  connective,  muscle,  and  nerve 
tissue.  When  the  tissues  are  subjected  to  a  microscopic  analysis,  it  is 
found  that  they  are  not  homogeneous  in  structure,  but  composed  of  still 
simpler  elements,  termed  cells  and  fibers.  The  investigation  of  the  inter- 
nal structure  of  the  organs,  the  physical  properties  and  structure  of  the 
tissues,  as  well  as  the  structure  of  their  component  elements,  the  cells  and 
fibers,  constitutes  a  department  of  anatomic  science  known  as  histology,  or 
as  it  is  prosecuted  largely  with  the  microscope,  microscopic  anatomy. 

Htmian  anatomy  is  that  department  of  anatomic  science  which  has 
for  its  object  the  investigation  of  the  construction  of  the  human  body. 

GENERAL  STRUCTURE  OF  THE  ANIMAL  BODY 

The  body  of  every  animal,  from  fish  to  man,  may  be  divided  into — 

1.  An  axial  portion,  consisting  of  the  head,  neck,  and  trunk;  and — 

2,  An  appendicular  portion,  consisting  of  the  anterior  and  posterior  limbs 
or  extremities. 

The  Axial  Portion. — ^The  axial  portion  of  all  mammals,  to  which  class 
man  zoologically  belongs,  as  well  as  of  all  birds,  reptiles,  amphibians,  and 
osseous  fish,  is  characterized  by  the  presence  of  a  bony,  segmented  axis, 
which  extends  in  a  longitudinal  direction  from  before  backward,  and  which 
is  known  as  the  vertebral  column  or  backbone.  In  virtue  of  the  existence 
of  this  column  all  the  classes  of  animals  just  mentioned  form  one  great 
division  of  the  animal  kingdom,  the  Vertehrata. 

Each  segment,  or  vertebra,  of  this  axis  consists  of — 
I.    A  solid  portion,  known  as  the  body  or  centrum,  and 


INTRODUCTION  3 

2.    A  bony  arch  arising  from  the  dorsal  aspect  and  surmounted  by  a  spine- 
like process. 

At  the  anterior  extremity  of  the  body  of  the  animal  the  vertebrae  are 
variously  modified  and  expanded,  and,  with  the  addition  of  new  elements, 
form  the  skull;  at  the  posterior  extremity  they  rapidly  diminish  in  size, 
and  terminate  in  man  in  a  short,  tail-like  process.  In  many  animals,  how- 
ever, the  vertebral  column  extends  for  a  considerable  distance  beyond  the 
trunk  into  the  tail.  The  vertebral  column  may  be  regarded  as  the  founda- 
tion element  in  the  plan  of  organization  of  all  the  higher  animals  and  the 
center  around  which  the  rest  of  the  body  is  developed  and  arranged  with  a 
certain  degree  of  conformity.  In  all  vertebrate  animals  the  bodies  of 
the  segments  of  the  vertebral  column  form  a  partition  which  serv^es  to 
divide  the  trunk  of  the  body  into  two  cavities — viz.,  the  dorsal  and  the 
ventral. 

The  Dorsal  Cavity. — The  dorsal  cavity  is  found  not  only  in  the  trunk, 
but  also  in  the  head.  Its  walls  are  formed  partly  by  the  arches  which  arise 
from  the  posterior  or  dorsal  surface  of  the  vertebrae  and  partly  by  the  bones 
of  the  skull.  If  a  longitudinal  section  can  be  made  through  the  center  of 
the  vertebral  column,  and  including  the  head,  the  dorsal  ca\dty  will  be 
observed  running  through  its  entire  extent.  Though  for  the  most  part  it  is 
quite  narrow,  at  the  anterior  extremity  it  is  enlarged  and  forms  the  cavity 
of  the  skull.  This  cavity  is  lined  by  a  membranous  canal,  the  neural  canal, 
in  which  are  contained  the  brain  and  the  neural  or  spinal  cord.  Through 
openings  in  the  sides  of  the  dorsal  cavity  nerves  pass  out  which  connect  the 
brain  and  spinal  cord  with  all  the  structures  of  the  body. 

The  Ventral  Cavity. — ^The  ventral  cavity  is  confined  mainly  to  the  trunk 
of  the  body.  Its  walls  are  formed  by  muscles  and  skin,  strengthened  in 
most  animals  by  bony  arches,  the  ribs.  Within  the  ventral  cavity  is  con- 
tained a  musculo-membranous  tube  or  canal  known  as  the  alimentary  or 
food  canal,  which  begins  at  the  mouth  on  the  ventral  side  of  the  head,  and, 
after  passing  through  the  neck  and  trunk,  terminates  at  the  posterior  ex- 
tremity of  the  trunk  at  the  anus.  It  may  be  di\aded  into  mouth,  pharynx, 
esophagus,  stomach,  small  and  large  intestines. 

In  all  mammals  the  ventral  cavity  is  divided  by  a  musculo-membranous 
partition  into  two  smaller  cavities,  the  thorax  and  abdomen.  The  former 
contains  the  lungs,  heart  and  its  great  blood-vessels,  and  the  anterior  part  of 
the  alimentary  canal,  the  gullet  or  esophagus;  the  latter  contains  the  con- 
tinuation of  the  alimentary  canal — that  is,  the  stomach  and  intestines 
— and  the  glands  in  connection  with  it,  the  liver  and  pancreas.  In  the 
posterior  portion  of  the  abdominal  cavity  are  found  the  kidneys,  ureters, 
and  bladder,  and  in  the  female  the  organs  of  reproduction.  The  thoracic 
and  abdominal  cavities  are  each  lined  by  a  thin  serous  membrane,  known, 
respectively,  as  the  pleural  and  peritoneal  membranes,  which,  in  addition, 
are  reflected  over  the  surfaces  of  the  organs  contained  within  them. 

The  Surfaces  of  the  Body. — The  external  surface  of  the  body  is  covered 
by  the  skin.  This  is  composed  of  an  inner  portion,  the  derma,  and  an 
outer  portion,  the  epidermis.  The  former  consists  of  connective-tissue 
fibers,  blood-vessels,  nerves,  etc.;  the  latter  of  layers  of  scales  or  cells.  Em- 
bedded within  the  skin  are  numbers  of  glands,  which  exude,  in  the  different 
classes  of  animals,  sweat,  oily  matter,  etc.     Projecting  from  the  surface  of 


4  TEXT-BOOK  OF  PHYSIOLOGY 

the  skin  are  hairs,  bristles,  feathers,  claws.     Beneath  the  skin  are  found 
muscles,  bones,  blood-vessels,  nerves,  etc. 

The  internal  surface — the  surface  of  the  alimentary  tract  and  associated 
cavities — is  covered  by  mucous  membrane. 

The  Appendicular  Portion. — ^The  appendicular  portion  of  the  body 
consists  of  two  pairs  of  symmetric  limbs,  which  project  from  the  sides  of 
the  trunk,  and  which  bear  a  determinate  relation  to  the  vertebral  column. 
They  consist  fundamentally  of  bones,  surrounded  by  muscles,  blood-vessels, 
nerves  and  lymphatics.  The  limbs,  though  having  a  common  plan  of 
organization,  are  modified  in  form  and  adapted  for  prehension  and  loco- 
motion in  accordance  with  the  needs  of  the  animal. 

Anatomic  Systems. — All  the  organs  of  the  body  which  have  certain 
peculiarities  of  structure  in  common  are  classified  by  anatomists  into 
systems — e.g.,  the  bones,  collectively,  constitute  the  bony  or  osseous  system; 
the  muscles,  the  nerves,  the  skin,  constitute,  respectively,  the  muscle,  the 
nerve,  and  the  tegumentary  systems. 

Physiologic  Apparatus. — More  important  from  a  physiologic  point 
of  view  than  a  classification  of  organs  based  on  similarities  of  structure 
is  the  natural  association  of  two  or  more  organs  acting  together  for  the 
accomplishment  of  some  definite  object,  and  to  which  the  term  physiologic 
apparatus  has  been  applied.  While  in  the  community  of  organs  which 
together  constitute  the  animal  body  each  one  performs  some  definite  function, 
and  the  harmonious  cooperation  of  all  is  necessary  to  the  life  of  the  individual, 
everywhere  it  is  found  that  two  or  more  organs,  though  performing  totally 
distinct  functions,  are  cooperating  for  the  accomplishment  of  some  larger 
or  compound  function  in  which  their  individual  functions  are  blended — e.g., 
the  mouth,  stomach,  and  intestines,  with  the  glands  connected  with  them, 
constitute  the  digestive  apparatus,  the  object  or  function  of  which  is  the  com- 
plete digestion  of  the  food.  The  capillary  blood-vessels  and  lymphatic 
vessels  of  the  body,  and  especially  those  in  relation  to  the  vilh  of  the  small 
intestine,  constitute  the  absorptive  apparatus,  the  function  of  which  is  the 
introduction  of  new  material  into  the  blood.  The  heart  and  blood-vessels 
constitute  the  circulatory  apparatus,  the  function  of  which  is  the  distribution 
of  blood  to  all  portions  of  the  body.  The  lungs  and  trachea,  together  with 
the  diaphragm  and  the  walls  of  the  chest,  constitute  the  respiratory  apparatus, 
the  function  of  which  is  the  introduction  of  oxygen  into  the  blood  and  the 
elimination  from  it  of  carbon  dioxid  and  other  injurious  products.  The 
kidneys,  the  ureters,  and  the  bladder  constitute  the  urinary  apparatus.  The 
skin,  with  its  sweat-glands,  constitutes  the  perspiratory  apparatus,  the  func- 
tions of  both  being  the  excretion  of  waste  products  from  the  body.  The 
liver,  the  pancreas,  the  mammary  glands,  as  well  as  other  glands,  each  form 
a  secretory  apparatus  which  elaborates  some  specific  material  necessary 
to  the  nutrition  of  the  individual.  The  functions  of  these  different  physio- 
logic apparatus — e.g.,  digestion,  absorption  of  food,  elaboration  of  blood, 
circulation  of  blood,  respiration,  production  of  heat,  secretion,  and  excretion 
— are  classified  as  nutritive  functions,  and  have  for  their  final  object  the 
preservation  of  the  individual  and  therefore  the  species. 

The  nerves  and  muscles  constitute  the  nervo-muscle  apparatus,  the 
function  of  which  is  the  production  of  motion.     The  eye,  the  ear,  the  nose, 


INTRODUCTION  5 

the  tongue,  and  the  skin,  with  their  related  structures,  constitute,  respec- 
tively, the  visual,  auditory,  olfactory,  gustatory,  and  tactile  apparatus,  the 
function  of  which,  as  a  whole,  is  the  reception  of  impressions  and  the  trans- 
mission of  nerve  impulses  to  the  brain,  where  they  give  rise  to  visual,  audi- 
tory, olfactory,  gustatory,  and  tactile  sensations  and  volitional  impulses. 

The  brain,  in  association  with  the  sense  organs,  forms  an  apparatus 
related  to  mental  processes.  The  larynx  and  its  accessory  organs— the 
lungs,  trachea,  respiratory  muscles,  the  mouth  and  resonant  cavities  of  the 
face — form  the  vocal  and  articulating  apparatus,  by  means  of  which  voice 
and  articulate  speech  are  produced.  The  functions  exhibited  by  the  ap- 
paratus just  mentioned — viz.,  motion,  sensation,  language,  mental  and 
moral  manifestations — are  classified  2^?,  functions  of  relation,  as  they  serve  to 
bring  the  individual  into  conscious  relationship  with  the  external  world. 

The  ovaries  and  the  testes  are  the  essential  reproductive  organs,  the 
fonner  producing  the  germ-cell,  the  latter  the  sperm  element.  Together 
with  their  related  structures — the  fallopian  tubes,  uterus,  and  vagina  in  the 
female,  and  the  urogenital  canal  in  the  male — they  constitute  the  repro- 
ductive apparatus  characteristic  of  the  two  sexes.  Their  cooperation  results 
in  the  union  of  the  germ-cell  and  sperm  element  and  the  consequent  develop- 
ment of  a  new  being.  The  function  of  reproduction  serves  to  perpetuate 
the  species  to  which  the  individual  belongs. 

The  animal  body  is  therefore  not  a  homogeneous  organism,  but  one 
composed  of  a  large  number  of  widely  dissimilar  but  related  organs.  As 
all  vertebrate  animals  have  the  same  general  plan  of  organization,  there  is  a 
marked  similarity  both  in  form  and  structure  among  corresponding  parts 
of  different  animals.  Hence  it  is  that  in  the  study  of  human  anatomy  a 
knowledge  of  the  form,  construction,  and  arrangement  of  the  organs  in 
different  types  of  animal  life  is  essential  to  its  correct  interpretation;  it 
follows  also  that  in  the  investigation  and  comprehension  of  the  complex 
problems  of  human  physiology  a  knowledge  of  the  functions  of  the  organs 
as  they  manifest  themselves  in  the  different  types  of  animal  life  is  indispen- 
sable. As  many  of  the  functions  of  the  human  body  are  not  only  complex, 
but  the  organs  exhibiting  them  are  practically  inaccessible  to  investigation, 
we  must  supplement  our  knowledge  and  judge  of  their  functions  by  analogy, 
by  attributing  to  them,  within  certain  limits,  the  functions  revealed  by 
experimentation  upon  the  corresponding  organs  of  lower  animals.  This 
experimental  knowledge,  corrected  by  a  study  of  the  clinical  phenomena  of 
disease  and  the  results  of  post-mortem  investigations,  forms  the  basis  of 
modern  human  physiology. 


CHAPTER  II 
CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY 

Since  it  has  been  demonstrated  that  every  exhibition  of  functional 
activity  is  associated  with  changes  of  structure,  it  has  been  apparent  that  a 
knowledge  of  the  chemic  composition  of  the  body,  not  only  when  in  a  state 
of  rest,  but  to  a  far  greater  degree  when  in  a  state  of  activity,  is  necessary  to 
a  correct  understanding  of  the  intimate  nature  of  physiologic  processes. 
Though  the  analysis  of  the  dead  body  is  comparatively  easy,  the  determina- 
tion of  the  successive  changes  in  composition  of  the  living  body  is  attended 
with  many  difficulties.  The  living  material,  the  bioplasm,  is  not  only 
complex  and  unstable  in  composition,  but  extremely  sensitive  to  all  physical 
and  chemic  influences.  The  methods,  therefore,  which  are  employed  for 
analysis  destroy  its  composition  and  vitality,  and  the  products  which  are 
obtained  are  peculiar  to  dead  rather  than  to  living  material. 

Chemic  analysis,  therefore,  may  be  directed — 

1.  To  the  determination  of  the  composition  of  the  dead  body. 

2.  To  the  determination  of  the  successive  changes  in  composition  which 
the  living  bioplasm  undergoes  during  functional  activity. 

A  chemic  analysis  of  the  dead  body,  with  a  view  to  disclosing  the  sub- 
stances of  which  it  is  composed,  their  properties,  their  intimate  structure, 
their  relationship  to  one  another,  constitutes  what  might  be  termed  chemic 
anatomy.  An  investigation  of  the  living  material  and  of  the  successive 
changes  it  undergoes  in  the  performance  of  its  functions  constitutes  what 
has  been  termed  chemic  physiology  or  physiologic  chemistry. 

By  chemic  analysis  the  animal  body  can  be  reduced  to  a  number  of 
liquid  and  solid  compounds  which  belong  to  both  the  inorganic  and  organic 
worlds.  These  compounds,  resulting  from  a  proximate  analysis,  have 
been  termed  proximate  principles.  That  they  may  merit  this  term,  how- 
ever, they  must  be  obtained  in  the  form  under  which  they  exist  in  the  living 
condition.  The  organic  compounds  consist  of  representatives  of  the  carbo- 
hydrate, fat,  and  protein  groups  of  organic  bodies;  the  inorganic  compounds 
consist  of  water,  various  acids,  and  inorganic  salts. 

The  compounds  or  proximate  principles  thus  obtained  can  be  further 
resolved  by  an  ultimate  analysis  into  a  small  number  of  chemic  elements 
which  are  identical  with  elements  found  in  many  other  organic  as  well  as 
inorganic  compounds.  The  different  chemic  elements  which  are  thus 
obtained,  and  the  percentages  in  which  they  exist  in  the  body,  are  as  follows 
— viz.,  oxygen,  72  per  cent.;  hydrogen,  9.10;  nitrogen,  2.5;  carbon,  13.50; 
phosphorus,  1.15;  calcium,  1.30;  sulphur,  0.147;  sodium,  o.io;  jjotassium, 
0.026;  chlorin,  0.085;  Auorin,  iron,  silicon,  magnesium,  iodine,  in  small  and 
variable  amounts. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  7 

THE  CARBOHYDRATES 

The  carbohydrate  compounds,  which  enter  into  the  composition  of  the 
animal  body,  are  mainly  starches  and  sugar.  In  many  respects  they  are 
closely  related,  and  by  appropriate  means  are  readily  converted  into  one 
another.  In  composition  they  consist  of  the  elements  carbon,  hydrogen, 
and  oxygen.  As  their  name  implies,  the  hydrogen  and  oxygen  are  present  in 
these  compounds  in  the  proportion  in  which  they  exist  in  water,  or  as  2  :  i. 
The  molecule  of  the  carbohydrates  above  mentioned  consists  of  either  six 
atoms  of  carbon  or  a  multiple  of  six;  in  the  latter  case  the  quantity  of 
hydrogen  and  oxygen  taken  up  by  the  carbon  is  increased,  though  the 
ratio  remains  unchanged. 

The  carbohydrates  may  be  divided  into  three  groups — viz.,  (i)  amylases, 
including  starch,  dextrin,  glycogen,  and  cellulose;  (2)  dextroses,  including 
dextrose,  levulose,  galactose;  (3)  saccharoses,  including  saccharose,  lactose, 
and  maltose.  According  to  the  number  of  carbon  atoms  entering  into  the 
second  group  (six),  they  are  frequently  termed  monosaccharids;  those  of 
the  third  group,  disaccharids — twice  six;  those  of  the  first  group,  poly- 
saccharids — multiples  of  six. 

Though  but  few  of  the  members  of  the  carbohydrate  group  are  con- 
stituents of  the  human  body,  many  are  constituents  of  the  foods;  on 
account  of  their  importance  in  this  respect,  and  their  relation  to  one 
another,  the  chemic  features  of  the  more  generally  consumed  carbohydrates 
will  be  stated  in  this  connection. 

Chemic  Composition. — A  chemic  analysis  of  the  carbohydrates  shows 
that  they  consist  of  carbon  hydrogen  and  oxygen  though  the  percentage 
of  these  elements  varies  somewhat  in  different  members  of  the  group. 
The  average  percentage  composition  of  several  carbohydrates  is  as  follows: 

C.  H.  O. 

Starch 44.44  6.17  49-39 

Dextrose 40.00  6.66  53-34 

Saccharose 42.10  6.44  51-46 

I.  AMYLOSES,  (CgHioOJn. 

Starch  is  widely  distributed  in  the  vegetable  world,  being  abundant 
in  the  seeds  of  the  cereals,  leguminous  plants,  and  in  the  tubers  and  roots 
of  many  vegetables.  It  occurs  in  the  form  of  microscopic  granules  which 
vary  in  size,  shape,  and  appearance,  according  to  the  plant  from  which 
they  are  obtained.  Each  granule  presents  a  nucleus,  or  hilum,  around 
which  is  arranged  a  series  of  eccentric  rings,  alternately  light  and  dark. 
The  granule  consists  of  an  envelope  and  stroma  of  cellulose,  containing 
in  its  meshes  the  true  starch  material — granulose.  Starch  is  insoluble  in 
cold  water  and  alcohol.  When  heated  with  water  up  to  7o°C.,  the  granules 
swell,  rupture,  and  liberate  the  granulose,  which  forms  an  apparent  solution; 
if  present  in  sufficient  quantity,  it  forms  a  gelatinous  mass  termed  starch 
paste.  On  the  addition  of  iodin,  starch  strikes  a  characteristic  deep  blue 
color;  the  compound  formed — iodid  of  starch — is  weak,  the  color  dis- 
appearing on  heating,  but  reappearing  on  cooling. 

Boiling  starch  with  dilute  sulphuric  acid  (25  per  cent.)  converts  it  into 
dextrose.  In  the  presence  of  vegetable  diastase  or  animal  ferments,  starch 
is  converted  into  maltose  and  dextrose,  two  forms  of  sugar. 


8  TEXT-BOOK  OF  PHYSIOLOGY 

Dextrin  is  a  substance  formed  as  an  intermediate  product  in  the  trans- 
formation of  starch  into  sugar  (maltose) .  There  are  at  least  two  principal 
varieties — erythrodextrin,  which  strikes  a  red  color  with  iodin,  and  achroodex- 
trin,  which  is  without  color  when  treated  with  this  reagent.  In  the  pure 
state  dextrin  is  a  yellow-white  powder,  soluble  in  water.  In  the  presence 
of  animal  ferments  erythrodextrin  is  converted  into  maltose. 

Glycogen  is  a  constituent  of  the  animal  liver,  and,  to  a  slight  extent, 
of  muscles,  0.5  to  0.9  per  cent.,  and  of  tissues  generally.  In  the  tissues  of  the 
embryo  it  is  especially  abundant.  When  obtained  in  a  pure  state  it  is  an 
amorphous,  white  powder.  It  is  soluble  in  water,  forming  an  opales- 
cent solution.  With  iodin  it  strikes  a  port-wine  color.  In  some  respects 
it  resembles  starch,  in  others  dextrin.  Like  vegetable  starch,  glycogen  or 
animal  starch  can  be  converted  by  dilute  acids  and  ferments  into  sugar 
(dextrose). 

Cellulose  is  the  basic  material  of  the  more  or  less  solid  framework 
of  plants.  It  is  soluble  in  ammoniacal  solution  of  cupric  oxid,  from  which 
it  can  be  precipitated  by  acids.  It  is  an  amorphous  powder.  Cellulose  is 
very  stable  toward  dilute  acids  and  alkalies.  It  is  this  property  which  is 
made  use  of  in  the  technical  preparation  of  cellulose,  in  order  to  free  it  from 
the  other  substances  present  in  the  plant  material.  When  cellulose  is 
treated  with  strong  sulphuric  acid,  and  after  disintegration  diluted  with 
water  and  then  boiled,  it  is  completely  hydrolyzed  to  dextrose. 

2.  DEXTROSES,  C,H„0,. 

Dextrose,  glucose,  or  grape-sugar  is  found  in  grapes,  most  sweet 
fruits,  and  honey,  and  as  a  normal  constituent  of  liver,  blood,  muscles, 
and  other  animal  tissues.  In  the  disease  diabetes  mellitus  it  is  found  also 
in  the  urine. 

When  obtained  from  any  source,  it  is  soluble  in  water  and  in  hot  alcohol, 
from  which  it  crystallizes  in  six-sided  tables  or  prisms.  As  usually  met  with, 
it  is  in  the  form  of  irregular,  warty  masses.  It  is  sweet  to  the  taste.  When 
examined  with  the  polariscope,  it  will  be  found  that  dextrose  turns  the  plane 
of  polarized  light  to  the  right.  It  is  therefore  termed  dextro-rotatory  and 
has  received  its  name  from  this  fact. 

It  has  for  a  long  time  been  known  that  when  sugar,  cupric  hydroxid,  and 
an  alkali — e.g.,  sodium  or  potassium  hydroxid- — are  present  in  solution,  the 
sugar  will  abstract  from  the  cupric  hydroxid  a  portion  of  its  oxygen,  thus  re- 
ducing it  to  a  lower  stage  of  oxidation  giving  rise  to  cuprous  oxid.  Sugar  has 
a  similar  action  on  both  silver  and  bismuth  salts.  On  this  property  of  sugar  a 
standard  solution  of  cupric  hydroxid  was  suggested  by  Fehling  which  may 
be  employed  for  both  qualitative  and  quantitative  tests  for  the  presence  of 
sugar  in  solution. 

Fehling' s  Test  Solution. — This  is  a  solution  of  cupric  hydroxid  made 
alkaline  by  an  excess  of  sodium  or  potassium  hydroxid  with  the  addition 
of  sodium  and  potassium  tartrate.  It  is  made  by  dissolving  cupric  sulphate 
34.64  grams,  potassium  hydroxid  125  grams,  sodium  and  potassium  tartrate 
173  grams,  in  distilled  water  sufficient  to  make  one  liter. 

The  reaction  is  expressed  by  the  following  equation : 

CuSO,  +  2KOH  =  Cu(OH)2  +  K2SO,. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  9 

The  object  of  the  sodium  and  potassium  tartrate  is  to  dissolve  the  cupric 
hydroxid  and  hold  it  in  solution. 

For  qualitative  analysis  it  is  only  necessary  to  boil  a  cubic  centimeter  of 
this  solution,  diluted  with  4  c.c.  of  water,  in  a  test-tube;  then  add  the  suspected 
solution  and  again  heat  to  the  boiling-point.  If  sugar  be  present,  the  cupric 
hydroxid  is  reduced  to  the  condition  of  a  cuprous  oxid,  which  shows  itself  as  a 
red  or  orange-yellow  precipitate.  The  color  of  the  precipitate  depends  on  the 
relative  excess  of  either  copper  or  sugar,  being  red  with  the  former,  orange 
or  yellow  with  the  latter.  The  delicacy  of  this  test  is  shown  by  the  fact  that 
a  few  minims  of  this  solution  will  detect  in  i  c.c.  of  water  the  -^-^  of  a  milli- 
gram of  sugar.     (Dextrose.) 

For  quantitative  analysis,  10  c.c.  of  Fehling's  solution,  diluted  with 
40  c.c.  of  water,  are  heated  in  a  porcelain  capsule,  to  which  the  suspected 
solution  is  cautiously  added  from  a  buret  until  the  blue  color  entirely  dis- 
appears. The  strength  of  this  solution  is  such  that  10  c.c.  is  decolorized 
by  50  milligrams  of  sugar  {dextrose).  Thus  if  0.8  c.c.  of  the  suspected  solu- 
tion, e.g.,  urine,  decolorizes  10  c.c.  of  FehHng's  solution,  then  it  contains  50 
milligrams  of  sugar,  from  which  the  percentage  of  sugar  in  the  urine  can  be 
determined. 

The  Fermentation  Test. — All  the  sugars  with  the  exception  of  lactose 
undergo  reduction  to  simpler  compounds,  mainly  alcohol  and  carbon  dioxid, 
under  the  action  of  the  yeast  plant,  Saccharomyces  cerevisia.  The  change 
with  dextrose  is  expressed  in  the  following  equation: 

C.HjoOg  =  2C2H6O+2CO2. 

Dextrose    =  Alcohol   +  Carbon 
Dioxid. 

About  95  per  cent,  of  the  dextrose  is  so  changed,  the  remaining  5  per 
cent,  yielding  secondary  products — succinic  acid,  glycerin,  etc.  As  a 
means  of  testing  any  solution  for  the  presence  of  sugar  this  method  may 
be  adopted.  It  is  generally  very  satisfactory.  From  the  quantity  of 
carbon  dioxid  and  alcohol  thus  produced  the  quantity  of  sugar  in  the  solu- 
tion may  be  determined. 

Levulose,  or  fruit-sugar,  is  found  in  association  with  dextrose  as 
a  constituent  of  many  fruits.  It  is  sweeter  than  dextrose  and  more  soluble 
in  both  water  and  dilute  alcohol.  From  alcoholic  solutions  it  crystallizes 
in  fine,  silky  needles,  though  it  usually  occurs  in  the  form  of  a  syrup. 

Levulose  is  distinguished  from  dextrose  by  its  property  of  turning 
the  plane  of  polarized  light  to  the  left;  the  extent  to  which  it  does  so,  how- 
ever, varies  with  the  temperature  and  concentration  of  the  solution.  For  this 
reason  it  is  turned  levulo-rotatory  and  has  received  its  name  from  this  fact. 

Under  the  influence  of  the  yeast  plant  it  slowly  undergoes  fermen- 
tation, yielding  the  same  products  as  dextrose.  It  also  has  a  reducing 
action  on  cupric  hydroxid. 

Galactose  is  obtained  by  boiling  milk-sugar  (lactose)  with  dilute  sul- 
phuric acid.  In  many  chemic  relations  it  resembles  dextrose.  It  is  less 
soluble  in  water,  however,  crystallizes  more  easily,  and  has  a  greater  dextro- 
rotatory power.     It  also  undergoes  fermentation  with  the  yeast  plant. 

3.  SACCHAROSES,  C.^Hj^G,!. 

Saccharose,  or  cane-sugar,  is  widely  distributed  throughout  the  vege- 
table world,  but  is  especially  abundant  in  sugar-cane,  sorghum  cane,  sugar- 


10  TEXT-BOOK  OF  PHYSIOLOGY 

beet,  Indian  corn,  etc.  It  crystallizes  in  large  monoclinic  prisms.  It  is 
soluble  in  water  and  in  dilute  alcohol.  Saccharose  has  no  reducing  power  on 
cupric  hydroxid,  and  hence  its  presence  cannot  be  detected  by  Fehling's 
solution.  It  is  dextro-rotatory.  Boiled  with  dilute  mineral,  as  well  as 
with  organic  acids,  saccharose  combines  with  water  and  undergoes  a  change 
in  virtue  of  which  it  rotates  the  plane  of  polarized  light  to  the  left,  and  hence 
the  product  was  termed  invert  sugar.  This  latter  has  been  shown  to  be  a 
mixture  of  equal  quantities  of  levulose  and  dextrose.  This  inversion  of 
saccharose  through  hydration  and  decomposition  is  expressed  in  the  fol- 
lowing equation: 

Saccharose  +  Water  =  Levulose  +  Dextrose 


Invert  Sugar. 


Saccharose  is  not  directly  fermentable  by  yeast,  but  through  the  specific 
action  of  a  ferment,  invertin  or  invertase,  secreted  by  the  yeast  plant,  or  the 
inverting  ferment  of  the  small  intestine,  it  undergoes  inversion,  as  pre- 
viously stated,  after  which  it  is  readily  fermented,  yielding  alcohol  and 
carbon  dioxid. 

Lactose  is  the  form  of  sugar  found  exclusively  in  the  milk  of  the  mam- 
malia, from  which  it  can  be  obtained  in  the  form  of  hard,  white,  rhombic 
prisms  united  with  one  molecule  of  water.  It  is  soluble  in  water,  insoluble 
in  alcohol  and  ether.  It  is  dextro-rotatory.  It  reduces  cupric  hydroxid, 
but  to  a  less  extent  than  dextrose.  Dilute  acids  decompose  it  into  equal 
quantities  of  dextrose  and  galactose.  Lactose  is  not  fermentable  with 
yeast,  but  in  the  presence  of  the  lactic  acid  bacillus  it  is  decomposed  into 
lactic  acid,  and  finally  into  butyric  acid,  as  expressed  in  the  following 
equation : 

C12H22O11  +    HjO    =  4C3H8O3 
Lactose      +  Water    ■=  Lactic  Acid. 

2C3H6O3    =       C.HsO,     +     2CO2     +     2R. 
Lactic  Acid  =  Buytric  Acid  +     Carbon      -t-      Free 

Dioxid        Hydrogen. 

Maltose  is  a  transformation  product  of  starch,  and  arises  whenever 
the  latter  is  acted  on  by  malt  extract  or  the  diastatic  ferments  in  saliva  and 
pancreatic  juice.     The  change  is  expressed  by  the  following  equation: 

sCgHioOj     +     HjO     =     CijHjzOi, 

Starch.  Water.  Maltose. 

Maltose  crystallizes  in  the  form  of  white  needles,  which  are  soluble  in 
water  and  in  dilute  alcohol.  It  is  dextro-rotatory.  In  the  presence  of 
ferments  and  dilute  acids  maltose  undergoes  hydration  and  decomposition, 
giving  rise  to  two  molecules  of  dextrose.  It  has  a  reducing  action  on  cupric 
hydroxid.  Fermentation  is  readily  caused  by  yeast,  but  whether  directly 
or  indirectly  by  inversion  is  somewhat  uncertain, 

Osazones. — All  the  sugars  which  possess  the  power  of  reducing  cupric 
hydroxid  are  capable  of  combining  with  phenyl-hydrazin,  with  the  formation 
of  compounds  termed  osazones.  The  osazones  so  formed  are  crystalline 
in  structure,  but  have  different  melting-points,  varying  degrees  of  solubility 
and  optic  properties,  all  of  which  serve  to  detect  the  various  sugars  and  to 
distinguish  one  from  the  other.     Of  the  different  osazones,  phenyl-gluco- 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  ii 

sazone  is  the  most  characteristic,  and  occurs  in  the  form  of  long,  yellow 
needles.  It  may  be  obtained  from  dextrose  by  the  following  method: 
To  50  c.c.  of  a  dextrose  solution  add  2  gm.  of  phenyl-hydrazin  and  2  gm. 
of  sodium  acetate,  and  boil  for  an  hour.  On  cooling,  the  osazone  crystal- 
lizes in  the  form  of  long,  yellow  needles. 

THE  FATS 

The  fats  constitute  a  group  of  organic  bodies  found  in  the  tissues  of 
both  vegetables  and  animals.  In  the  vegetable  world  they  are  largely 
found  in  fruits,  seeds,  and  nuts,  where  they  probably  originate  from  a 
transformation  of  the  carbohydrates.  In  the  animal  body  the  fats  are 
found  largely  in  the  subcutaneous  tissue,  in  the  marrow  of  bones,  in  and 
around  various  internal  organs  and  in  milk.  In  these  situations  fat  is 
contained  in  small,  round  or  polygon-shaped  vesicles,  which  are  united  by 
areolar  tissue  and  surrounded  by  blood-vessels.  At  the  temperature  of  the 
body  the  fat  is  liquid,  but  after  death  it  soon  solidifies  from  the  loss  of  heat. 

The  fats  are  compounds  consisting  of  carbon,  hydrogen,  and  oxygen. 
The  percentage  composition  of  fat  (stearin)  is  as  follows:  Carbon,  76.86; 
hydrogen,  12.36;  oxygen,  10.78.  The  fat  found  in  animals  is  a  mixture,  in 
varying  proportions  in  different  animals,  of  three  neutral  fats — stearin, 
palmitin,  and  olein.  Each  fat  is  a  derivative  of  glycerin  and  the  particular 
acid  indicated  by  its  name — e.g.,  stearic  acid,  in  the  case  of  stearin,  etc. 
The  reaction  which  takes  place  in  the  combination  of  glycerin  and  the 
acid  is  expressed  in  the  following  equation: 

C3H3(OH)3     +     -sH.CisHsjO^    =    C,B.,{C,,B.,,0,),     +     3H2O. 
Glycerin.  Stearic  Acid.  Stearin.  Water. 

Hence,  strictly  speaking,  the  fats  are  compound  ethers,  in  which  the 
hydrogen  of  the  organic  acid  is  replaced  by  the  trivalent  radical,  tritenyl, 

Stearin,  C3H5(C^8H3502)3,  is  the  chief  constituent  of  the  more  solid 
fats.  It  is  solid  at  ordinary  temperatures,  melting  at  55°C.,  then  solidify- 
ing again  as  the  temperature  rises,  until  at  7i°C.  it  melts  permanently.  It 
crystallizes  in  square  tables. 

Palmitin,  C3H.(CigH3iO,)3  is  a  semifluid  fat,  solid  at  45°C.  and 
melting  at  62''C.     It  crystallizes  in  fine  needles,  and  is  soluble  in  ether. 

Olein,  C3H-(Ci8H3302)3,  is  a  colorless,  transparent  fluid,  liquid  at 
ordinary  temperatures,  only  solidifying  at  o°C.  It  possesses  marked 
solvent  powers,  and  holds  stearin  and  palmitin  in  solution  at  the  temperature 
of  the  body. 

Saponification. — When  subjected  to  the  action  of  superheated  steam, 
a  neutral  fat  is  saponified — i.e.,  decomposed  into  glycerin  and  the  particular 
acid  indicated  by  the  name  of  the  fat  used:  e.g.,  stearic,  palmitic,  or  oleic. 
The  reaction  is  expressed  as  follows: 

C3H,(C„H3302)3+3H,0  =  C3H5(OH)3  +  3Ci3H3,02 
Olein.  Water.         Glycerin,         Oleic  Acid. 

The  fat  acids  thus  obtained  are  characterized  by  certain  chemic  fea- 
tures, as  follows: 

Stearic  acid  is  a  firm,  white  solid,  fusible  at  69°C.  It  is  soluble  in 
ether  and  alcohol,  but  not  in  water. 


12  TEXT-BOOK  OF  PHYSIOLOGY 

Palmitic  acid  occurs  in  the  form  of  white,  glistening  scales  or  needles, 
melting  at  62°C. 

Oleic  acid  is  a  clear,  colorless  liquid,  tasteless  and  odorless  when  pure. 
It  crystallizes  in  white  needles  at  o°C. 

If  this  saponification  takes  place  in  the  presence  of  an  alkali — e.g., 
potassium  hydroxid  or  sodium  hydroxid — the  acid  produced  combines 
at  once  with  the  alkali  to  form  a  salt  known  as  a  soap,  while  the  glycerin 
remains  in  solution.     The  reaction  is  as  follows: 

3KHO    +  sC.sH^^O,  =    3KC,,1I,,0,     +    3H,0 
Potassium  Hydroxid.      Oleic  Acid.        Potassium  Oleate.  Water. 

All  soaps  are,  therefore,  salts  formed  by  the  union  of  alkalies  and  fat 
acids.  The  sodium  soaps  are  generally  hard,  while  the  potassium  soaps 
are  soft.  Those  made  with  stearin  and  palmitin  are  harder  than  those 
made  with  olein.  If  the  soap  is  composed  of  lead,  zinc,  copper,  etc.,  it  is 
insoluble  in  water. 

Emulsification. — ^When  a  neutral  oil  is  vigorously  shaken  with  water 
or  other  fluid,  it  is  broken  up  into  minute  globules  that  are  more  or  less 
permanently  suspended;  the  permanency  depending  on  the  nature  of  the 
liquid.  The  most  permanent  emulsions  are  those  made  with  soap  solutions. 
The  process  of  emulsification  and  the  part  played  by  soap  can  be  readily 
observed  by  placing  on  a  few  cubic  centimeters  of  a  solution  of  sodium 
carbonate  (0.25  per  cent.)  a  small  quantity  of  a  perfectly  neutral  oil  to  which 
has  been  added  2  or  3  per  cent,  of  a  fat  acid.  The  combination  of  the  acid 
and  the  alkali  at  once  forms  a  soap.  The  energy  set  free  by  this  combination 
rapidly  divides  the  oil  into  extremely  minute  globules.  A  spontaneous 
emulsion  is  thus  formed. 

THE  PROTEINS 

The  proteins  constitute  a  group  of  organic  bodies  which  are  found  in  both 
vegetable  and  animal  tissues.  Though  present  in  all  animal  tissues,  they 
are  especially  abundant  in  muscles  and  bones,  where  they  constitute  20 
per  cent,  and  30  per  cent,  respectively.  Though  genetically  related,  and 
possessing  many  features  in  common,  the  different  members  of  the  protein 
group  are  distinguished  by  characteristic  physical  and  chemic  properties 
which  serve  not  only  for  their  identification,  but  for  their  classification  into 
more  or  less  well-defined  groups. 

Chemic  Composition. — A  chemic  analysis  of  proteins  shows  that 
they  consist  of  carbon,  hydrogen,  oxygen,  nitrogen  and  sulphur,  though 
the  percentage  of  each  of  these  elements  varies  somewhat  in  the  different 
proteins. 

A  certain  number  of  proteins  contain  phosphorus  while  almost  all 
of  them  contain  different  inorganic  salts  in  varying  amounts.  The  average 
percentage  composition  of  several  proteins  is  shown  in  the  following  analyses: 

C.        H.       N.  O.       S. 

Egg-albumin 52.9  7.2  15.6  23.9  0.4    (Wiirtz). 

Serum-albumin 53.0  6.8  16.0  22.29  i  .77  (Hammers  ten). 

Casein 52.3  7-o7  iS-9i  22.03  0.82  (Chittenden  and  Painter). 

Myosin ..    52.82  7.11  16.77  21.90  i  .27  (Chittenden  and  Cummins). 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  13 

The  molecular  composition  of  the  proteins  is  not  definitely  known 
and  the  formulae  which  have  been  suggested  are  therefore  only  approxi- 
mative. Leow  assigns  to  albumin  the  formula  C72Hji2Ni8022S,  while 
Schiitzenberger  raises  the  numbers  to  C240H392N65O75S3,  either  of  which 
shows  that  the  protein  molecule  is  extremely  complex. 

Structure  of  the  Protein  Molecule. — ^From  the  large  size  of  the  protein 
molecule  as  indicated  by  its  chemic  composition  it  might  be  inferred  that 
its  structure  was  equally  complex.  This,  modern  imestigation  has  shown 
to  be  the  case. 

When  any  one  of  the  typical  proteins,  found  in"  animal  or  vegetable 
tissues,  is  hydrolyzed  by  acids,  alkalies  and  animal  ferments  under  appro- 
priate conditions,  it  can  be  resolved  through  a  series  of  descending  stages 
into  relatively  simple  nitrogen-holding  bodies  termed  amino-acids  and 
diamino-acids ,  of  which  somewhat  more  than  twenty  have  been  isolated 
and  their  properties  determined.  The  principal  amino-acids  are  as  follows: 
Glycocoll,  alanin,  leucin,  isoleucin,  amino-isovalerianic  acid,  serin,  aspartic 
acid,  glutamic  acid,  phenylalanin,  tyrosin,  prolin,  tryptophan.  The  principal 
diamino-acids  are  as  follows:     Ornithin,  lysin,  histidin,  arginin,  cystin. 

The  protein  molecule  is  therefore  structurally  complex.  The  manner 
in  which  these  elementary  compounds  are  arranged,  united  or  grouped 
in  any  given  protein,  is  practically  unknown.  More  or  less  successful 
attempts  have  been  made  at  the  reconstruction  of  the  protein  molecule  by 
synthetic  methods,  by  the  union  of  two  or  more  of  the  amino-acids.  A 
number  of  such  compounds  have  been  formed  by  the  union  of  from  two  to 
ten  or  more  amino-acids,  all  of  them  exhibiting  many  of  the  protein  reac- 
tions. Such  bodies  are  termed,  according  to  their  complexity,  peptids  and 
polypeptids. 

Physical  Properties. — As  a  class  the  proteins  are  characterized  by  the 
following  properties: 

1.  Indiffusibility. — None  of  the  proteins  normally  assume  the  crystalline 

form,  and  hence  they  are  not  capable  of  diffusing  through  parchment 
or  an  animal  membrane.  Peptone,  a  product  of  the  digestion  of 
proteins,  is  an  exception  as  regards  its  diffusibility.  As  met  with  in 
the  body,  all  proteins  are  amorphous,  but  vary  in  consistence  from  the 
liquid  to  the  sohd  state.  The  colloid  character  of  the  proteins  permits 
of  their  separation  and  purification  from  crystalloid  diffusible  com- 
pounds by  the  process  of  dialysis. 

2.  Solubility. — Some  of  the  proteins  are  soluble  in  water,  others  in  solutions 

of  the  neutral  salts  of  varying  degrees  of  concentration,  in  strong  acids 
and  alkalies.     All  are  insoluble  in  alcohol  and  ether. 

3.  Coagulability.— Under  the  influence   of  heat  and   animal   ferments, 

some  of  the  proteins  readily  pass  from  the  soluble  liquid  state  to  the 
insoluble  solid  state,  attended  by  a  permanent  alteration  in  their  chemic 
composition.  To  this  change  the  term  coagulation  has  been  given. 
The  various  proteins,  however,  coagulate  at  different  temperatures. 
Proteins  are  capable  of  precipitation  without  losing  their  solubility 
by  ammonium  sulphate,  sodium  chlorid,  and  magnesium  sulphate. 

4.  Fermentability. — In  the  presence  of  specific  microorganisms — bacteria 

— the  proteins,  owing  to  their  complexity  and  instability,  are  prone 


14  TEXT-BOOK  OF  PHYSIOLOGY 

to  undergo  disintegration  and  reduction  to  simpler  compounds.     This 
decomposition  or  putrefaction  occurs  most  readily  when  the  conditions 
most  favorable  to  the  growth  of  bacteria  are  present — viz.,  a  temperature 
varying  from  25°C.  to  4o°C.,  moisture,  and  oxygen.     The  intermediate 
as  well  as  the  terminal  products  of  the  decomposition  of  the  proteins  are 
numerous,  and  vary  with  the  composition  of  the  protein  and  the  specific 
physiologic  action  of  the  bacteria.     Among  the  intermediate  products 
is  a  series  of  alkaloid  bodies,  some  of  which  possess  marked  toxic 
properties,  known  as  ptomains.     The  toxic  symptoms  which  frequently 
follow  the  ingestion  of  foods  in  various  stages  of  putrefaction  are  to 
be  attributed  to  these  compounds.     The  terminal  products  are  repre- 
sented by  hydrogen  sulphid,  ammonia,  carbon  dioxid,  fats,  phosphates, 
nitrates,  etc. 
Classification. — The   animal   proteins   by   virtue   of   their  structural 
composition,  their  physical  and  chemic  properties,  permit  of  a  provisional 
arrangement   into   three   groups    as   follows:   Simple   proteins,   conjugate 
proteins  and  protein  derivatives. 

SIMPLE  PROTEINS 

The  simple  proteins  are  so  called  because  of  the  fact  that  when  they 
are  hydrolyzed  they  yield  only  a-amino-  and  diamino-acids.  The  members 
of  this  group  are  as  follows: 

PROTAMINS. 

These  proteins  are  derived  for  the  most  part  from  the  heads  of  the  sper- 
matozoa of  fish.  They  take  their  names  from  the  species  of  fish  from 
which  they  are  obtained,  e.g.,  salmin  (salmon),  sturin  (sturgeon),  scom- 
brin  (mackerel),  etc.  Inasmuch  as  they  respond  to  Piotrowski's  test  in  a 
characteristic  way  they  are  regarded  as  true  proteins.  When  subjected  to 
hydrolysis  they  can  be  resolved  into  the  diamino  bodies,  lysin,  arginin  and 
histidin,  of  which  they  constitute  about  90  per  cent.,  and  a  small  number  of 
the  mono-amino-acids.  Because  of  the  fact  that  the  diamino  bodies,  lysin, 
histidin  and  arginin  contain  6  atoms  of  carbon  they  are  known  as  the  hexone 
bases.  Inasmuch  as  the  protamins  contain  practically  but  these  three 
bodies,  they  are  regarded  as  the  simplest  of  all  the  proteins.  Since  a  typical 
protein  always  yields  on  hydrolysis  the  hexone  bases,  in  addition  to  a  variable 
number  of  mono-amino-acids,  it  is  believed  that  the  usual  protein  is  com- 
posed of  a  nucleus  of  the  hexone  bases  to  which  is  attached  a  variable 
number  of  mono-amino-acids.  The  proportions  in  which  the  bases  exist 
in  the  nucleus  and  the  proportions  in  which  the  amino-acids  are  united  to 
the  nucleus,  vary  in  different  proteins. 

HISTONS. 

The  proteins  embraced  in  this  class  comprise  a  series  of  compounds 
which  are  somewhat  more  complex  than  the  protamins  and  less  complex 
than  the  typical  proteins;  for  on  hydrolysis  they  not  only  yield  the  hexone 
bases  but  in  addition  a  large  number  of  mono-amino-acids.  They  are,  there- 
fore, intermediate  in  structural  composition  between  the  protamins  and  the 
usual  proteins.  Their  protein  character  is  indicated  by  their  reaction  to 
Millon's  reagent  and  to  Piotrowski's  test.     They  are  precipitated  from 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  15 

neutral  solution  by  alkaloid  reagents.  The  histons  are  usually  found 
in  combination  with  nucleic  acid,  in  the  spermatozoa  of  most  animals  and 
especially  in  fish,  and  in  the  coloring  matter  (the  hemoglobin)  of  the  red 
corpuscles.  The  proteins  of  the  tissues  usually  contain  from  25  to  30  per 
cent,  of  histons. 

ALBUMINS. 

The  members  of  this  group  are  soluble  in  water,  in  dilute  saline  solu- 
tions, and  in  saturated  solutions  of  sodium  chlorid  and  magnesium  sulphate. 
They  are  coagulated  by  heat,  and  when  dried  form  an  amber-colored  mass. 

(a)  Serum-albumin. — This  most  important  protein  is  found  in  blood, 
lymph,  chyle,  and  some  tissue  fluids.  It  is  obtained  readily  by 
precipitation  from  blood-serum,  after  the  other  proteins  have  been 
removed,  on  the  addition  of  ammonium  sulphate.  When  freed 
from  saline  constituents,  it  presents  itself  as  a  pale,  amorphous  sub- 
stance, soluble  in  water  and  in  strong  nitric  acid.  It  is  coagulated 
at  a  temperature  of  73°C.,  as  well  as  by  various  acids — e.g.,  citric, 
picric,  nitric,  etc.     It  has  a  rotatory  power  of  —62.6°. 

(b)  Egg-albtunin. — Though  not  a  constituent  of  the  human  body,  egg- 
,      albumin  resembles  the  foregoing  in  many  respects.     When  obtained 

in  the  solid  form  from  the  white  of  the  egg,  it  is  a  yellow  mass  without 
taste  or  odor.  Though  similar  to  serum-albumin,  it  differs  from 
it  in  being  precipitated  by  ether,  in  coagulating  at  54°C.,  and  in 
having  a  lower  rotatory  power,  —35.5°. 

(c)  Lact-albumin. — As  its  name  imphes,  this  protein  is  found  in  milk. 
It  can  be  precipitated  from  milk-plasma  by  sodium  sulphate  after 
the  precipitation  of  the  other  proteids  by  half  saturation  with  am- 
monium sulphate.     It  slowly  coagulates  at  77°C. 

{d)  Myo-albumin. — This  protein  is  found  in  muscle-plasma  from 
which  it  subjects  the  plasma  to  fractional  heat  coagulation.  At 
73°C.  myo-albumin  coagulates. 

GLOBULINS. 

{a)  Serirni-globulin  or  Paraglobulin. — This  protein,  as  its  name 
implies,  is  found  in  blood-serum,  though  it  is  present  in  other  animal 
fluids.  When  precipitated  by  magnesium  sulphate  or  carbon  dioxid, 
it  presents  itself  as  a  flocculent  substance,  insoluble  in  water,  soluble 
in  dilute  acids  and  alkalies,  and  coagulating  at  75°C. 

(b)  Fibrinogen. — This  protein  is  found  in  blood-plasma  in  association 
with  serum-globulin  and  serum-albumin.  It  is  also  present  in 
lymph-tissue  fluids  and  in  pathologic  transudates.  It  can  be  ob- 
tained from  blood-plasma  which  has  been  previously  treated  with 
magnesium  sulphate  on  the  addition  of  a  saturated  solution  of  sodium 
chlorid.  It  is  soluble  in  dilute  acids  and  alkalies,  and  coagulates 
at  56°C. 

(c)  Paramyosinogen  or  Myosin. — This  protein  is  a  constituent  of  the 
muscle-plasma  from  which  it  can  be  precipitated  by  a  temperature 
of  47°C. 

(d)  Myosinogen  or  Myogen. — This  protein  is  the  chief  constituent  of 
the  muscle-plasma  and  is  of  great  nutritive  value.  During  the 
living  condition  it  is  liquid,  but  after  death  it  readily  undergoes  a 


1 6  TEXT-BOOK  OF  PHYSIOLOGY 

chemic  change  and  contributes  to  the  formation  of  an  insoluble 
protein  known  as  myogen  fibrin.     It  is  soluble  in  dilute  hydrochloric 
acid  and  dilute  alkalies.     It  coagulates  at  56°C. 
(e)  Crystallin  or  Globulin. — This  is  obtained  by  passing  a  stream 
of  CO2  through  a  watery  extract  of  the  crystalline  lens. 

SCLERO-PROTEINS  (ALBUMINOIDS). 

The  sclero-proteins  constitute  a  group  of  substances  similar  to  the  pro- 
teins in  many  respects,  though  differing  from  them  in  others.  When  ob- 
tained from  the  tissues,  in  which  they  form  an  organic  basis,  they  are  found 
to  be  amorphous,  colloid,  and  when  decomposed  yield  products  similar 
to  those  of  the  true  proteins.  The  principal  members  of  this  group  are  as 
follows: 

(a)  Collagen,  Ossein. — These  are  two  closely  allied,  if  not  identical, 
substances,  found  respectively  in  the  white  fibrous  connective  tissue 
and  in  bone.  When  the  tendons  of  muscles,  the  ligaments,  or  de- 
calcified bone  are  boiled  for  several  hours,  the  collagen  and  ossein 
are  converted  into  soluble  gelatin,  which,  when  the  solution  cools, 
becomes  solid. 

(b)  Chondrigen. — This  is  supposed  to  be  the  organic  basis  of  the  more 
permanent  cartilages.  When  the  latter  are  boiled,  they  yield  a 
substance  which  gelatinizes  on  cooling,  and  to  which  the  name 
chondrin  has  been  given.  Chondrin,  however,  is  not  a  pure  gelatin, 
but  has  associated  with  it  a  compound  protein  known  as  chrondro- 
mucoid. 

(c)  Elastin  is  the  name  given  to  the  substance  composing  the  fibers  of 
the  yellow,  elastic  connective  tissue. 

(d)  Keratin  is  the  substance  found  in  all  horny  and  epidermic  tissues, 
such  as  hairs,  nails,  scales,  etc.  It  differs  from  most  proteins  in  con- 
taining a  high  percentage  of  sulphur. 

PHOSPHO  -PROTEINS. 

The  two  members  of  this  group  are  distinguished  by  yielding  on  decom- 
position a  protein  which  contains  phosphorus.  It  was  formerly  regarded 
as  a  nuclein. 

(a)  Caseinogen. — This  is  the  principal  protein  of  milk,  in  which  it 
exists  in  association  with  calcium  in  a  form  known  as  calcium- 
caseinogenate.  It  is  precipitated  by  acetic  acid  and  by  magnesium 
sulphate.  It  is  coagulated  by  rennin,  though  the  nature  of  the  process 
is  not  very  clear.  It  was  formerly  taught  that  under  the  action  of 
rennin,  an  enzyme  of  the  gastric  mucous  membrane,  caseinogen  was 
separated  into  a  solid  portion,  casein  or  tyrein,  and  a  soluble  portion. 
The  cleavage  action  of  rennin  thus  indicated  has  not  been  verified  by 
subsequent  investigations.  It  is  more  in  accordance  with  the  facts  to 
assume  that  the  process  is  a  double  one  and  that  the  action  of  rennin  is 
to  change  the  caseinogen  to  a  soluble  form,  termed  paracasein,  after 
which  the  lime  salts  present  react  with  the  paracasein  in  such  a 
manner  as  to  cause  it  to  assume  the  solid  condition.  Calcium 
phosphate  seems  to  be  the  natural  alkali  necessary  to  this  process,  for 
if  it  be  removed  by  dialysis,  or  precipitated  by  the  addition  of  potassium 
oxalate,  the  casein  will  not  be  curdled  by  rennin,  but  it  will  neverthe- 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  17 

less,  undergo  a  change,  for  if  the  solution  be  treated  as  above  and  then 
boiled  to  destroy  the  rennin,  it  will  curdle  upon  the  addition  of  calcium 
salts. 
{b)  Vitellin. — ^Vitellin  is  a  constituent  of  the  vitellis  or  yolk  of  eggs. 
It  differs  from  other  proteins  in  the  fact  that  it  is  semicrystalline 
in  character.  Though  usually  regarded  as  a  nucleo-protein  it  is 
not  definitely  known  whether  or  not  it  contains  phosphorus  in  its 
composition. 

CONJUGATED  OR  COMBINED  PROTEINS 

The  conjugated  proteins  are  compounds  in  which  the  protein  molecule  is 
combined  with  some  other  molecule  or  molecules,  the  chemic  nature  of 
which  varies  considerably  in  the  different  members  of  the  group,  e.g., 
coloring  matter,  carbohydrates  and  nuclein.  The  chemic  character  of 
the  non- protein  substance  furnishes  the  basis  for  the  following  classification: 

CHROMO-PROTEINS. 

(a)  Hemoglobin. — Hemoglobin  is  the  coloring  matter  of  the  red  cor- 
puscles, of  which  it  constitutes  about  30  per  cent,  of  the  total  weight. 
It  possesses  the  power  of  absorbing  oxygen  as  it  passes  through  the 
lung  capillaries  and  of  yielding  it  up  to  the  tissues  as  it  passes  through 
the  tissue  capillaries.  In  the  arterial  blood  it  is  known  as  oxy- 
hemoglobin, and  in  the  venous  blood  as  deoxy-  or  reduced-hemoglobin. 
When  hydrolyzed  by  acids  or  alkalies,  hemoglobin  undergoes  a 
cleavage  into  a  protein,  globin,  and  a  coloring  matter,  hemochromogen, 
containing  iron,  which  is  easily  oxidized  to  hematin. 

(b)  Myohematin. — Myohematin  is  a  protein  supposed  to  be  present  in 
m.uscle.  It  has  never  been  isolated,  hence  its  chemic  features  are 
unknown.  Spectroscopic  examination  indicates  that  it  is  capable  of 
absorbing  and  again  yielding  up  oxygen.  For  this  reason  it  is  believed 
to  be  a  derivative  of  hemoglobin. 

GLUCO-PROTEINS. 

(a)  Mucin. — Mucin  is  the  protein  which  gives  the  mucus,  secreted  by 
the  epithelial  cells  of  the  mucous  membranes  and  related  glands, 
its  viscid,  tenacious  character.  It  is  also  a  constituent  of  the  inter- 
cellular substance  of  the  connective  tissues.  It  is  readily  precipitated 
by  acetic  acid.  When  heated  with  dilute  acids,  mucin  undergoes  a 
cleavage  into  a  simpler  protein  and  a  carbohydrate  termed  mucosa, 
which  is  capable  of  reducing  Fehling's  solution. 

(b)  Mucoids. — The  mucoids  resemble  the  mucins  though  differing  from 
them  in  solubility  and  in  not  being  precipitable  from  alkaline  solutions 
by  acetic  acid.  They  are  found  in  the  vitreous  humor,  white  of  egg, 
cartilage,  tendon,  bone,  and  in  other  situations.  Chondromucoid 
differs  from  other  connective-tissue  mucoids  in  the  large  amount  of 
chondroitin-sulphuric  acid  obtained  upon  decomposition.  When 
decomposed  through  the  action  of  acid,  the  chondroitin-sulphuric 
acid  yields  sulphuric  acid  and  a  nitrogenous  body — chondroitin,  which 
in  turn  yields  acetic  acid  and  a  new  nitrogenous  substance — chon- 
drosin,  which  has  a  more  strongly  reducing  action  on  Fehling's  solution 


1 8  TEXT-BOOK  OF  PHYSIOLOGY 

than  dextrose.  They  differ  slightly  one  from  the  other  in  proper- 
ties and  chemic  composition.  They  yield  on  decomposition  a 
carbohydrate. 

NUCLEO-PROTEINS. 

The  nucleo-proteins  are  obtained  from  the  nuclei  and  cell-substance 
of  tissue-cells.  Chemically  they  are  characterized  by  the  presence  of 
phosphorus  in  relatively  large  amounts.  When  hydrolyzed,  they 
separate  into  a  protein  and  a  nuclein.  The  nucleins  derived  from 
cell  nuclei  can  be  still  further  separated  into  a  simpler  protein  and 
nucleic  acid,  which  latter  in  turn  yields  phosphoric  acid,  carbohy- 
drates— mostly  pentoses,  pyrimidine  bases — thymine,  cytosine,  and 
uracyl,  and  the  so-called  purin  bases,  xanthin,  hypoxanthin,  adenin, 
and  guanin.  All  nucleins  which  yield  the  purin  bases  are  termed 
true  nucleins. 

DERIVATIVES  OF  PROTEIN 

The  protein  derivatives  include  a  variety  of  substances  which  arise 
through  a  process  of  hydrolysis  of  simple  proteins  under  the  action  of  enzymes 
and  in  the  presence  of  acids  and  alkalies. "  The  number  of  derivatives 
obtained  between  the  first  cleavage  of  the  protein  molecule  and  its  final 
cleavage  to  amino-acids  is  large  and  will  be  presented  at  length  in  the  para- 
graph relating  to  protein  digestion.     The  chief  derivatives  are  as  follows: 

INFRA-PROTEINS. 

{a)  Acid-albumin. — This  is  formed  when  a  native  albumin  is  digested 
with  dilute  hydrochloric  acid  (0.2  per  cent.)  or  dilute  sulphuric  acid 
for  some  minutes.  It  is  precipitated  by  neutralization  with  sodium 
hydroxid  (o.i  percent,  solution).  After  the  precipitate  is  washed, 
it  is  found  to  be  insoluble  in  distilled  water  and  in  neutral  saline  solu- 
tions.    In  acid  solutions  it  is  not  coagulated  by  heat. 

(b)  Alkali-albumin. — This  is  formed  when  a  native  albumin  is  treated 
with  a  dilute  alkali — e.g.,  o.i  per  cent,  of  sodium  hydroxid — for  five 
or  ten  minutes.  On  careful  neutralization  with  dilute  hydrochloric 
acid,  it  is  precipitated.  It  is  also  insoluble  in  distilled  water  and  in 
saline  solutions;  it  is  not  coagulable  by  heat. 

PROTEOSES,  PEPTONES  AND  POLYPEPTIDS. 

During  the  progress  of  the  digestive  process,  as  it  takes  place  in  the  stom- 
ach and  intestines,  there  is  produced  by  the  action  of  the  gastric  and  pan- 
creatic juices,  out  of  the  proteins  of  the  food,  a  series  of  new  proteins, 
known  as  proteoses,  peptones  and  polypeptids.  The  chemic  properties  of 
these  substances  will  be  considered  in  connection  with  the  process  of  digestion. 

COAGULATED  PROTEINS. 

Although  these  proteins  are  not  found  as  constituents  of  the  animal 
organism,  they  possess  much  interest  on  account  of  their  relation  to  prepared 
foods  and  to  the  digestive  process.  They  are  produced  when  solutions  of 
egg-albumin,  serum-albumin,  or  globulins  are  subjected  to  a  temperature 
of  ioo°C.  or  to  the  prolonged  action  of  alcohol.  They  are  insoluble  in 
water,  in  dilute  acids,  and  in  neutral  saline  solutions. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  19 

In  this  same  group  may  be  included  also  those  coagulated  proteins 
which  are  produced  by  the  action  of  animal  ferments  on  soluble  proteins — 
e.g.,  fibrin,  myosin,  casein. 

(a)  Fibrin. — Fibrin  is  derived  from  one  of  the  blood  proteins — viz., 
fibrinogen.  It  is  not  present  under  normal  circumstances  in  the 
circulating  blood,  but  makes  its  appearance  after  the  blood  is  with- 
drawn from  the  vessels  and  at  the  time  of  coagulation.  It  can  also  be 
obtained  by  whipping  the  blood  with  a  bundle  of  twigs,  on  which  it 
accumulates.  When  freed  from  blood  by  washing  under  water,  it  is 
seen  to  consist  of  bundles  of  white  elastic  fibers  or  threads.  It  is  in- 
soluble in  water,  in  alcohol,  and  ether.  In  dilute  acids  it  swells,  be- 
comes transparent,  and  finally  is  converted  into  acid  albumin.  In 
dilute  alkalies  a  similar  change  takes  place,  but  the  resulting 
product  is  an  alkali-albumin.  Fibrin  possesses  the  property  of 
decomposing  hydrogen  dioxid,  H^O^ — i.e.,  liberating  oxygen,  which 
accumulates  in  the  form  of  bubbles  on  the  fibrin.  On  incinera- 
tion fibrin  yields  an  ash  which  contains  calcium  phosphate  and  mag- 
nesium phosphate. 

Two  views  are  held  as  to  the  origin  of  fibrin:  first  that  it  is  the  result 
of  the  action  of  a  special  enzyme,  termed  thrombin  on  fibrinogen, 
though  the  nature  of  the  action  is  not  very  clear;  second  that  it  is 
the  result  of  a  definite  combination,  physio-chemic  in  character,  of 
fibrinogen  with  thrombin  which,  however,  is  not  regarded  as  an  enzyme, 
inasmuch  as  it  is  not  destroyed  by  boiling,  but  a  definite  compound 
partaking  of  the  nature  of  an  organic  colloid.     The  amount  of  fibrin 
formed  from  fibrinogen  will  be  proportional  to  the  amount  of  thrombin 
present. 
{b)  Myosin  fibrin  and  myogen  fibrin  are  two  insoluble  proteins  developed 
out  of  the  two  chief   proteins  of  muscle-plasma.     Their  develop- 
ment after  death  is  believed  to  be  the  cause  of  the  stiffening  of  the 
muscles.     It  is  not  definitely  known  whether  this  is  the  result  of  the 
action  of  a  special  enzyme  or  not. 
(c)  Casein. — Casein  is  derived  from  the  chief  protein  of  milk — caseinogen 
-^by  the  action  of  a  special  ferment  known  as  rennin  or  chymosin. 
This  ferment  is  a  constituent  of  gastric  juice. 
The  Color  Reactions  of  Proteins. — When  proteins  are  present  in 
solution,   they  may   be  detected  by   the   following   color  reactions — viz., 

1.  Xanthoproteic.     The   solution   is   boiled   with   nitric   acid   for   several 

minutes,  when  the  protein  assumes  a  light  yellow  color.  After 
the  solution  has  cooled,  the  addition  of  ammonia  changes  the  color 
to  an  orange  or  amber-red,  due  to  the  presence  of  phenylalanin  and 
ty  rosin. 

2.  The  rose-red  reaction.     The   solution  is  boiled   with  acid   nitrate   of 

mercury  (Millon's  reagent)  for  a  few  minutes,  when  the  coagulated 
protein  turns  a  purple-red  color.  This  color  is  attributed  to  the 
presence  of  tyrosin. 

3.  The  blue-violet  reaction.     To  the  solution  is  added  an  excess  of  sodium 

hydroxide,  at  least  an  equal  volume,  and  then  drop  by  drop,  a  very 
dilute  solution  of  copper  sulphate.  A  blue-violet  color  is  produced, 
which  deepens  somewhat  on  heating,  but  no  further  change  ensues. 


20  TEXT-BOOK  OF  PHYSIOLOGY 

This  is  also  known  as  Piotrowski's  test:  As  this  same  color  is  de- 
veloped with  the  substance  biuret,  it  is  also  known  as  the  biuret 
reaction.  Biuret  is  formed  by  heating  urea  to  i8o°C  and  driving  ofif 
ammonia. 

Precipitation  Tests. — ^Proteins  in  solution  may  be  precipitated  by 
nitric  acid,  acetic  acid  and  potassium  ferrocyanid,  picric  acid,  copper 
sulphate,  tannin,  alcohol,  etc.  As  stated  in  a  foregoing  paragraph,  certain 
of  the  proteins,  e.g.,  fibrinogen,  caseinogen  and  myosinogen,  will  undergo, 
by  the  action  of  an  animal  ferment  a  change  of  state  by  virtue  of  which 
they  become  solid.  To  this  process  the  term  ferment  coagulation  is  applied. 
The  solidification  of  proteins  by  the  action  of  heat  is  designated  heat 
coagulation. 

INORGANIC  CONSTITUENTS 

The  inorganic  compounds  and  mineral  constituents  obtained  from  the 
solids  and  fluids  of  the  body  are  very  numerous,  and,  in  some  instances, 
quite  abundant.  Though  many  of  the  compounds  thus  obtained  are 
undoubtedly  derivatives  of  the  tissues  and  necessary  to  their  physical  and 
physiologic  activity,  others,  in  all  probability,  are  decomposition  products, 
or  transitory  constituents  introduced  with  the  food.  Of  the  inorganic 
compounds,  the  following  are  the  most  important: 
WATER. 

Water  is  the  most  important  of  the  inorganic  constituents,  as  it  is  in- 
dispensable to  life.  It  is  present  in  all  the  tissues  and  fluids  without  excep- 
tion, varying  from  99  per  cent,  in  the  saliva  to  80  per  cent,  in  the  blood,  75 
per  cent,  in  the  muscles  to  2  per  cent,  in  the  enamel  of  the  teeth.  The  total 
quantity  contained  in  a  body  weighing  75  kilograms  (165  pounds)  is  52.5 
kilograms  (115  pounds).  Much  of  the  water  exists  in  a  free  condition,  and 
forms  the  chief  part  of  the  fluids,  giving  to  them  their  characteristic  degree  of 
fluidity..  Possessing  the  capability  of  holding  in  solution  a  large  number  of 
inorganic  as  well  as  some  organic  compounds,  and  being  at  the  same  time 
diffusible,  it  renders  an  interchange  of  materials  between  all  portions  of  the 
body  possible.  It  aids  in  the  absorption  of  new  material  into  the  blood  and 
tissues,  and  at  the  same  time  it  transfers  waste  products  from  the  tissues  to 
the  blood,  from  which  they  are  finally  eliminated,  along  with  the  water  in 
which  they  are  dissolved.  A  portion  of  the  water  is  chemically  combined 
with  other  tissue  constituents  and  gives  to  the  tissues  their  characteristic 
physical  properties.  The  consistency,  elasticity,  and  pliability  are,  to  a 
large  extent,  conditioned  by  the  amount  of  water  they  contain.  The  total 
quantity  of  water  eliminated  by  the  kidneys,  lungs,  and  skin  amounts  to 
about  3  kilograms  (6J  pounds)  daily. 
CALCIUM  COMPOUNDS. 

Calcium  phosphate,  Ca3(POj2>  has  a  very  extensive  distribution 
throughout  the  body.  It  exists  largely  in  the  bones,  teeth,  and  to  a  slight  ex- 
tent in  cartilage,  blood,  and  other  tissues.  Milk  contains  0.27  per  cent. 
The  solidity  of  the  bones  and  teeth  is  almost  entirely  due  to  the  presence  of 
this  salt,  and  is,  therefore,  to  be  regarded  as  necessary  to  their  structure. 
It  enters  into  chemic  union  with  the  organic  matter,  as  shown  by  the  fact 
that  it  cannot  be  separated  from  it  except  by  chemic  means,  such  as  immer- 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  21 

sion  in  hydrochloric  acid.  Though  insoluble  in  water,  it  is  held  in  solution 
in  the  blood  and  milk  by  the  protein  constituents,  and  in  the  urine  by  the 
acid  phosphate  of  soda.  The  total  quantity  of  calcium  phosphate  which 
enters  into  the  formation  of  the  body  has  been  estimated  at  2.5  kilograms. 
The  amount  eliminated  daily  from  the  body  has  been  estimated  at  0.4  gm., 
a  fact  which  indicates  that  nutritive  changes  do  not  take  place  with  much 
rapidity  in  those  tissues  in  which  it  is  contained. 

Calcium  carbonate,  CaCOg,  is  present  in  practically  the  same  situa- 
tions in  the  body  as  the  phosphate,  and  plays  essentially  the  same  r61e.  It 
is,  however,  found  in  the  crystalline  form,  aggregated  in  small  masses  in  the 
internal  ear,  forming  the  otoliths,  or  ear  stones.  Though  insoluble,  it  is  held 
in  solution  by  the  carbonic  acid  diffused  through  the  fluids. 

Calcium  fluorid,  CaF2,  is  found  in  bones  and  teeth. 

SODIUM  COMPOUNDS. 

Soditmi  chlorid,  NaCl,  is  present  in  all  the  tissues  and  fluids  of  the 
body,  but  especially  in  the  blood,  0.6  per  cent.,  lymph,  0.5,  and  pancreatic 
juice,  0.25  per  cent.  The  entire  quantity  in  the  body  has  been  estimated 
at  about  200  gm.  Sodium  chlorid  is  of  much  importance  in  the  body  as  it 
determines  and  regulates  to  a  large  extent  the  phenomena  of  diffusion 
which  are  there  constantly  taking  place.  The  ingested  water  is  absorbed 
into  the  blood  largely  in  consequence  of  the  percentage  of  this  salt  which  it 
contains.  The  normal  percentage  of  sodium  chlorid  in  the  blood-plasma 
assists  in  maintaining  the  shape  and  structure  of  the  red  blood-corpuscles 
by  determining  the  amount  of  water  entering  into  their  composition.  The 
same  is  true  of  other  tissue  elements. 

Sodium  chlorid  also  influences  favorably  the  general  nutritive  process, 
though  the  manner  in  which  it  acts  is  not  very  clear.  During  its  existence 
in  the  body  it  undergoes  chemic  transformations  or  decompositions,  yielding 
its  chlorin  to  form  the  potassium  chlorid  of  the  blood-corpuscles  and  muscles 
and  to  form  the  hydrochloric  acid  of  the  gastric  juice. 

Sodiimi  phosphate,  NagHPO^,  is  found  in  all  solids  and  fluids  of  the 
body,  to  which,  with  but  few  exceptions,  it  imparts  an  alkaline  reaction. 
This  is  especially  true  of  blood,  lymph,  and  tissue  fluids  generally.  It  is 
essential  to  physiologic  action  that  all  tissue  elements  should  be  bathed  by 
an  alkaline  medium. 

Soditmi  carbonate,  NagCOg,  is  generally  found  in  association  with  the 
preceding  salt.  As  it  is  an  alkaline  compound,  it  also  assists  in  giving  to 
the  blood  and  lymph  their  characteristic  alkalinity.  In  carnivorous  animals 
the  sodium  phosphate  is  the  more  abundant,  while  in  the  herbivorous 
animals  the  sodium  carbonate  is  the  more  abundant. 

Sodium  sulphate,  NajSO^,  is  present  in  many  of  the  tissues  and  fluids, 
especially  in  the  urine.  Though  introduced  in  the  food,  it  is  also,  in  all 
probability,  formed  in  tiie  body  from  the  decomposition  and  oxidation  of 
the  proteids. 

POTASSIUM  COMPOUNDS. 

Potassium  chlorid,  KCl,  is  met  with  in  association  with  sodium  chlorid 
in  almost  all  situations  in  the  body.  It  preponderates,  however,  in  the 
tissue  elements,  especially  in  the  muscle  tissue,  nerve  tissue,  and  red  cor- 
puscles.    The  plasma  with  which  these  structures  are  bathed  contains  but  a 


22  TEXT-BOOK  OF  PHYSIOLOGY 

very  small  amount  of  this  salt,  but,  as  previously  stated,  a  relatively  large 
amount  of  sodium  chlorid.  Though  introduced  to  some  extent  in  the  food, 
it  is  very  likely  that  it  is  also  formed  through  the  decomposition  of  the  so- 
dium chlorid. 

Potassium  phosphate,  KjHPO^,  is  found  in  association  with  sodium 
phosphate  in  all  the  fluids  and  solids.  As  it  has  similar  chemic  properties, 
its  functions  are  practically  the  same. 

Potassium  carbonate,  KjCOg,  is  generally  found  with  the  preceding 
salt. 

MAGNESIUM  COMPOUNDS. 

Magnesium  phosphate,  Mg3(POj2>  is  found  in  all  tissues,  in  associa- 
tion with  calcium  phosphate,  though  in  much  smaller  quantity. 

Magnesium  carbonate,  MgCOg,  occurs  only  in  traces  in  the  blood. 

IRON  COMPOUNDS. 

Iron  is  a  constituent  of  the  coloring-matter  of  the  blood.  Traces,  how- 
ever, are  also  found  in  lymph,  bile,  gastric  juice,  and  in  the  pigment  of  the 
eyes,  skin  and  hair.  The  amount  of  iron  contained  in  a  body  weighing  70 
kilograms  is  about  2.2  gm.  It  exists  under  various  forms — e.g.,  ferric  oxid, 
and  in  combination  with  organic  compounds. 

lodin  is  found  in  a  number  of  organs — thyroid  gland,  lungs,  ovaries, 
liver,  hypophysis,  small  intestines  as  well  as  in  the  blood  and  bile.  The 
iodine  content  of  the  thyroid  gland  is  eight  to  ten  times  that  of  all  the  other 
organs.  The  active  ingredient  of  the  thyroid  colloid  is  the  iodin-containing 
substance  called  thyroidin.  It  is  contained  in  that  portion  of  the  protein 
which  is  soluble  in  physiological  salt  solution  and  precipitated  upon  half 
saturation  with  ammonium  sulphate.  This  portion  is  apparently  a  globulin 
— called  thyreoglobulin,  and  contains  an  easily  separated  carbohydrate  group. 
The  thyroid  glands  of  new-born  children  are  iodin  free. 

Chemic  analysis  thus  shows  that  the  chemic  elements  into  which  the 
compounds  may  be  resolved  by  an  ultimate  analysis  do  not  exist  in  the 
body  in  a  free  state,  but  only  in  combination,  and  in  characteristic  pro- 
portions, to  form  compounds  whose  properties  are  the  resultant  of  those  of 
the  elements.  Of  the  four  principal  elements  which  make  up  97  per  cent, 
of  the  body,  O,  H,  N  are  extremely  mobile,  elastic,  and  possessed  of  great 
atomic  heat.  C,  H,  N  are  distinguished  for  the  narrow  range  of  their 
affinities,  and  for  their  chemic  inertia.  C  possesses  the  greatest  atomic 
cohesion.     O  is  noted  for  the  number  and  intensity  of  its  combinations. 

As  the  properties  of  the  compounds  formed  by  the  union  of  elements 
must  be  the  resultants  of  the  properties  of  the  elements  themselves,  it  follows 
that  the  ternary  compounds,  starches,  sugars,  and  fats  must  possess  more 
or  less  inertia,  and  at  the  same  time  instability;  while  in  the  more  complex 
proteids,  in  which  sulphur  and  phosphorus  are  frequently  combined  with 
the  four  principal  elements,  molecular  instability  attains  its  maximum. 
As  all  the  foregoing  compounds  possess  in  varying  degrees  the  properties  of 
inertia  and  instability,  it  follows  that  living  matter  must  possess  correspond- 
ing properties,  and  the  capability  of  undergoing  unceasingly  a  series  of 
chemic  changes,  both  of  composition  and  decomposition,  in  response  to  the 
chemic  and  physical  influences  by  which  it  is  surrounded,  and  which  underlie 
all  the  phenomena  of  life. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY  23 

PRINCIPLES  OF  DISSIMILATION 

In  addition  to  the  previously  mentioned  compounds — viz.,  carbo- 
hydrates, fats,  proteids,  and  inorganic  salts — there  is  obtained  by  chemic 
analysis  from  the  tissues  and  fluids  of  the  body : 

1.  A  number  of  organic  acids,  such  as  acetic,  lactic,  oxalic,  butyric,  pro- 
pionic, etc.,  in  combination  with  alkaline  and  earthy  bases. 

2.  Organic  compounds,  such  as  alcohol,  glycerin,  cholesterin. 

3.  Pigments,  such  as  those  found  in  bile  and  urine. 

4.  Crystallizable  nitrogen-holding  bodies,  such  as  urea,  uric  acid,  xanthin, 
hippuric  acid,  creatin,  creatinin,  etc. 

While  some  few  of  these  compounds  may  possibly  be  regarded  as  neces- 
sary to  the  physiologic  integrity  of  the  tissues  and  fluids,  the  majority  of 
them  are  to  be  regarded  as  products  of  dissimilation  of  the  tissues  and  foods 
in  consequence  of  functional  activity,  and  represent  stages  in  their  reduction 
to  simpler  forms  previous  to  being  eliminated  from  the  body. 


CHAPTER  III 
PHYSIOLOGY  OF  THE  CELL 

A  histologic  analysis  of  the  organs  and  tissues  of  the  animal  body  shows 
that  they  can  be  resolved  into  ultimate  elements,  termed  cells,  which  may, 
therefore,  be  regarded  as  the  primary  units  of  structure.  Though  cells 
vary  considerably  in  shape,  size,  and  chemic  composition  in  the  different 
tissues  of  the  adult  body,  they  are,  nevertheless,  descendants  from  typical 
cells,  known  as  embryonic  or  undifferentiated  cells,  the  first  offspring  of  the 
fertilized  ovum.  Ascending  the  line  of  embryonic  development,  it  will  be 
found  that  every  organized  body  originates  in  a  single  cell — the  ovum.  As 
the  cell  is  the  elementary  unit  of  all  tissues,  the  function  of  each  tissue  must 
be  referred  to  the  function  of  the  cell.  Hence  the  cell  may  be  defined  as  the 
primary  anatomic  and  physiologic  unit  of  the  organic  world,  to  which  every 
exhibition  of  life,  whether  normal  or  abnormal,  is  to  be  referred. 

Structure  of  Cells. — Though  cells  vary  in  shape  and  size  and  internal 
structure  in  different  portions  of  the  body,  a  typical  cell  may  be  said  to  con- 
sist mainly  of  a  gelatinous  substance  forming  the  body  of  the  cell,  termed 
cytoplasm  or  bioplasm,  in  which  is  embedded  a  smaller  spheric  body,  the 
nucleus.  Within  the  nucleus  there  is  frequently  seen  a  still  smaller  body, 
the  nucleolus.  The  shape  of  the  adult  cell  varies  according  to  the  tissue  in 
which  it  is  found;  when  young  and  free  to  move  in  a  fluid  medium,  the  cell 
assumes  a  spheric  form,  but  when  subjected  to  pressure,  may  become 
cylindric,  fusiform,  polygonal,  or  stellate.  Cells  vary  in  size  within  wide 
limit,  ranging  from  7.7/1  (^^Vo"  of  an  inch,  the  diameter  of  a  red  blood- 
corpuscle),  to  135//  {yto  of  an  inch,  the  diameter  of  the  large  cells  in  the 
gray  matter  of  the  spinal  cord).     (See  Fig.  i.) 

The  cytoplasm  consists  of  a  soft,  semifluid,  gelatinous  material,  varying 
somewhat  in  appearance  in  different  tissues.  Though  frequently  homogene- 
ous, it  often  exhibits  a  finely  granular  appearance  under  medium  powers 
of  the  microscope.  Young  cells  consist  almost  entirely  of  clear  cytoplasm. 
Mature  cells  contain,  according  to  the  tissue  in  which  they  are  found, 
material  of  an  entirely  different  character — e.g.,  small  globu'es  of  fat, 
granules  of  glycogen,  mucigen,  pigments,  digestive  ferments,  etc.  Under 
high  powers  of  the  microscope  the  cytoplasm  is  found  to  be  pervaded  by 
a  network  of  fibers,  termed  spongioplasm,  in  the  meshes  of  which  is  con- 
tained a  clearer  and  more  fluent  substance,  the  hyaloplasm.  The  relative 
amount  of  these  two  constituents  varies  in  different  cells,  the  proportion  of 
hyaloplasm  being  usually  greater  in  young  cells.  The  arrangement  of  the 
fibers  forming  the  spongioplasm  also  varies,  the  fibers  having  sometimes  a 
radial  direction,  'n  others  a  concentric  disposition,  but  most  frequently  being 
distributed  evenly  in  all  directions.  In  many  cells  the  outer  portion  of 
the  cell  protoplasm  undergoes  chemic  changes  and  is  transformed  into  a 
thin,  transparent,  homogeneous  membrane — the  cell  membrane — which 
completely  incloses  the  cell  substance.     The  cell  membrane  is  permeable 

24 


PHYSIOLOGY  OF  THE  CELL 


25 


to  water  and  watery  solutions  of  various  inorganic  and  organic  substances. 
It  is,  however,  not  an  essential  part  of  the  cell. 

The  nucleus  is  a  small  vesicular  body  embedded  in  the  cytoplasm  near 
the  center  of  the  cell.  In  the  resting  condition  of  the  cell  it  consists  of  a 
distinct  membrane,  composed  of  amphipyrenin,  inclosing  the  nuclear  con- 
tents. The  latter  consists  of  a  homogeneous  amorphous  substance — the 
nuclear  matrix — in  which  is  embedded  the  nuclear  network.  It  can  often 
be  seen  that  a  portion  of  one  side  of  the  nucleus,  called  the  pole,  is  free  from 
this  network.  The  main  cords  of  the  network  are  arranged  as  V-shaped 
loops  about  it.  These  main  cords  send  out  secondary  branches  or  twigs, 
which,  uniting  with  one  another,  complete  the  network.  The  nuclear  cords 
are  composed  of  granules  of  chromatin — so  called  because  of  its  affinity  for 


Nuclear  membrane.    , 


Linin. 


Nuclear  fluid  (matrix) .  - .  (. 


Nucleolus.  - 


Chromatin-cords 
(nuclear  network). 


Nodal  enlargements  . 
of  the  chromatin. 


-  -Cell  membrane. 


Exoplasm. 


Microsomes. 


Centrosoma. 


Spongioplasm. 
--   Hyaloplasm. 


Foreign  inclosures. 

Fig.  I. — Diagram  of  a  Cell.     Microsomes  and  spongioplasm  are  only  partly  drawn. — {Stohr. 


certain  staining  materials — held  together  by  an  achromatin  substance 
known  as  linin.  Besides  the  nuclear  network,  there  are  embedded  in  the 
nuclear  matrix  one  or  more  small  bodies  composed  of  pyrenin,  known  as 
nucleoli.  At  the  pole  of  the  nucleus,  either  within  or  just  without  in  the 
cytoplasm,  is  a  small  body,  the  cenirosome,  or  pole  corpuscle. 

Chemic  Composition  of  the  Cell. — The  composition  of  living  bioplasm 
is  difficult  of  determination,  for  the  reason  that  all  chemic  and  physical 
methods  employed  for  its  analysis  destroy  its  vitality,  and  the  products 
obtained  are  peculiar  to  dead  rather  than  to  living  matter.  Moreover,  as 
bioplasm  is  the  seat  of  extensive  chemic  changes,  it  is  not  easy  to  determine 
whether  the  products  of  analysis  are  crude  food  constituents  or  cleavage  or 
disintegration  products.  Nevertheless,  chemic  investigations  have  shown 
that  even  in  the  living  condition  bioplasm  is  a  highly  complex  compound — 
the  resultant  of  the  intimate  union  of  many  different  substances.  About 
75  per  cent,  of  bioplasm  consists  of  water  and  25  per  cent,  of  solids,  of 
which  the  more  important  compounds  are  various  nucleo-proteins  (char- 
acterized by  their  large  percentage  of  phosphorus),  globulins,  lipoids,  such 
as  lecithin  (a  phosphorized  fat)  and  cholesterin  (a  monatomic  alcohol)  and 
possibly  fat  and  carbohydrates.     Inorganic  salts,  especially  the  potassium, 


26  TEXT-BOOK  OF  PHYSIOLOGY 

sodium,  and  calcium  chlorids  and  phosphates,  are  almost  invariable  and 
essential  constituents. 

MANIFESTATIONS  OF  CELL  LIFE 

Growth,  the  Maintenance  of  Nutrition,  and  Reproduction. — All 

cells  exhibit  three  fundamental  properties  of  life — viz.,  growth,  the  mainte- 
nance of  their  nutrition,  and  reproduction.  Growth  is  an  increase  in  size. 
When  newly  reproduced  all  cells  are  extremely  small,  but  in  consequence  of 
their  organization  and  the  character  of  their  surrounding  medium,  they 
gradually  grow  until  they  attain  the  size  characteristic  of  the  adult  state. 

Nutrition  may  be  defined  as  the  sum  of  the  processes  concerned  in  the 
maintenance  of  the  physiologic  condition  of  the  cell  and  includes  both  growth 
and  repair.  So  long  as  this  is  accomplished,  the  cells  and  the  tissues  which 
are  formed  by  them  continue  to  exhibit  their  functions  or  their  characteristic 
modes  of  activity.  Both  growth  and  nutrition  are  dependent  on  the  power 
which  living  material  possesses  of  not  only  absorbing  nutritive  material  from 
the  surrounding  medium,  the  lymph,  but  of  subsequently  assimilating  it, 
organizing  it,  transforming  it  into  material  like  itself  and  endowing  it  with 
its  own  physiologic  properties. 

In  the  physiologic  condition  the  living  material  of  the  cell,  the  bioplasm, 
is  the  seat  of  a  series  of  chemic  changes  which  vary  in  degree  from  moment  to 
moment  in  accordance  with  the  degree  of  functional  activity,  and  on  the 
continuance  of  which  all  life  phenomena  depend.  Some  of  these  chemic 
changes  are  related  to  or  connected  with  the  molecules  of  the  living  material, 
while  others  are  connected  with  the  food  material  supplied  to  them.  Of 
the  chemic  changes  occurring  within  the  molecules  some  are  destructive, 
dissimilative  or  disintegrative  in  character,  whereby  the  molecule  is  in  part 
eventually  reduced  through  a  series  of  descending  chemic  stages  to  simpler 
compounds  which,  apparently  of  no  use  in  the  cell,  are  eliminated  from  it. 
It  is,  therefore,  said  that  the  living  material  undergoes  molecular  disintegra- 
tion as  a  result  of  functional  activity.  To  these  changes  the  term  kataholism 
is  also  applied.  Other  of  these  changes  are  constructive,  assimilative  or 
integrative  in  character,  whereby  a  part  at  least  of  the  food  material  furnished 
by  the  blood-plasma  is  transformed  through  a  series  of  ascending  chemic 
stages  into  living  material,  and  whereby  it  is  repaired  and  its  former  physio- 
logic condition  restored.  It  is,  therefore,  said  that  the  living  material  under- 
goes molecular  integration  as  a  preparation  for  functional  activity.  To  these 
changes  the  term  anaholism  is  also  applied.  During  the  course  of  its  physio- 
logic activities  the  cell  bioplasm  produces  materials  of  an  entirely  different 
character  which  vary  with  the  cell,  such  as  fat,  glycogen,  mucigen,  pigments, 
ferments,  etc.,  which  are  generally  spoken  of  as  metabolic  products. 

Living  material  has  also  a  temperature  varying  in  degree  in  different 
.  species  of  animals  as  well  as  in  different  parts  of  the  same  animal.  Here  as 
elsewhere  the  temperature  is  due  to  heat  liberated  from  organic  compounds 
through  disruption  and  subsequent  oxidation  to  simpler  compounds. 
Though  some  of  the  heat  liberated  may  come  from  the  tissue  molecules,  the 
larger  part  by  far  comes  from  the  food  molecules — sugar,  fat,  and  protein, 
constituents  of  the  fluids  circulating  in  the  tissue  spaces.  These  foods  carry 
into  the  body  potential  energy,  ultimately  derived  from  the  sun.     When  they 


PHYSIOLOGY  OF  THE  CELL  27 

are  disrupted  and  oxidized  the  potential  energy  is  transformed  into  kinetic 
energy  which  manifests  itself  for  the  most  part  as  heat.  To  the  sum  total 
of  all  the  chemic  changes  occurring  in  tissues  and  foods  the  term  metabolism 
is  given. 

There  is,  however,  much  difference  of  opinion  as  to  the  extent  to  which 
the  living  material  is  metabolized  and  to  the  actual  disposition  of  the  food 
materials,  and  especially  the  proteins,  or  their  cleavage  products,  the  amino- 
acids.  Thus  Voit  contended  that  the  tissue  molecules  are  comparatively 
stable  in  composition  and  under  ordinary  conditions  of  nutrition  do  not 
undergo  any  material  change  during  either  rest  or  activity,  and  that  metabo- 
lism is  confined  to  the  food  materials  occupying  spaces  in  and  around 
the  living  cell.  The  cause  which  initiates  this  metabolism  is  unknown, 
but  is  supposed  to  reside  in  the  cell,  if  it  is  not  a  property  of  the  cell  itself. 
Because  of  the  fact  that  but  a  very  small  amount  of  sugar  or  fat  enters  into 
the  composition  of  bioplasm  it  is  generally  admitted  that  these  foods  are 
metabolized  in  the  tissue  spaces  and  in  the  manner  just  alluded  to.  The 
problem,  however,  is  different  in  the  case  of  the  proteins.  Voit  contended, 
as  previously  stated,  that  the  proteins  of  the  tissue  molecules,  which  he 
distinguished  as  tissue  proteins,  do  not  metabolize  to  any  appreciable  extent 
and  confined  practically  all  protein  metabolism  to  the  food  proteins,  now 
distinguished  as  amino-acids,  circulating  in  the  tissue  spaces  and  which  he 
characterized  as  circulating  proteins.  Even  in  starvation  the  tissue  pro- 
teins, as  such,  do  not  metabolize  until  they  have  been  disintegrated  in  con- 
sequence of  chemic  changes  and  transformed  into  circulating  proteins. 

PflUger,  however,  asserted  that  the  circulating  proteins  cannot  be  metabo- 
lized as  such  but  that  they  must  first  be  built  up  into  tissue  proteins.  The 
metabolism  of  protein  is,  therefore,  confined,  in  this  view,  to  the  molecules  of 
the  living  material.  It  is  possible,  however,  that  both  views  are  correct  and 
that  in  the  physiologic  condition  the  activity  of  the  tissues  is  attended  by  a 
partial  destruction  of  the  protein  molecules  which  is  followed  in  turn  by  con- 
struction out  of  amino-acids  during  the  subsequent  rest,  but  that  the  greater 
part  of  the  protein  metabolism  takes  place  outside  the  cell,  though  in 
contact  with  it. 

Though  the  cell  is,  therefore,  the  seat  of  two  opposing  processes,  assimila- 
tion and  dissimilation,  it  retains  under  normal  conditions  an  average  physio- 
logic state,  and  so  long  as  this  is  the  case  it  is  in  a  condition  of  nutritive 
equilibrium  and  capable  of  performing  its  various  functions. 

Though  the  foregoing  statements  are  applied  to  the  individual  cell  they 
are  equally  applicable  to  the  body  as  a  whole,  inasmuch  as  the  organs  and 
tissues  of  which  it  consists  arie  composed  of  cells.  The  body  grows  in  size 
and  maintains  its  nutrition,  by  the  introduction  of  food  materials  which  are 
utilized  in  part,  for  the  repair  of  the  tissues  which  have  undergone  molecular 
disintegration  in  consequence  of  activity,  and  in  part  for  the  liberation  of 
energy.  As  a  result  of  the  disintegration  or  the  metabolism  of  tissue  and 
food  materials,  products  such  as  carbon  dioxid,  urea,  etc.,  are  formed  which, 
apparently  of  no  further  use,  are  discharged  from  the  body  by  eliminating 
organs  as  the  kidney,  lungs,  skin,  etc.  Assimilation  and  dissimilation  are 
constantly  taking  place.  If  the  food  assimilated  and  metabolized  exactly 
replaces  the  tissues  dissimilated  and  the  food  metabolized  the  body  will  retain 
a  condition  of  nutritive  equilibrium. 


28 


TEXT-BOOK  OF  PHYSIOLOGY 


To  the  metabolism  that  takes  place  in  the  tissue  protein  the  term  endog- 
enous protein  metabolism  is  given  while  to  that  which  takes  place  in  the 
tissue  fluids  the  term  exogenous  protein  metabolism  is  given. 

Reproduction. — Cells  reproduce  themselves  in  the  higher  animals  in 
two  ways — by  direct  division  and  by  indirect  division,  or  karyokinesis.  In 
the  former  the  nucleus  becomes  constricted,  and  divides  without  any  special 
grouping  of  the  nuclear  elements.  It  is  probable  that  this  occurs  only  in 
disintegrating  cells,  and  never  in  a  physiologic  multiplication.  In  division 
by  karyokinesis  (Fig.  2)  there  is  a  progressive  rearranging  and  definite 
grouping  of  the  nucleus,  the  result  of  which  changes  is  the  division  of  the 
centrosome,  the  chromatin,  and  the  rest  of  the  nucleus  into  two  equal  por- 
tions, which  form  the  nuclei.  Following  the  division  of  the  nuclei,  the 
protoplasm  divides.     The  process  may  be  divided  into  three  phases: 


Close  Skein 
(viewed  from 
the  side). 
Polar  field. 


Loose  Skein  (viewed 
from  above— i.  e.,  from 
the  pole). 


Mother  Stars  (viewed  from  the  side). 


^  Polar 
radia- 
tion. 

—    Spindle. 


^^ii* 


m 


Mother  Star  (viewed      Daughter  Star 

from  above).  Division  of  the  Protoplasm. 

Fig.  2. — Karyokinetic  Figtjres  Observed  in  the  Epithelium  of  the  Oral  Cavity  of 
A  Salamander.  The  picture  in  the  upper  right-hand  corner  is  from  a  section  through  a  dividing 
egg  of  Siredon  pisciformis.  Neither  the  centrosomes  nor  the  first  stages  of  the  development  of 
the  spindle  can  be  seen  by  this  magnification.     X  560. — (Stohr.) 

I.  Prophase. — The  centrosome,  at  first  small  and  lying  within  the  nucleus, 
increases  in  size  and  moves  into  the  protoplasm,  where  it  lies  near  the 
nucleus,  surrounded  by  a  clear  zone,  from  which  delicate  threads 
radiate  through  an  area  known  as  the  attraction  sphere.  The  nucleus 
enlarges  and  becomes  richer  in  chromatin.  The  lateral  twigs  of  the 
chromatin  cords  are  drawn  in,  while  the  main  cords  become  much 
contorted.  These  cords  have  a  general  direction  transverse  to  the  long 
axis  of  the  cell,  and  parallel  to  the  plane  of  future  cleavage.  They  are 
seen  as  V-shaped  segments  or  loops,  chromosomes,  having  their  closed 
ends  directed  toward  a  common  center,  the  polar  field,  while  the  other 
ends  interdigitate  on  the  opposite  side  of  the  nucleus — the  antipole. 
The  polar  field  corresponds  to  the  area  occupied  by  the  centrosome.  This 
arrangement  is  known  as  the  close  skein;  but  as  the  process  goes  on,  the 
chromosomes  become  thicker,  shorter  and  less  contorted,  producing  a 


PHYSIOLOGY  OF  THE  CELL  29 

much  looser  arrangement,  known  as  the  loose  skein.  During  the 
formation  of  the  loose  skein,  the  centrosome  divides  into  two  portions, 
which  move  apart  to  positions  at  the  opposite  ends  of  the  long  axis  of 
the  nucleus.  At  the  same  time  delicate  achromatin  fibers  make  their 
appearance,  arranged  in  the  form  of  a  double  cone,  the  apices  of  which 
correspond  in  position  to  the  centrosomes.  This  is  known  as  the 
nuclear  spindle.  During  the  prophase  the  nuclear  membrane  and  the 
nucleoli  disappear. 

2.  The  Metaphase. — The  two  centrosomes  are  at  opposite  ends  of  the  long 

axis  of  the  nucleus,  each  surrounded  by  an  attraction  sphere,  now 
called  the  polar  radiation.  The  chromosomes  become  yet  shorter  and 
thicker,  and  move  toward  the  equator  of  the  nucleus,  where  they  lie 
with  their  closed  ends  toward  the  axis,  presenting  the  appearance, 
when  seen  from  the  poles,  of  a  star — the  so-called  mother  star,  or  mon- 
aster. While  moving  toward  the  equator  of  the  nucleus,  and  often 
earlier,  each  chromosome  undergoes  longitudinal  cleavage,  the  sister 
loops  remaining  together  for  a  time.  Upon  the  completion  of  the 
monaster,  one  loop  of  each  pair  passes  to  each  pole  of  the  nucleus, 
guided,  and  perhaps  drawn  by  the  threads  of  the  nuclear  spindle.  The 
separation  of  the  sister  segments  begins  at  their  apices,  and  as  the  open 
ends  are  drawn  apart  they  remain  connected  by  delicate  achromatin 
filaments  drawn  out  from  the  chromosomes.  This  separation  of  the 
daughter  chromosomes,  and  their  movement  toward  the  daughter 
centrosomes,  is  called  metakinesis.  As  they  approach  their  destination, 
we  have  the  appearance  of  two  stars  in  the  nucleus — the  daughter  stars, 
or  diasters. 

3.  Anaphase. — The  daughter  stars  undergo,  in  reverse  order,  much  the 

same  changes  that  the  mother  star  passed  through.     The  chromo- 
somes become  much  convoluted,  and  perhaps  united  to  one  another, 
the  lateral  twigs  appear,  and  the  chromatin  resumes  the  appearance 
of  the  resting  nucleus.     The  nuclear  spindle,  with  most  of  the  polar 
radiation,  disappears,  and  the  nucleoli  and  the  nuclear  membrane 
reappear,   thus  forming  two  complete  daughter  nuclei.     Meanwhile 
the  protoplasm  becomes  constricted  midway  between  the  young  nuclei. 
This  constriction  gradually  deepens  until  the  original  cell  is  divided, 
with  the  formation  of  two  complete  cells. 
Physiologic  Properties  of  Bioplasm. — All  living  bioplasm  possesses 
properties  which  serve  to  distinguish  and  characterize  it — ^viz.,  irritability, 
conducti-\dty,  and  motility. 

Irritability ,  or  the  power  of  reacting  in  a  definite  manner  to  some  form 
of  external  excitation,  whether  mechanic,  chemic,  or  electric,  is  a  funda- 
mental property  of  all  living  bioplasm.  The  character  and  extent  of  the 
reaction  will  vary,  and  will  depend  both  on  the  nature  of  the  bioplasm  and 
the  character  and  strength  of  the  stimulus.  If  the  bioplasm  be  muscle, 
the  response  will  be  a  contraction;  if  it  be  gland,  the  response  will  be  a 
secretion;  if  it  be  nerve,  the  response  will  be  a  sensation  or  some  other  form 
of  nerve  activity. 

Conductivity,  or  the  power  of  transmitting  molecular  disturbances  arising 
at  one  point  to  all  portions  of  the  irritable  material,  is  also  a  characteristic 
feature  of  all  bioplasm.     This  power,  however,  is  best  developed  in  that  form 


30  TEXT-BOOK  OF  PHYSIOLOGY 

of  bioplasm  found  in  nerves,  which  serves  to  transmit,  with  extreme  rapidity, 
molecular  disturbances  arising  at  the  periphery  to  the  brain,  as  well  as  from 
the  brain  to  the  periphery.  Muscle  bioplasm  also  possesses  the  same  power 
in  a  high  degree. 

Motility,  or  the  power  of  executing  apparently  spontaneous  movements, 
is  exhibited  by  many  forms  of  cell  bioplasm.  In  addition  to  the  molecular 
movements  which  take  place  in  certain  cells,  other  forms  of  movement  are 
exhibited,  more  or  less  constantly,  by  many  cells  in  the  animal  body — e.g., 
the  waving  of  cilia,  the  ameboid  movements  and  migrations  of  white  blood- 
corpuscles,  the  activities  of  spermatozoa,  the  projection  of  pseudopodia, 
etc.  These  movements,  arising  without  any  recognizable  cause,  are  fre- 
quently spoken  of  as  spontaneous.  Strictly  speaking,  however,  all  proto- 
plasmic movement  is  the  resultant  of  natural  causes,  the  true  nature  of 
which  is  beyond  the  reach  of  present  methods  of  investigation. 


CHAPTER  IV 

HISTOLOGY  OF  THE  EPITHELIAL  AND   CONNECTIVE  TISSUES 

I.  EPITHELIAL  TISSUE 

The  epithelial  tissue  consists  of  one  or  more  layers  of  cells  resting  on  a 
homogeneous  membrane,  the  other  side  of  which  is  abundantly  supplied 
with  blood-vessels  and  nerves.  The  form  of  the  epithelial  cell  varies  in 
different  situations,  and  may  be  flattened,  cuboid,  spheroid  or  columnar. 
(See  Figs.  4,  5,  and  6.)  The  form  of  the  cell  in  all  instances  is  related  to 
some  specific  function.  When  arranged  in  layers  or  strata,  the  cells  are 
cemented  together  by  an  intercellular  substance. 

The  epithelial  tissue  forms  a  continuous  covering  for  the  surfaces  of  the 
body.  The  external  investment  (the  skin)  and  the  internal  investment  (the 
mucous  membrane,  which  lines  the  entire  alimentary  canal  as  well  as  as 
sociated  body  ca\dties)  are  both  formed,  in  all  situations,  by  the  homogeneous 
basement  membrane,  covered  with  one  or  more  layers  of  cells.     The  glands 


Fig.  3. — Epithelial  Cells  of  Rabbit,  Isolated.  X  560.  i.  Squamous  cells  (mucous 
membrane  of  mouth).  2.  Columnar  cells  (corneal  epithelium).  3.  Columnar  cells,  with  cuticular 
border,  5  (intestinal  epithelium).     4.  Ciliated  cells;  h,  cilia  (bronchial  epithelium). — {Stohr.) 

of  the  skin,  the  lungs  and  the  glands  in  connection  with  the  alimentary  canal 
and  the  uro-genital  apparatus  are  formed  of  the  same  elemental  structures. 
All  materials,  therefore,  whether  nutritive,  secretory,  or  excretory,  must  pass 
through  epithelial  cells  before  they  can  enter  into  the  formation  of  the  blood 
or  be  eliminated  from  it.  The  nutrition  of  the  epithelial  tissue  is  maintained 
by  the  nutritive  material  derived  from  the  blood  diffusing  itself  into  and 
through  the  basement  membrane.  Chemically,  the  epithelial  cells  of  the 
epidermis — hair,  nails,  etc. — are  composed  of  a  sclero-protein  (keratin),  a 
small  quantity  of  water,  and  inorganic  salts.  In  other  situations,  especially 
on  the  mucous  membranes,  the  cells  consist  largely  of  mucin,  in  association 
with  other  proteins.  The  consistency  of  epithelium  varies  in  accordance  with 
external  influences,  such  as  the  presence  or  absence  of  moisture,  pressure, 
friction,  etc.  This  is  well  seen  in  the  skin  of  the  palms  of  the  hands  and  the 
soles  of  the  feet — situations  where  it  acquires  its  greatest  density.  In  the 
ahmentary  canal,  in  the  lungs,  and  in  other  cavities,  where  the  reverse  condi- 
tions prevail,  the  epithelium  is  extremely  soft.     Epithelial  tissues  also  possess 

31 


Z2  TEXT-BOOK  OF  PHYSIOLOGY 

varying  degrees  of  cohesion  and  elasticity— physical  properties  which  enable 
them  to  resist  considerable  pressure  and  distention  without  having  their 
physiologic  integrity  destroyed.  Inasmuch  as  these  tissues  are  poor  con- 
ductors of  heat,  they  assist  in  preventing  too  rapid  radiation  of  heat  from 
the  body,  and  cooperate  with  other  mechanisms  in  maintaining  the  normal 
temperature.  The  physiologic  activity  of  all  epithelial  tissue  depends  on 
a  due  supply  of  nutritive  material  derived  from  the  blood,  which  not 
only  maintains  its  nutrition,  but  affords  those  materials  out  of  which  are 
formed   the   secretions   of   the  glands,   whether  of   the   skin   or  mucous 

membrane. 

^"'"'^-^i--^  The    cells    lining    the    blood- 

-^^  '^"---  ^K^'^  vessels,  the  lymph-vessels,  the  peri- 

~- -        «,  \^  -i;'  toneal,    pleural,    pericardial,    and 

'   i-''  5*       ^  other    closed    cavities   are  usually 


c     .  ^0 


Fig.    4. —  Stratified    Squamous  Fig.     S. —  Stratified     Ciliated 

Epithelium     (Larynx     of     Man).  Epithelium.     X  560.     From  the  res- 

X  240.     I.  Columnar  cells.     2.  Prickle-  piratory     nasal     mucous     membrane 

cells.     3.  Squamous  cells. — (Stohr.)  of  man.     i.  Oval  cells.     2.   Spindle- 

shaped    cells.     3.   Columnar    cells. — ■ 
(Stohr.) 

termed  endothelial  cells.     The  cells  in  these  situations  are  flat,  irregular  in 
shape,  with  borders  more  or  less  wavy  or  sinuous  in  outline. 

Functions  of  Epithelial  Tissue. — In  succeeding  chapters  the  form, 
chemic  composition,  and  functions  of  epithelial  cells  will  be  considered  in 
connection  with  the  functions  of  the  organs  of  which  they  constitute  a  part. 
In  this  connection  it  may  be  stated  in  a  general  way  that  the  functions  of 
the  epithelial  tissues  are: 

1.  To  serve  on  the  surface  of  the  body  as  a  protective  covering  to  the  under- 

lying structures  which  collectively  form  the  true  skin,  thus  protecting 
them  from  the  injurious  influences  of  moisture,  air,  dust,  microorgan- 
isms, etc.,  which  would  otherwise  impair  their  vitality.  Wherever  con- 
tinuous pressure  is  applied  to  the  skin,  as  on  the  palms  of  the  hands  and 
soles  of  the  feet,  the  epithelium  increases  in  thickness  and  density,  and 
thus  prevents  undue  pressure  on  the  nerves  of  the  true  skin.  The 
density  of  the  epidermis  enables  it  to  resist,  within  limits,  the  injurious 
influence  of  acids,  alkalies,  and  poisons. 

2.  To  promote  absorption.     Inasmuch  as  the  skin  and  mucous  membranes 

cover  the  surfaces  of  the  body,  it  is  obvious  that  all  nutritive  material 
entering  the  body  must  first  traverse  the  epithelial  tissue.  Owing  to 
their  density,  however,  the  epithelial  cells  covering  the  skin  play  but  a 
feeble  role  as  absorbing  agents  in  man  and  the  higher  animals.     The 


THE  CONNECTIVE  TISSUES  33 

epithelium  of  the  mucous  membrane  of  the  ahmentary  canal,  particu- 
larly that  of  the  small  intestine,  is  especially  adapted,  from  its  situa- 
tion, consistency,  and  properties,  to  play  the  chief  role  in  the  absorp- 
tion of  new  materials  from  the  canal.  The  epithelium  lining  the 
air-vesicles  of  the  lungs  is  engaged  in  promoting  the  absorption  of 
oxygen  and  the  exhalation  of  carbon  dioxid. 
3.  To  form  secretions  and  excretions.  Each  secretory  gland  connected 
with  the  surfaces  of  the  body  is  lined  by  epithelial  cells,  which  are  actively 
concerned  in  the  formation  of  the  secretion  peculiar  to  the  gland. 
Each  excretory  organ  is  similarly  provided  with  epithelial  cells,  which 
are  engaged  either  in  the  production  of  the  constituents  of  the  excretion 
or  in  their  removal  from  the  blood. 

2.  THE  CONNECTIVE  TISSUES 

The  connective  tissues,  in  their  collective  capacity,  constitute  a  frame- 
work which  pervades  the  body  in  all  directions,  and,  as  the  name  implies, 
serve  as  a  bond  of  connection  between  the  individual  parts,  at  the  same  time 
affording  a  basis  of  support  for  the  muscle,  nerve,  and  gland  tissues.  The 
connective-tissue  group  includes  a  number  of  varieties,  among  which  may 
be  mentioned  the  areolar,  adipose,  retiform,  white  fibrous,  yellow  elastic, 
cartilaginous  and  osseous.  Notwithstanding  their  apparent  diversity,  they 
possess  many  points  of  similarity.  They  have  a  common  origin,  developing 
from  the  same  embryonic  material;  they  have  much  the  same  structure, 
passing  imperceptibly  into  one  another,  and  perform  practically  the  same 
functions. 

Areolar  Tissue. — This  variety  is  found  widely  distributed  throughout 
the  body.  It  serves  to  unite  the  skin  and  mucous  membrane  to  the  struc- 
tures on  which  they  rest;  to  form  sheaths  for  the  support  of  blood-vessels, 
nerves,  and  lymphatics;  to  unite  into  compact  masses  the  muscular  tissue 
of  the  body,  etc.  Examined  with  the  naked  eye,  it  presents  the  appearance 
of  being  composed  of  bundles  of  fine  fibers  interlacing  in  every  direction. 
In  the  embryonic  state  the  elements  of  this  form  of  connective  tissue  are 
united  by  a  ground  substance,  gelatinous  in  character.  In  the  adult  state  this 
substance  shrinks  and  largely  disappears,  leaving  intercommunicating 
spaces  of  varying  size  and  shape,  from  which  the  tissue  takes  its  name. 
When  subjected  to  the  action  of  various  reagents,  and  examined  micro- 
scopically, the  bundles  can  be  shown  to  consist  of  extremely  delicate,  color- 
less, transparent,  wavy  fibers,  which  are  cemented  together  by  a  ground 
substance  composed  largely  of  mucin.  Other  fibers  are  also  observed, 
which  are  distinguished  by  a  straight  course,  a  sharp,  well-defined  out- 
line, a  tendency  to  branch  and  unite  with  adjoining  fibers,  and  to  curl  up 
at  their  extremities  when  torn.  From  their  color  and  elasticity  they  are 
known  as  yellow  elastic  fibers.  Distributed  throughout  the  meshes  of  the 
areolar  tissue  are  found  flattened,  irregularly  branched,  or  stellate  corpus- 
cles, connective-tissue  corpuscles,  plasma  cells,  and  granule  cells. 

Adipose  Tissue. — This  tissue,  which  exists  very  generally  throughout 

the  body,  though  found  most  abundantly  beneath  the  skin,  around  the 

kidneys,  and  in  the  bones,  is  practically  but  a  modification  of  areolar  tissue. 

In  these  situations  it  presents  itself  in  small  masses  or  lobules  of  varying 

3 


34 


TEXT-BOOK  OF  PHYSIOLOGY 


size  and  shape,  surrounded  and  penetrated  by  the  fibers  of  connective 
tissue.  (See  Fig.  6.)  Microscopic  examination  shows  that  these  masses 
consist  of  small  vesicles  or  cells,  round,  elliptical  or  polyhedral  in  shape, 
depending  somewhat  on  pressure.  (See  Fig.  7.)  Each  vesicle  consists  of  a 
thin,  colorless,  protoplasmic  membrane,  thickened  at  one  point,  in  which  a 
nucleus  can  usually  be  detected.  This  membrane  incloses  a  globule  of  fat, 
which  during  life  is  in  the  liquid  state.  It  is  composed  of  olein,  stearin,  and 
palmitin.  The  origin  of  the  fat  is  to  be  referred  to  a  synthesis  of  a  fat- 
acid  and  glycerin  6n  the  part  of  the  cell  material  or  of  an  enzyme  contained 
therein.  As  fat  granules  accumulate,  at  the  expense  of  the  cell  proto- 
plasm, they  gradually  coalesce,  until  there  remains  but  a  thin  stratum 
of  the  protoplasm,  which  forms  the  wall  of  the  vesicle.     Adipose  tissue 


In 

super- 
posed 
layers. 


Fig.  6. — Adipose  Tissue. — (Stohr.) 


Fig.  7. — Fat -CELLS  from  the 
Axilla  of  Man.  i.  The  equator 
of  the  cell  in  focus.  2.  The  ob- 
jective somewhat  elevated.  3,  4. 
Forms  changed  by  pressure,  p. 
Traces  of  protoplasm  in  the  vicinity 
of  the  flat  nucleus  k. — (Stohr.) 


may,  therefore,  be  regarded  as  areolar  tissue,  in  which,  and  at  the  expense 
of  some  of  its  elements,  fat  is  stored  for  the  future  needs  of  the  organism. 
A  diminution  of  food,  especially  of  fat  and  carbohydrates,  is  promptly 
followed  by  an  absorption  of  fat  by  the  blood-vessels  and  by  its  transference 
to  the  tissues,  where  it  is  either  utilized  for  tissue  construction  or  for  oxida- 
tion purposes.  In  the  situations  in  which  adipose  tissue  is  found  it  serves, 
by  its  chemic  and  physical  properties,  to  assist  in  the  prevention  of  a  too 
rapid  radiation  of  heat  from  the  body,  to  give  form  and  roundness,  and 
to  diminish  angularities,  etc. 

Retiform  and  adenoid  tissue  are  also  modifications  of  areolar  tissue. 
The  meshes  of  the  former  contain  but  little  ground  substance,  its  place  being 
taken  by  fluids;  the  meshes  of  the  latter  contain  large  numbers  of  lymph 
corpuscles. 

Fibrous  Tissue. — This  variety  of  connective  tissue  is  widely  distributed 
throughout  the  body.  It  constitutes  almost  entirely  the  ligaments  around 
the  joints,  the  tendons  of  the  muscles,  the  membranes  covering  organs  such 
as  the  heart,  liver,  nerve  system,  bones,  etc.  All  fibrous  tissue,  wherever 
found,  can  be  resolved  into  elementary  bundles,  which  on  microscopic  exami- 
nation are  seen  to  consist  of  delicate,  wavy,  transparent,  homogeneous 
fibers,  which  pursue  an  independent  course,  neither  branching  nor  uniting 
with  adjoining  fibers.  (See  Fig.  8.)  A  small  amount  of  ground  substance 
serves  to  hold  them  together.     Fibrous  tissue  is  tough  and  inextensible,  and 


THE  CONNECTIVE  TISSUES 


35 


in  consequence  is  admirably  adapted  to  fulfil  various  mechanical  functions 
in  the  body.  It  is,  however,  quite  pliant,  bending  easily  in  all  directions. 
When  boiled,  fibrous  tissue  yields  gelatin,  a  derivative  of  collagen. 

Elastic  Tissue. — The  fibers  of  elastic  tissue  are  usually  associated  in 
varying  proportions  with  the  w^hite  fibrous  tissue;  but  in  some  structures — 
as  the  ligamentum  nuchse,  the  ligamenta  subflava,  the  middle  coat  of  the 
larger  blood-vessels — the  elastic  fibers  are  almost  the  only  elements  present, 
and  give  to  these  structures  a  distinctly  yellow  appearance.  The  fibers 
throughout  their  course  give  off  many  branches,  which  unite  with  adjoining 
branches  to  form  a  more  or  less  close  network.  As  the  name  implies,  these 
fibers  are  highly  elastic,  and  are  capable  of  being  extended  as  much  as  60 
per  cent,  of  their  length  before  breaking.     (See  Fig.  9.) 


Fig.  8. — Connective-tissue 
Bundles  of  Various  Thick- 
nesses OF  THE  Intermuscular 
Connective  Tissue  of  Man. 
X  240. — (Stohr.) 


Fig.  9. — Elastic  Fibers  of  the 
Subcutaneous  Areolar  Tissue  of 
a  Rabbit. — {AiteiSchdfer.) 


Cartilaginous  Tissue. — This  form  of  connective  tissue  differs  from  the 
preceding  varieties  chiefly  in  its  density.  As  a  rule,  it  is  firm  in  consistency, 
though  somewhat  elastic.  It  is  opaque,  bluish-white  in  color,  though  in  thin 
sections  translucent.  All  cartilaginous  tissues  consist  of  connective-tissue 
cells  embedded  in  a  solid  ground  substance.  According  to  the  amount 
and  texture  of  the  ground  substance,  three  principal  varieties  may  be 
distinguished: 

I.  Hyaline  cartilage,  in  which  the  cells,  relatively  few  in  number,  are  embedded 
in  an  abundant  quantity  of  ground  substance  (Fig.  10,  a.)  The  body  of 
the  cells  is  in  many  instances  distinctly  marked  off  from  the  surround- 
ing substance  by  concentric  Hnes  of  fibers,  w^hich  form  a  capsule  for  the 
cell.  Repeated  division  of  the  cell  substance  takes  place,  until  the  whole 
capsule  is  completely  occupied  by  daughter  cells.  The  ground  sub- 
stance is  pervaded  by  minute  channels,  which  communicate  on  one  hand 
with  the  spaces  around  the  cells,  and  on  the  other  with  lymph-spaces  in 
the  connective  tissue  surrounding  the  cartilage.  By  means  of  these 
channels,  nutritive  fluid  can  permeate  the  entire  structure.  Hyaline 
cartilage  is  found  on  the  ends  of  the  long  bones,  where  it  enters  into  the 
formation  of  the  joints;  between  the  ribs  and  sternum,  forming  the  costal 
cartilage,  as  well  as  in  the  nose  and  larynx. 


36  TEXT-BOOK  OF  PHYSIOLOGY 

2.  White  fibro-cartilage,  the  ground  substance  of  which  is  pervaded  by  white 
fibers,  arranged  in  bundles  or  layers,  between  which  are  scattered  the 
usual  encapsulated  cells.  (See  Fig.  io,c.)  White  fibro-cartilage  is 
tough,  resistant,  but  flexible,  and  is  found  in  joints  where  strength  and 
fixedness  are  required.  Hence  it  is  present  between  the  vertebrse, 
forming  the  intervertebral  discs,  between  the  condyle  of  the  lower  jaw 
•  and  the  glenoid  fossa,  in  the  knee-joint,  around  the  margins  of  the  joint 
cavities,  etc.  In  these  situations  it  assists  in  maintaining  the  apposition 
of  the  bones,  in  giving  a  certain  degree  of  mobility  to  the  joints,  and  in 
diminishing  the  effects  of  shock  and  pressure  imparted  to  the  bones. 


ABC 
Fig.  lo. — The  Three  Types  of  Cartilage:  A,  Hyaline;  B,  Elastic;  C,  Fibrous. — {Rad- 
asch).     a,  b,  Outer  and  inner  layers  of  perichondrium;  c,  young  cartilage  cells;  d,  older  cartilage 
cells;  e,  f,  capsule;  g,  lacuna. 

3.   Yellow  fibro-cartilage,  the  ground  substance  of  which  is  pervaded  by 

opaque,  yellow  elastic  fibers,  which  form,  by  the  interlacing  of  their 

branches,  a  complicated  network,  in  the  meshes  of  which  are  to  be  found 

the  usual  corpuscles.     (See  Fig.  io,B.)     As  these  fibers  are  elastic,  they 

impart  to  the  cartilage  a  very  considerable  degree  of  elasticity.     Yellow 

fibro-cartilage  is  well  adapted,  therefore,  for  entering  into  the  formation  of 

the  external  ear,  epiglottis.  Eustachian  tube,  etc. — structures  which 

require  for  their  functional  activity  a  certain  degree  of  flexibility  and 

elasticity. 

Osseous  Tissue. — Osseous  tissue,  as  distinguished  from  bone,  is  a 

member  of  the  connective-tissue  group,  the  ground  substance  of  which  is 

permeated  with  insoluble  lime  salts,  of  which  the  phosphate  and  carbonate 

are  the  most  abundant.     Immersed  in  dilute  solutions  of  hydrochloric  acid, 

they  can  be  converted  into  soluble  salts  and  dissolved  out.     The  osseous 

matrix  left  behind  is  soft  and  pliable.     When  boiled,  it  yields  gelatin. 

A  thin,  transverse  section  of  a  decalcified  bone,  when  examined  micro- 
scopically, reveals  a  number  of  small,  round,  or  oval  openings,  which  repre- 
sent transverse  sections  of  canals  which  run  through  the  bone,  for  the  most 
part  in  a  longitudinal  direction,  though  frequently  anastomosing  with  one 
another.  These  so-called  Haversian  canals  in  the  living  state  contain  blood- 
vessels and  lymphatics.     (See  Fig.  11.) 


THE  CONNECTIVE  TISSUES 


37 


Around  each  Haversian  canal  is  a  series  of  concentric  laminae,  composed 
of  white  fibers.  Between  every  two  laminae  are  found  small  cavities  (lacunae), 
from  which  radiate  in  all  directions  small  canals  (canahculi),  which  com- 
municate freely  with  one  another.  The  Haversian  canals,  with  their  associ- 
ated lacunae  and  canaliculi,  form  a  system  of  intercommunicating  passages, 
which  circulate  lymph  destined  for  the  nourishment  of  bone.  Each  lacuna 
contains  the  bone  corpuscle,,  which  bears  a  close  resemblance  to  the  usual 
branched  connective-tissue  corpuscle,  and  whose  function  appears  to  be  the 
maintenance  of  the  nutrition  of  the  bone. 

Periosteum. 

Outer  ground  lamellae. 

Haversian  canals. 


Haversian  jameilce. 

Interstitial  lamellas. 
Inner  ground  lamellae. 

Marrow. 


Fig.  II. — From  a  Cross-section  of  a  Metacarp  of  Man.     X  50.     The  Haversian  canals 
contain  a  little  marrow  (fat-cells).     Resorption  line  at  h. — {Stohr). 

The  surface  of  every  bone  in  the  living  state  is  invested  with  a  fibrous 
membrane,  the  periosteum,  except  where  it  is  covered  with  cartilage.  The 
inner  surface  of  this  membrane  is  loose  in  texture,  and  supports  a  fine  plexus 
of  capillary  blood-vessels  and  numerous  protoplasmic  cells — the  osteoblasts. 
As  this  layer  is  directly  concerned  in  the  formation  of  bone,  it  is  spoken|^of 
as  the  osteogenetic  layer. 

A  section  of  a  bone  shows  that  it  is  composed  of  two  kinds  of  tissue — 
compact  and  cancellated.  The  compact  is  dense,  resembling  ivory,  and  is 
found  on  the  outer  portion  of  the  bone;  the  cancellated  is  spongy,  and  appears 
to  be  made  up  of  thin,  bony  plates,  which  intersect  one  another  in  all  direc- 
tions, and  is  found  in  greatest  abundance  in  the  interior  of  the  bones.  The 
shaft  of  a  long  bone  is  hollow.  This  central  cavity,  which  extends  from  one 
end  of  the  bone  to  the  other,  as  well  as  the  interstices  of  the  cancellated  tissue, 
is  filled  in  the  living  state  with  marrow.  The  marrow  or  medulla  is  composed 
of  a  connective-tissue  framework  supporting  blood-vessels.  In  its  meshes 
are  to  be  found  characteristic  bone  cells  or  osteoblasts,  the  function  of  which 
is  supposed  to  be  the  formation  of  bone.  In  the  long  bones  the  marrow  is 
yellow,  from  the  presence  in  the  connective-tissue  corpuscle  of  fat  globules. 
In  the  cancellated  tissue,  near  the  extremities  of  the  long  bones,  this  fatty 
deposition  does  not  take  place  to  the  same  extent,  and  the  marrow  appears 
red.  The  cells  of  the  red  marrow  are  believed  to  give  birth  indirectly  to 
the  red  blood-corpuscles. 

Physical  and  Physiologic  Properties  of  Connective  Tissues^— 
Among  the  physical  properties  may  be  mentioned  consistency,  cohesion,  and 
elasticity.     Their  consistency  varies  from  the  semiliquid  to  the  solid  state,  and 


38  TEXT-BOOK  OF  PHYSIOLOGY 

depends  mainly  on  the  quantity  of  water  which  enters  into  their  composition. 
Their  cohesion,  except  in  the  softer  varieties,  is  very  considerable,  and  offers 
great  resistance  to  traction,  pressure,  torsion,  etc.  In  all  the  movements  of 
the  body,  in  the  contraction  of  muscles,  in  the  performance  of  work,  the 
consistence  and  cohesion  of  these  tissues  play  most  important  roles.  Wher- 
ever the  various  forms  of  connective  tissue  are  found,  their  chemic  composi- 
tion and  structure  are  in  relation  to  their  functions.  If  traction  be  the  pre- 
ponderating force,  the  structure  becomes  fibrous  as  in  ligaments  and  tendons, 
and  the  cohesion  greatest  in  the  longitudinal  direction.  If  pressure  be 
exerted  in  all  directions,  as  upon  membranes,  the  fibers  interlace  and  offer 
a  uniform  resistance.  When  pressure  is  exerted  in  a  definite  direction,  as 
on  the  extremities  of  the  long  bones,  the  tissue  becomes  expanded  and  can- 
cellated. The  lamellae  of  the  cancellated  tissue  arrange  themselves  in 
curves  which  correspond  to  the  direction  of  the  greatest  pressure  or  traction. 
Extensibility  is  not  a  characteristic  feature,  except  in  those  forms  containing 
an  abundance  of  yellow  elastic  fibers.  The  elasticity  is  an  essential  factor 
in  many  physiologic  actions.  It  not  only  opposes  and  limits  forces  of  trac- 
tion, pressure,  torsion,  etc.,  but  on  their  cessation  returns  the  tissues  or 
organs  to  their  original  condition.  Elasticity  thus  assists  in  maintaining 
the  natural  form  and  position  of  the  organs  by  counterbalancing  and  oppos- 
ing temporarily  acting  forces. 

The  Skeleton. — The  connective  tissues  in  their  entirety  constitute  a 
framework  which  presents  itself  under  two  aspects:  (i)  As  a  solid,  bony 
skeleton,  situated  in  the  trunk  and  limbs,  affording  attachment  for  muscles 
and  viscera;  (2)  as  a  fine,  fibrous  skeleton,  found  everywhere  throughout 
the  body,  connecting  the  various  viscera  and  affording  support  for  the 
epithelial,  muscle,  and  ner\'e  tissues. 


CHAPTER  V 
THE  PHYSIOLOGY  OF  MOVEMENT 

Of  the  four  phenomena  presented  by  an  animal,  that  which  more  im- 
mediately interests  the  physiologist  is  movement,  for  the  reason  that  it  is 
not  only  the  animal's  most  characteristic  form  of  activity,  and  that  which 
serves  to  distinguish  it  in  the  main  from  forms  of  vegetable  life,  but  its 
solution  affords  an  explanation  of  many  physiologic  processes  occurring 
within  the  human  body.  It  is  also  for  this  reason  that  movement  constitutes 
for  the  most  part  the  subject-matter  of  physiologic  experimentation. 

The  movements  of  the  body  may  for  convenience  be  divided  into  two 
groups,  viz.,  external  and  internal. 

The  External  or  Skeleto-muscle  Movements. — ^The  external  move- 
ments are  exhibited  mainly  by  the  head  and  extremities  and  may  be  either 
special  as  when  the  animal  changes  the  relation  of  one  part  of  the  body  to 
another,  or  general,  as  when  it  changes  its  position  relatively  to  the  en- 
vironment as  in  the  various  acts  of  locomotion.  The  external  movements 
are  the  result  of  the  cooperation  of  the  bones  of  the  skeleton  and  the  muscles 
which  are  attached  to  them.  The  skeleton  imparts,  nevertheless,  a  certain 
degree  of  rigidity  and  fixity  to  the  body;  were  it  not  for  this  the  body  would 
be  but  a  shapeless  mass  and  incapable  of  performing  any  of  its  characteristic 
external  movements. 

The  joints  of  the  skeleton  permit  various  parts  of  the  body  to  move 
and  to  change  their  relation  to  each  other.  Without  the  presence  of  joints 
such  as  are  found  in  the  vertebral  column  and  in  the  limbs,  external  move- 
ments would  be  impossible.  The  muscles  impart  movements  to  various 
parts  of  the  body.  This  they  do  by  suddenly  shortening  and  widening 
whereby  their  extremities  are  approximated.  The  majority  of  the  muscles 
of  the  body  are  attached  to  the  bones  in  such  a  manner  that  when  their  form 
is  altered,  they  change  not  only  the  relation  of  the  bones  with  reference 
to  each  other,  but  perhaps  also  the  individual's  relation  to  surrounding  ob- 
jects. The  muscles  thus  become  the  active  organs  in  both  motion  and 
locomotion,  in  contradistinction  to  the  bones  which  may  be  regarded  as  the 
passive  organs  in  the  performance  of  the  corresponding  movements. 

In  the  execution  of  the  movements  the  animal,  of  necessity,  meets  with 
various  forms  of  resistance,  viz.,  gravity,  cohesion,  friction,  etc.,  which  tend 
to  oppose  the  movement.  When  its  different  parts  are  appHed  or  directed, 
either  volitionally  and  in  a  determinate  manner,  or  non-volitionally  and  in 
an  indeterminate  or  reflex  manner,  to  the  overcoming  of  these  opposing 
forces  in  the  environment,  the  animal  may  be  said  to  be  doing  work. 

In  the  animal  as  in  the  physical  machine,  work  is  accomplished  by  the 
intermediation  of  levers.  In  the  animal  machine,  the  levers  are  found  in 
the  bones  of  the  skeleton  and  more  particularly  in  the  long  bones  of  the 
extremities,  the  fulcra  of  which,  the  points  around  which  they  move,  lie  in 
the  joints. 

39 


40  TEXT-BOOK  OF  PHYSIOLOGY 

That  a  lever  may  be  effective  as  an  instrument  for  the  accompHshment  of 
work  it  must  not  only  be  capable  of  moving  around  its  fulcrum,  but  it  must 
at  the  same  time  be  acted  on  by  two  opposing  forces,  one  passive,  the  other 
active.  In  the  movements  of  the  bony  levers  of  the  animal  body,  the 
passive  forces  to  be  overcome  are  largely  those  connected  with  the  environ- 
ment, e.g.,  gravity,  cohesion,  friction,  etc.,  the  active  forces  by  which 
these  are  opposed  and  overcome  through  the  mediation  of  the  bony  levers, 
are  found  in  the  muscles  attached  to  them.  The  muscles  are  therefore  to 
be  regarded  as  the  seat  of  those  active  energies  that  impart  movement  to 
the  levers. 

The  Internal  or  Vasculo-  and  Viscero-muscle  Movements  and 
Gland  Activities. — The  internal  movements  are  exhibited  by  the  viscera, 
the  vascular  apparatus  and  by  glands,  and,  though  less  obvious,  are  no  less 
characteristic. 

The  viscera  undergo  variations  in  size  from  moment  to  moment  as  a 
result  of  the  contraction  and  relaxation  of  the  non-striated  muscle  fibers, 
composing  in  part  their  walls  and  thus  regulate  the  reception  and  discharge 
of  their  contents. 

The  vascular  apparatus,  and  its  adjunct,  the  lymph-vessel  apparatus,  is 
engaged  in  the  distribution  of  blood  and  nutritive  material  throughout  the 
body.  The  cavities  of  the  heart  are  alternately  increased  and  diminished 
in  size  by  the  alternate  relaxation  and  contraction  of  the  muscle-fibers 
composing  the  walls  of  the  heart.  During  the  relaxation  the  cavities  fill 
with  blood  and  during  the  contraction  the  blood  is  driven  through  the  vessels 
in  opposition  to  the  friction  presented  by  their  walls;  while  the  vessels  them- 
selves and  especially  the  arteries,  by  virtue  of  the  presence  of  elastic  fibers 
and  the  non-striated  muscle-fibers  in  their  walls,  increase  and  decrease  in 
caliber  from  moment  to  moment  and  thus  regulate  the  amount  of  blood 
flowing  through  them  in  accordance  with  the  physiologic  needs  of  the  organ 
to  which  they  are  distributed. 

The  glands  and  more  especially  their  epithelial  investments  are  the  seat 
of  certain  molecular  movements  the  result  of  which  is  the  production  and 
discharge  of  a  secretion  destined  to  play  a  more  or  less  important  part  in  the 
maintenance  of  the  activities  of  the  body. 

In  the  performance  of  their  functions  these  organs  also  meet  resistances, 
e.g.,  cohesion,  friction,  elasticity,  etc.,  and  when  they  are  applied  to  the 
overcoming  of  these  resistances  or  forces,  as  they  are  in  the  performance  of 
their  functions,  it  can  also  be  said  that  they  too  are  doing  work.  The  co- 
operation of  external  and  internal  organs  is  necessary,  however,  not  only 
for  the  maintenance  of  the  life  of  the  animal  but  also  for  the  accomplishment 
of  external  work. 

Tissue  Stimuli. — ^The  various  tissues  of  the  body,  mentioned  in  fore- 
going paragraphs,  though  irritable  do  not  possess  spontaneity  of  action, 
but  require  for  the  manifestation  of  their  characteristic  forms  of  activity 
the  application  of  a  stimulus. 

Thus  the  skeletal  muscles  and  glands  though  capable  of  being  excited 
to  activity  by  various  artificial  stimuli,  require  for  the  exhibition  of  their 
normal  activity  the  arrival  of  the  physiologic  stimulus,  the  nerve  impulse^ 
developed  in  and  transmitted  to  them  by  the  nerve-tissue. 

The  visceral  and  vascular  muscles  though  apparently  capable  of  being 


THE  PHYSIOLOGY  OF  MOVEMENT 


41 


CSC. 


Fig.  12. — Diagram  Showing  the  Relaton  of  Skeletal,  Muscle  and  Nerve 
Tissues.  (G.  Bachman.)  f.a.  Eones  of  the  forearm  representing  the  skeletal  tissue;  e.j. 
the  elbow-joint,  the  fulcrum  of  the  lever  formed  by  the  bones  of  the  forearm;  W.  a  weight 
acting  in  a  downward  direction  and  representing  the  passive  force  of  gravity;  sk.m.  a 
skeletal  muscle  acting  in  an  upward  direction  and  the  source  of  the  active  power  to  be  ap- 
plied to  the  lever;  sp.c.  transection  of  the  spinal  cord  showing  the  relation  of  the  white  and 
the  gray  matter;  m.c.  a  motor  cell  in  the  anterior  horn  of  the  gray  matter;  ef.n.  an  effer- 
ent nerve-fiber  connecting  the  motor  cell  from  which  it  arises  with  the  skeletal  muscle  and 
contained  in  the  ventral  roots  of  the  spinal  nerves;  af.n.  an  afferent  nerve-fiber  arising  from 
the  ganglion  cell  along  its  course  and  connecting  the  skin,  s.,  on  the  one  hand  with  the  spinal 
cord  on  the  other  hand  and  contained  in  the  dorsal  roots  of  the  spinal  nerves;  c.s.c. 
coronal  section  of  the  cerebrum  showing  the  relation  of  the  gray  to  the  white  matter;  v.c. 
a  volitional  or  motor  cell;  d.a.  a  descending  axon  or  nerve-fiber  connecting  the  volitional 
cell  from  which  it  arises  with  the  motor  cell  in  the  spinal  cord;  s.c.  a  sensor  cell;  a.a.  an 
ascending  axon  or  nerve-fiber  connecting  a  receptive  cell  from  which  it  arises  (not  shown  in 
the  diagram)  with  the  sensor  cell  in  the  gray  matter  of  the  cerebrum.  The  nerve-fibers 
which  pass  outward  from  the  spinal  cord  to  the  glands,  blood-vessels,  and  the  muscle 
walls  of  the  viscera,  have  for  the  sake  of  simplicity  been  omitted  from  the  diagram. 


42 


TEXT-BOOK  OF  PHYSIOLOGY 


excited  to  activity  by  agencies  other  than  the  nerve  impulse  are  nevertheless 
augmented  or  inhibited  in  their  activity  from  moment  to  moment  by  nerve 
impulses. 

It  is  evident  therefore  that  the  activities  of  the  organs  and  tissues  which 
are  engaged  in  promoting  the  work  of  the  body  are  excited  to  action  and 
controlled  by  the  nerve-tissue,  a  fact  which  presupposes  an  anatomic  con- 
nection between  them. 

For  an  understanding  of  the  mode  of  excitation  of  the  motor  organs  and 
the  manner  in  which  they  cooperate  in  the  performance  of  any  given  move- 
ment, a  brief  preliminary  account  of  the  general  arrangement  and  mode  of 
action  of  the  nerve-tissue  will  be  found  helpful. 

The  General  Relation  of  the  Nerve-tissue  to  Peripheral  Organs. — 
The  nerve-tissue  is  arranged  partly  in  masses  contained  within  the  cavities 


,sp.c. 


Fig.  13. — Diagram  Showing  the  Structures  Involved  in  the  Production  of  Reflex 
Actions. — (G.  Bachman.)  r.s.  Receptive  surface;  af.n.  afferent  nerve;  e.c.  emissive  or  motor 
cells  in  the  anterior  horn  of  the  gray  matter  of  the  spinal  cord,  sp.c;  ef.n.  efferent  nerves  distributed 
to  responsive  organs,  e.g.,  directly  to  skeletal  muscles,  sk.m.,  and  indirectly  through  the  inter- 
mediation of  sympathetic  ganglia,  sym.g.,  to  blood-vessels,  b.v.,  and  to  glands,  g.  The  nerves 
distributed  to  the  walls  of  the  viscera  are  not  represented;  v.r.  and  d.r.,  ventral  and  dorsal  roots 
and  spinal  nerves. 


of  the  head  and  spinal  column  (the  encephalon  or  brain  and  spinal  cord), 
forming  the  central  organs  of  the  nerve  system,  and  partly  in  the  form  of 
cords  or  nerves  (the  cranial  and  spinal  nerves),  forming  the  peripheral  organs 
of  the  nerve  system.  The  latter  connect  the  former  not  only  with  muscles, 
glands,  blood-vessels,  and  viscera,  but  with  the  skin,  mucous  membranes, 
etc.,  as  well. 

(The  relation  of  the  nerve-tissue  to  the  skeletal  muscles,  to  glands,  to 
blood-vessels,  and  viscera  are  shown  in  Figs.  12,  13.) 

The  Spinal  Cord. — ^The  spinal  cord  is  more  especially  the  seat  of  origin 
of  the  nerve  energy  that  immediately  excites  and  controls  the  activity  of 
the  motor  organs,  and  a  knowledge  of  its  structure,  of  its  relations  to  these 
organs,  and  the  manner  in  which  it  is,  in  turn,  excited  to  activity  is  necessary 
to  an  understanding  of  the  problem  of  movement. 


THE  PHYSIOLOGY  OF  MOVEMENT  43 

The  spinal  cord  is  narrow  and  cylindric  in  shape  and  occupies  the  spinal 
canal  from  the  level  of  the  first  vertebra  as  far  down  as  the  second  or  third 
lumbar  vertebra.  It  presents  both  on  its  ventral  and  dorsal  surfaces  a 
deep  longitudinal  fissure  which  partly  divide  the  cord  into  halves,  a  right 
and  a  left. 

The  Spinal  Nerves. — ^To  each  side  of  the  spinal  cord  there  are  attached 
thirty-one  nerves,  which  as  they  pass  out  through  foramina  in  the  walls  of 
the  spinal  column  are  termed  spinal  nerves.  Each  spinal  nerve  is  con- 
nected with  the  spinal  cord  by  two  roots,  termed  from  their  relation  to  the 
ventral  and  dorsal  surfaces,  the  vetitral  and  dorsal  roots.  A  short  distance 
from  the  spinal  cord  these  two  roots  unite  to  form  the  spinal  nerve  proper, 
after  which  the  fibers  composing  it  pass  outward  to  be  distributed  to  various 
peripheral  organs. 

A  transverse  section  of  the  spinal  cord  shows  that  each  half  is  composed 
externally  of  white  matter,  and  internally  of  gray  matter.  The  gray  matter 
in  each  half  is  arranged  in  the  form  somewhat  of  a  crescent  united  in  the 
median  line  by  a  transverse  band  or  commissure,  the  whole  forming  a 
figure  resembling  the  letter  H.  Though  varying  in  shape  in  different 
regions  of  the  cord,  the  gray  matter  in  all  situations  presents  on  either  side 
an  anterior  or  ventral  and  a  posterior  or  dorsal  horn  (Fig.  13). 

In  the  ventral  horns  of  the  gray  matter  are  located  large  nerve-cells 
which  gives  origin  to  nerve-fibers;  these  fibers  early  in  embryologic  develop- 
ment become  connected  with  structures  for  which  they  are  by  heredity 
destined.  With  the  growth  of  the  embryo  and  the  development  of  the 
limbs  there  is  a  corresponding  growth  and  development  of  the  nerve-fibers 
until  they  attain  the  size  characteristic  of  the  adult.  The  fibers  collectively 
constitute  the  ventral  roots.  Histologic  investigation  has  shown  that  some 
of  the  ventral  roots  contain  two  groups  of  nerve-fibers  one  of  large  and  one 
of  small  size.  The  fibers  of  large  size  pass  forward  and  become  directly 
connected  with  the  skeletal  muscle.  The  fibers  of  small  size  pass  forward 
but  for  a  short  distance,  after  which  they  leave  the  ventral  root  and  enter  a 
group  of  nerve-cells  (the  so-called  sympathetic  ganglion)  around  the  den- 
drites of  which  their  terminal  branches  are  disposed;  from  these  cells  new 
non-medullated  fibers  arise  which  pass  backward  into  the  spinal  nerve  and 
in  association  with  the  fibers  of  large  size  pass  to  vascular  muscleand  gland 
epithelium  more  especially  of  the  skin  and  mucous  membrane.  In  some 
situations  the  fibers  of  small  size  pass  through  or  alongside  of  these  ganglia 
to  other  ganglia  more  or  less  distant  in  which  they  terminate  in  a  similar 
manner;  from  the  cells  of  these  ganglia  new  non-medullated  nerve-fibers 
arise  which  pass  directly  to  visceral  and  vascular  muscles  and  perhaps  to  gland 
epithelium.  The  two  portions  of  these  nerves  of  small  size  are  known  re- 
spectively, as  pre-  and  post-ganglionic. 

The  nerve-fibers  of  small  size  found  in  the  ventral  roots  of  the  spinal 
nerves  and  in  some  of  the  encephalic  nerves  and  distributed  by  way  of 
the  ganglia  to  visceral  and  vascular  muscles  and  to  gland  epithelium  con- 
stitute a  system  of  nerves  termed  the  autonomic  system  (see  Chapter  XXVI). 

The  dorsal  root  fibers  originate  from  cells  situated  outside  of,  but 
derived  from  the  spinal  cord.  The  cells  in  this  situation  at  first  develop  two 
processes  from  opposite  ends,  which  at  a  later  period  shift  their  position, 
unite  and  form  a  single  process  after  which,  a  division  into  two  branches 


44  TEXT-BOOK  OF  PHYSIOLOGY 

takes  place,  one  of  which  passes  toward  and  into  the  spinal  cord  and  becomes 
related  in  part  with  the  nerve-cells  in  the  ventral  horn  of  the  gray  matter, 
the  other  of  which  passes  to  the  periphery  and  becomes  connected  with 
the  skin  and  mucous  membrane  covering  the  surfaces  of  the  body  (Fig.  13). 
The  surfaces  of  the  body  thus  become  associated  anatomically  and  hence 
physiologically  with  the  motor  organs. 

With  the  growth  and  development  of  the  brain  and  especially  of  the 
cerebrum,  nerve-cells  make  their  appearance  in  the  outer  or  cortical  por- 
tion. From  the  cells  of  certain  specialized  regions  nerve-fibers  pass  down- 
ward as  far  as  the  medulla  oblongata,  where  for  the  most  part,  they  cross 
over  to  the  opposite  side  and  then  descend  the  spinal  cord  to  give  off  branches 
or  fibers  at  different  levels  which  become  related  histologically  with  the 
nerve-cells  in  the  ventral  horns  of  the  gray  matter.  The  cerebrum  is  thus 
brought  into  relation  through  the  intermediation  of  the  spinal  cord,  with 
skeletal,  vascular  and  visceral  muscles  and  gland  epithelium  (Fig.  12). 

Coincident  with  the  development  of  the  nerve-fibers  descending  from  the 
cerebrum,  to  the  spinal  cord,  nerve-cells  are  developing  in  the  more  central 
regions  of  the  gray  matter  of  the  spinal  cord  from  which  nerve-fibers  arise 
that  cross  to  the  opposite  side  of  the  cord  and  then  ascend  directly  or  in- 
directly to  the  cerebrum  where  their  terminal  branches  come  into  physiologic 
relationship  with  groups  of  specialized  nerve-cells  in  different  regions  of  the 
cortex.  Of  the  fibers  of  the  dorsal  roots  some,  as  previously  stated,  pass 
forward  to  the  gray  cells  in  the  ventral  horn;  others,  however,  become 
associated  with  the  more  centrally  located  nerve -cells  above  alluded  to. 
The  surfaces  of  the  body  are  thus  brought  into  relationship  through  the 
intermediation  of  the  spinal  cord  with  the  cortex  of  the  cerebrum. 

The  statements,  made  in  the  foregoing  paragraphs  in  reference  to  the 
spinal  nerves,  hold  true  for  the  twelve  cranial  nerves  which  correspond 
physiologically  at  least  with  the  ventral  and  dorsal  roots  of  the  spinal  nerves. 
Their  relation  to  the  cortex  of  the  cerebrum  is  similar. 

The  nerve-fibers  connecting  the  cerebrum  with  the  motor  organs  con- 
stitute the  outgoing  or  efferent  side  of  the  nerve  system.  The  nerve-fibers 
connecting  the  surfaces  of  the  body  with  the  cerebrum  constitute  the  in- 
going or  afferent  portion  of  the  nerve  system. 

The  Efferent  Spinal  Nerve-cells. — It  has  been  experimentally  demon- 
strated that  each  nerve-cell  in  the  ventral  horn  not  only  generates  but  under 
given  conditions  discharges  a  form  of  energy  termed  a  7terve  impulse,  which 
is  transmitted  by  the  nerve-fiber  arismg  from  it  and  by  way  of  the  ventral 
roots  of  the  spinal  nerves  directly  to  skeletal  muscles  and  indirectly  through 
the  sympathetic  ganglia  and  their  branches  to  glands,  blood-vessels  and 
walls  of  viscera.     (See  Fig.  13.) 

The  arrival  of  the  nerve  impulse  at  once  calls  forth  the  form  of  activity 
characteristic  of  the  structure  stimulated.  Thus  the  muscle,  for  example, 
passes  from  the  passive  to  the  active  state,  that  is,  the  muscle  becomes  shorter 
and  thicker,  and  the  bone  to  which  it  is  attached  is  moved.  This  is  at 
once  followed  by  a  return  of  the  muscle  to  the  passive  state;  that  is,  it 
lengthens,  becomes  narrower,  and  resumes  its  original  form;  the  bone  at 
the  same  time  returns  to  its  former  position.  Coincident  with  this  change 
of  shape  there  is  a  liberation  of  heat  and  electricity.  The  nerve  impulse 
which  occasions  this  transformation  of  potential  into  kinetic  energy  is  the 


THE  PHYSIOLOGY  OF  MOVEMENT  45 

normal  or  the  physiologic  stimulus.  The  glands  in  response  to  the  nerve 
impulse  pour  out  a  secretion,  the  blood-vessels  and  viscera  change  their 
caliber;  all  these  tissues  responding  to  the  nerve  impulse  in  a  characteristic 
manner  are  said  to  be  irritable. 

The  nerve-cells  in  the  ventral  horns  of  the  gray  matter  of  the  spinal  cord 
are  therefore  the  sources  of  the  energy  requisite  for  the  physiologic  excitation 
of  the  motor  organs.  If  they  are  destroyed  either  experimentally  or  by 
pathologic  processes,  the  energy  is  no  longer  discharged  and  the  motor  organs 
become  incapable  of  performing  their  functions  in  a  physiologic  manner. 

The  nerve-cells,  though  extremely  irritable,  do  not  possess  spontaneity 
of  action,  but  require  for  their  excitation  the  arrival  and  stimulating  action 
of  other  nerve  impulses.  These  may  come  (i)  from  the  periphery  through 
afferent  nerve- fibers  by  way  of  the  dorsal  roots  of  the  spinal  nerves;  and 
(2)  from  motor  nerve-cells  in  the  cortex  of  the  cerebral  portion  of  the  brain, 
through  descending  axons  or  nerve-fibers. 

In  the  first  instance  the  resulting  movements  taking  place  in  response 
to  a  peripheral  or  surface  stimulation  and  independently  of  volitional  or 
emotional  activity  are  termed  reflex  movements;  in  the  second  instance  the 
resulting  movements  taking  place  in  response  to  volitional  or  emotional 
activities  are  termed  volitional  or  emotional  movements. 

In  the  case  of  reflex  movements,  the  nerve  impulses  are  primarily  devel- 
oped in  specialized  organs  located  in  the  skin  or  mucous  membranes  and  as 
a  result  of  the  impact  of  various  external  agents,  which  for  this  reason  are 
termed  stimuli.  The  nerve  impulses  thus  developed  are  transmitted  by  the 
afferent  nerves  to  the  efferent  or  motor  nerve-cells  which  are  in  turn  excited 
to  activity  as  a  result  of  which,  motor  organs  are  aroused  to  action. 

In  the  case  of  both  the  volitional  skeletal-muscle  movements  and  the 
affective  or  emotional  viscero-muscle  and  vasculo-muscle  movements  as 
well  as  the  activities  of  glands,  the  nerve  impulses  which  cause  the  move- 
ments are  discharged  from  certain  motor  or  efferent  nerve-cells  in  the  gray 
matter  of  the  cortex  of  the  cerebrum  and  transmitted  by  descending  axons 
or  nerve-fibers  direct  to  the  nerve-cells  in  the  spinal  cord,  by  which  they  in 
turn  are  excited  to  activity. 

The  movements  due  to  cerebral  or  psychic  activity  are,  however,  the 
immediate  or  the  more  or  less  remote  effects  of  sensations  which  have  been 
evoked  in  the  sense  areas  of  the  brain,  by  the  arrival  of  nerve  impulses  coming 
through  ascending  axons,  or  nerve-fibers  from  peripheral  sense  organs,  e.g., 
skin,  eye,  ear,  nose,'  tongue,  and  which  have  been  developed  by  the  impact  of 
objects  in  the  external  world. 

The  only  organ  that  can  be  properly  said  to  be  excited  to  action  by  a 
volitional  act  is  the  skeletal  muscle;  the  glands,  blood-vessels,  and  viscera 
and  the  autonomic  nerves  which  control  them  are  apparently  only  influenced 
in  their  activity  by  emotional  states.  Why  this  difference  should  exist  it 
is  difficult  to  state. 

The  nerve-cells  and  their  related  nerve-fibers,  responding  by  the  develop- 
ment and  conduction  of  nerve  impulses  are  also  said  to  be  irritable.  The 
transformation  of  energy,  however,  manifests  itself  mainly  as  electricity  and 
molecular  motion.  The  animal  body  in  its  entirety  may  therefore  be  regarded 
as  a  machine  for  the  transformation  of  potential  energy  into  kinetic 
energy,  viz.,  heat  and  electricity,  movements  of  muscles  and  bony  levers, 


46  TEXT-BOOK  OF  PHYSIOLOGY 

secretion,  sensation  and  other  forms  of  nerve  activity.  When  muscles  and 
bones  are  applied  to  the  overcoming  of  opposing  forces,  mechanic  work  is 
accomplished.  In  the  following  chapters  some  of  the  problems  connected 
with  the  activities  of  the  primary  mechanisms,  the  skeletal,  muscle  and 
nerve  tissues  will  be  first  considered  and  subsequently  some  of  the  problems 
connected  with  the  activities  of  the  secondary  mechanisms. 


CHAPTER  VI 
THE  PHYSIOLOGY  OF  THE  SKELETON 

The  skeleton  in  its  entirety  determines  the  plan  of  organization  of  the 
animal  body  and  imparts  to  it  its  characteristic  features.  In  its  entirety  it 
serves  for  the  attachment  of  muscles,  the  support  of  viscera  and  by  reason  of 
the  relation  of  the  bones  one  to  another,  permits  of  a  great  variety  of  move- 
ments. The  skeleton  may  be  divided  into  an  axial  and  an  appendicular 
portion. 

The  Axial  Portion. — ^The  axial  portion  consists  of  the  bones  of  the  head, 
of  the  vetebral  column  and  the  ribs.  The  vertebral  column  is  the  foundation 
element  and  the  center  around  which  the  appendicular  portions  are  de- 
veloped and  arranged  with  a  certain  degree  of  conformity.  It  is  composed 
of  a  series  of  superimposed  bones,  the  vertebrae,  which  increase  in  size  from 
above  downward  as  far  as  the  brim  of  the  pelvic  cavity.  Superiorly,  it 
supports  the  skull;  laterally,  it  affords  attachment  for  the  ribs,  which  in 
turn  support  the  weight  of  the  upper  extremities;  below,  it  rests  upon  the 
pelvic  bones,  which  transmit  the  weight  of  the  body  to  the  inferior  extremities. 
The  bodies  of  the  vertebrae  are  united  one  to  another  by  tough  elastic  discs 
of  fibro-cartilage,  which,  collectively,  constitute  about  one-quarter  of  the 
length  of  the  vertebral  column.  The  vertebrae  are  held  together  by  ligaments 
situated  on  the  anterior  and  posterior  surfaces  of  their  bodies,  and  by  short, 
elastic  ligaments  between  the  neural  arches  and  processes.  These  structures 
combine  to  render  the  vertebral  column  elastic  and  flexible,  and  enable  it  to 
resist  and  diminish  the  force  of  shocks  communicated  to  it.  In  all  the  static 
and  dynamic  states  of  the  body  if  plays  a  most  essential  role.  The  character 
and  the  arrangement  of  the  bones  of  the  axial  portion  endow  the  animal 
mechanism  with  a  certain  degree  of  fixity  combined  with  slight  mobility. 

The  Appendicular  Portion. — ^The  appendicular  portion  consists 
of  the  bones  of  the  arms  and  legs,  the  scapular  and  pelvic  arches.  By 
reason  of  its  character  and  anatomic  arrangement,  the  animal  body  is  en- 
dowed with  extreme  mobility,  enabling  the  animal  to  execute  a  great  variety 
of  rapid  and  extensive  movements  which,  however,  vary  in  degree  in  different 
animals  in  accordance  with  their  organization  and  the  nature  of  their 
environment. 

For  the  manifestation  of  the  activities  of  the  animal  it  is  essential  that  the 
relation  of  the  various  portions  of  the  bony  skeleton  to  one  another  shall  be 
such  as  to  permit  of  movement  while  yet  retaining  close  apposition.  This 
is  accomplished  by  the  mechanical  conditions  which  have  been  evolved  at 
the  points  of  union  of  bones,  and  which  are  technically  known  as  articulations 
or  joints. 

A  consideration  of  the  body  movements  involves  an  account  of  (i)  the 
static  conditions,  or  those  states  of  equilibrium  in  which  the  body  is  at  rest 
— e.g.,  standing,  sitting;  (2)  the  dynamic  conditions,  or  those  states  of 
activity  characterized  by  movement — e.g.,  walking,  running,  etc.  In  this 
connection,  however,  only  those  physical  and  physiologic  pecuharities  of  the 

47 


48  TEXT-BOOK  OF  PHYSIOLOGY 

skeleton,  especially  in  its  relation  to  joints,  will  be  referred  to,  which  underlie 
and  determine  both  the  static  and  dynamic  states  of  the  body. 

Structure  of  Joints. — The  structures  entering  into  the  formation  of 
joints  are: 

1.  Bones,  the  articulating  surfaces  of  which  are  often  more  or  less  expanded, 

especially  in  the  case  of  long  bones,  and  at  the  same  time  variously 
modified  and  adapted  to  one  another  in  accordance  with  the  character 
and  extent  of  the  movements  which  there  take  place. 

2.  Hyaline  cartilage,  which  is  closely  applied  to  the  articulating  end  of  each 

bone.  The  smoothness  of  this  form  of  cartilage  facilitates  the  move- 
ments of  the  opposing  surfaces,  while  its  elasticity  diminishes  the  force 
of  shocks  and  jars  imparted  to  the  bones  during  various  muscular  acts. 
In  a  number  of  joints,  plates  or  discs  of  white  fibro-cartilage  are  inserted 
between  the  surfaces  of  the  bones. 

3.  A  synovial  membrane,  which  is  attached  to  the  edge  of  the  hyaline  cartilage, 

entirely  inclosing  the  cavity  of  the  joint.  This  membrane  is  composed 
largely  of  connective  tissue,  the  inner  surface  of  which  is  lined  by  endo- 
thelial cells,  which  secrete  a  clear,  colorless,  viscid  fluid — the  synovia. 
This  fluid  not  only  fills  up  the  joint-cavity,  but,  flowing  over  the 
articulating  surfaces,  diminishes  or  prevents  friction. 

4.  Ligaments — tough,  inelastic  bands,  composed  of  white  fibrous  tissue — 

which  pass  from  bone  to  bone  in  various  directions  on  the  different 
aspects  of  the  joint.     As  white  fibrous  tissue  is  inextensible  but  pliant, 
ligaments  assist  in  keeping  the  bones  in  apposition,  and  prevent  dis- 
placement while  yet  permitting  of  free  and  easy  movements. 
Classification  of  Joints. — All  joints  may  be  divided,  according  to  the 

extent  and  kind  of  movements  permitted  by  them,  into  (i)  diarthroses;  (2) 

amphiarthroses;  (3)  synarthroses. 

A.  Diarthroses. — In  this  division  of  the  joints  are  included  all  those  which 
permit  of  free  movement.  In  the  majority  of  instances  the  articulating 
surfaces  are  mutually  adapted  to  each  other.  If  the  articulating  sur- 
face of  one  bone  is  convex,  the  opposing  but  corresponding  surface  is 
concave.  Each  surface,  therefore,  represents  a  section  of  a  sphere  or 
a  cylinder,  which  latter  arises  by  rotation  of  a  line  around  an  axis  in 
space.  According  to  the  number  of  axes  around  which  the  movements 
take  place  all  diarthrodial  joints  may  be  divided  into: 

I.  Uniaxial  Joints. — In  this  group  the  convex  articulating  surface  is  a 
segment  of  a  cylinder  or  cone,  to  which  the  opposing  surface  more  or 
less  completely  corresponds.  In  such  a  joint  the  single  axis  of  rotation, 
though  nearly,  is  not  exactly  at  right  angles  to  the  long  axis  of  the  bone, 
and  hence  the  movements — flexion  and  extension — which  take  place 
are  not  confined  to  one  plane.  Joints  of  this  character — e.g.,  the  elbow, 
knee,  ankle,  the  phalangeal  joints  of  the  fingers  and  toes — are,  therefore, 
termed  ginglymi,  or  hinge-joints.  Owing  to  the  obliquity  of  their 
articulating  surfaces,  the  elbow  and  ankle  are  cochleoid  or  screw-ginglymi. 
Inasmuch  as  the  axes  of  these  joints  on  the  opposite  sides  of  the  body 
are  not  coincident,  the  right  elbow  and  left  ankle  are  right-handed 
screws;  the  left  elbow  and  right  ankle,  left-handed  screws.  In  the 
knee-joint  the  form  and  arrangement  of  the  articulating  surfaces  are 
such  as  to  produce  that  modification  of  a  simple  hinge  known  as  a 


THE  PHYSIOLOGY  OF  THE  SKELETON  49 

spiral  hinge,  or  helicoid.  As  the  articulating  surfaces  of  the  condyles 
of  the  femur  increase  in  convexity  from  before  backward,  and  as  the 
inner  condyle  is  longer  than  the  outer,  and,  therefore,  represents  a 
spiral  surface,  the  line  of  translation  or  the  movement  of  the  leg  is  also 
a  spiral  movement.  During  flexion  of  the  leg  there  is  a  simultaneous 
inward  rotation  around  a  vertical  axis  passing  through  the  outer  condyle 
of  the  femur;  during  extension  a  reverse  movement  takes  place.  More- 
over, the  slightly  concave  articulating  surfaces  of  the  tibia  do  not  revolve 
around  a  single  fixed  transverse  axis,  as  in  the  elbow-joint,  for  during 
flexion  they  slide  backward,  during  extension  forward,  around  a  shifting 
axis,  which  varies  in  position  with  the  point  of  contact. 

In  some  few  instances  the  axis  of  rotation  of  the  articulating  surface 
is  parallel  with,  rather  than  transverse  to  the  long  axis  of  the  bone,  and 
as  the  movement  takes  place  around  a  more  or  less  conic  surface, 
the  joint  is  termed  a  trochoid  or  pulley — e.g.,  the  odonto-atlantal  and 
the  radio-ulnar.  In  the  former  the  collar  formed  by  the  atlas  and  its 
transverse  ligament  rotates  around  the  vertical  odontoid  process  of  the 
axis.  In  the  latter  the  head  of  the  radius  revolves  around  its  own  long 
axis  upon  the  ulna,  giving  rise  to  the  movements  of  pronation  and 
supination  of  the  hand.  The  axis  around  which  these  two  movements 
take  place  is  continued  through  the  head  of  the  radius  to  the  styloid 
process  of  the  ulna. 

2.  Biaxial  Joints. — In  this  group  the  articulating  surfaces  are  unequally 

curved,  though  intersecting  each  other.  When  the  surfaces  lie  in  the 
same  direction,  the  joint  is  termed  an  ovoid  joint — e.g.,  the  radio-carpal 
and  the  atlanto-occipital.  As  the  axes  of  these  surfaces  are  vertical  to 
each  other,  the  movements  permitted  by  the  former  joint  are  flexion, 
extension,  adduction,  and  abduction,  combined  with  a  slight  amount 
of  circumduction;  the  latter  joint  permits  of  flexion  and  extension  of  the 
head,  with  inclination  to  either  side.  AVhen  the  surfaces  do  not  take  the 
same  direction,  the  joint,  from  its  resemblance  to  the  surfaces  of  a 
saddle,  is  termed  a  saddle-joint — e.g.,  the  trapezio-metacarpal.  The 
movements  permitted  by  this  joint  are  also  flexion,  extension,  adduction, 
abduction,  and  circumduction. 

3.  Polyaxial  Joints. — In  this  group  the  convex  articulating  surface  is  a 

segment  of  a  sphere,  which  is  received  by  a  socket  formed  by  the 
opposing  articulating  surface.  In  such  a  joint,  termed  an  enarthrodial 
or  ball-and-socket  joint — e.g.,  the  shoulder-joint,  hip-joint — the  distal 
bone  revolves  around  an  indefinite  number  of  axes,  all  of  which  intersect 
one  another  at  the  center  of  rotation.  For  simplicity,  however,  the 
movement  may  be  described  as  taking  place  around  axes  in  the  three 
ordinal  planes — viz.,  a  transverse,  a  sagittal,  and  a  vertical  axis.  The 
movements  around  the  transverse  axis  are  termed  flexion  and  extension; 
around  the  sagittal  axis,  adduction  and  abduction;  around  the  vertical 
axis,  rotation.  When  the  bone  revolves  around  the  surface  of  an 
imaginary  cone,  the  apex  of  which  is  the  center  of  rotation  and  the  base 
the  curve  described  by  the  hand,  the  movement  is  termed  circumduction. 
B.  Amphiarthroses. — In  this  division  are  included  all  those  joints  which 
permit  of  but  slight  movement — e.g.,  the  intervertebral,  the  interpubic, 
and  the  sacroiliac  joints.  The  surfaces  of  the  opposing  bones  are 
4 


50  TEXT-BOOK  OF  PHYSIOLOGY 

united  and  held  in  position  largely  by  the  intervention  of  a  firm,  elastic 
disc  of  fibro-cartilage.  Each  joint  is  also  strengthened  by  ligaments. 
C.  Synarthroses. — In  this  division  are  included  all  those  joints  in  which  the 
opposing  surfaces  of  the  bones  are  immovably  united,  and  hence  do  not 
permit  of  any  movement — e.g.,  the  joints  between  the  bones  of  the  skull. 
Levers. — In  the  animal  machine,  as  in  physical  machines  generally, 
work  is  accomplished  by  the  intermediation  of  levers.  The  bones  col- 
lectively constitute  a  system  of  levers  the  fulcra  of  which  lie  in  the  joints. 
The  long  bones  more  especially,  are  the  levers  which  are  employed  by  the 
muscles  to  overcome  the  opposing  forces  or  resistances.  The  structure  and 
the  chemic  composition  of  the  bones,  consisting  as  they  do  of  inorganic 
matter  67  per  cent,  and  of  organic  matter  33  per  cent,  endow  them  with  both 
rigidity  and  elasticity,  physical  properties  which  admirably  adapt  them  to 
the  character  of  the  work  necessitated  by  the  environment  and  the  organiza- 
tion of  the  animal.  The  rigidity  of  bone  is  considerable  as  compared  with 
other  hard  and  rigid  materials.  The  breaking  limit,  in  terms  of  the  weight 
in  kilos  required  to  tear  across  a  rod  i  square  millimeter  in  cross-section 
of  various  materials  is  as  follows:  Cast  iron  13;  bone  12;  oak  6.5;  granite  1.9. 
The  elasticity  is  about  one-sixth  that  of  wrought  iron  and  twice  that  of  oak 
parallel  to  the  grain  (MacAlister).  In  youth  bones  are  quite  elastic;  in 
old  age  they  are  fragile  because  of  a  diminution  of  osseous  tissue  and  an 
increased  porosity,  and,  therefore,  at  both  periods  less  capable  of  function- 
ating as  effectively  as  in  the  middle  period  of  life.  The  animal  body  presents 
many  illustrations  of  the  three  orders  of  levers,  their  advantages  and  dis- 
advantages from  the  mechanical  point  of  view. 

The  nature  of  the  opposing  forces,  however,  is  of  such  a  character  that 
the  animal  and  especially  man,  is  but  to  a  slight  degree  capable  of  over- 
coming them  with  the  natural  anatomic  and  physiologic  levers.  With  the 
invention  of  tools  (physical  levers)  of  all  kinds  and  their  utilization  by 
man  the  effectiveness  of  the  anatomic  levers  in  the  performance  of  work 
has  been  enormously  increased.  Through  their  cooperation  the  progress 
of  man  in  the  arts  of  civilization  has  been  made  possible. 

The  axial  portion  of  the  skeleton  possesses  largely,  joints  of  the  amphi- 
arthrodial  character  which  endow  the  vertebral  column  with  certain  forms 
of  movement  which  are  necessary  to  the  performance  of  many  body  activities. 
While  the  range  of  movement  between  any  two  vertebrae  is  slight,  the  sum 
total  of  movement  of  the  entire  series  of  vertebrse  is  considerable.  In 
different  regions  of  the  column  the  character,  as  well  as  the  range  of  move- 
ment, varies  in  accordance  with  the  form  of  the  vertebrae  and  the  inclination 
of  their  articular  processes.  In  the  cervical  and  lumbar  regions  extension 
and  flexion  are  freely  permitted,  though  the  former  is  greater  in  the  cervical, 
the  latter  in  the  lumbar  region,  especially  between  the  fourth  and  fifth 
vertebrae.  Lateral  flexion  takes  place  in  all  portions  of  the  column,  but  is 
particularly  marked  in  the  cervical  region.  A  rotatory  movement  of  the 
column  as  a  whole  takes  place  through  an  angle  of  about  twenty-eight 
degrees.     This  is  most  evident  in  the  lower  cervical  and  dorsal  regions. 

The  appendicular  portion  possesses  largely,  joints  of  the  diarthrodial 
character  which  permit  of  free  movement.  The  character  of  the 
movement  depends  mainly  on  the  shape  and  adjustment  of  the  bones  at 
their  points  of  union. 


CHAPTER  VII 
GENERAL  PHYSIOLOGY   OF  MUSCLE-TISSUE 

The  Muscle-tissue. — The  muscle-tissue,  which  closely  invests  the 
bones  of  the  body  and  which  is  familiar  to  all  as  the  flesh  of  animals,  is  the 
immediate  cause  of  the  active  movements  of  the  body.  This  tissue  is  grouped 
in  masses  of  varying  size  and  shape,  which  are  technically  known  as  muscles. 
The  majority  of  the  muscles  of  the  body  are  attached  to  the  bones  of  the 
skeleton  in  such  a  manner  that,  by  an  alteration  in  their  form,  they  can 
change  not  only  the  position  of  the  bones  with  reference  to  one  another,  but 
can  also  change  the  individual's  relation  to  surrounding  objects.  They  are 
therefore,  the  active  organs  of  both  motion  and  locomotion,  in  contradistinc- 
tion to  the  bones  and  joints,  which  are  but  passive  agents  in  the  performance 
of  the  corresponding  movements.  In  addition  to  the  muscle  masses  which 
are  attached  to  the  skeleton,  there  are  also  other  collections  of  muscle- 
tissue  surrounding  cavities  such  as  the  stomach,  intestine,  blood-vessels,  etc., 
which  impart  to  their  walls  motility,  and  so  influence  the  passage  of  material 
through  them. 

Muscles  produce  movement  of  the  structures  to  which  they  are  attached 
by  the  property  with  which  they  are  endowed  of  changing  their  shape, 
shortening  or  contracting  under  the  influence  of  a  stimulus  transmitted  to 
them  from  the  nerve  system.     Muscles  are  divided  into: 

1.  Skeletal  muscles,  comprising  those  muscles  which  are  attached  to  the 
various  bones  of  the  skeleton. 

2.  Visceral  muscles,  comprising  those  muscles  which  are  found  in  and 
which  compose  a  portion  of  the  walls  of  the  hollow  viscera. 

As  the  skeletal  muscles  are  capable  of  being  excited  to  activity  by  nerve 
impulses  descending  from  the  cerebrum  as  a  result  of  volition  they  are 
(frequently  termed  voluntary  muscles.  By  reason  of  their  appearance  as 
seen  under  the  microscope  they  are  termed  also  striped  or  striated  muscles. 
As  the  \dsceral  muscles  are  not  capable  of  being  excited  to  action  by  volition 
they  are  frequently  termed  involuntary  muscles.  By  reason  of  their  ap- 
pearance as  seen  under  the  microscope  they  are  termed  also  non-striated 
or  smooth  muscles. 

Though  for  the  most  part  the  skeletal  muscles  are  red  in  color,  there  are 
certain  muscles  in  man  and  other  animals  which  are  pale  in  color  and  in 
many  muscles,  pale  fibers  are  extensively  distributed  among  the  red  fibers. 

THE  SKELETAL  OR  VOLUNTARY  MUSCLE 

All  skeletal  muscles  consist  of  a  central  fleshy  portion,  the  body  or  belly, 
provided  at  either  extremity  with  a  tendon  in  the  form  of  a  cord  or  mem- 
brane.    The  body  is  the  active,  contractile  region,  the  source  of  the  move- 


52 


TEXT-BOOK  OF  PHYSIOLOGY 


merit;  the  tendon  is  the  inactive  region,   the  passive  transmitter  of  the 
movement  to  the  bones. 

A  skeletal  muscle  is  a  complex  organ  consisting  of  a  framework  of 
connective  tissue,  supporting  muscle-fibers,  blood-vessels,  nerves,  and 
lymphatics.  The  general  body  of  the  muscle  is  covered  by  a  dense  layer 
of  connective  tissue,  the  epimysium,  which  blends  with  and  partly  forms 
the  tendon.  From  the  under  surface  of  this  covering,  septa  of  connective 
tissue  pass  inward,  dividing  and  grouping  the  fibers  into  larger  and  smaller 
bundles,  termed  fasciculi.  The  fasciculi,  invested  by  a  special  sheath,  the 
perimysium,  are  prismatic  in  shape  and  on  cross-section  present  an  irregular 
outline.  The  muscle-fibers  composing  the  fasciculi  are  separated  one  from 
another  and  supported  by  a  very  delicate  connective  tissue,  the  endomysium. 
The  connective  tissue  thus  surrounding  and  penetrating  the  muscle  binds 

the  fibers  into  a  distinct  organ  and 
affords  support  to  all  remaining  struc- 
tures (Fig.  14). 

Histology  of  the  Skeletal  Mus- 
cle-fiber.— The  muscle-fiber  is  the 
ultimate  anatomic  unit  of  the  muscle 
system.  The  fibers  for  the  most  part 
are  arranged  parallel  one  to  another 
and  in  a  direction  corresponding  to 
the  long  axis  of  the  muscle.  They 
vary  in  length  from  30  to  40  milHmeters 
and  in  breadth  from  20  to  30  micro- 
milHmeters.  There  are  exceptional 
fibers,  however,  which  have  a  much 
greater  length.  As  the  fibers  have 
but  a  limited  length  in  the  vast  major- 
ity of  muscles,  each  end,  more  or  less 
pointed  or  beveled,  is  united  to  adjoin- 
ing fibers  by  cement.  In  this  way  the 
length  of  the  muscles  is  built  up. 

When  examined  with  the  micro- 
scope, the  muscle-fiber  is  seen  to  be 
cylindric  or  prismatic  in  shape  and 
to  consist  of  a  thin  transparent  mem- 
brane, the  sarcolemma,  in  which  is 
contained  the  true  muscle  substance  or  sarcous  substance.  The  sarco- 
lemma is  elastic  and  adapts  itself  to  all  changes  of  form  the  sarcous  sub- 
stance undergoes.  Beneath  the  sarcolemma  there  are  several  nuclei 
surrounded  by  granular  material;  a  muscle-fiber  may  therefore  be  re- 
garded as  a  large  multinucleated  cell.  Each  fiber  also  presents  a  series 
of  transverse  bands  alternately  dim  and  bright  which  give  to  it  a  striated 
appearance.  If  the  bright  bands  are  examined  with  high  magnifying 
powers,  each  one  is  seen  to  be  crossed  by  a  fine  dark  line  which  at  the 
time  of  its  discovery  by  Krause  was  regarded  as  the  optic  expression  of  a 
membrane  attached  laterally  to  the  sarcolemma.  According  to  Rollet, 
it  is  composed  of  a  series  of  granules  so  closely  applied  as  to  give  rise  to  the 
appearance  of  a  continuous  line  (Fig.  15). 


i^ 


Fig.  14. — From  a  Cross-section  of  the 
Adductor  Muscle  of  a  Rabbit.  P.  Peri- 
mysium, containing  two  blood-vessels,  at  g; 
m,  muscle-fibers;  many  are  shrunken  and  be- 
tween them  the  endomysium,  p,  can  be  seen; 
at  X  the  section  of  muscle-fiber  has  fallen  out. 
X  60.— (Stohr.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE 


53 


The  muscle-fiber  also  presents  a  longitudinal  striation  which  indicates 
that  it  is  composed  of  finer  elements  placed  side  by  side,  termed  fibrillas. 
The  fibrillae  extend  throughout  the  entire  length  of  the  fiber,  though  they 
are  not  of  uniform  thickness.  That  portion  of  the  fibrilla  correspond- 
ing in  position  to  the  dim  band  is  thick,  prismatic,  or  rod-like  in  shape,  and 
termed  a  sarcostyle;  that  portion  corresponding  in  position  to  the  bright 
band  is  extremely  thin  and  narrow  and  presents  at  its  middle  a  slight  en- 
largement or  nodule.  The  fibrillae  are  embedded  in  a  clear  transparent  fluid 
which,  from  its  supposed  nutritive  character,  is  termed  sarcoplasm,  or 
interfibrillar  substance.  The  diminution  in  caliber  of  the  fibrillae  at  different 
levels  would  permit  of  the  accumulation  and  storage  of  a  larger  amount  of 
this  nutritive  material  than  could  otherwise  be  the  case.  It  is  for  this 
reason  that  the  fiber  at  these  points  presents  a  brighter  appearance. 

When  the  muscle-fiber  is  examined 
by  polarized  light,  the  dim  band  ap- 
pears bright  and  the  bright  band  appears 
dim  against  a  dark  background,  indicat- 
ing that  the  former  is  doubly  refracting 
or  anisotropic,  the  latter  singly  refracting 
or  isotropic. 


Fig.  16.—A.  Diagram  of 
arrangement  of  the  contrac- 
tile substance  according  to 
the  view  of  Rollett;  the 
granular  figures  represent 
the  contractile  elements,  the 
intervening  light  areas  the 
sarcoplasm.  B.  Small 
muscle-fiber  of  man,  the 
corresponding  parts  in  the 
two  figures  are  indicated; 
/,  i,  I,  respectively  the  trans- 
verse, the  intermediate,  and 
lateral  discs,  ti.  Muscle 
nuclei. — (Piersol.) 


Fig.  15. — Muscle-fiber 
OF  A  Rabbit,  a.  Dark 
band.  b.  Light  band.  c.  In- 
termediate line.  71.  Nucleus. 
• — {Landois  and  Stirling) 


This  interpretation  of  the  structure  of  the  muscle-fiber  has  been  subjected 
to  criticism  in  recent  years  by  Heidenhain.  This  observer  regards  the  trans- 
verse line  in  the  bright  band  as  did  its  discoverer  Krause  as  a  true  membrane 
which  is  attached  laterally  to  the  sides  of  the  sarcolemma.  The  fibrillae  he 
also  regards  as  continuous  but  of  uniform  thickness,  and  passing  directly 
through  the  transverse  membrane  by  which  they  are  supported  and  main- 
tained in  their  normal  relation.  In  this  view  the  fibrilla  consists  of  alternate 
regions  of  a  doubly  refracting  and  a  singly  refracting  material.  The  sarco- 
plasm is,  therefore,  confined  to  the  interfibrillar  spaces.    Fig.  17. 

The  fiber  of  the  pale  muscle  is  similar  histologically  to  the  fiber  of  the 
red  muscle.  It,  however,  does  not  contain  so  much  granular  protoplasm  as 
does  the  fiber  of  the  red  muscle  and  hence  does  not  intercept  the  light  to  the 
same  extent.  The  greater  the  quantity  of  granular  protoplasm  the  darker 
the  muscle. 


54 


TEXT-BOOK  OF  PHYSIOLOGY 


cd 


bl 


The  Blood-supply. — Muscles  in  the  physiologic  condition  require  for 
the  maintenance  of  their  activity  a  large  amount  of  nutritive  material.  This 
is  obtained  directly  from  the  lymph  and  indirectly  from  the  blood  furnished 
by  the  blood-vessels.  The  vascular  supply  to  the  muscles  is  very  great  and 
the  disposition  of  the  capillary  vessels  with  reference  to  the  muscle-fiber  is 
very  characteristic.  The  arterial  vessels,  after  entering  the  muscle,  are 
supported  by  the  peri-mysium;  in  this  situation  they  give  off  short  transverse 
branches,  which  immediately  break  up  into  a  capillary  network  of  rectangu- 
lar shape  within  which  the  muscle-fibers  are  contained. 

The  muscle-fiber,  in  intimate  relation  with  the  capillary,  is  bathed  with 
lymph  derived  from  it.  Its  contractile  substance,  how- 
ever, is  separated  from  the  lymph  by  its  own  investing 
membrane,  through  which  all  interchange  of  nutritive  and 
waste  material  must  take  place. 

The  nutritive  material  passes  through  the  capillary 
wall  into  the  lymph-space,  then  through  the  sarcolemma 
into  the  interior  of  the  fiber,  where  it  comes  into  relation 
with  the  living  muscle  material.  The  waste  products 
arising  in  the  muscle  as  a  result  of  nutritive  changes  pass 
in  the  reverse  direction  first  into  the  lymph  and  then  into 
the  blood,  by  which  they  are  carried  away  to  eliminating 
organs.  Lymphatics  are  present  in  muscle,  but  confined 
to  the  connective  tissue,  in  the  spaces  of  which  they  take 
their  origin. 

The  Nerve-supply. — The  nerves  which  carry  the 
stimuli  to  a  muscle  enter  near  its  middle  point.  Many 
of  the  fibers  pass  directly  to  the  muscle-fibers  with  which 
they  are  connected;  others  are  distributed  to  blood-vessels. 
Every  muscle-fiber  is  supplied  with  a  special  nerve-fiber 
except  in  those  instances  where  the  nerve-trunks  entering 
a  muscle  do  not  contain  as  many  fibers  as  the  muscle.  In 
such  cases  the  nerve-fibers  divide  near  their  termination 
until  the  number  of  branches  equals  the  number  of  muscle- 
fibers.  The  individual  muscle-fiber  is  penetrated  near 
its  center  by  the  nerve  where  it  terminates;  the  ends 
being  practically  free  from  nerve  influence.  The  stimulus 
that  comes  to  the  muscle-fiber  acts  primarily  upon  its 
center,  the  effect  of  which  then  travels  in  both  directions 
to  the  ends.  The  manner  in  which  the  nerve-fibers  termi- 
nate in  muscle  will  be  more  fully  described  in  connection 
with  the  histology  of  the  nerve-tissue. 


Fig.  17. — Dia- 
gram OF  Muscle 
Striations. — {Modi- 
fied from  "Stokr's 
Histology.")  The 
fibrillae  consists  of 
alternate  dark  bands, 
d.6.,.and  light  bands, 
l.h.  l.b.  is  crossed 
by  Krause's  mem- 
brane, m.;  a  similar 
membrane  crosses 
the  dim  band  accord- 
ing to  Heidenhain. 


CHEMIC  COMPOSITION  OF  MUSCLE 

The  chemic  composition  of  living  muscle  is  but  imperfectly  understood 
owing  to  the  fact  that  shortly  after  death  some  of  its  constituents  undergo  a 
spontaneous  coagulation  and  for  the  reason  that  the  methods  employed  for 
analysis  also  tend  to  alter  its  composition.  To  human  muscle,  the  following 
average  percentage  composition  has  been  given : 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  55 

Water 73-5 

Proteins,    including    those    of   sarcolemma,    connective    tissue, 

pigments 18.02 

Gelatin 1-99 

Fat 2  .27 

Extractives 0.22 

Inorganic  salts 3  •  12  <Halliburton.) 

When  fresh  muscle  is  freed  from  fat  and  connective  tissue,  frozen,  rubbed 
up  in  a  mortar,  and  expressed  through  linen,  a  sHghtly  yellow  syrupy  alkaline 
or  neutral  liquid  is  obtained  which  has  been  termed  muscle-plasma.  This 
fluid  at  normal  temperatures  coagulates  spontaneously,  the  phenomena 
resembling  in  many  respects  those  observed  in  the  coagulation  of  blood- 
plasma.  The  coagulum  subsequently  contracts  and  squeezes  out  an  acid 
muscle-serum.  The  coagulated  protein  partakes  of  the  nature  of  fibrin  and 
belongs  to  the  class  of  globuHns.  Inasmuch  as  it  is  not  present  in  living 
muscle  and  only  makes  its  appearance  under  conditions  not  strictly  physio- 
logic, it  is  regarded  as  a  derivative  of  pre-existing  proteins.  An  analysis  of 
muscle-plasma  has  shown  the  presence  of  at  least  two  proteins  which  are 
distinguished  by  their  varying  solubilities  in  different  salt  solutions,  and  by 
the  varying  temperatures  at  which  they  coagulate.  One  of  these  proteins 
coagulates  at  about  47°C.  and  because  of  its  chemic  relations  has  been 
termed  myosin  or  paramyosinogen;  the  other  coagulates  at  about  56°C. 
and  for  similar  reasons  has  been  termed  myogen  or  myosinogen.  The 
latter  is  three  or  four  times  more  abundant  than  the  former.  If  the  tempera- 
ture of  the  cooled  plasma  be  permitted  to  rise,  both  myosin  and  myogen 
undergo  a  change  of  state  termed  coagulation.  The  substances  resulting 
are  known  as  myosin  fibrin  and  myogen  fibrin.  It  is  not  known  whether 
these  changes  are  due  to  the  action  of  an  enzyme  or  not.  A  similar  change 
in  myosin  and  myogen  occurs  after  death,  giving  rise  to  the  condition  known 
as  death  stiffening  or  rigor  mortis.  The  coagulation  of  these  proteins  in  this 
instance  is  probably  caused  by  the  presence  and  accumulation  of  metabolic 
products.  From  the  muscle-serum,  according  to  Halliburton,  may  also  be 
obtained  at  68°C.  a  globuHn  body  termed  myoglobulin  and  a  small  quantity 
of  myo-albumin.  Among  the  proteins  may  be  mentioned  hemoglobin,  which 
gives  the  color  to  the  muscles.  Spectroscopic  investigation  reveals  the 
presence  of  a  special  pigment,  myohematin,  which  is  supposed  to  have  a 
respiratory  function,  inasmuch  as  its  spectral  absorption  bands  change  by 
o.xidation  and  reduction. 

Among  the  extractives  containing  nitrogen  may  be  mentioned  creatin, 
creatinin,  xanthin,  carnin,  urea,  uric  acid,  carnic  acid,  etc.  Among  the 
extractives  free  of  nitrogen,  glycogen,  dextrose,  inosite,  lactic  acid  and  fat,  are 
the  most  important.  Inorganic  salts  are  relatively  abundant,  of  which 
potassium  is  the  most  abundant  among  the  bases,  and  phosphoric  acid 
among  the  acids. 

THE  PHYSICAL  AND  PHYSIOLOGIC  PROPERTIES  OF  MUSCLE -TISSUE 

Consistency. — The  consistency  of  muscle-tissue  during  life  varies 
considerably  in  accordance  with  different  states  of  the  muscle.  In  a  state  of 
tension  it  is  hard  and  resistant;  in  the  absence  of  tension  it  is  soft  and  fluctu- 
ating to  the  sense  of  touch.     Tension  alone  gives  rise  to  hardness. 


56 


TEXT-BOOK  OF  PHYSIOLOGY 


— Extension  Curve  of 
Muscle. — {Gad.) 


Cohesion. — The  cohesion  of  a  muscle  is  largely  dependent  on  the  quan- 
tity of  connective  tissue  it  contains.  A  band  of  fresh  human  muscle  i 
square  centimeter  in  cross-section  has  been  found  able  to  resist  a  weight  of 
14  kilograms  without  rupture  (MacAlister).  Cohesion  resists  the  forces  of 
traction  and  pressure. 

Elasticity. — Muscle,  in  common  with  many  other  organic  as  well  as 
inorganic  substances,  is  capable  of  being  ex- 
tended beyond  its  normal  length  by  the  action 
of  external  forces  and  of  resuming  the  normal 
length  when  these  forces  cease  to  act.     All  such 
bodies  are  said  to  be  elastic;  and  the  greater  the 
variations  between  the  natural  and  acquired 
lengths,   the  greater  is  their  elasticity  said  to 
be.     Muscles,  therefore,  possessing  extensibility 
and  retractility  are  said  to  be  elastic.     If  the 
muscle  of  a  frog,  preferably  the  sartorius,  the 
fibers  of  which  are  arranged  in  a  practically 
parallel  manner,  be  fastened  at  one  extremity 
by  a  clamp,  and  then  extended  by  a  series  of 
successive  weights  which  differ  by  a  common 
increment,  it  will  be  found  that  the  extensi- 
bility of  muscle  does  not  follow  the  law  of 
elasticity  as  determined  for  inorganic  bodies;  p^^ 
i.e.,  directly  proportional  to  the  weight  and  to  the 
length  of  the  body  extended;  but  that  while  in- 
creasing in  length  with  each  successive  weight,  the  increase  is  always  in  a 
diminishing  ratio.     Thus,  for  example,  as  shown  in  Fig.  18:     The  exten- 
sion produced  by  5  grams  is  5  millimeters,  that  produced  by  10  grams  is 
only  4  millimeters  more,  and  so  on  with  additional  weights  until  the  in- 
crease in  passing  from  25  to  30  grams  is  only  i  millimeter.     The  exten- 
sibility   is    thus   shown    to   be   proportionately 
greater  with  small  than  with  larger  weights.     It 
is,   however,    actually  greater   with    the  larger 
weights.     The  extension  curve  A  B  formed  by 
joining  the  ends  of  the  muscle  approximates  that 
of   a  parabola.     The    behavior  of  the  muscle 
in  returning  to  its  original  length  also  shows  a 
variation  from  the  behavior  of  inorganic  bodies. 
With  the  successive  removal  of  the  weights,  the 
elasticity  of  the  muscle  asserts  itself  with  gradu- 
ally increasing  energy  until  its  normal  length  is 
nearly,  if  not  entirely,  regained  (Fig.  19).     It  is 
usually  stated  that  the  elasticity  of  muscle  is  in- 
complete, but  it  must  be  borne  in  mind  that  the  experiments  have  usually 
been  made  on  muscles  removed   from  the  body,  deprived  of  blood  and 
nerve  influences,  and  hence  under  abnormal  conditions.     It  is  highly  prob- 
able that  in  the  living  body  muscles  possess  perfect  elasticity  which  enables 
them  completely  to  return  to   their  normal  length  after  extension.     The 
extension  and  retraction  or  elastic  recoil  of  muscle  depends  on  the  main- 
tenance of  physiologic  conditions.     If  the  nutrition  is  impaired  by  fatigue, 


Fig.  19. — Curve  of  Elas- 
ticity Produced  by  Continu- 
ous Extension  and  Recoil 
of  a  Frog's  Muscle,  o  x.  Ab- 
scissa before;  x',  after  exten- 
sion.— (Landois  and  Stirling.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  57 

deficient  blood-supply,  or  any  pathologic  condition,  the  elasticity  is  at  once 
impaired. 

Tonicity. — Muscle  tonus  may  be  defined  as  a  state  of  tension  of  a 
muscle  due  to  a  slight  but  continuous  contraction  of  its  individual  fibers  in 
consequence  of  which  it  tends  to  become  shorter,  and  would  do  so,  were  it 
not  restrained  by  its  attachments.  As  a  result  of  this  tension  its  efficiency 
as  a  quickly  responsive  motor  organ  is  increased.  Though  the  skeletal 
musculature  of  the  body  as  a  whole  is  in  a  state  of  tonus,  individual  muscles 
vary  in  the  degree  of  their  tonus  from  time  to  time  in  consequence  of  varia- 
tions in  the  causes  that  give  rise  to  it.  That  such  a  tonus  or  tension  exists 
is  apparently  shown  by  the  fact  that  when  a  muscle  in  a  living  animal  is 
divided  the  two  portions  will  retract  and  separate  a  certain  distance.  This 
would  indicate  that  the  muscle  even  in  a  state  of  relative  rest  is  in  a  slight 
degree  of  contraction. 

This  condition  of  tonus  is  attributed  to  the  continuous  arrival  of  nerve 
impulses  through  efferent  nerves  discharged  by  ner\^e-cells  in  the  spinal  cord. 
The  tonus  was  therefore  at  one  time  attributed  to  an  automatic  activity  of 
the  spinal  cord.  Brondgeest,  however,  showed  that  this  was  not  the  case, 
but  that  the  activity  of  the  spinal  cord  and  hence  the  tonus  of  the  muscles  is 
partly  reflex  in  origin  inasmuch  as  it  largely  disappears  on  division  of  the 
posterior  or  dorsal  roots  of  the  spinal  nerves.  The  afferent  impulses  excit- 
ing the  cord  reflexly  may  come  from  the  skin  in  which  they  are  developed 
by  the  impressions  made  by  external  stimuli,  or  from  the  tendons  and 
muscles  themselves  in  which  they  are  developed  by  the  slight  degree  of 
extension  and  variations  in  extension  to  which  these  are  subjected  from 
moment  to  moment.  That  this  latter  is  a  considerable  factor  in  the  pro- 
duction of  the  tonus  is  shown  by  the  effects  which  follow  division  of  the 
afferent  nerves  coming  from  any  given  muscle  group;  with  the  division  of 
the  nerves  the  muscles  relax  and  lose  their  usual  tone.  It  is  also  probable 
that  the  activity  of  the  cord  is  partly  the  result  of  impulses  descending  the 
cord  in  consequence  of  cerebral  and  sense  organ  activities. 

Muscle  tonus  or  elastic  tension  plays  an  important  role  in  muscle  con- 
traction; being  always  on  the  stretch  the  muscle  loses  no  time  in  acquiring 
that  degree  of  tension  necessary  to  immediate  action  on  the  bone  to  which 
it  is  attached.  The  working  power  of  a  muscle  is  also  considerably  increased 
by  the  presence  within  limits  of  some  resistance  to  the  act  of  contraction. 
According  to  Marey,  the  amount  of  work  is  considerably  increased  when  the 
muscle  energy  is  transmitted  by  an  elastic  body  to  the  mass  to  be  moved, 
while  at  the  same  time  the  shock  of  the  contraction  is  lessened.  The  posi- 
tion of  a  passive  limb  is  the  resultant  also  of  the  elastic  tension  of  antago- 
nistic groups  of  muscles.  Again  as  a  result  of  the  slight  but  continuous 
stimulation  from  the  spinal  cord  the  metabolic  changes  in  the  muscle  material 
are  maintained  at  a  certain  level,  with  a  corresponding  production  of  heat. 
A  function  of  the  tonicity  would  thus  be  the  production  of  heat,  other  functions 
which  the  tone  subserves  being  more  or  less  secondary. 

Irritability,  Contractility. — These  are  terms  employed  to  denote 
that  property  of  muscle-tissue  by  virtue  of  which  it  responds  by  a  change  of 
form,  becoming  shorter  and  thicker  on  the  application  of  a  stimulus.  On 
the  withdrawal  of  the  stimulus  the  muscle  again  undergoes  a  reverse  change 
of  form,  becoming  longer  and  narrower,  and  returning  to  its  original  condi- 


S8  TEXT-BOOK  OF  PHYSIOLOGY 

tion.  All  muscles  which  possess  this  capability  are  said  to  be  irritable  and 
contractile;  and  all  agents  which  call  forth  this  response  of  the  muscle  are 
termed  stimuli.  The  rapid  change  of  form  which  a  highly  irritable  muscle 
undergoes  in  response  to  the  action  of  a  stimulus  of  short  duration  is  usually 
termed  a  twitch  or  pulsation.  With  appropriate  apparatus  it  can  be  shown 
that  the  muscle  at  the  time  of  the  twitch  becomes  warmer  and  exhibits  electric 
phenomena.  The  muscle  is  therefore  an  apparatus  for  the  conversion  of 
potential  into  kinetic  energy:  viz.,  heat,  electricity,  and  mechanic  motion. 

Though  usually  associated  with  the  activity  of  the  nerve  system,  and  to 
some  extent  dependent  on  it,  irritability  is  nevertheless  an  independent 
endowment  of  the  muscle  and  persists  for  a  longer  or  shorter  period,  as 
shown  by  many  experiments,  after  all  nerve  connections  have  been  destroyed. 
Among  the  proofs  which  may  be  presented  in  support  of  this  view  are  the 
following:  The  introduction  of  the  drug,  curara,  into  the  body  of  an  animal 
produces  in  a  short  time  complete  paralysis.  Experiment  has  shown  that 
curara  suspends  the  conductivity  of  the  intramuscular  terminations  of  the 
nerve-fiber  and  thus  separates  the  muscle  entirely  from  the  nerve.  Though 
the  animal  is  incapable  of  executing  a  single  movement,  its  muscles  respond 
promptly  on  the  application  of  a  stimulus.  Moreover,  portions  of  muscles 
exhibit  irritability  although  containing  no  trace  of  nerve  structure.  This  is 
the  case  with  the  ends  of  the  sartorius  muscle  of  the  frog  and  the  anterior 
end  of  the  retractor  muscle  of  the  eyeball  of  the  cat.  These  and  other 
facts  demonstrate  the  independence  of  muscle  irritability. 

In  the  living  body  nutritive  activity  and  irritability  are  maintained  by  a 
due  supply  of  oxygen,  and  of  nutritive  material,  the  removal  of  waste  prod- 
ucts, and  a  normal  temperature.  The  muscles  of  the  cold-blooded  animals, 
for  example  the  frog,  retain  their  irritability  for  a  much  longer  period  after 
death  than  the  muscles  of  the  warm-blooded  animals.  This  is  the  case  also 
with  the  individual  muscles  after  removal  from  the  body  of  the  animal. 
The  reason  for  this  is  found  in  all  probability  in  the  difference  in  the  rate 
of  their  nutritive  activities  and  in  the  quantity  of  nutritive  material  stored  up 
in  their  cells.  The  duration  of  the  irritability  of  isolated  muscles  can  be 
considerably  prolonged  by  keeping  them  in  a  moist  atmosphere. 

Conductivity. — All  muscle  protoplasm  possesses  conductivity.  The 
change  excited  in  a  muscle-fiber  by  the  arrival  of  a  nerve  impulse  is  at  once 
conducted  with  great  rapidity  in  opposite  directions  to  the  ends  of  the  fiber; 
the  advance  of  the  excitation  process  is  immediately  succeeded  by  the  con- 
traction process,  the  change  of  form  which  constitutes  the  contraction. 
With  the  disappearance  of  the  former,  the  latter  also  disappears  and  the 
muscle  resumes  its  previous  passive  condition.  There  is  no  evidence,  how- 
ever, that  the  excitation  process  travels  transversely — that  is,  into  adjoining 
fibers — being  prevented  from  doing  so  by  the  presence  of  the  limiting 
membranes,  the  sarcolemmata.  The  fact  that  each  muscle-fiber  receives 
its  own,  or  at  least  a  branch  of  a  nerve-fiber,  and  hence  its  own  nerve  impulse 
or  stimulus,  would  also  indicate  that  the  excitation  process  cannot  be  con- 
ducted longitudinally  into  adjoining  fibers,  or  at  least  with  sufficient  rapidity 
for  the  purposes  of  ordinary  muscle  actions.  Nevertheless  if  a  long  muscle, 
such  as  the  sartorius,  from  a  curarized  frog  be  stimulated  at  one  end  with 
an  induced  electric  current,  the  excitation  and  the  contraction  processes  will 
be  conducted  with  extreme  rapidity  to  the  opposite  end  of  the  muscle.     The 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  59 

rapidity  of  conduction  in  human  muscles  has  been  estimated  at  from  10  to  13 
meters  per  second,  and  in  frog's  muscle  at  from  3  to  4.5  meters  per  second. 
The  contraction  process,  the  thickening  of  the  muscle,  is  termed  the  cmi- 
traction  wave.  As  it  is  the  result  of  the  excitation  process  and  immediately 
succeeds  it,  its  rate  of  conduction  must  be  the  same  as  that  given  above. 
With  appropriate  apparatus  the  duration  of  the  wave  at  any  given  point  has 
been  shovv^n  to  be,  in  the  frog's  muscle,  about  one-tenth  of  a  second  and  its 
length  three-tenths  of  a  meter. 

Muscle  Stimuli. — Though  consisting  of  a  highly  irritable  tissue,  muscles 
do  not  possess  spontaneity  of  action.  They  require  for  the  manifestation 
of  their  characteristic  activity  the  application  of  a  stimulus.  In  the  living 
body  all  contractions,  at  least  of  the  skeletal  muscles,  occurring  under 
normal  or  physiologic  conditions  are  caused  by  the  action  of  "nerve  im- 
pulses" transmitted  by  the  nerves  from  the  central  nerve  system  to  the 
muscles.  The  nerve  impulse  is  the  normal  or  physiologic  stimulus.  After 
removal  from  the  body  and  freed  from  nerve  connections  muscles  can  be 
excited  to  action  by  various  agents  of  a  mechanic,  chemic,  thermic,  or  electric 
nature.     These  are  artificial  or  non-physiologic  stimuli. 

1.  Mechanic  Stimuli. — Cutting,  pinching,  sharply  tapping  the  muscle  will 

cause  it  to  contract,  providing  the  stimulus  has  sufficient  intensity. 
With  each  stimulation  a  short,  fleeting  contraction  ensues.  If  repeated 
with  sufl&cient  rapidity,  a  series  of  continuous  but  irregular  pulsations 
are  produced. 

2.  Chemic  Stimuli. — ^\"arious  chemic  substances  in  solution  will  excite  single 

or  continuous  pulsations  if  the  strength  of  the  solution  is  not  such  as  to 
destroy  at  once  the  irritability.  They  owe  their  efficiency  as  stimuli  to 
the  rapidity  with  which  they  alter  the  composition  of  the  muscle-sub- 
stance. Among  these  may  be  mentioned  solutions  of  potassium  and 
sodium  salts,  weak  solutions  of  the  mineral  and  organic  acids,  ammo- 
nium vapor,  distilled  water,  glycerin,  and  sugar. 

3.  Thermic  Stimuli. — The  application  of  a  heated  object,  such  as  a  hot 

wire,  causes  the  muscle  to  contract  rapidly. 

4.  Electric  Stimuli. — The  most  efficient  stimulus  and  the  one  least  injurious 

to  the  tissue  is  the  electric  current.     Either  the  constant  or  the  induced 

current  may  be  used.^ 

The  Constant  Current. — If  the  ends  of  the  wires  in  connection  with  an 
electric  cell  be  provided  with  non-polarizable  electrodes  and  the  latter  placed 
on  opposite  ends  of  a  freshly  prepared  sartorius  muscle  of  a  frog  which  has 
been  previously  curarized,  it  will  be  found  on  closing  or  making  the  circuit 
that  the  muscle  will  exhibit  a  short,  quick  pulsation.  During  the  actual 
passage  of  the  current,  especially  if  it  is  weak,  there  may  be  no  apparent 
change  in  the  muscle.  If  the  current  is  strong,  the  muscle  may,  on  the 
contrary,  remain  in  a  state  of  continuous  contraction.  With  the  opening  or 
breaking  of  the  current  the  muscle  at  once  relaxes,  or  perhaps  again  contracts 
and  then  relaxes.  The  extent  of  the  contraction  depends  mainly  on  the 
strength  of  the  current,  being  greater  with  strong,  less  with  weak  currents. 

'  Since  the  study  of  the  physiologic  properties  of  both  muscle-tissue  and  nerve-tissue 
involves  the  employment  of  electricity  as  a  stimulus,  it  is  necessary  for  the  student  to 
familiarize  himself  with  certain  forms  of  apparatus  by  which  it  is  generated,  controlled,  and 
applied.  To  avoid  interrupting  the  continuity  of  the  text  this  information  is  embodied 
in  an  appendix.     The  facts  therein  contained  should  be  mastered  at  this  time  by  the  student. 


6o 


TEXT-BOOK  OF  PHYSIOLOGY 


When  the  current  is  sufficiently  strong  to  ehcit  both  making  and  breaking 
contractions,  it  is  found  that  the  contraction  occurring  on  the  make  or  closure 
of  the  circuit  is  regularly  greater  than  that  occurring  on  the  break  or  opening 
of  the  circuit.  Moreover,  it  has  been  shown  in  many  ways  that  the  con- 
traction occurring  on  the  closure  of  the  circuit  has  its  origin  at  the  point 
where  the  current  is  leaving  the  muscle — i.e.,  in  the  immediate  neighborhood 
of  the  negative  pole  or  cathode — and  propagates  itself  to  the  opposite  extrem- 
ity; while  the  contraction  occurring  on  the  opening  of  the  circuit  has  its 
origin  at  the  point  where  the  current  is  entering  the  muscle,  i.e.,  in  the 
neighborhood  of  the  positive  pole  or  anode. 

These  facts  can  be  readily  demonstrated  by  destroying  the  irritability 
and  contractility  of  one  extremity  of  a  muscle  with  parallel  fibers  such  as  the 


Fig.  20. — Diagram  to  Show  the  Effect  of  Local  Injury  on  the  Irritability  of  a 
Muscle. — {After  Starling.)  C  Z  a.n  electric  cell  from  which  wires  pass  to  non-polarizable  elec- 
trodes, anode  and  kathode,  in  contact  with  a  muscle,  the  injured  end  of  which  is  more  deeply 
shaded.     The  arrows  indicate  the  direction  of  the  current. 


sartorius.  On  applying  non-polarizable  electrodes  to  the  muscle  as  in  Fig. 
20,  A,  it  will  be  found  that  when  the  circuit  is  made  a  contraction  occurs 
which  must,  of  course,  have  developed  at  the  irritable  cathodic  region,  for 
on  the  break  of  the  circuit  the  muscle  remains  at  rest.  When  the  electrodes 
are  applied  as  in  Fig.  20,  B,  and  the  circuit  made  the  muscle  remains  at 
rest,  but  on  the  break  of  the  circuit  a  contraction  occurs  which  must  have 
developed  at  the  irritable  anodic  region. 

The  Induced  Current. — If  the  primar}^  coil  of  the  inductorium  be  con- 
nected with  an  electric  cell  and  the  secondary  coil  be  connected  with  a 
muscle,  it  will  be  found  that  the  current  induced  in  the  secondary  circuit, 
both  on  the  make  and  break  of  the  primary,  will  also  cause  the  muscle  to 
pulsate  sharply  and  rapidly  if  the  two  coils  are  sufficiently  near  each  other. 
Observation,  however,  makes  it  evident  that  the  pulsation  occurring  with 
the  break  of  the  primary  circuit  is  more  energetic  than  that  occurring  with 
the  make,  a  result  the  opposite  of  that  obtained  with  the  constant  current. 
This  is  not  due  to  any  difference  in  the  electricity,  however,  but  to  peculiari- 
ties in  the  construction  of  the  inductorium.  When  the  primary  circuit  is 
interrupted  with  sufficient  frequency,  as  it  can  be  by  throwing  into  the  circuit 
Neef's  hammer  or  some  other  form  of  interrupter,  the  contractions  excited 
by  the  induced  currents  may  be  made  to  succeed  one  another  so  rapidly  that 
they  become  fused  together,  producing  a  spasm  or  tetanus  of  the  muscle. 
The  rapidity  with  which  the  induced  current  appears  and  disappears,  its 
brief  duration,  the  ease  with  which  its  strength  can  be  regulated,  combine 
to  render  it  a  most  efficient  stimulus  for  either  muscle  or  nerve. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE 


6i 


PHENOMENA  FOLLOWING  A  MUSCLE  STIMULATION 
PHYSICAL  PHENOMENA 

Physiologic  investigation  has  made  it  apparent  that  when  a  nerve  impulse 
reaches  a  muscle,  it  occasions  a  disruption  of  certain  complex  energy- 
holding  compounds  and  their  subsequent  oxidation  to  simpler  compounds. 
Coincidently  with  the  chemic  changes  there  is  a  transformation  of  the 
potential  energy  of  the  molecules  into  kinetic  energy  which  manifests 
itself  under  three  forms,  heat,  electricity  and  mechanic  motion,  or  a  change 
of  shape  of  the  muscle.  These  phenomena  vary  in  extent  in  accordance  with 
the  intensity  of  the  impulse  as  well  as  the  frequency  of  its  repetition. 
Though  the  chemic  changes  are  the  first  effects  of  the  action  of  the  nerve 


Fig.  21. — Showing  the  Changes  in  a  Muscle  and  Muscle-fiber  during 

Contraction. 

impulse  and  the  ones  on  which  other  phenomena  depend,  it  will  be  found 
convenient  to  consider  the  most  evident  effect,  the  physical  change  in  the 
shape  of  the  muscle,  first. 

Change  of  Shape. — The  most  obvious  change  in  a  muscle  following  the 
arrival  of  a  nerve  impulse  is  that  relating  to  its  form.  The  muscle  not  only 
becomes  shorter,  but  at  the  same  time  thicker.  The  extent  to  which  it  may 
shorten  when  unopposed  may  amount  to  30  per  cent,  or  more  of  its  original 
length.  The  increase  in  thickness  practically  compensates  for  the  diminu- 
tion in  length,  for  there  is  no  observable  diminution  in  volume.  The  change 
in  form  of  the  entire  muscle  results  from  a  corresponding  change  of  form  of 
its  individual  fibers  as  determined  by  microscopic  examination,  each  of 
which  becomes  shorter  and  thicker.  The  successive  changes  in  both  the 
muscle  and  the  individual  fibers  are  represented  in  Fig.  21. 

When  the  contraction  begins  both  the  dim  and  bright  bands  diminish  in 
length,  but  at  the  same  time  increase  in  breadth.  This  continues  until 
the  contraction  reaches  its  maximum.  The  diminution  in  the  length  of  the 
bright  band  is  greater  proportionally  than  the  diminution  in  the  length  of  the 
dim  band,  a  fact  which  gave  rise  to  the  supposition  on  the  part  of  Englemann 
that  there  is  at  the  time  of  the  contraction  a  passage  of  fluid  material  from 
the  bright  into  the  dim  band  or  from  the  sarcoplasm  into  the  sarcostyles. 
When  the  relaxation  begins,  a  reverse  change  in  the  dim  and  bright  bands 
sets  in  and  continues  until  they  regain  their  former  shape  and  volume.  Coin- 
cidently there  is  a  passage  of  fluid  material  from  the  sarcostyles  to  the 


62 


TEXT-BOOK  OF  PHYSIOLOGY 


sarcoplasm.  As  the  contraction  wave  reaches  its  maximum  the  optic  proper- 
ties of  the  bright  and  dim  bands  change.  The  former  now  becomes  darker 
and  less  transparent  until  at  the  crest  of  the  wave  it  assumes  the  appearance 
of  a  distinct  dark  band;  the  latter  now  becomes  clear  and  bright  in  compari- 
son. This  change  in  the  appearance  of  the  fiber  is  due  to  an  increase  in 
refrangibility  of  the  bright,  and  a  decrease  in  the  refrangibility  of  the  dim 
band,  coincident  with  the  passage  of  the  fluid  from  the  former  into  the  latter. 
There  is  at  the  height  of  the  contraction  a  complete  reversal  in  the  positions 
of  the  striations.  At  a  certain  stage  between  the  beginning  and  the  crest  of 
the  wave  the  striae  almost  entirely  disappear,  giving  to  the  fiber  an  appearance 


0            5 

10^ 

s 

1 

5              2 

0             2 

5            3< 

"~> 

-..^ 

^ 

b 

Ob' 


B' 


Fig.  22. — Extension  Curves:     B  B',  of  the  resting;  b  B',  of  the  contracting  muscle. 

of  homogeneity.  There  is,  however,  no  change  in  refractive  power  as  shown 
by  the  polarizing  apparatus.  When  the  contraction  wave  has  reached  the 
stage  of  greatest  intensity,  there  is  a  reversal  of  the  above  phenomena  as  the 
fiber  returns  to  its  former  condition,  that  of  relaxation. 

Change  of  Elasticity. — During  the  contraction  of  a  muscle  there  is  a 
greater  or  less  alteration  in  its  elasticity,  as  shown  by  the  fact  that  it  is  ex- 
tended to  a  greater  degree  by  the  same  weight  in  the  active  than  in  the  passive 
condition.  The  degree  to  which  the  extensibility  is  increased  and  the  elas- 
ticity decreased  is  dependent  on  the  amount  of  the  resisting  force.  These 
facts,  as  determined  experimentally,  are  represented  in  Fig.  22.  Let  A  B  and 
A  b  represent  the  length  of  the  normal  unweighted  muscle,  in  passive  and 
active  states  respectively;  the  line  B  B',  the  extension  cur\'e  of  the  passive 
muscle  produced  by  successive  weights,  5,  10, 15,  20,  25, 30  grams,  differing  by 
a  common  increment;  the  line  b  B',  the  extension  curve  of  the  active  con- 
tracted muscle  when  weighted  with  the  same  weights;  A'  B'  the  length  of 
the  muscle  when  the  weight  is  sufficiently  great  to  prevent  shortening. 
It  will  be  observed  from  these  facts  that  while  the  muscle  is  extended  in 
both  the  passive  and  active  states  by  corresponding  weights,  the  extension 
during  the  latter  is  progressively  greater,  until  with  a  given  weight  the  length 
of  the  muscle  is  the  same.  Under  such  circumstances,  there  being  no  short- 
ening of  the  muscle,  the  force  of  its  contraction  manifests  itself  physically 
simply  as  tension.  In  the  successive  actions  of  the  muscle  represented  in 
the  same  figure  there  is  to  be  obser\^ed  also  a  combination  of  a  change  of 
length  and  a  change  of  tension,  the  ratio  of  the  one  to  the  other  being  deter- 
mined by  the  amount  of  the  supported  weights.     When  the  weight  is  slight 


GENER.AL  PHYSIOLOGY  OF  MUSCLE-TISSUE  63 

in  amount,  the  shortening  of  the  muscle  reaches  a  maximum  and  the  tension 
a  minimum;  when  the  weight  is  large  in  amount,  the  reverse  conditions  obtain. 

GRAPHIC  REPRESENTATION  OF  THE  CHANGE  OF  SHAPE 

The  contraction  of  a  muscle  as  it  takes  place  in  the  living  body  and 
under  normal  physiologic  conditions  is  a  complex  act  persisting  for  a  variable 
length  of  time  in  accordance  with  the  number  of  stimuli  transmitted  to  it 
in  a  given  unit  of  time,  and  as  determined  experimentally  is  the  resultant 
of  the  fusion  of  a  greater  or  less  number  of  separate  and  individual  contrac- 
tions or  pulsations.  To  this  enduring  contraction  the  term  tetanus  has  been 
given.  With  the  aid  of  appropriate  apparatus  it  has  become  possible  to 
obtain  and  record  single  muscle  contractions,  to  analyze  and  decompose  them 
into  their  constituent  elements,  or  to  combine  them  in  such  a  manner  as  to 
produce  practically  a  normal  physiologic  tetanus.  As  in  the  experimental 
study  of  the  phenomena  of  muscle  contraction  it  frequently  becomes  neces- 
sary to  remove  the  muscle  from  the  body  of  the  animal,  the  muscles  of  warm- 
blooded animals  are  not  well  adapted  for  this  purpose,  owing  to  the  rapid 
alteration  in  composition  they  undergo,  with  a  consequent  loss  of  irritability, 
when  deprived  of  their  normal  blood-supply.  The  excised  muscles  of  cold- 
blooded animals,  such  as  the  frog — in  which,  owing  to  the  relatively  slow 
rate  of  the  nutritive  activities,  the  irritability  and  contractility  endure  for 
a  relatively  long  period  of  time,  even  though  deprived  of  blood — are  particu- 
larly valuable  for  experimental  studies.  The  muscles  generally  employed 
are  the  gastrocnemius,  the  sartorius,  and  the  hyoglossus.  If  kept  at  a  normal 
temperature  and  moistened  with  0.6  per  cent,  solution  of  sodium  chlorid, 
such  a  muscle  will  contract  for  a  long  period  of  time  on  the  application  of 
any  form  of  stimulus,  but  especially  the  electric. 

Method  of  Recording  a  Muscle  Contraction. — Inasmuch  as  the 
changes  in  the  form  of  a  muscle  during  a  single  contraction  take  place 
with  extreme  rapidity,  their  succession,  peculiarities,  and  time  relations 
cannot  be  determined  with  any  degree  of  accuracy  by  the  unaided  eye. 
This  difl&culty  can  largely  be  overcome  by  the  employment  of  the  graphic 
method,  the  principle  of  which  consists  in  recording  the  movements  by 
means  of  a  pen  on  some  appropriate  moving  and  receiving  surface.  To 
accomplish  this  object  the  muscle  is  attached  at  one  extremity  by  a  clamp 
to  a  firm  support,  and  at  the  other  extremity  to  a  weighted  lever,  which  is, 
however,  sufficiently  light  to  enable  it  to  take  up,  reproduce,  and  magnify 
its  movements.  The  end  of  the  lever  provided  with  a  point  is  applied  to 
a  smooth  surface,  such  as  glazed  paper  on  a  cyHnder  or  plate,  covered 
with  lampblack.  If  the  surface  is  stationary,  the  contraction  is  recorded  as  a 
vertical  line;  if  it  is  put  in  movement  at  a  uniform  rate  by  clockwork,  the 
contraction  is  recorded  in  the  form  of  a  curv^e,  the  width  of  the  arms  of  which 
will  depend  on  the  rate  of  movement.  The  time  relations  of  the  phases  of 
the  contraction  can  be  obtained  by  placing  beneath  the  lever  a  writing  point 
attached  to  an  electro-magnet  thrown  into  action  by  a  tuning-fork  vibrating 
in  hundredths  of  a  second.  In  order  to  determine  the  rapidity  with  which 
the  contraction  follows  the  stimulation,  it  is  essential  that  the  moment  of 
the  latter  be  also  recorded.  This  is  accomplished  by  an  automatic  key,  the 
opening  or  closing  of  which  develops  the  stimulus  which  excites  the  muscle. 


64 


TEXT-BOOK  OF  PHYSIOLOGY 


A  combination  of  these  different  appliances  constitutes  a  myograph  and  the 
curve  of  contraction  a  myogram.     (See  Fig.  23.) 

It  is  necessary  for  the  purpose  of  placing  the  excised  muscle  under  me- 
chanic conditions  similar  to  those  found  in  the  body  and  for  the  registration  of 
its  movements  under  varying  conditions  to  give  the  lever  mass.  This  is  accom- 
plished by  attaching  weights  to  it  beneath  the  muscle. 


Fig.  23. — Myograph.     K.  Recording  cylinder.     M.  Moist  chamber.     L. 
Recording  lever.     W.  Weight.     I.  Induction  coil. 

The  Isotonic  Myogram. — With  the  object  of  obtaining  a  curve  of 
the  successive  changes  in  the  length  of  a  muscle  during  a  single  contraction 
and  at  the  same  time  avoiding  changes  in  tension  and  therefore  an  accelera- 
tion of  the  lever,  the  weight  attached  to  the  lever  should  be  applied  close  to  its 
axis,  in  accordance  with  the  isotonic  method.  The  curve  of  contraction  thus 
obtained  is  known  as  an  isotonic  myogram.^ 

With  the  muscle  arranged  as  previously  described  and  stimulated 
directly  with  a  single  induced  electric  current,  the  contraction  will  be  re- 
corded in  the  form  of  a  curve  similar  to  that  represented  in  Fig.  24,  in  which 
the  horizontal  line  represents  the  abscissa  of  time;  a,  the  moment  of  stimula- 
tion; and  bed,  the  degree  of  shortening  and  the  subsequent  relaxation  at  each 
successive  moment.  The  undulating  line  shows  the  time  relations,  the 
distance  from  crest  to  crest  representing  hundredths  of  a  second.  The 
curve  may  be  divided  into  three  portions: 

*  The  weighting  of  the  lever  or  the  loading  of  the  muscle  is  accompHshed  in  several  ways:  (i) 
The  weight  is  attached  to  the  lever  just  beneath  or  in  the  immediate  neighborhood  of  the  point  of 
attachment  of  the  muscle.  This  is  known  as  the  "  loaded  method"  and  has  the  effect  of  extending 
the  muscle  beyond  its  normal  length  previous  to  the  moment  of  its  stimulation  and  contraction. 
(2)  The  weight  is  attached  to  the  lever  at  the  same  point  as  in  the  foregoing  method,  but  by 
means  of  a  support  beneath  the  lever,  the  weight  is  prevented  from  extending  the  muscle  previous 
to  the  moment  of  its  stimulation  and  contraction.  This  is  known  as  the  "after-loaded"  method. 
In  either  case  a  certain  momentum  is  imparted  to  the  weight,  which  continues  after  the  muscle 
has  ceased  to  act,  both  when  shortening  and  relaxing,  and  so  imparts  to  the  recording  lever  addi- 
tional movements  which  vitiate  the  true  character  of  the  curve.  (3)  The  weight  is  attached  to  a 
small  pulley  on  the  axis  of  the  lever  and  therefore  at  some  distance  from  the  point  of  attachment 
of  the  muscle.  The  advantage  of  this  method  lies  in  the  fact  that  the  initial  tension  of  the  muscle 
induced  by  the  load  remains  practically  constant  throughout  the  contraction  period  and  hence 
acceleration  of  the  movement  of  the  lever  is  prevented.     This  is  known  as  the  "  isotonic  method." 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  65 

1.  A  short  but  measurable  portion  between  the  point  of  stimulation  and 

the  first  evidence  of  the  shortening,  a  b,  known  as  the  "latent  period," 
The  duration  of  this  period  for  the  skeletal  muscle  of  the  frog  was 
originally  determined  to  be  o.oi  second,  but  with  the  employment 
of  more  accurate  apparatus  it  has  been  reduced  to  0.0025  to  0.004  second. 
During  this  period  it  is  supposed  that  certain  chemic  changes  are 
taking  place  preparatory  to  the  exhibition  of  the  movement.  The 
duration  of  the  latent  period  is  influenced  by  a  variety  of  conditions, 
e.g.,  temperature,  fatigue,  strength  of  stimulus,  etc. 

2.  An  ascending  portion,  b  c,  the  contraction  or  period  of  increasing  energy. 

The  contraction  as  shown  by  the  character  of  the  curve  begins  slowly, 
then  proceeds  rapidly,  and  again  slowly  as  the  shortening  reaches  its 
maximum.  The  contraction  may  be  said  to  end  when  the  tangent 
to  the  curve  becomes  parallel  with  the  abscissa. 


Fig.  24. — The  Isotonic  Myogram. 

3.  A  descending  portion,  c  d,  the  relaxation  or  period  of  decreasing  energy. 

The  relaxation  as  shown  by  the  character  of  the  curve  begins  slowly, 

then  proceeds  rapidly,  and  again  slowly  as  the  muscle  attains  its 

original  length.     The  termination  of  the  relaxation  is  at  the  point  where 

the  curve  cuts  the  abscissa.     The  curve  beyond  this  point  may  be 

complicated  by  the  presence  of  one  or  more  residual  or  after- vibrations, 

which  are  probably  due  to  the  inertia  of  the  lever  as  well  as  to  changes 

in  the  muscle  elasticity. 

The  duration  of  the  period  of  shortening  is  about  0.04  second,  and 

of  the  period  of  relaxation  0.05  second.     A  single  pulsation  of  the  isolated 

muscle  of  the  frog  therefore  occupies,  from  the  moment  of  stimulation  to 

termination,  the  tenth  of  a  second.     Muscles  of  many  other  animals  have 

a  contraction  period  the  duration  of  which  varies  considerably  from  this. 

Thus,  in  man  the  time  of  a  single  contraction  is  one-twentieth  of  a  second, 

in  some  insects  one  three-hundredth  of  a  second,  and  in  the  turtle  one  second. 

Pale  muscles  have  a  shorter  period  than  the  red. 

Influences  Modifying  the  Effect  of  the  Stimulus. — The  contraction 
process  in  its  entirety  as  well  as  in  its  individual  parts  is  considerably  modi- 
fied by  both  external  and  internal  conditions,  among  which  may  be  mentioned 
the  following: 

i.C  haracler  of  the  Stimulus. — As  the  contraction  is  the  response  of  the 

muscle  to  a  stimulus,  it  may  be  inferred  that  the  vigor  of  the  former  is 

proportional,  within  limits,  to  the  strength  of  the  latter.     Thus  using 

as  a  stimulus  the  single  induced  current,  it  has  been  found  that  if  the 

5 


66  TEXT-BOOK  OF  PHYSIOLOGY 

strength  of  the  current  is  progressively  increased,  the  height  of  the  con- 
traction will  correspondingly  increase  until  a  certain  maximum  height  is 
attained  (Fig.  25,  A,  a  b);  then  notwithstanding  a  continued  increase 
in  the  strength  of  the  stimulus,  this  height  will  not  be  exceeded  for  some 
time.  But  if  the  strength  of  the  stimulus  be  yet  further  increased,  there 
comes  a  moment  when  the  contractions  again  increase  in  vigor  and  a 
second  maximum  height  is  attained  (Fig.  25,  B,  d  e).  Beyond  this  no 
further  increase  in  height  is  obser\^ed.  The  second  maximum  has  been 
attributed  to  the  presence  in  the  muscle  of  two  different  substances 
differently  affected  by  changes  in  temperature,  by  fatigue  and  by 
various  drugs. 


A.  B. 

Fig.  25. — ^Tracing  Showing  the  Effects  of  a  Gradual  Increase  in  the  Strength 
OF  the  Stimulus  on  the  Height  of  the  Contraction,  a.  Minimal  contraction;  ab.  pro- 
gressive increase  in  the  height;  b  c.  first  maximum  (a  number  of  contractions  have  been  omitted 
for  economy  of  space) ;  d  e.  second  maximum. 

It  has  also  been  shown  that  the  rate  at  which  the  muscle  is  stimulated 
with  a  given  stimulus  of  uniform  strength  will  influence  the  char- 
acter of  the  contraction  process.  If  the  intervals  between  the  successive 
stimulations  be  such  as  permit  the  muscle  to  recover  from  the  effects  of 
the  contraction,  it  may  contract  as  many  as  a  thousand  times  without 
showing  any  particular  variation  from  the  normal  form;  but  if  the 
inter\'als  are  shorter  than  that  just  stated  it  is  found  that  from  the 
beginning  of  the  stimulation  each  succeeding  contraction  slightly  exceeds 
in  height  the  preceding  contraction,  until  a  certain  maximum  is  reached 
and  maintained,  indicating  that  for  some  reason  the  irritability  and  the 
energy  of  the  contraction  have  been  increased.  This  gradual  increase 
in  the  height  of  the  contraction  has  been  termed  the  staircase  effect,  or 
the  treppe.  In  the  beginning  of  the  period  of  stimulation  there  is  some- 
times observed  a  decrease  in  the  height  of  the  contraction  following 
several  stimulations  before  the  staircase  effect  develops,  indicating 
a  temporary  decrease  in  the  irritability.  These  staircase  contractions 
have  been  observ^ed  in  the  muscle  of  both  warm-blooded  and  cold- 
blooded animals.  The  cause  for  this  increase  in  irritability  upon  which 
the  effect  depends  is  attributed  to  the  presence  of  certain  chemic  sub- 
stances in  the  muscle  arising  as  a  result  of  its  katabolism,  such  as 
carbon  dioxid,  mono-potassium  phosphate,  and  paralactic  acid.  These 
compounds,  when  present  in  small  amounts  or  in  larger  amounts  for  a 


GENER.'\L  PHYSIOLOGY  OF  MUSCLE-TISSUE 


67 


short  time,  augment  the  action  of  the  muscle  and  give  rise  to  the  treppe 
effect.  (Lee.)  In  time,  however,  if  the  stimulation  be  continued,  the 
irritability  declines,  the  height  of  the  contraction  diminishes  and 
finally  the  muscle  ceases  to  respond  to  any  stimulus. 
Variations  in  the  Temperature. — The  temperature  at  which  all  phases  of 
the  contraction  process,  as  represented  by  the  myogram,  attain  their 
physiologic  maximum  value  is  about  30°C.  If  the  temperature 
of  the  muscle  falls  to  2o°C.  there  is  a  corresponding  decline  in  activity, 


Fig.  26.- 


-SiNGLE  Contractions  of  the  Gastrocnemtus  Muscle  at  Different  Tempera- 
tures.    Time  tracing  200  per  second. — (Brodie.) 


as  shown  by  an  increase  of  the  latent  period,  a  decrease  in  the  height 
of  curve — i.e.,  in  the  shortening  of  the  muscle — an  increase  both  in 
the  contraction  and  relaxation  periods.  As  the  temperature  approaches 
o°C,  the  height  of  the  curve  again  suddenly  increases,  indicating,  for 
some  unknown  reason,  an  increase  in  the  irritability.  This  is,  however, 
scarcely  a  physiologic  condition.  At  a  temperature  of  40°C.  to  5o°C. 
the  muscle  suddenly  contracts  and  passes  into  the  condition  of  heat 
rigor  or  rigor  caloris.  The  protein  constituents  of  the  muscle  are 
coagulated  and  the  irritability  destroyed.     (Fig.  26.) 


Fig.  27. — Contractions  of  a  Gastrocnemius  Muscle 
WITH  Different  Loads. — {Brodie.) 

Variations  in  the  Load. — The  extent  to  which  a  muscle  is  loaded  or 
weighted  will  not  only  determine  the  height  of  the  contraction,  but  also 
the  time  relations  of  all  its  phases.  This  is  apparent  from  an  exami- 
nation of  Fig.  27,  in  which  it  is  shown  that  with  an  increase  in  load 
there  is  a  decrease  in  the  height  of  the  contraction,  an  increase  in  the 
latent  period,  and  a  general  increase  in  the  duration  of  both  the  periods 
of  rising  and  falling  energy. 

Rapidly  Repeated  Stimulation. — Prolonged  or  excessive  activity  of  our  own 
muscles  is  accompanied  by  a  feeling  of  stiffness  or  soreness  and  lassi- 


68  TEXT-BOOK  OF  PHYSIOLOGY 

tude.  There  is  at  the  same  time  a  diminution  in  the  speed  and  vigor 
of  the  contractions  and  the  power  of  doing  work.  To  this  condition 
the  term  fatigue  has  been  given.  The  cause  of  the  fatigue  is  attributed 
to  a  diminution  in  the  amount  of  the  energy-yielding  compounds  as 
well  as  to  the  production  and  accumulation  of  waste  products  resulting 
from  katabolic  activity.  Among  the  waste  products,  mono-potassium 
phosphate,  paralactic  acid,  and  carbon  dioxid  are  the  most  important. 
These  substances,  when  present  in  small  amounts  or  in  larger  amounts 
for  a  short  time,  increase  the  irritability  of  the  muscle,  but  when  they 
accumulate  more  rapidly  than  they  are  removed,  as  is  the  case  during 
excessive  activity,  they  exert  a  depressive  influence  on  the  irritability  of 
the  muscle  and  thus  diminish  its  contractile  power  and  its  capacity 
for  doing  work.  The  more  rapidly  they  are  removed,  the  sooner  is 
a  fatigued  muscle  restored  to  its  normal  condition.  The  condition 
of  fatigue  with  its  attendant  phenomena  is  shown  by  stimulating 
through  its  nerve  an  excised  frog  muscle  with  induced  electric  currents 


Fig.  28. — Fatigue  Curves.     Every  Twentieth  Contraction  Recorded. 

at  intervals  of  one  second.  In  a  variable  period  of  time  the  muscle 
shows  an  increase  in  the  duration  of  the  latent  period,  a  diminution 
of  the  height  of  the  contraction,  in  the  power  of  doing  work,  and  an 
increase  in  the  time  required  for  relaxation.  (Fig.  28.)  If  the  stimu- 
lation is  continued  the  contractions  gradually  decline  as  the  muscle 
becomes  exhausted.  When  a  muscle  will  no  longer  respond  to  stimu- 
lation through  its  related  nerve,  it  can  be  made  to  respond  to  direct 
stimulation  with  the  electric  current.  This  taken  in  connection  with 
the  fact  that  stimulation  of  a  nerve-trunk  even  for  several  hours  does 
not  fatigue  it,  leads  to  the  inference  that  the  cause  of  the  cessation  of 
contraction  does  not  lie  wholly  in  the  muscle  but  partly  in  the  nerve 
endings  in  the  muscle.  These  structures  it  is  believed  fatigue  more 
readily  than  the  muscle  structures,  and  hence  fail  to  conduct  the  nerve 
impulse  to  the  muscle.  By  this  means  it  is  protected  from  absolute 
exhaustion. 
Nutrition. — The  irritability  of  a  muscle  which  conditions  the  con- 
traction process  is  dependent  on  the  maintenance  of  its  nutrition; 
hence  a  continuous  and  sufficient  supply  of  nutritive  material  and 
a  rapid  removal  of  waste  products  are  essential  conditions  for  the  ex- 
hibition of  normal  contractions.  A  diminution  of  blood  supply  or 
an  accumulation  of  waste  products  sooner  or  later  impairs  the  irritability 
and  diminishes  the  vigor  and  extent  of  the  contraction.  Various 
drugs — e.g.,  veratrin,  barium,  etc. — introduced  into  the  circulation  and 
finding  their  way  into  the  muscle  modify  the  contraction  process,  as 
shown  by  a  very  great  increase  in  the  duration  of  the  relaxation  period. 


GENER-\L  PHYSIOLOGY  OF  MUSCLE-TISSUE  69 

The  Isometric  Myogram.— With  the  object  of  obtaining  a  curve  of 
the  increase  and  decrease  in  the  tension  of  a  muscle  during  a  single  con- 
traction, with  the  exclusion  as  far  as  possible  of  a  change  in  length,  the  muscle 
may  be  made  to  contract  against  a  strong  spring  or  similar  resistance  practically 
though  not  absolutely  sufl&cient  to  prevent  shortening.  To  this  method 
the  term  isometric  has  been  given,  and  the  curve  so  obtained  is  an  isometric 
myogram  or  a  tonogram.  The  recording  portion  of  the  lever  is  prolonged 
some  distance  so  that  the  very  slight 
upward  movement  at  its  axis,  close  to 
which  the  muscle  is  attached,  will  be 
considerably  magnified.  That  the 
ordinate  value  of  an  isometric  curve 
may  be  known,  the  apparatus  must  be 
graduated  by  subjecting  the  spring  to 
a  series  of  weights  playing  over  a  pul- 
ley supported  by  the  muscle  clamp. 

The  curve  of  the  variation  in  ten- 
sion obtained  bv  the  isometric  method  ^i*^-  29-— a.  Diagram  of  Isotonic;  b, 
is  shown  in  Fig.^29,,  b,  in  which  the  two  °'/S™  ^^''"''"^^  Cv^v^s.-iLandois 
curves  are  contrasted.     The  form  of  the 

curve  indicates  that  the  muscle  attains  its  maximum  of  tension  more 
rapidly  than  its  maximum  of  shortening;  that  the  tension  endures  for  a 
certain  period  of  time  unchanged;  that  the  fall  in  tension  takes  place  more 
rapidly  than  the  muscle  lengthens. 

The  Myogram  Due  to  the  Make  and  the  Break  of  a  Galvanic  Current. 
— The  contraction  of  the  muscle  which  has  heretofore  been  recorded  has 
been  caused  by  the  momentary  action  of  an  induced  current.  The  con- 
traction of  the  muscle  which  is  caused  by  the  action  of  a  constant  or  galvanic 
current  presents  features  which  are  somewhat  different  and,  as  it  serves  to 
illustrate  the  difference  in  the  effects  of  a  constant  or  galvanic  and  an  induced 
or  interrupted  current,  a  myogram  of  a  contraction  due  to  the  make  and 


Fig.  30. — Myogram  Due  to  the  Action  of  a  Galvanic  Current,  Applied  Directly  to  a 
Muscle,  when  the  Circuit  was  Closed  (c)  and  when  it  was  Opened  (o). 

break  of  a  galvanic  current  is  introduced  at  this  place.  The  effects  which 
are  observed  in  a  muscle  during  the  passage  of  both  feeble  and  strong 
currents  have  been  alluded  to  in  a  previous  section.  (See  page  57.)  In 
Fig.  30  these  effects  are  graphically  represented.  It  will  be  observed  that 
on  the  closure  of  the  circuit  at  c  the  muscle  at  once  contracted  and  so  long  as 
the  current  was  flowing,  the  muscle  remained  in  a  more  or  less  contracted 
state  known  as  galvanotonus;  on  opening  the  circuit  at  o  the  muscle  again  con- 


70 


TEXT-BOOK  OF  PHYSIOLOGY 


tracted,  after  which  it  gradually  relaxed  and  returned  to  its  original  con- 
dition. The  record  shows  also  that  during  the  actual  passage  of  the  cur- 
rent the  muscle  substance  was  being  stimulated  by  it. 

The  Work  Accomplished  by  a  Muscle  during  the  Time  of  a  Single 
Contraction.— By  work  is  meant  the  overcoming  of  opposing  forces. 
In  the  physiologic  activities  of  the  body  the  muscles  at  each  contraction  not 
only  overcome  the  resistances  of  antagonistic  muscles,  the  weight  of  the 
limbs,  the  friction  of  joints,  etc.,  but  in  addition  overcome  various  external 
resistances  connected  with  the  environment — e.g.,  gravity,  cohesion,  friction, 
elasticity,  etc.  The  muscles  may  therefore  be  regarded  as  machines  for 
the  accomplishment  of  work.  Experimentally  the  work  done  by  an  iso- 
lated muscle  may  be  calculated  if  the  height  of  the  contraction  is  first  obtained 

and  then  multiplied  by  the  weight 
raised.  The  influence  of  the 
weight  on  the  height  of  the  con- 
traction is  shown  in  Fig.  31,  in 
which  only  the  height  of  the  con- 
traction or  the  degree  of  shorten- 
ing and  hence  the  lift  of  the  weight 
is  represented.  From  this  tracing 
it  will  be  observed  that  the  extent 
to  which  a  muscle  will  shorten  in 
response  to  a  maximal  stimulus 
is  greatest  when  it  is  unweighted; 
but  as  weights  differing  by  a  com- 
mon   increment    are  added,    the 


Fig.   31. — I'RACiNG   Showing   the   Gradual 
Diminution  in  the  Height  of  the  Contrac- 


tion AS  the  Weight  was  Increased  by  a  Com-    height   of   the   contraction   dimin- 
MON   Increment    of  10  Grams  from  o  to  180    .      °  ,-i      -^i  •  •   i.^.  -i. 

ishes  until  with  a  given  weight  it 


Grams.    Magnification  of  the  Lever,  4. 


is  nil. 


A  careful  study  of  the  results  of  this  experiment  will  show  that  the 
work  done  gradually  increased  as  the  load  was  increased  from  o  to  70 
grams,  when  it  amounted  to  210  gram-millimeters;  but  that  after  this, 
even  though  the  weight  lifted  was  greater,  the  height  to  which  it  was  lifted 
was  less,  and  hence  the  work  done  gradually  decreased,  until  it  amounted 
to  nothing. 

The  following  table  will  also  show  the  work  done  by  a  frog's  muscle 
according  to  Rosenthal. 


Weight, 
o  grams 

50  grams 
100  grams 
150  grams 
200  grams 
250  grams 


Height. 
14  mm. 

9  mm. 

7  mm. 

5  mm. 

2  mm. 

o  mm. 


Work  Done. 

o  gram-millimeters. 
450  gram-millimeters. 
700  gram-millimeters. 
750  gram-millimeters. 
400  gram-millimeters. 

o  gram-millimeters. 


From  the  preceding  figures  it  is  evident  that  the  mechanical  work  of  a 
muscle  increases  with  increasing  weights  up  to  a  certain  maximum,  and 
then  declines  to  zero.  Equally  when  the  muscle  contracts  to  its  maxi- 
mum without  being  weighted,  and  when  it  does  not  contract  at  all  from 
being  overweighted,  no  work  is  done.  Between  these  two  extremes  the 
muscle  performs  varying  amounts  of  work. 


GENER.\L  PHYSIOLOGY  OF  MUSCLE-TISSUE  71 

Absolute  Muscle  Force. — The  maximum  amount  of  force  which  a 
muscle  puts  forth  during  a  contraction  is  naturally  measured  by  the  amount  of 
work  done;  but  as  this  varies  with  the  degree  to  which  the  muscle  is  weighted, 
another  measure  has  been  adopted,  to  which  the  term  absolute  muscle 
force  or  static  force  has  been  given.  The  absolute  force  is  measured  by  the 
weight  which  a  muscle  can  raise  and  hold  at  its  natural  length  after  it  has 
been  extended  by  this  weight.  In  Fig.  22  this  weight  would  be  on  the  ab- 
scissa B  b'  v/here  it  is  cut  by  the  extension  curve  of  the  active  contracted 
muscle.  The  absolute  force  is  also  measured  by  the  weight  which  is  just 
sufficient  to  prevent  the  muscle  from  shortening  when  stimulated.  '  This  is 
best  determined  by  the  method  of  after-loading  in  which  the  muscle  is  not 
extended  by  the  weight  previous  to  the  contraction.  It  has  been  found 
that  the  absolute  force  of  a  muscle  is  direcdy  dependent  on  the  number  and 
not  the  length  of  the  fibers  it  contains  and  proportional  to  the  physiologic 
transverse  section  of  the  muscle.  The  transverse  section  of  a  muscle  is 
obtained  by  dividing  its  volume  (obtained  by  dividing  its  actual  weight  by 
the  specific  weight  of  muscle-tissue,  1.058)  by  the  average  length  of  the 
fibers.  Assuming  that  the  muscle  weighs  609  grams,  its  volume  would  be 
576  c.c;  and  if  it  be  further  assumed  that  the  fibers  have  an  average  length 
of  4  centimeters  the  transverse  section  would  contain  114  sq.  centimeters 
each  of  which  would  have  a  length  of  4  centimeters. 

For  purposes  of  comparison  it  is  customary  to  refer  the  absolute 
force  to  the  units  of  area — viz.,  one  square  centimeter,  Rosenthal  esti- 
mates the  force  for  the  square  centimeter  of  the  muscle  of  the  frog  at  from 
2  to  8  kilograms;  for  the  muscles  of  man  at  6  to  8  kilograms;  Koster  at 
about  10  kilograms  for  the  muscles  of  the  leg  and  7  to  8  kilograms  for  the 
muscles  of  the  arm. 

Summation  Effects.— If  a  series  of  successive  stimuli  be  applied 
to  a  muscle,  the  effect  will  vary  according  to  the  rapidity  with  which  they 
follow  one  another.  As  previously  stated,  if  the  interval  preceding  each 
stimulus  be  sufficiently  long  to  enable  the  muscle  to  recover  from  the  effects 
of  the  previous  contraction,  there  will  be  no  change  in  the  form  or  the  char- 
acter of  the  contraction  for  a  long  time  except  a  slight  increase,  in  the  early 
period,  of  the  irritability  as  shown  by  the  increased  height  of  the  curve 
or  shortening  of  the  muscle.  If,  however,  a  second  stimulus  be  applied 
to  a  muscle  during  the  period  of  relaxation,  a  second  contraction  immediately 
follows  which  is  added  to  or  superposed  on  the  first;  the  effect  produced  will 
be  greater  than  that  produced  by  either  stimulus  separately.     (See  Fig.  32.) 

A  third  stimulus  applied  during  the  relaxation  of  the  second  contraction 
produces  a  third  contraction  which  adds  itself  to  the  second,  and  so  on. 
The  increment  of  increase  in  the  extent  of  the  successive  contractions  gradu- 
ally diminishes,  however,  until  the  muscle  reaches  a  maximum  of  contrac- 
tion. The  superposition  of  the  second  contraction  on  the  first,  the  third  on 
the  second,  and  so  on,  is  termed  summation  of  contractions  or  effects.  Experi- 
ment has  shown  that  the  greatest  effect  of  a  second  stimulus — that  is,  the 
highest  contraction — is  produced  when  the  stimulus  is  applied  during  the 
last  third  of  the  period  of  rising  energy,  when  the  sum  of  the  two  contractions 
is  almost  twice  as  great  as  the  first  contraction  would  have  been.  (Fig.  32,) 
The  effects  following  both  maximal  and  submaximal  stimuli  indicate  that 
the  muscle  cannot  attain  its  maximum  of  shortening  except  through  a 


72 


TEXT-BOOK  OF  PHYSIOLOGY 


summation  of  several  stimuli.  If  a  second  maximal  stimulus  enters  a  muscle 
during  the  latent  period  following  the  first,  the  effect  produced  will  be  no 
greater  than  that  produced  by  a  single  stimulus.  The  muscle  during  this 
period  is  said  to  be  refractory  or  non-responsive  to  a  second  stimulus.  If, 
however,  the  stimuli  are  submaximal  they  add  themselves  together,  and 
though  the  effect  is  but  a  single  contraction,  it  is  larger  than  either  would 
have  produced  separately.     This  is  termed  the  summation  of  stimuli. 

Still  further,  if  a  series  of  submini- 
mal stimuli,  each  of  which  is  alone  in- 
sufficient to  produce  a  contraction  of 
the  muscle,  be  applied  in  rapid  succes- 
sion, a  contraction  frequently  results. 
This  is  termed  the  summation  of  submini- 
mal stimuli. 

Tetanus. — Tetanus  may  be  defined 
as  a  more  or  less  continuous  contraction 
of  a  muscle  which  arises  when  the  time 
intervals  between  the  stimuli  are  shorter 
than  the  time  of  the  contraction  proc- 
ess. Tetanus  will  be  incomplete  or 
complete  according  to  the  number  of 
stimuli  that  reach  the  muscle  in  a  sec- 
ond of  time.  When  a  muscle  is  stimu- 
lated directly  or,  better,  indirectly 
through  its  related  nerve  by  a  series  of 
induced  currents  at  the  rate  of  four  or 
six  per  second,  it  undergoes  a  corre- 
sponding number  of  contractions  of 
about  equal  extent.  If  the  rate  of 
stimulation  is  increased  up  to  the  point 
when  the  interval  between  each  stimulus 
is  less  than  the  duration  of  the  entire 
contraction  process,  the  muscle  does  not 
have  time  to  relax  completely  before  the 
Fig.  32.— Tracing  Showing  the  Ef-  arrival  of  the  succeeding  Stimulus,  and 
FECTs  OF  Two  StrccEssrv'E  STIMULI,  a.  a'  hence  remains  in  a  more  or  less  con- 
wiTH  Gradually  Diminishing  Inter-  t^p^tpH  cfnt*^  rlnrino-  wVnVVi  ^f  PYViiKitQ  a 
VAL  on  a  Muscle  Contraction.     To  be  ^racted   State,  durmg  WhlCti  It  exniDltS  a 

read  from  below  upward.  Series  of  alternate  partial   contractions 

and  relaxations.  To  this  condition  of 
muscle  activity  the  term  incomplete  tetanus  or  clonus  is  applied.  A 
graphic  record  of  an  incomplete  tetanus  is  given  in  Fig.  33. 

In  such  a  tracing  it  is  observed  that  the  second  stimulation,  occurring 
before  the  muscle  had  time  to  relax,  gave  rise  to  a  second  contraction, 
which  was  superposed  on  the  first;  the  same  result  followed  the  third  stimu- 
lus, the  fourth,  the  fifth,  and  so  on.  Owing  largely  to  this  summation 
of  the  contractions  there  is  a  gradual  rise  in  the  height  of  the  contraction 
curve.  This  condition  of  the  muscle,  viz.,  continued  contraction,  com- 
bined with  diminished  power  of  relaxation,  is  termed  contracture.  The 
tracing  also  shows  that  as  the  stimulus  continues,  the  base  line,  that  con- 
necting the  lowest  points  of  the  contractions,  gradually  rises  and  takes  the  form 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE 


73 


of  a  curve  which  increases  in  height  as  the  stimulus  continues.  The  apex 
line,  that  connecting  the  highest  points  of  the  contractions,  also  rises  at  the 
same  time,  indicating  a  continuous  increase  in  the  height  of  the  contractions. 
The  length  of  time  a  muscle  will  exhibit  incomplete  tetanus  depends  on  a 
variety  of  circumstances,  e.g.,  character  of  muscle,  rate  and  strength  of 
stimulation,  etc.,  but  mainly  on  the  rapidity  with  which  the  muscle  becomes 
fatigued.  With  the  oncoming  of  fatigue  the  muscle  begins  to  relax,  and 
ultimately  returns  to  its  normal  condition,  notwithstanding  the  continued 


Fig.  33. — Curves  Showing  the  Analysis  of  Tetanus  of  a  Frog's  Muscle  (Gastroc- 
nemius). The  numbers  under  the  curves  indicate  the  number  of  shocks  per  second  applied  to 
the  muscle.  There  is  almost  complete  tetanus  with  twenty-five  per  second,  and  it  is  a  little  lower 
than  the  previous  one  because  the  muscle  was  slightly  fatigued. — (Stirling.) 

stimulation.  If  the  stimulation  be  withdrawn,  the  muscle  does  not  at  once 
return  to  its  original  length  but  remains  more  or  less  contracted  for  a  variable 
time.  This  contraction  after  stimulation  is  known  as  the  contraction- 
remainder. 

If  the  stimulation  be  still  further  increased  in  frequency,  the  individual 
contractions  become  fused  together  and  the  curve  described  by  the  lever 
becomes  a  continuous  line.  (See  Fig.  33.)  Notwithstanding  the  fact 
that  the  individual  contractions  are  no  longer  visible,  it  can  be  shown  by 


Fig.  34. 


-Development  of  Fatigue  and  Contraction.     Muscle  stimulated  once  a  second  by 
a  strong  induced  ciurrent. 


Other  methods  that  the  muscle  is  undergoing  a  series  of  slight  alternate  con- 
tractions and  relaxations  or  vibrations  at  least.  After  a  varying  length  of 
time  the  muscle  becomes  fatigued,  relaxes,  and  returns  to  its  natural  con- 
dition even  though  the  stimulation  be  continued.  The  number  of  stimuli 
per  second  necessary  to  develop  complete  tetanus  will  depend  under  normal 
circumstances  on  the  period  of  duration  of  the  individual  contractions. 
The  longer  this  period,  the  less  the  number  of  stimuli  required,  and  the 


74  TEXT-BOOK  OF  PHYSIOLOGY 

reverse.  Hence  the  number  of  stimuli  will  vary  for  different  classes  of  animals 
and  for  different  muscles  in  the  same  animal,  e.g.,  2  or  3  for  the  muscles  of 
the  tortoise,  10  for  the  muscles  of  the  rabbit,  15  to  20  for  the  frog,  70  to 
80  for  birds,  330  to  340  for  insects. 

An  effect  which  follows  frequent  stimulation  of  a  muscle,  e.g.,  50  to 
60  times  per  minute,  and  especially  when  the  muscle  is  somewhat  fatigued 
or  cold  is  shown  in  Fig.  34.  It  is  evidently  a  combination  of  contracture  and 
fatigue.  It  will  be  observed  that  at  the  beginning  of  the  stimulation  there 
is  a  staircase ,  effect,  a-b,  combined  with  diminished  relaxation.  This  in 
turn  is  followed  by  a  decline  in  the  height  of  the  contractions,  b-c,  and  a  fall 
of  the  base  line  which  may  be  attributed  to  fatigue  conditions.  After  a  short 
time  there  is  a  second  rise  of  the  base  line,  d,  and  a  rapid  development  of 
contracture.  The  muscle  at  this  period  is  in  a  condition  of  incomplete 
tetanus  which  gradually  passes  into  complete  tetanus  attended  by  fatigue. 

The  tetani  of  muscles  may  be  classified  in  accordance  with  their  causes 
as  follows: — 

-P,i      .1     .    f  Volitional. 

1.  Physiologic  I  ^^^^^^ 

2.  Experimental. 

3.  Pharmacologic. 

■n  i.1.  1     •     f  Bacterial. 

4.  Pathologic  I  ^^^^^^ 

1.  Physiologic  Tetanus,  i.  Volitional. — Because  of  the  fact  that 
during  the  continuance  of  a  volitional  movement  the  muscle  is  in  a  state  of 
continuous  contraction,  it  may  be  accepted  that  volitional  contractions  are 
states  of  tetanus,  more  or  less  complete;  for  the  shortest  possible  volitional 
contraction,  however  quickly  it  takes  place,  has  always  a  longer  duration 
than  a  single  contraction  caused  by  an  induced  electric  current.  As  the 
volitional  contraction  is  similar  to  that  observed  when  a  muscle  or  its  related 
nerve  is  stimulated  by  rapidly  repeated  induced  currents,  it  is  assumed  that 
the  nerve-cells  in  the  spinal  cord  are  discharging  in  a  rhythmic  manner  a 
certain  number  of  nerve  impulses  per  second  in  consequence  of  the  arrival 
of  nerve  impulses  coming  from  the  cerebral  cortex,  the  result  of  volitional 
acts.  In  other  words  the  volitional  tetanus  is  the  result  of  a  discontinuous 
stimulation.  The  number  of  stimuli  transmitted  to  a  muscle  during  a 
volitional  tetanus  has  been  estimated  by  the  employment  of  the  graphic 
method  at  from  8  to  13  per  second,  10  being  about  the  average.  When  a 
volitional  contraction  is  recorded  the  myogram  not  infrequently  exhibits  a 
series  of  small  wave-like  elevations  which  indicate  that  the  muscle  is  not  in 
a  state  of  complete  tetanus  but  is  undergoing  slight  alternate  contractions 
and  relaxations.  Unless  the  contraction  process  in  human  muscle  differs 
from  that  of  frogs  it  is  difi&cult  to  see  how  10  or  even  20  stimuli  per  second 
can  give  rise  to  even  an  incomplete  tetanus  when  the  single  contraction  is  -^ 
of  a  second  in  duration. 

2.  Reflex. — A  tetanus  of  muscle,  physiologic  in  character,  arises  during 
the  performance  of  many  muscle  movements  in  consequence  of  peripherally 
acting  causes  and  may  therefore  be  termed  a  reflex  tetanus.  The  duration 
of  a  tetanus  thus  induced,  like  the  duration  of  a  volitional  tetanus,  will  vary 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  75 

with  the  duration  of  the  exciting  cause.  Reflex  tetani  are  presented  by  the 
muscles  of  the  lower  jaw  during  mastication,  by  the  intercostal  muscles 
during  breathing,  by  the  muscles  of  the  limbs  during  walking,  etc.  In  these 
and  other  instances  there  are  reasons  for  believing  that  for  a  variable  period 
of  time  the  muscles  are  in  a  state  of  continuous  contraction  from  the  dis- 
charge of  nerve  impulses  from  the  nerve  cells  in  the  spinal  cord  as  the  result 
of  the  arrival  of  nerve  impulses  coming  from  a  peripheral  surface. 

2.  Experimental  Tetanus. — The  tetanus  of  muscle  developed  in 
accordance  with  the  method  described  in  foregoing  paragraphs,  i.e.,  by  the 
employment  of  instrumental  procedures,  may  be  termed  experimental 
tetanus.  Its  mode  of  development  serves  to  illustrate  and  explain  the 
method  by  which  individual  contractions  are  summated  and  continuous 
contractions  made  possible  for  the  performance  of  volitional  acts. 

3.  Pharmacologic  Tetanus. — The  administration  of  certain  drugs,  e.g., 
strychnin,  in  sufficient  amounts,  is  followed  in  a  short  time  by  a  series  of 
intermittent  spasms  in  which  all  the  muscles  of  the  body  are  involved.  At 
the  beginning  of  the  spasms  the  muscles  are  thrown  into  tonic  or  complete 
tetanus,  during  the  continuance  of  which  the  muscles  are  hard  and  firm.  In 
a  short  time  this  tonic  state  begins  to  subside,  giving  way  to  tremors  or  a 
series  of  irregular  contractions  resembling  incomplete  tetanus  or  clonus.  A 
tetanus  of  this  character  may  be  termed  pharmacologic.  Though  the  onset 
of  the  tetanus  is  occasioned  largely  by  peripheral  stimulation,  the  seat  of 
action  of  strychnin  is  central  and  for  the  most  part  focalized  in  the  spinal 
cord.  The  exact  seat  of  its  action  is  not  definitely  determined  but  there  are 
reasons  for  believing  that  it  is  on  the  end-tufts  of  afferent  nerves  in  the  spinal 
cord  or  on  the  intercalated  neuron  between  them  and  the  nerve-cells  in  the 
anterior  horns  of  the  gray  matter,  the  irritability  of  which  is  raised  and  the 
resistance  to  the  transmission  of  nerve  impulses  coming  from  the  periphery 
diminished.  As  a  result  the  nerve  impulses  are  transmitted  to  the  nerve- 
cells  more  readily,  not  only  in  a  horizontal  but  also  in  a  longitudinal  direction, 
and  the  effects  they  produce  enormously  increased. 

4.  Pathologic  Tetanus,  i.  Bacterial. — The  introduction  of  a  specific 
bacillus  into  a  wound  in  any  region  of  the  body  is  followed  after  a  period  of 
incubation  of  from  three  or  four  days  to  a  week  by  a  tetanus  in  which  nearly 
all  the  muscles  of  the  body  are  involved,  characterized  by  a  tonic  contraction 
and  clonic  exacerbations.  A  tetanus  of  this  character  may  be  termed 
pathologic.  The  persistent  tonic  contraction  is  the  result  of  a  more  or  less 
continuous  discharge  of  nerve  impulses  from  the  nerve-cells  of  the  spinal  cord 
which  have  been  rendered  abnormally  irritable  by  the  action  of  a  toxin, 
produced  by  the  bacilli,  and  having  a  selective  action  on  these  structures. 
The  clonic  exacerbations  are  evoked  from  time  to  time  by  various  forms  of 
peripheral  stimulation. 

2.  Reflex. — A  tetanus  of  individual  muscles  more  or  less  continuous  in 
character  is  occasionally  the  result  of  peripheral  irritations  of  a  pathologic 
character.  A  tonic  contraction  of  the  masseter  muscles,  for  example,  firmly 
closing  the  jaws  for  weeks  and  months  at  a  time  is  caused  in  some  instances 
by  an  impacted  wisdom  tooth  or  an  ulcerative  condition  of  the  mouth. 
Since  the  removal  of  the  cause  is  followed  by  a  relaxation  of  the  muscle,  this 
form  of  tetanus,  known  as  trismus,  may  be  regarded  as  pathologic  in  char- 
acter and  reflex  in  origin. 


76  TEXT-BOOK  OF  PHYSIOLOGY 

The  Muscle  Sound. — If  a  stethoscope  or  a  myophone  with  telephone 
connections  be  placed  on  a  muscle  while  in  a  condition  of  volitional  tetanus 
and  at  the  same  time  kept  in  a  certain  degree  of  tension,  there  will  be  devel- 
oped in  the  observer  a  sensation  of  sound  or  tone  which  is  spoken  of  as  a 
muscle  sound  or  tone.  It  is  also  readily  heard  in  the  masseter  muscle  when 
the  side  of  the  face  is  placed  on  a  receiving  body  such  as  a  pillow,  and  the 
masseter  muscles  made  to  contract  volitionally.  This  tone  is  attributed  to 
a  vibration  or  an  alternate  contraction  or  relaxation  of  the  muscle  or  to  an 
intermittent  rhythmic  variation  in  tension,  the  result  of  the  rate  of  stimula- 
tion. This  tone  corresponds  to  a  vibration  frequency  of  from  1 8  to  20  per 
second  and  is  accepted  as  one  of  the  proofs  that  the  physiologic  volitional 
tetanus  is  not  continuous  but  discontinuous  in  character.  If  a  muscle  is 
tetanized  with  induced  currents,  the  tone  increases  in  pitch  for  a  limited 
time  as  the  frequency  of  the  current  per  second  increases  up  to  a  certain 
maximum,  which  for  frogs  is  about  200  and  for  mammals  about  1000. 

CHEMIC  PHENOMENA 

The  chemic  changes  which  underlie  the  transformation  of  energy  in  the 
living  muscle  even  when  in  a  state  of  relative  rest  are  active  and  complex, 
though  but  little  is  known  as  to  their  exact  character.  As  shown  by  an 
analysis  of  the  blood  flowing  to  and  from  the  resting  muscle,  it  has,  while 
flowing  through  the  capillaries,  lost  oxygen  and  gained  carbon  dioxid.  The 
amount  of  oxygen  absorbed  by  the  muscle  (9  per  cent.)  is  greater  than  the 
amount  of  carbon  dioxid  (6.7  per  cent.)  given  off.  Notwithstanding  the 
relation  of  the  oxygen  absorbed  to  the  carbon  dioxid  produced,  there  is  no 
parallelism  between  these  two  processes,  as  the  carbon  dioxid  will  be  given 
off  in  the  absence  of  free  oxygen  or  in  an  atmosphere  of  nitrogen. 

If  the  muscle  be  stimulated  through  its  related  nerve  all  the  chemic 
changes  are  increased  as  shown  both  by  an  increased  absorption  of  oxygen 
and  an  increased  production  of  carbon  dioxid,  though  the  ratio  existing 
between  them  differs  considerably  from  that  of  the  resting  muscle.  Thus, 
according  to  Ludwig,  an  active  muscle  absorbs  12.26  per  cent,  of  oxygen 
and  gives  off  10.8  per  cent,  carbon  dioxid.  At  the  same  time  the  muscle- 
tissue  changes  from  a  neutral  to  an  acid  reaction,  from  the  development  of 
sarcolactic  acid  and  possibly  phosphoric  acid.  The  degree  of  the  acidity 
depends  partly  on  the  duration  of  the  contraction  periods.  Chemic  analysis 
of  a  tetanized  muscle  shows  that  it  contains  less  glycogen  than  a  resting 
muscle,  and  that  it  contains  a  larger  amount  of  water.  Coincident  with 
the  muscle  contraction,  the  blood-vessels  become  widely  dilated,  leading  to  a 
large  increase  in  the  blood-supply  and  a  rapid  removal  of  the  products  of 
decomposition. 

Rigor  Mortis. — A  short  time  after  death  the  muscles  pass  into  a  condi- 
tion of  extreme  rigidity  or  contraction  known  as  death  stiffening  or  rigor 
mortis,  which  lasts  from  o*ne  to  five  days.  In  this  state  they  offer  great 
resistance  to  extension.  At  the  same  time  their  tonicity  disappears,  their 
cohesion  diminishes,  and  their  irritability  ceases.  The  time  of  the  appear- 
ance of  this  post-mortem  rigidity  varies  from  a  quarter  of  an  hour  to  seven 
hours.  Its  onset  and  duration  are  influenced  by  the  condition  of  the  muscle 
irritability  at  the  time  of  death.     When  the  irritability  is  impaired  from  any 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  77 

cause,  such  as  chronic  disease  or  defective  blood-supply,  the  rigidity  appears 
promptly  but  is  of  short  duration.  After  death  from  acute  diseases  it  is  apt 
to  be  delayed,  but  will  continue  for  a  longer  period.  The  rigidity .  first 
appears  in  the  muscles  of  the  lower  jaw  and  neck;  next  in  the  muscles 
of  the  abdomen  and  upper  extremities;  finally  in  the  trunk  and  lower  ex- 
tremities. It  disappears  in  practically  the  same  order.  Chemic  changes 
of  a  marked  character  accompany  this  process.  The  muscle  becomes  acid 
in  reaction  from  the  development  of  sarcolactic  acid  and  there  is  a  large 
increase  in  the  amount  of  carbon  dioxid  given  off.  The  immediate  cause 
of  the  rigidity  appears  to  be  coagulation  of  the  myosin  and  myogen  within 
the  sarcolemma  with  the  formation  of  two  insoluble  proteins,  myosin  fibrin 
and  myogen  fibrin.  In  the  early  stages  of  the  coagulation  restitution  is 
possible  by  the  circulation  of  arterial  blood  through  the  vessels.  The  final 
disappearance  of  this  post-mortem  rigidity  is  due  probably  to  the  action  of 
acids  which  render  the  myosin  and  myogen  fibrins  soluble,  and  possibly 
to  the  action  of  various  microorganisms  which  give  rise  to  putrefactive 
changes. 

Source  of  the  Muscle  Energy. — The  nature  of  the  materials  which  are 
the  immediate  sources  of  the  muscle  energy  has  been  the  subject  of  much 
discussion.  The  absence  of  any  noticeable  increase  in  the  quantity  of 
urea  or  other  nitrogen-holding  compounds  excreted  renders  it  probable 
that  the  energy  does  not  come  from  the  metabolism  of  protein  materials. 
The  marked  production  of  carbon  dioxid  and  sarcolactic  acid  points  to  the 
decomposition  of  some  unstable  compound,  of  a  carbohydrate  character, 
rich  in  carbon  and  oxygen.  As  the  result  of  many  investigations  it  has  come 
to  be  believed  that  glycogen  is  the  compound  which  furnishes  the  energy 
under  physiologic  conditions,  inasmuch  as  this  substance,  generally  present  in 
muscle,  disappears  during  activity.  A  muscle  which  has  been  tetanized 
contains  less  glycogen  than  the  corresponding  muscle  at  rest.  A  muscle 
which  has  been  separated  from  the  nervous  system  by  division  of  its  nerves 
and  thus  prevented  from  contracting  accumulates  glycogen.  Bunge  is 
of  the  opinion  that  though  the  carbohydrates  are  the  main,  they  are  not 
the  only  sources  of  muscle  energy.  If  there  is  a  deficiency  or  absence  of 
carbohydrate  food,  the  muscle  will  utilize  fat  and  protein,  for  experiment 
has  shown  that  the  available  glycogen  is  entirely  consumed  by  the  second  or 
third  day.  The  mechanism  by  which  the  energy  is  liberated,  whether 
by  direct  oxidation  or  decomposition  is  uncertain.  The  general  trend  of 
experimental  investigation  points  to  the  disruption  of  some  carbohydrate, 
perhaps  glucose,  derived  from  the  stored  glycogen  and  the  oxidation  of  the 
intermediate  products  to  carbon  dioxid  and  water.  The  oxidizable  com- 
pound appears  to  be  lactic  acid.  For  if  the  muscle  be  made  to  contract 
in  an  atmosphere  deficient  in  oxygen,  the  amount  of  lactic  acid  produced 
is  relatively  large  and  the  amount  of  carbon  dioxid  relatively  small.  If 
the  surrounding  atmosphere  be  rich  in  oxygen,  the  reverse  conditions 
obtain.  Under  physiological  conditions,  when  the  muscle  is  supplied 
with  blood  containing  its  customary  percentage  of  oxygen  it  is  probable 
that  the  products  set  free  by  the  disruption  of  the  sugar  molecule  are  rapidly 
oxidized  to  CO2  and  H2O,  with  the  liberation  of  their  contained  energy. 
But  the  fact  that  muscle  will  contract  in  an  atmosphere  free  of  oxygen, 
that  no  free  oxygen  can  be  obtained  from  muscle,  would  support  the  idea 


78  TEXT-BOOK  OF  PHYSIOLOGY 

that  the  mechanism  is  one  of  decomposition.  Hermann  suggests  that 
the  energy  of  a  contraction  is  Hberated  by  the  splitting  and  subsequent 
, re-formation  of  a  complex  body  belonging  neither  to  the  carbohydrates 
nor  fats,  but  to  the  proteins — to  this  hypothetic  body  the  term  inogen 
is  given.  This  complex  molecule,  the  product  of  the  nutritive  activity 
of  the  muscle-cell  in  undergoing  decomposition,  would  yield  carbon  di- 
oxid,  sarcolactic  acid,  and  a  protein  residue  resembling  myosin.  On  the 
cessation  of  the  contraction  the  muscle-cell  recombines  the  protein  residue 
with  oxygen,  carbohydrates,  and  fats,  and  again  forms  the  energy-holding 
compound,  inogen.  The  phenomena  of  rigor  mortis  support  this  view. 
At  the  moment  of  this  contraction  the  muscle  gives  off  COj  in  large  amount, 
develops  sarcolactic  acid  and  myosin.  There  is  thus  a  close  analogy  be- 
tween the  two  processes;  in  other  words,  a  contraction  is  a  partial  death  of 
the  muscle.  If  this  view  is  correct,  then  the  oxygen  is  required  mainly  for 
heat  production  through  oxidation  processes. 

THERMIC  PHENOMENA 

The  potential  energy  liberated  in  a  muscle  on  the  arrival  and  subsequent 
action  of  a  nerve  impulse,  manifests  itself  partly  as  heat  and  partly  as 
mechanic  motion  or  a  change  of  shape  of  the  muscle.  Though  heat  pro- 
duction is  taking  place  even  during  the  passive  condition,  it  is  largely  in- 
creased by  muscle  activity.  The  amount  of  heat  produced  will  vary  however 
with  a  variety  of  conditions,  as  strength  of  stimulus,  tension,  work  done,  etc. 

Stimulus. — It  has  been  experimentally  determined  that  the  skeletal 
muscle  of  the  frog,  the  gastrocnemius,  shows  after  a  single  contraction  a  rise 
in  temperature  of  from  o.ooi°C.  to  o.oo5°C.  and  after  tetanization  an 
increase  of  from  o.i4°C.  to  o.i8°C.  It  has  also  been  shown  that  an  increase 
in  the  strength  of  the  stimulus  from  a  minimal  to  a  maximal  value  increases 
the  amount  of  heat  liberated.  This  is  the  direct  result  of  increased  chemic 
change  naturally  following  increased  stimulation. 

Tension. — The  greater  the  tension  of  a  muscle,  the  greater,  other  condi- 
tions being  the  same,  is  the  amount  of  heat  liberated.  If  the  muscle  is 
securely  fastened  at  both  extremities  so  that  shortening  is  practically  im- 
possible during  the  stimulation,  the  maximum  of  heat  production  is  reached. 
In  the  tetanic  state  the  great  increase  in  temperature  is  due  to  the  ten- 
sion of  antagonistic  and  strongly  contracted  muscles.  In  both  instances, 
mechanic  motion  being  prevented,  the  liberated  energy  is  transformed  into 
heat. 

Mechanic  Work. — ^If  the  muscle  is  permitted  to  shorten  and  raise  a 
weight,  some  of  the  energy  liberated  takes  the  form  of  mechanic  motion.  If 
the  weight  is  removed  at  the  height  of  the  contraction,  external  work  is 
accomphshed.  The  greater  the  weight  raised,  within  limits,  the  greater  is 
the  percentage  of  energy  which  takes  the  direction  of  mechanic  motion.  The 
percentage  of  the  total  energy  liberated  which  is  thus  utilized,  has  been 
estimated  at  from  25  to  40  per  cent.  In  accordance  with  the  law  of  the  con- 
servation of  energy,  the  heat  produced,  stated  in  calories,  plus  the  energy 
required  in  the  raising  of  the  weight,  expressed  in  kilogrammeters  of  work, 
must  equal  the  potential  energy  transformed. 

A  muscle  during  a  tetanic  contraction  of  short  duration  accomplishes 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  79 

more  work  than  during  a  single  contraction,  the  weight  in  each  case  being 
the  same.  In  the  former  condition  the  height  of  contraction  through  sum- 
mation, and  hence  the  work  done,  is  greater  than  in  the  latter.  The  work 
done  by  a  short  tetanic  contraction  may  be  two  or  three  times  that  of  a  single 
contraction,  but  after  the  muscle  reaches  its  maximum  degree  of  shortening 
and  then  continues  in  a  state  of  tetanus,  no  further  work  is  done.  Internal 
work  is  done,  however,  i.e.,  the  continuous  liberation  of  energy,  as  shown  by 
an  increase  in  the  temperature. 

When  a  weight  which  is  lifted  by  a  muscle  during  a  single  contraction  is 
allowed  to  act  on  the  muscle  during  the  relaxation,  no  external  work  is 
accomplished.  All  the  energy  set  free  manifests  itself  as  heat.  Internal 
work  is  done,  as  shown  by  the  fact  that  the  muscle  becomes  fatigued. 

Work  Done  Daily. — The  muscle  system  in  its  entirety  is  to  be  regarded 
as  a  machine  for  the  transformation  of  potential  into  kinetic  energy,  and  in 
so  doing  accomplishes  work.  Through  the  intermediations  of  the  bones  of 
the  skeleton  which  play  the  part  of  levers  the  individual  not  only  changes  his 
position  in  space,  but  overcomes  to  some  extent  the  resistances  offered  by 
the  environment.  The  employment  of  artificial  levers,  tools,  as  distinguished 
from  natural  levers,  bones,  materially  adds  to  the  effectiveness  of  the  muscle 
machine.  The  amount  of  work  which  a  man  of  average  physical  develop- 
ment weighing  72  kilos  can  perform  in  eight  hours  has  been  variously  esti- 
mated. It  will  naturally  vary  according  to  the  character  of  the  occupation. 
If  the  work  done  be  calculated  from  the  number  of  kilograms  raised  one 
meter,  the  average  laboring-man  performs  about  300,000  kilogrammeters  of 
work. 

ELECTRIC  PHENOMENA 

Electric  Currents  from  Injured  Muscles. — The  energy  liberated  as 
the  result  of  the  action  of  a  nerve  impulse  is  not  only  transformed  into  heat 
and  mechanic  motion,  but  to  some  extent  also  into  electric  energy.  The 
presence  of  points  of  different  potential  on  the  surface  of  the  muscle,  the 
necessary  condition  for  the  development  of  electric  currents,  is  tested  by 
means  of  non-polarizable  electrodes  connected  by  wires  with  a  sensitive 
galvanometer  or  capillary  electrometer.  When  such  electrodes  are  brought 
in  contact  with  a  muscle  properly  prepared,  there  is  at  once  developed  and 
conducted  to  the  galvanometer  an  electric  current  the  intensity  and  direction 
of  which  are  indicated  by  the  deflection  of  the  galvanometer  needle.  The 
existence  of  this  current  is  most  conveniently  demonstrated  with  single 
muscles  the  fibers  of  which  are  parallel — e.g.,  the  sartorius,  or  the  semimem- 
branosus of  the  frog.  If  the  tendinous  ends  of  either  of  these  muscles  be 
removed  by  a  section  made  at  right  angles  to  the  long  axis,  a  muscle  prism  is 
obtained  which  presents  a  natural  longitudinal  surface  and  two  artificial 
transverse  surfaces.  A  line  drawn  around  the  surface  of  such  a  muscle 
prism  at  a  point  midway  between  the  two  transverse  sections  constitutes  the 
equator. 

When  the  natural  longitudinal  and  artificial  transverse  surfaces  are 
connected  with  the  wires  of  a  galvanometer  the  terminals  of  which  are  pro- 
vided with  non-polarizable  electrodes,  an  electric  current  is  at  once  de- 
veloped. In  all  instances  the  current,  as  shown  by  the  deflection  of  the 
needle,  originates  at  the  transverse  surface,  passes  through  the  muscle  to 


8o 


TEXT-BOOK  OF  PHYSIOLOGY 


the  longitudinal  surface,  thence  through  the  galvanometer  to  the  transverse 
surface.  The  longitudinal  surface  is,  therefore,  electropositive,  the  trans- 
verse surface  electronegative.  The  two  points  exhibiting  the  greatest 
difference  of  potential,  and  hence  the  most  powerful  current,  lie  in  the 
equator  and  in  the  center  of  the  transverse  surface.  Currents  of  gradually 
diminishing  intensity  are  obtained  when  the  electrode  placed  on  the  longi- 
tudinal surface  is  removed  toward  either  end.  Feeble  currents  are  developed 
when  two  points  situated  at  unequal  distances,  either  on  corresponding  or 
opposite  sides  of  the  equator,  are  connected;  in  either  case  the  current 
flows  from  the  point  lying  nearest  the  equator  to  the  point  farthest  from  it. 

Similar  currents  are  obtained  when  two  points 
on  the  cross-section  situated  at  unequal  dis- 
tances from  the  central  axis  are  connected, 
in  which  case  the  direction  of  the  currents  will 
be  from  the  point  lying  nearest  the  periphery 
toward  the  center.  On  the  contrary,  no  cur- 
rent is  developed  when  two  points  on  the  longi- 
tudinal surface  equally  distant  from  the  equa- 
tor, or  two  points  on  the  transverse  surface 
equally  distant  from  the  central  axis,  are  con- 
nected. Such  points  are  said  to  be  isoelectric. 
These  facts  are  shown  in  Fig.  35.  The  natural 
ends  of  the  muscle,  enclosed  by  sarcolemma 
and  tendon,  do  not  exhibit,  if  carefully  pre- 
served from  injury,  the  negativity  characteris- 
tic of  the  artificial  transverse  ends. 

Similar  electric  conditions  are  exhibited  by 
the  muscles  of  man  and  other  mammals,  by 
the  muscles  of  birds,  reptiles,  amphibia,  etc. 
The  currents  developed  by  connecting  the 
equator  on  the  longitudinal  surface  with  the 
axis  of  the  transverse  surface  have  an  electro- 
motive force  in  the  frog  muscle  of  from  0.037 
to  0.075  of  a  Daniell  cell. 

The  electric  currents  in  the  muscle  are 
intimately  associated  with  the  chemic  changes 
underlying  its  nutrition,  and  hence  their  in- 
tensity rises  and  falls  with  all  the  conditions  which  maintain  or  impair 
muscle  nutrition  and  irritability.  The  currents  observed  in  the  injured 
muscle  during  the  inactive  state  have  been  termed  currents  of  rest.  Du 
Bois-Reymond  regarded  them  as  pre-existent,  intimately  connected  with 
the  living  condition  of  the  muscle,  and  essential  to  the  performance  of  its 
functions,  and  to  be  explained  by  the  view  that  the  entire  muscle  is  com- 
posed of  molecules  each  of  which  exhibits  the  same  difference  of  potential 
on  its  longitudinal  and  transverse  surfaces  as  the  muscle  prism  itself. 
Hermann  denies  the  existence  of  currents  in  normal  resting  muscle  and 
attributes  them  to  injuries  of  the  surface,  due  to  methods  of  preparation, 
in  consequence  of  which  the  tissue  dies  and  becomes  electronegative  to  the 
uninjured  area,  which  remains  electropositive.  These  currents  Hermann 
terms  "demarcation  currents." 


Fig.  35. — Diagram  to  Illus- 
trate THE  Current  in  Muscle. 
The  arrowheads  indicate  the  direc- 
tion; the  thickness  of  the  lines  in- 
dicates the  strength  of  the  currents. 
— (Landois  and  Stirling.) 


tEneral  physiology  of  muscle-tissue 


Fig.  36. — The  Negative  Variation  of  the 
Demarcation  Current,  A.  The  contraction 
wave,  which  as  it  passes  beneath  the  electrode  at  B 
causes  a  diminution  of  potential. 


Negative  Variation  of  the  Muscle  Current. — If  a  muscle  exhibiting  a 
current  of  injury  be  excited  to  activity  by  tetanizing  induced  currents 
applied  to  the  opposite  end  of  the  muscle,  it  will  be  observ^ed  that  as  the 
contraction  wave  passes  over  the  muscle  there  is  a  movement  of  the  gal- 
vanometer needle  toward  the  zero  point,  indicating  a  diminution  of  the 
potential  on  the  longitudinal  surface.  To  this  diminution  in  the  strength 
of  the  current  the  term  negative  variation  was  given.  On  the  withdrawal 
of  the  stimulus  the  needle  again  returns  in  a  short  time  to  its  former  posi- 
tion. The  diminution  of  potential  on  the  longitudinal  surface  of  the  muscle 
is  now  attributed  to  the  passage  of 
the  excitation  and  contraction 
processes,  to  a  temporary  disinte- 
gration of  the  muscle  substance 
(Fig.  36).  With  their  disappear- 
ance and  the  subsequent  restora- 
tion of  the  nutrition  of  the  mus- 
cle, the  former  electric  condition 
returns. 

The  primary  deflection  of  the 
galvanometer  needle  is  due  to  the 
demarcation  current  which  arises 
as  a  result  of  the  difference .  in 
electric  potential  produced  by  the 
destructive  chemic  changes  taking 

place  at  the  cut  end  of  the  muscle.  The  negative  variation  is  caused  by  the 
fact  that  the  activity  of  the  muscle,  with  its  attendant  chemic  changes,  will 
always  be  greater  in  the  uninjured  equatorial  region,  and  hence  will  always 
tend  to  counterbalance  the  original  source  of  difference  in  electric  potential. 

Electric  Currents  from  Non-injured  Muscles. — Though  perfectly 
normal  resting  muscle,  according  to  Hermann,  is  isoelectric,  nevertheless 
electric  currents  are  developed  during  activity  to  which  he  has  given  the  term 
action  currents,  and  which  are  attributed  to  the  propagation  of  the  contrac- 
tion wave. 

Action  Currents. — When  two  isoelectric  points  on  the  longitudinal 
surface  of  a  muscle  are  connected  with  a  galvanometer  and  a  single  stimulus 
applied  directly  to  one  extremity,  it  can  be  shown  that  as  the  contraction 
wave  passes  beneath  A,  Fig.  37,  the  muscle-tissue  at  that  point  becomes 
electronegative  toward  B  and  a  current  at  once  passes  through  the  galvan- 
ometer from  B  to  A,  as  shov/n  by  the  deflection  of  the  needle  toward  A.  xAs 
the  contraction  wave  passes  beneath  B  it  in  turn  becomes  electronegative, 
and  a  temporary  condition  of  equal  potential  is  estabhshed  when  the  needle 
returns  to  the  zero  point.  In  a  very  short  time  the  nutrition  of  A  is  restored 
and  becomes  electropositive  toward  B,  w^hen  a  current  will  pass  through  the 
galvanometer  in  the  opposite  direction  from  A  to  B,  as  shown  by  the  move- 
ment of  the  needle  toward  B,  Fig.  38.  As  the  contraction  wave  passes 
beyond  B  its  nutrition  is  restored  and  becomes  of  equal  potential  with  A. 
The  term  phasic  is  applied  to  these  currents.  The  first  current  flows  in  the 
muscle  in  the  direction  of  progress  of  the  contraction  wave — first  phase;  the 
second  current  flows  in  the  reverse  direction — second  phase;  the  current  is 
therefore  diphasic.  When  a  muscle  is  tetanized,  there  is  but  a  single  current 
6 


82  TEXT-BOOK  OF  PHYSIOLOGY 

observed,  which,  however,  endures  so  long  as  the  tetanic  contraction  is 
maintained.  To  this  current  the  term  decremential  is  given.  When  a 
muscle  is  excited  to  action  by  the  nerve  impulse  which  enters  at  its  center, 
two  contraction  waves  are  developed,  one  in  each  half  of  the  muscle,  and 
hence  there  are  two  sets  of  diphasic  action  currents. 


Fig.   37. — The  Condition  Leading  to  the  Development  of  the  First  Action 

Current. 

The  presence  of  action  currents  in  the  muscle  of  the  living  body  during  a 
single  contraction  was  demonstrated  by  Hermann  in  the  muscles  of  the 
forearm.  The  arrangement  of  the  experiment  was,  briefly,  as  follows: 
The  forearm  was  surrounded  by  two  twine  electrodes  saturated  with  zinc 


Fig.  38. — The  Condition  Leading  to  the  Development  of  the  Second  Action 

Current. 


solution,  one  being  placed  at  the  physiologic  middle — the  nervous  equator — 
the  other  at  the  wrist.  Both  electrodes  were  then  connected  with  the  galvan- 
ometer. When  the  brachial  plexus  was  stimulated  in  the  axillary  space,  the 
deflections  of  the  galvanometer  needle,  when  analyzed  with  the  repeating 
rheotome,  indicated  phasic  currents  with  a  single  contraction.     In  the  first 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  83 

phase — atterminal — the  wrist  became  positive  and  the  current  passed  in  the 
muscle  toward  its  termination;  and  in  the  second— abterminal — it  became 
negative  and  the  current  now  passed  in  the  reverse  direction.  The  action 
currents  which  are  observed  in  the  frog's  muscle  were  thus  shown  to  be 
present  in  the  living  human  muscle,  with  this  difference,  however:  that  the 
second  phase — abterminal — instead  of  being  weaker  in  man,  is  equally 
strong  with  the  atterminal.  This  experiment  also  revealed  the  fact  that  the 
rapidity  of  propagation  of  the  excitation  wave  was  much  greater  in  man, 
amounting  to  about  twelve  meters  per  second.  Hermann  therefore  denies 
the  pre-existence  of  electric  currents  and  regards  them  as  due  to  localized 
temporary  disintegration  of  the  muscle  in  consequence  of  activity,  as  they 
disappear  on  the  restoration  of  the  muscle  to  its  normal  condition. 

SPECIAL  ACTION  OF  MUSCLE  GROUPS 

The  individual  muscles  of  the  axial  and  appendicular  portions  of  the 
body  are  named  with  reference  to  their  shape,  action,  structure,  etc.;  e.g., 
deltoid,  flexor,  penniform,  etc.  In  different  localities  a  group  of  muscles 
having  a  common  function  is  named  in  accordance  with  the  kind  of  motion 
it  produces  or  to  which  it  gives  rise:  e.g.,  groups  of  muscles  which  alternately 
diminish  or  increase  the  angular  distance  between  two  bones  are  known 
respectively  as  flexors  and  extensors;  such  muscle  groups  are  usually  found 
in  association  with  ginglymus  joints.  Muscles  which  rotate  the  bone  to 
which  they  are  attached  around  its  own  axis  without  producing  any  great 
change  of  position  are  known  as  rotators,  and  are  found  in  association  with 
enarthrodial  or  ball-and-socket  joints.  Muscles  which  impart  an  angular 
movement  to  the  extremities  to  and  from  the  median  line  of  the  body  are 
termed  adductors  and  abductors  respectively. 

In  addition  to  the  actions  of  individual  groups  of  muscles  in  producing 
special  movements,  in  some  regions  of  the  body,  several  groups  of  muscles 
are  coordinated  for  the  accomplishment  of  certain 
definite  functions;  e.g.,  the  functions  of  respiration,  ^ 

mastication,   etc.      The  coordination  of   axial  and    — 5^ '^ 

appendicular  muscles  enables  the  individual  to  as- 
sume certain  postures,  such  as  standing,  sitting,  and 
lying;  to  engage  in  various  acts  of  locomotion,  as 
walking,  running,  dancing,  swimming. 

Levers. — The  function  or  special  mode  of  action 
of  individual  muscles  can  be  understood  only  when 
the  bones  with  which  they  ace  connected  are  regarded 
as  levers  whose  fulcra  or  fixed  points  lie  in  the  joints 

where  the  movement  takes  place,  and  the  muscles  as  sources  of  power  for 
imparting  movement  to  the  levers  with  the  object  of  overcoming  resistance. 

In  mechanics  levers  of  three  kinds  or  orders  are  recognized  according 
to  the  relative  positions  of  the  fulcrum  or  axis  of  motion,  the  applied 
power,  and  the  weight  to  be  moved.     (See  Fig.  39.) 

In  levers  of  the  first  order  the  fulcrum,  F,  lies  between  the  weight  or 
resistance,  W,  and  the  power  or  moving  force,  P.  The  distance  P  F  is 
known  as  the  power  arm  and  the  distance  W  F  as  the  weight  arm.  As 
examples  of  this  form  of  lever  found  in  the  human  body  may  be  mentioned: 


W          A 

-pl'^ 

F          • 

«  /n\ 

A            W 

1 

pl^/ 

• 

^f-5) 

W 

p 

A^^^ 

Fig.  39.— The  Three  Or- 

ders OF 

Levers. 

84  TEXT-BOOK  OF  PHYSIOLOGY 

1.  The  elevation  of  the  trunk  from  the  flexed  position.     The  axis  of  move- 

ment, the  fulcrum,  lies  in  the  hip-joint;  the  weight,  that  of  the  trunk, 
acting  as  if  concentrated  at  the  center  of  gravity,  which  lies  close  to  the 
tenth  dorsal  vertebra;  the  power,  the  muscles  attached  to  the  tuberosity 
of  the  ischium.  The  opposite  movement  is  equally  one  of  the  first 
order,  but  the  relative  positions  of  P  and  W  are  reversed. 

2.  The  head  in  its  movement  backward  and  forward  on  the  atlas. 

In  levers  of  the  second  order  the  weight  lies  between  the  power  and  the 
fulcrum.     As  illustration  of  this  form  of  lever  may  be  mentioned: 

1.  The  depression  of  the  lower  jaw,  in  which  movement  the  fulcrum  is  the 

temporomaxillary  articulation;  the  resistance,  the  tension  of  the  elevator 
muscles;  the  power,  the  contraction  of  the  depressor  muscles. 

2.  The  raising  of  the  body  on  the  toes,  in  which  movement  the  fulcrum  is 

the  toes,  the  weight  that  of  the  body  acting  through  the  ankle,  the 
power  the  gastrocnemius  muscle  applied  to  the  heel  bone. 
In  levers  of  the  third  order  the  power  is  applied  at  a  point  lying  between 

the  fulcrum  and  the  weight.     As  example  of  this  form  of  lever  may  be 

mentioned: 

1.  The  flexion  of  the  forearm,  in  which  the  fulcrum  is  the  elbow-joint,  the 

power  the  biceps  and  brachialis  anticus  muscles  applied  at  their  points 
of  insertion,  the  weight  that  of  the  forearm  and  hand. 

2.  The  extension  of  the  leg  on  the  thigh. 

When  levers  are  employed  in  mechanic  operations,  the  object  aimed  at 
is  the  overcoming  of  a  great  resistance  by  the  application  of  a  small  force 
acting  through  a  great  distance,  so  as  to  obtain  mechanic  advantage.  In 
the  mechanism  of  the  human  body  the  reverse  generally  obtains,  viz.,  the 
overcoming  of  a  small  resistance  by  the  application  of  a  large  force  acting 
through  a  short  distance.  As  a  result  there  is  a  gain  in  the  extent  and  rapid- 
ity of  the  movement  of  the  lever.  The  power,  however,  owing  to  its  point 
of  application,  acts  at  a  great  mechanic  disadvantage  in  many  instances, 
especially  in  levers  of  the  third  order. 

Postures. — Owing  to  its  system  of  joints,  levers,  and  muscles  the  human 
body  can  assume  a  series  of  positions  of  equilibrium,  such  as  standing  and 
sitting,  to  which  the  term  posture  has  been  given.  In  order  that  the  body 
may  remain  in  a  state  of  stable  equilibrium  in  any  posture,  it  is  essential 
that  the  vertical  line  passing  through  its  center  of  gravity  shall  fall  within 
the  base  of  support. 

Standing  is  that  position  of  equilibrium  in  which  a  line  drawn  through 
the  center  of  gravity  of  the  entire  body  falls  within  the  base  of  support. 
This  position  is  maintained  largely  by  the  mechanical  conditions  of  the 
joints,  apparently  for  the  purpose  of  reducing  to  a  minimum  muscular 
action,  so  that  it  can  be  prolonged  for  some  time  without  giving  rise  to 
fatigue.     In  the  military  position,  which  may  be  assumed  as  the  normal 
position,  all  the  joints  must  be  in  such  a  condition  of  extension  and  fixation 
that  the  body  will  represent  a  rigid  column  resting  on  the  astragalus  and 
supported  by  the  arch  of  the  foot.     This  is  accomplished : 
I.  By  balancing  the  head  on  the  apex  of  the  vertebral  column.     This  is 
done  by  the  action  of  the  muscles  on  the  back  of  the  neck.     The  mus- 
cular effort  is,  however,  very  slight,  as  the  center  of  gravity  of  the  head 
lies  but  a  short  distance  in  front  of  the  articulation. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  85 

2.  By  making  the  vertebral  column  erect  and  rigid.     This  is  brought  about 
by  the  action  of  the  common  extensor  muscles  of  the  trunk.     In  this 
condition  the  center  of  gravity  lies  just  in  front  of  the  tenth  dorsal 
vertebra.     The  head,  trunk,  and  upper  extremities  are  now  supported 
by  the  hip-joints;  and  in  order  that  this  support  may  give  to  the  body 
a  certain  degree  of  stable  equilibrium,  independent  of  muscular  action, 
the  line  of  gravity  falls  behind  the  line  uniting  the  center  of  rotation  of 
the  two  joints.     In  consequence  the  body  would  fall  backward  were  it 
not  prevented  by  the  tension  of  the  iliofemoral  ligament  and  the  fascia 
lata. 
The  line  of  gravity,  continued  downward,  passes  through  the  knee-joint 
posterior  to  the  axis  of  rotation,  and  hence  the  body  would  now  fall  back- 
ward were  it  not  prevented  by  the  tension  of  the  lateral  ligaments  and  the 
contraction  of  the  quadriceps  femoris  muscle.     Though  the  body  is  supported 
by  the  astragalus,  the  line  of  gravity  does  not  pass  through  the  line  uniting 
the  two  joints,  for  in  so  doing  constant  muscular  effort  would  be  required  to 
maintain  stable  equilibrium;  passing  a  short  distance  in  advance  of  this  line, 
there  would  be  a  tendency  of  the  body  to  fall  forward,  which  is  prevented  by 
the  extensor  muscles  of  the  foot.     When  the  body  is  in  the  erect  or  military 
position,  the  center  of  gravity  lies  between  the  sacrum  and  last  lumbar 
vertebra.     Standing  is  thus  an  act  of  balancing,  and  requires  not  only  the 
static  conditions  of  joints,  but  the  dynamic  conditions  of  various  groups  of 
muscles,  and  hence  is  not  a  position  of  absolute  ease  and  cannot  be  main- 
tained for  any  length  of  time  without  experiencing  discomfort  and  fatigue. 
Sitting  erect  is  an  attitude  of  equilibrium  in  which  the  body  is  balanced 
on  the   tubera   ischii,  when   the   head  and  trunk  together  form  a  rigid 
column. 

Locomotion  is  the  act  of  transferring  the  body  as  a  whole  through  space, 
and  is  accomplished  by  the  combined  action  of  its  own  muscles.  The  acts 
involved  consist  of  walking,  running,  jumping,  etc. 

Walking  is  a  complicated  act  involving  almost  all  the  voluntary  muscles 
of  the  body  either  for  purposes  of  progression  or  for  balancing  the  head  and 
trunk,  and  may  be  defined  as  a  progression  in  a  forward  horizontal  direction 
due  to  the  alternate  action  of  both  legs.  In  walking  one  leg  becomes  for  the 
time  being  the  active  or  supporting  leg,  carrying  the  trunk  and  head;  the 
other  the  passive  but  progressing  leg,  to  become  in  turn  the  active  leg  when 
the  foot  touches  the  ground.  Each  leg  is  therefore  alternately  in  an  active 
and  in  a  passive  state. 

Running  is  distinguished  from  walking  by  the  fact  that  at  a  given  moment 
both  feet  are  off  the  ground  and  the  body  is  raised  in  the  air. 

THE  VISCERAL  OR  INVOLUNTARY  MUSCLE 

The  visceral  muscle,  as  the  name  implies,  is  found  in  the  walls  of  hollow 
viscera,  where  it  is  arranged  in  the  form  of  a  membrane  or  sheet.  It  is 
present  in  the  walls  of  the  alimentary  canal,  blood-vessels,  respiratory  tract, 
ureter,  bladder,  vas  deferens,  uterus,  fallopian  tubes,  iris,  etc.  In  some 
situations  it  is  especially  thick  and  well  developed — e.g.,  uterus  and  pyloric 
end  of  the  stomach;  in  others  it  is  thin  and  slightly  developed. 

The  Histology  of  the  Visceral  Muscle-fiber. — When  examined  with 
the  microscope,  the  muscle  sheet  is  seen  to  be  composed  of  fibers,  narrow. 


86 


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elongated,  and  fusiform  in  shape.  As  a  rule,  they  are  extremely  small, 
measuring  only  from  40  to  250  micromillimeters  in  length  and  from  4  to  8 
micromillimeters  in  breadth.  The  center  of  each  fiber  presents  a  narrow, 
elongated  nucleus.  The  muscle-protoplasm  which  makes  up  the  body  of 
the  fiber  appears  to  be  enclosed  by  a  delicate  elastic  membrane  resembling 
in  some  respects  the  sarcolemma  of  the  skeletal  muscle.  In  some  animals 
the  visceral  fiber  presents  a  longitudinal  striation  suggesting  the  existence 
of  fibrillae  surrounded  by  sarcoplasm  (Fig.  40).  The  fibers  are  united 
longitudinally  and  transversely  by  a  cement  material.  The  muscle  is 
increased  in   thickness  by  the  superposition  of  successive  layers.     At  vary- 


FiG.  40. — Two  Smooth  Muscle-fibers  FROM  Small  Intestine  OF  Frog.  X  240.  Isolated 
with  35  per  cent,  potash-lye.  The  nuclei  have  lost  their  characteristic  form  through  the  action 
of  the  lye. — (Stohr.) 

ing  intervals  the  fibers  are  grouped  into  bundles  or  fasciculi  by  septa  of 
connective  tissue  (Fig.  41).  Blood-vessels  ramify  in  the  connective  tissue 
and  furnish  the  necessary  nutritive  material. 

The  visceral  muscle  receives  stimuli  from  the  spinal  cord,  not  directly, 
however,  as  in  the  case  of  the  skeletal  muscle,  but  indirectly  through  the 
intermediation  of  ganglion  cells,  which  may  be  located  at  some  distance 
from  the  muscle  or  near  the  walls  of  the  viscera.  Non-medullated  fibers 
from  the  ganglion  pass  directly  into  the  muscle,  where  they  frequently  unite 
to  form  a  general  plexus.  From  this  plexus  fine  branches  take  their  origin 
and  ultimately  become  physiologically  associated  with  the  muscle-fiber. 

Physiologic  Properties. — The  visceral  muscles  which  have  been  sub- 
jected to  experiment  are  mainly  those  of  the  stomach,  intestine,  bladder, 

ureter,  and  iris.  From  the  results  of 
the  experiments  which  have  been  pub- 
lished, it  is  evident  that  all  visceral 
muscles  possess  elasticity,  tonicity,  irri- 
tability, and  conductivity. 

The  elasticity  of  the  bladder  mus- 
cle of  the  cat  was  strikingly  shown  in 
the  experiments  published  by  Dr.  Colin 
C.  Stewart.  When  this  muscle  was 
weighted  with  weights  differing  by  a 
common  increment,  it  was  extended  on 
the  addition  of  each  weight,  though  to  a  progressively  less  extent.  On  the 
removal  of  the  weights  the  muscle  eventually  returned  to  its  former  length. 
The  records  of  the  extension  were  similar  to,  if  not  identical  with,  those  of 
the  skeletal  muscle. 

The  tonicity  of  visceral  muscles  is  as  pronounced  in  many  situations  as 
is  the  tonicity  of  skeletal  muscles.  Each  muscle  is  continuously  in  a  state 
of  contraction  intermediate  between  that  of  complete  contraction  and  that 
of  relaxation.  In  how  far  this  is  due  to  local  and  inherent  causes  or  to 
stimuli  reflected  from  the  nervous  system  as  a  result  of  peripherally  acting 
causes  is  not  in  individual  instances  readily  determinable.     From  time  to 


Connective-tissue, 
septum. 


Nucleus, 

Smooth   muscle-fiber 
in  transverse  section. 


Fig.  41. — Section  of  the  Circular 
Layer  of  the  Muscular  Coat  of  the 
Human  Intestine. — {Stohr.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  87 

time  the  tonicity  varies,  increasing  and  decreasing  in  response  to  these  various 
stimuli  and  in  accordance  with  the  functional  activities  of  the  organs  in 
which  the  muscle  is  found. 

The  irritability  manifests  itself  by  a  change  of  form,  and  doubtless  by 
the  liberation  of  heat  on  the  application  of  any  form  of  stimulus — mechanic, 
chemic,  thermic,  electric. 

The  conductivity  is  less  marked  in  the  visceral  than  in  the  skeletal 
muscle,  and,  contrary  to  what  is  observed  in  the  latter,  the  conduction  extends 
laterally  as  well  as  longitudinally  from  fiber  to  fiber.  This  is  shown  by 
stimulation  of  the  exposed  intestine.  Shortly  after  the  stimulus  is  appHed 
the  muscle  contracts  longitudinally — i.e.,  in  a  direction  at  right  angles  to  the 
long  axis  of  the  intestine,  partially  obliterating  its  lumen.  From  this  point 
the  conduction  process  indicated  by  the  contraction  wave  passes  in  opposite 
directions  for  some  distance  along  the  canal.  As  to  whether  this  is  accom- 
plished by  protoplasmic  processes  extending  from  fiber  to  fiber,  or  whether 
the  uniting  membrane  differs  in  conducting  power  from  the  sarcolemma,  is 
as  yet  a  matter  of  doubt.  From  the  fact  that  the  upper  two-thirds  of  the 
ureter,  though  free  of  nerve-cells,  exhibits  lateral  conduction,  it  is  evident 
that  it  may  take  place  independent  of  the  nervous  system. 

The  Contraction  of  the  Visceral  Muscle. — The  general  character  of 
the  contraction  may  be  witnessed  on  opening  the  abdomen  of  a  recently 
killed  animal,  especially  the  rabbit.  Shortly  after  exposure  to  the  air  the 
walls  of  the  intestine  begin  to  contract  in  a  most  vigorous  manner.  The 
contraction  wave  beginning  at  various  points  is  propagated  in  both  direc- 
tions, running  along  the  intestinal  wall  for  a  variable  distance.  A  succession 
of  similar  waves  may  be  observed  for  some  minutes.  To  the  alternate 
contraction  and  relaxation  of  the  muscle-fibers,  which  are  circularly  ar- 
ranged, the  term  peristalsis  is  usually  given.  The  excised  stomach  of  a  dog 
kept  under  suitable  conditions  will  exhibit  similar  movements.  The  same 
holds  true  of  the  bladder  muscle  of  the  cat,  the  muscle  of  the  ureter,  etc. 
Careful  observation  shows  a  certain  periodicity  in  the  movements.  Inas- 
much as  the  cause  is  not  apparent,  these  contractions  are  termed  spontane- 
ous or  automatic. 

Graphic  Record  of  the  Contraction. — For  experimental  purposes 
narrow  transverse  sections  of  the  stomach  of  the  frog  or  the  entire  bladder 
muscle  of  the  cat,  excised  or  in  situ,  according  to  the  method  of  Prof.  Colin 
C.  Stewart,  may  be  employed.  If  kept  moist,  they  will  retain  their  irritability 
for  some  hours.  The  changes  of  form  may  be  recorded  with  the  usual 
muscle  lever.  When  thus  prepared,  the  muscle  may  exhibit  for  several 
hours  a  series  of  pulsations,  rhythmic  in  character.  With  spontaneously 
acting  mammalian  muscle  the  contraction  and  relaxation  periods  are  of 
equal  duration.  With  the  amphibian  muscle  they  are  of  unequal  duration, 
as  a  rule.  In  both  classes  of  animals  the  character  of  the  record,  a  suc- 
cession of  large  and  small  contractions,  would  indicate  that  the  general 
rhythmic  movement  is  compounded  of  two  or  three  secondary  rhythms 
which  differ  in  rate  and  character.  A  single  pulsation  may  be  recorded 
by  stimulating  the  bladder  muscle  with  the  induced  or  the  make  and  break 
of  the  constant  current.  A  cun^e  of  such  a  contraction  is  shown  in  Fig,  42, 
The  contraction  takes  place  more  rapidly  than  the  relaxation;  the  two 
phases   occupying  five  and  thirty-five  seconds  respectively.      The  latent 


TEXT-BOOK  OF  PHYSIOLOGY 


iiiirfiiiiiiiiMim.ii/riiiiii/rlinil/lll 

Fig.  42. — The  Curve 
OF  Contraction  of  the 
Bladder  Muscle  at 
Body-temperature  in 
Response  to  a  Single 
Induction  Current. 
The  time  -is  indicated 
in  seconds. — {Stewart.) 


period  covered  0.25  second.  With  other  muscles  the  time  relations  are 
shghtly  different.  Tetanization  of  the  bladder  muscle  of  the  cat  occurred 
when  the  stimuli  succeeded  each  other  with  a  certain  rapidity;  the  interval 
between  stimuli  approximating  a  period  somewhat  less  than  two  seconds. 
This  muscle  responds  to  variations  in  'temperature,  to  strength  of  stim- 
ulus, to  the  load,  in  a  manner  similar  to,  if  not  iden- 
tical with,  the  skeletal  muscle. 

The  Function  of  the  Visceral  Muscle. — In  a 
general  way  it  may  be  said  that  the  visceral  muscle 
determines  and  regulates  the  passage  through  the 
viscus  or  organ  of  the  material  contained  within  it. 
The  food  in  the  stomach  and  intestines  is  subjected 
to  a  churning  process  by  the  muscles,  in  consequence 
of  which  the  digestive  fluids  are  more  thoroughly  in- 
corporated and  their  characteristic  action  increased. 
At  the  same  time  the  food  is  carried  through  the 
canal,  the  absorption  of  the  nutritive  material  pro- 
moted, and  the  indigestible  residue  removed  from  the 
body.  The  blood  is  delivered  in  larger  or  smaller 
volumes  according  to  the  needs  of  the  tissues  through 
a  relaxation  or  contraction  of  the  muscle-fibers  of  the 
blood-vessels.  The  urine  is  forced  through  the  ureter 
and  from  the  bladder  by  the  contraction  of  their  re- 
spective muscles.  The  mode  of  action  of  the  individ- 
ual muscles  will  be  described  in  successive  chapters. 

Ciliary  Movement. — The  free  surface  of  the  epithelium  covering  the 
mucous  membrane  in  certain  regions  of  the  body  is  characterized  by  the 
presence  of  delicate  filamentous  processes  termed  cilia.  (See  Fig.  43.) 
Ciliated  epithelium  is  found  in  man  and  mammals  generally,  in  the  nose. 
Eustachian  tube,  larynx,  with  the  exception  of  the  vocal  membranes,  trachea 
and  bronchial  tubes  as  far  as  the  pulmonary  lobules,  Fallopian  tubes,  uterus, 
and  epididymis.  The  lumen  of  the  central  canal  of  the  spinal  cord  and  the 
cavities  of  the  brain  are  lined,  especially  in  childhood, 
by  cells  provided  with  similar  cilia.  Ciliated  epithe- 
lium is  also  found  in  all  classes  of  animals,  and  espe- 
cially in  the  invertebrates. 

The  cilia  found  in  the  human  body  vary  in  length 
from  0.003  J^'^-  to  0.005  ^^-  They  are  apparently 
structureless  and  colorless,  and  appear  to  have  their 
origin  in  and  to  be  a  prolongation  of  a  transparent 
material  on  the  outer  surface  of  the  cell  material.  The 
number  of  cilia  present  on  the  surface  of  any  individual 
cell  varies  approximately  from  five  to  twenty-five. 
When  ciliated  epithelial  cells,  freshly  removed  from 
the  mucous  membrane  and  moistened  with  normal  saline,  are  examined 
with  the  microscope,  it  will  be  found  that  the  cilia  are  in  continuous  and 
rapid  vibratile  movement,  so  much  so  that  the  individual  cilium  cannot 
be  distinguished.  In  time,  however,  their  vitality  declines  and  the  rapid- 
ity of  movement  diminishes.  When  the  movement  of  the  individual 
cilium  falls  to  about  eight  or  ten  per  second,  its  character  can  be  readily 


Fig.  43. — Ciliated  Epi- 
thelium. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE  89 

determined.  It  will  then  be  seen  that  the  movement  is,  as  a  rule,  alter- 
nately a  backward  and  a  forward  one,  the  cilium  lowering  and  then  rais- 
ing itself,  the  latter  taking  place  more  quickly  and  energetically  than  the 
former.  As  the  cilium  raises  itself  it  becomes  somewhat  flexed  in  a  direc- 
tion corresponding  to  that  of  the  general  movement.  The  movement, 
however,  varies  in  character  in  different  situations  and  in  different  animals. 
The  cause  of  the  movements  and  the  mechanism  of  their  coordination  are 
unknown.  They  are,  as  far  as  known,  independent  of  the  nerve  system. 
The  force  of  ciliary  motion  is  very  great.  A  load  of  twenty  grams  can  be 
supported  and  carried  forward  by  the  cilia  on  the  mucous  membrane  of  the 
mouth  and  esophagus  of  the  frog.  The  activity  of  the  cilia  is  associated  with 
the  nutrition  of  the  cell  of  which  they  are  a  part  and  rises  and  falls  with  it. 
Experimentally  it  has  been  found  that  the  rate  and  energy  of  the  movement 
are  greatest  at  a  temperature  of  about  35°  to  4o°C.,  especially  if  they  are 
bathed  with  normal  saline,  rendered  slightly  alkaline.  Low  temperatures, 
acids,  alkalies,  carbon  dioxid,  etc.,  retard  the  movement. 

The  function  of  the  cilia,  though  not  always  apparent,  is  associated  with 
the  function  of  the  passages  in  which  they  are  found.  As  the  surfaces  of 
these  passages  are  swept  by  a  current  of  considerable  power,  it  is  probable 
that  they  assist  in  the  passage  of  the  materials  which  ordinarily  traverse 
them.  Mucus  and  particles  of  dust  are  carried  upward  through  the  air- 
passages;  the  ovarian  cell  is  carried  from  the  ovary  toward  the  uterus;  the 
spermatozoa,  as  well  as  the  fluid  in  which  they  are  contained,  are  carried 
forward  through  the  epididymis  ducts. 


CHAPTER  VIII 

THE  GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE 

The  Nerve-tissue. — The  nerve-tissue,  which  unites  and  coordinates 
the  various  organs  and  tissues  of  the  body  and  brings  the  individual  into 
relationship  with  the  external  world,  is  conventionally  arranged  in  two 
systems,  termed  the  encephalo spinal  or  cerebrospinal  and  the  sympathetic. 

The  encephalospinal  system  consists  of: 

1.  The  brain  and  spinal  cord,  contained  within  the  cavities  of  the  cranium 

and  the  spinal  column  respectively,  and 

2.  The  cranial  and  spinal  nerves. 

The  sympathetic  system  consists  of: 

1.  A  chain  of  ganglia  situated  on  each  side  of  the  spinal  column  and  extend- 

ing from  the  base  of  the  skull  to  the  tip  of  the  coccyx. 

2.  Various  collections  of  ganglia  situated  in  the  head,  face,  thorax,  abdomen, 

and  pelvis.  All  these  ganglia  are  united  by  an  elaborate  system  of 
intercommunicating  nerves,  many  of  which  are  connected  with  the 
cerebrospinal  system.      (See  Chapter  XXVI.) 

HISTOLOGY  OF  NERVE-TISSUE 

The  Neuron. — The  nerve-tissue  has  been  resolved  by  the  investigations 
of  modern  histologists  into  single  morphologic  units,  to  which  the  term 
neurons  has  been  applied.  The  entire  nerve  system  has  been  shown  to  be 
but  an  aggregate  of  an  infinite  number  of  neurons,  each  of  which  is  histologic- 
ally distinct  and  independent.  Though  having  a  common  origin,  as  shown 
by  embryologic  investigations,  they  have  acquired  a  variety  of  forms  in 
different  parts  of  the  nerve  system  in  the  course  of  development.  The  old 
conception  that  the  nerve  system  consisted  of  two  distinct  histologic  elements, 
nerve-cells  and  nerve-fibers,  which  differed  not  only  in  their  mode  of  origin, 
but  also  in  their  properties,  their  relation  to  each  other,  and  their  functions, 
has  been  entirely  disproved. 

The  neuron,  or  neurologic  unit,  is  histologically  a  nerve-cell,  the  surface 
of  which  presents  a  greater  or  less  number  of  processes  in  varying  degrees 
of  differentiation.  As  represented  in  Fig.  44,  A,  the  neuron  may  be  said  to 
consist  of:  (i)  The  nerve-cell,  neurocyte,  or  corpus;  (2)  the  axon,  or  nerve 
process;  (3)  the  end-tufts,  or  terminal  branches.  Though  these  three  main 
histologic  features  are  everywhere  recognizable,  they  exhibit  a  variety  of 
secondary  features  in  different  situations  in  accordance  with  peculiarities  of 
function. 

The  Nerve-cell. — The  nerve-cell,  or  body  of  the  neuron,  presents  a 
variety  of  shapes  and  sizes  in  different  portions  of  the  nerve  system. 
Originally  ovoid  in  shape,  it  has  acquired,  in  course  of  development,  pecul- 
iarities of  form  which  are  described  as  pyramidal,  stellate,  pear-shaped, 
spindle-shaped,  etc.     The  size  of  the  cell  varies  considerably,  the  smallest 

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GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE 


91 


having  a  diameter  of  not  more  than  10  to  12  micro-millimeters,  the  largest 
not  more  than  150  micro-millimeters.  Each  cell  consists  of  granular, 
striated  cytoplasm,  containing  a  distinct  vesicular  nucleus  and  a  well-de- 
fined nucleolus.  A  characteristic  feature  of  the  cytoplasm  is  the  presence 
of  granules  first  described  by  Nissl,  which  stain  deeply  with  methylene  blue 
and  other  dyes.  For  this  reason  these  granules  are  spoken  of  as  chromo- 
phile  granules.  The  remainder  of  the  cytoplasm  is  penetrated  in  various 
directions  with  nerve  fibrils  which  are  continuous  with  similar  fibrils  run- 
ning through  the  axonic  process  as  well  as  the  dendrites.  The  physio- 
logic significance  of  Nissl's  granules  is  unknown.  The  nerve  fibrils  are 
probably  connected  with  the  transmission  of  nerve  impulses.  A  cell  mem- 
brane has  not  been  observed.     From  the  surface  of  the  adult  cell  portions 


Fig.  44. — A.  Efferent  Neuron;  B,  Afferfnt  Neuron. 

Nerves. 


Found.  IN  Both  Spinal  and  Cranul 


of  the  cytoplasm  are  projected  in  various  directions,  which  portions, 
rapidly  dividing  and  subdividing,  form  a  series  of  branches,  termed  den- 
drites or  dendrons.  In  some  situations  the  ultimate  branches  of  the  den- 
drites present  short  oclateral  presses,  known  as  lateral  buds,  or  gemmules, 
which  impart  to  the  branches  a  feathery  appearance.  This  character- 
istic is  common  to  the  cells  of  the  cortex  of  the  cerebrum  and  of  the 
cerebellum.  The  ultimate  branches  of  the  dendrites,  though  forming  an 
intricate  feltwork,  never  anastomose  with  one  another  nor  unite  with 
dendrites  of  adjoining  cells.  According  to  the  number  of  axons,  nerve-cells 
are  classified  as  monaxonic,  diaxonic,  polyaxonic.  Most  of  the  cells  of 
the  nerve  system  of  the  higher  vertebrates  are  monaxonic.  In  the  ganglia 
of  the  posterior  or  dorsal  roots  of  the  spinal  and  cranial  nerves,  however, 
they  are  diaxonic.  In  this  situation  the  axons,  emerging  from  opposite 
poles  of  the  cell,  either  remain  separate  and  pursue  opposite  directions,  or 
unite  to  form  a  common  stem,  which  subsequently  divides  into  two  branches, 
which  then  pursue  opposite  directions.  (See  Fig.  44,  B.)  The  nerve-cell 
maintains  its  own  nutrition,  and  presides  over  that  of  the  dendrites  and  the 


92  TEXT-BOOK  OF  PHYSIOLOGY 

axon  as  well.  If  the  latter  be  separated  in  any  part  of  its  course  from  the 
cell,  it  speedily  degenerates  and  dies. 

The  Axon. — The  axon,  or  nerve  process,  arises  from  a  cone-shaped  "pro- 
jection from  the  surface  of  the  cell,  and  is  the  first  outgrowth  from  its  cyto- 
plasm. At  a  short  distance  from  its  origin  it  becomes  markedly  differentiated 
from  the  dendrites  which  subsequently  develop.  It  is  characterized  by  a  sharp, 
regular  outline,  a  uniform  diameter,  and  a  hyalin  appearance.  In  structure, 
the  axon  appears  to  consist  of  fine  fibrillae  embedded  in  a  clear,  semi-fluid 
material,  the  neuroplasm.  The  axon  varies  in  length  from  a  few  millimeters 
to  one  meter.  In  the  former  instance  the  axon,  at  a  short  distance  from 
its  origin,  divides  into  a  number  of  branches,  which  form  an  intricate  felt- 
work  in  the  neighborhood  of  the  cell.  In  the  latter  instance  the  axon 
continues  for  an  indefinite  distance  as  an  individual  structure.  In  its 
course,  however,  especially  in  the  brain  and  spinal  cord,  it  gives  off  a  number 
of  collateral  branches,  which  possess  all  its  histologic  features.  The  long 
axons  serve  to  bring  the  body  of  the  cell  into  direct  relation  with  peripheral 
organs,  or  with  more  or  less  remote  portions  of  the  nerve  system,  thus  con- 
stituting association  or  commissural  fibers.  Physiologic  investigations  have 
established  the  fact  that  the  axon  is  the  conducting  agent  of  the  nerve 
impulses. 

The  Myelin. — At  a  short  distance  from  the  cell  the  more  or  less  elon- 
gated axon  becomes  invested  with  nucleated  oblong  cells,  which  subse- 
quently become  modified  and  constitute  the  medullary  or  myelin  sheath. 
When  fresh  the  myelin  is  clear  and  semi-fluid;  when  treated  with  varicnis 
reagents  it  becomes  opaque  and  imparts  a  white  appearance  to  nerves. 
The  function  of  the  myelin  is  unknown.  All  axons  that  possess  a  myelin 
investment  are  known  as  myelinated  nerve-fibers. 

The  Neurilemma. — The  myelin  in  many  situations  is  enclosed  by  a 
thin  transparent  elastic  membrane  known  as  the  neurilemma.  In  the 
spinal  cord  and  brain,  the  nerve  fibers  are  for  the  most  part  wanting  in 
this  membrane. 

At  intervals  of  about  seventy-five  times  its  diameter,  the  medullated  nerve- 
fiber  undergoes  a  remarkable  diminution  in  size,  due  to  an  interruption  of 
the  medullary  substance,  so  that  the  neurilemma  lies  directly  on  the  axis- 
cylinder.  These  constrictions,  or  nodes  of  Ranvier,  taking  their  name  from 
their  discoverer,  occur  at  regular  intervals  along  the  course  of  the  nerve, 
separating  it  into  a  series  of  segments.  The  portion  between  the  nodes  is 
termed  the  internodal  segment.  It  has  been  suggested  that  in  consequence 
of  the  absence  of  the  myelin  at  these  nodes,  a  free  exchange  of  nutritive 
material  and  decomposition  products  can  take  place  between  the  axis- 
cylinder  and  the  surrounding  plasma.  Beneath  the  neurilemma  in  each 
internodal  segment  there  is  a  large  nucleus  surrounded  by  a  small  amount  of 
granular  protoplasm. 

The  End  Tufts. — The  end-tufts  or  terminal  organs  are  formed  by  the 
splitting  of  the  axon  into  a  number  of  filaments,  which  remain  independent 
of  one  another  and  are  free  from  the  myelin  investment.  The  histologic 
peculiarities  of  the  terminal  organs  vary  in  different  situations,  and  in  many 
instances  are  quite  complex  and  characteristic.  In  peripheral  organs,  as 
muscles,  glands,  blood-vessels,  skin,  mucous  membrane,  the  tufts  are  in 
direct  histologic  and  physiologic  connection  with  their  cellular  elements.     In 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE  93 

the  brain  and  spinal  cord  the  tufts  are  in  more  or  less  intimate  relation 
with  the  dendrites  of  adjacent  neurons. 

The  neurons  in  their  totality  constitute  the  neuron  or  nerve  tissue. 
From  the  fact  that  they  are  arranged  both  serially  and  collaterally  into  a 
regular  and  connected  whole,  they  collectively  constitute  a  system  known  as 
the  neuron  or  nerve  system. 

The  neurons  composing  the  spinal  and  cranial  nerves  are  represented  in 
Fig.  44,  which  are  connected  peripherally  by  their  terminal  branches  with 
muscles  on  the  one  hand  and  with  epithelium  of  skin,  mucous  membrane,  etc., 
on  the  other  hand.  In  the  spinal  cord  the  terminal  branches  of  the  afferent 
neuron  come  into  histologic  and  physiologic  relation  with  the  dendrites  of  a 
second  neuron,  the  axonic  process  of  which  in  many  instances  ascends  the 
cord  to  different  levels  or  even  as  far  as  the  brain,  where  its  terminal  branches 
come  into  relation  with  the  dendrites  of  still  another  neuron,  the  axonic 
process  of  which  is  in  turn  connected  with  neurons  in  the  cortex  of  either 
the  cerebrum  or  cerebellum.  The  surfaces  of  the  body  are  thus  brought  into 
relation  with  the  cerebral  and  cerebellar  neurons.  The  neurons  arranged 
in  this  serial  manner  constitute  the  afferent  side  of  the  nerve  system. 

In  a  similar  way  the  efferent  neurons  of  the  spinal  and  cranial  nerves  are 
brought  into  relation  with  the  cortex  of  the  cerebrum.  Large  pyramidal- 
shaped  neurocytes  situated  in  specialized  regions  of  the  cortex  of  the  cere- 
brum send  their  axonic  processes  down  through  the  brain  and  cord.  As  they 
approach  their  destination  the  terminal  branches  become  related  histo- 
logically and  physiologically  with  the  dendrites  of  the  neurons  composing 
the  cranial  and  spinal  nen^es.  The  cortex  of  the  cerebrum  is  thus  brought 
into  relation  with  the  general  musculature  of  the  body.  The  neurons 
arranged  in  this  serial  manner  constitute  the  efferent  side  of  the  nerve  system. 

Neurons,  moreover,  are  grouped  into  more  or  less  complexly  organized 
masses,  termed  organs,  which  in  accordance  with  their  locations  may  be 
divided  for  convenience  into  central  and  peripheral  organs. 

The  Central  Organs  of  the  Nerve  System. — The  central  organs  con- 
sist of  the  encephalon  and  spinal  cord,  contained  within  the  cavities  of  the 
head  and  spinal  column  respectively.  They  consist  of  neurons  arranged 
in  a  very  complex  manner.  In  a  subsequent  chapter  the  anatomic  arrange- 
ment of  their  constituent  parts  will  be  detailed. 

The  Peripheral  Organs  of  the  Nerve  System. — These  consist  of  the 
cranial  and  spinal  nerves  and  the  sympathetic  ganglia.  Each  nerv-e 
consists  of  a  variable  number  of  ner\-e-fibers  united  into  firm  bundles  by 
connective  tissue  which  supports  blood-vessels  and  lymphatics.  The  bundles 
are  technically  known  as  nerve-trunks  or  nerves. 

The  nerve-trunks  connect  the  brain  and  cord  with  all  the  remaining 
structures  of  the  body.  Each  ner\'e  is  invested  by  a  thick  layer  of  lamel- 
lated  connective  tissue,  known  as  the  epineurium.  A  transverse  section  of  a 
nerv^e  shows  (see  Fig.  45),  that  it  is  made  up  of  a  number  of  small  bundles  of 
fibers,  each  of  which  possesses  a  separate  investment  of  connective  tissue — 
the  perineurium.  Within  this  membrane  the  nerv^e-fibers  are  supported  by 
a  fine  stroma — the  endoneurium.  After  pursuing  a  longer  or  shorter  course, 
the  nerve-trunk  gives  off  branches,  which  interlace  very  freely  with  neigh- 
boring branches,  forming  plexuses,  the  fibers  of  which  are  distributed  to 
associated  organs  and  regions  of  the  body.     From  their  origin  to  their 


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TEXT-BOOK  OF  PHYSIOLOGY 


termination,  however,  nerve-fibers  retain  their  individuahty,  and  never  be- 
come blended  with  adjoining  fibers. 

As  nerves  pass  from  their  origin  to  their  peripheral  terminations,  they 
give  off  a  number  of  branches,  each  of  which  becomes  invested  with  a 
lamellated  sheath — an  offshoot  from  that  investing  the  parent  trunk.  This 
division  of  nerve-bundles  and  sheath  continues  throughout  all  the  branchings 
down  to  the  ultimate  nerve  fibers,  each  of  which  is  surrounded  by  a  sheath 
of  its  own,  consisting  of  a  single  layer  of  endothelial  cells.  This  delicate 
transparent  membrane,  the  sheath  of  Henle,  is  separated  from  the  nerve- 
fiber  by  a  considerable  space,  in  which  is  contained  lymph  destined  for  the 
nutrition  of  the  fiber.     Near  their  ultimate  terminations  the  nerve-fibers 


Fig.  45. — Transverse    Section    of  a   Nerve   (Median),     ep.   Epineurium. 
pe.   Perineurium,     ed.  'Endone\ii'\\xva.—{Landois  and  Stirling.) 

themselves  undergo  division,  so  that  a  single  fiber  may  give  origin  to  a  num- 
ber of  branches,  each  of  which  contains  a  portion  of  the  parent  axis-cylinder 
and  myelin. 

Sympathetic  Ganglia. — A  sympathetic  ganglion  consists  essentially  of 
a  connective-tissue  capsule  with  an  interior  framework.  The  meshes  of 
this  framework  contain  nerve-cells  possessing  dendrites  and  branching 
axons.  The  majority  of  the  axons  are  devoid  of  myelin  and  are  therefore 
known  as  non-myelinated  nerve-fibers.  Owing  to  the  absence  of  the  myelin 
they  present  a  rather  pale  or  grayish  appearance.  In  all  instances,  with 
the  exception  of  the  ganglion  cells  of  the  heart,  the  axons  are  distributed  to 
non-striated  muscle  tissue  and  to  the  epithelium  of  glands. 

The  nerve-cells  of  the  ganglia  are  also  in  histologic  connection  with  the 
terminal  branches  of  certain  fine  medullated  nerve-fibers  which  leave  the 
spinal  cord  by  way  of  the  ventral  roots  of  the  spinal  nerves.  These  nerve- 
fibers  are  designated  pre- ganglionic  fibers,  while  those  emerging  from  the  cells 
are  designated  post- ganglionic  fibers. 

Blood-supply. — Nerves  being  parts  of  living  cells  require  for  the  main- 
tenance of  their  nutrition  a  certain  amount  of  blood.  This  is  furnished  by 
the  blood-vessels  ramifying  in  and  supported  by  the  connective-tissue  frame- 
work. Here  as  elsewhere  there  is  a  constant  exchange,  through  the  capillary 
wall  and  the  neurilemma,  of  nutritive  material  to  the  nerv^e  proper  and  of 
waste  materials  to  the  blood. 

The  Chemic  Composition  and  Metabolism. — Chemic  analysis  of 
nerve-tissue  has  shown  the  presence  of  water,  proteins  (two  globulins,  a 
nucleo-protein  and  neurokeratin),  certain  lipoids,  e.g.,  (a)  cholesterin  (a 
monotomic  alcohol  free  from  both  nitrogen  and  phosphorus),  (b)  several 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE  95 

cerebrosides  or  galactosides  (nitrogen-holding  bodies,  free  from  phos- 
phorous, compounds  of  a  glucoside  character,  as  shown  by  their  yielding  on 
hydrolysis  the  reducing  carbohydrate  galactose),  (c)  phosphatids  (com- 
pounds containing  both  nitrogen  and  phosphorus,  e.g.,  lecithin,  kephalin, 
sphingo-myelin) ,  inorganic  salts,  and  a  series  of  nitrogen-holding  bodies 
such  as  creatin,  xanthin,  urea,  leucin,  etc.  As  to  the  metabolism  that  is 
taking  place  in  nerve-cells  and  fibers,  practically  nothing  definite  is  known. 
That  such  changes,  however,  are  taking  place  would  be  indicated  first  by  the 
blood-supply,  and  second  by  the  fact  that  withdrawal  of  the  blood-supply  is 
followed  by  a  loss  of  irritability.  The  metabolism  of  the  central  organs  of 
the  nerve  system  is  more  active  and  extensive.  In  this  situation  any  with- 
drawal of  blood  from  compression  or  occlusion  of  blood-vessels  is  followed 
by  impairment  of  nutrition  and  loss  of  function. 

THE  RELATION  OF  THE  PERIPHERAL  ORGANS  OF  THE  NERVE 
SYSTEM  TO  THE  CENTRAL  ORGANS 

Spinal  Nerves. — The  nerves  in  connection  with  the  spinal  cord  are 
thirty-one  in  number  on  each  side.  If  traced  toward  the  spinal  column,  it 
will  be  found  that  the  nerve-trunk  passes  through  an  intervertebral  foramen. 
Near  the  outer  limits  of  the  foramina  each  nerve-trunk  divides  into  two 
branches,  generally  termed  roots,  one  of  which,  curving  slightly  forward  and 
upward,  enters  the  spinal  cord  on  its  anterior  or  ventral  surface,  while  the 
other,  curving  backward  and  upward,  enters  the  spinal  cord  on  its  posterior 
or  dorsal  surface.  The  former  is  termed  the  anterior  or  ventral  root;  the 
latter,  the  posterior  or  dorsal  root.  Each  dorsal  root  presents  near  its  union 
with  the  ventral  root  a  small  ovoid  grayish  enlargement  known  as  a  ganglion. 
Both  roots  previous  to  entering  the  cord  subdivide  into  from  four  to  six 
fasciculi. 

A  microscopic  examination  of  a  cross-section  of  the  spinal  cord  shows 
that  the  fibers  of  the  ventral  roots  can  be  traced  directly  into  the  body  of  the 
nerve-cells  in  the  ventral  horns  of  the  gray  matter.  The  fibers  of  the  dorsal 
roots  are  not  so  easily  traced,  for  they  diverge  in  several  directions  shortly 
after  entering  the  cord.  In  their  course  they  give  off  collateral  branches 
which,  in  common  with  the  main  fiber,  end  in  tufts  which  become  associated 
with  nerve-cells  in  both  the  ventral  and  dorsal  horns  of  the  gray  matter. 

Cranial  Nerves. — The  nerves  in  connection  with  the  base  of  the  brain 
are  known  as  cranial  nerves;  some  of  these  nerves  present  a  similar  ganglionic 
enlargement,  and  therefore  may  be  regarded  as  dorsal  nerves,  while  others 
may  be  regarded  as  ventral  nerves.  Their  relations  within  the  medulla 
oblongata  are  similar  to  those  within  the  spinal  cord. 

Efferent  and  Afferent  Nerves. — Nerves  are  channels  of  communication 
between  the  brain  and  spinal  cord,  on  the  one  hand,  and  the  skeletal  muscles, 
glands,  blood-vessels,  visceral  muscles,  skin,  mucous  membrane,  etc.,  on 
the  other.  Some  of  the  nerve-fibers  serve  for  the  transmission  of  nerve 
energy  from  the  brain  and  spinal  cord  to  certain  peripheral  organs,  and  so 
accelerate  or  retard,  augment  or  inhibit  their  activities;  others  serve  for  the 
transmission  of  nerve  energy  from  certain  peripheral  organs  to  the  brain  and 
spinal  cord  which  gives  rise  to  sensation  or  other  modes  of  nerve  activity. 
The  former  are  termed  efferent  or  centrifugal,  the  latter  afferent  or  centripetal 
nerves.     Experimentally  it  has  been  determined  that  the  anterior  or  ventral 


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Fig.  46. — Motor  Nerve-exdings 
OF  Intercostal  JMuscle-fibers  of 
A  Rabbit.     X  150. — (Stohr.) 


roots  contain  all  the  efferent  fibers,  the  posterior  or  dorsal  roots  all  the  afferent 
fibers. 

The  Peripheral  Endings  of  Nerves. — The  efferent  nerves  as  they 
approach  their  ultimate  terminations  lose  both  the  neurilemma  and  myelin 
sheaths.  The  axon  or  axis-cylinder  then  divides  into  a  number  of  branches 
which  become  directly  and  intimately  associated  with  tissue-cells.     The 

particular  mode  of  termination  varies  in 
different  situations.  These  terminations 
are  generally  spoken  of  as  end-organs, 
terminal  organs,  or  end-tufts. 

In  the  skeletal  muscle  the  nerv^e-fiber 
loses  both  neurilemma  and  myelin  sheath 
at  the  point  where  it  comes  in  contact  with 
the  muscle-fiber.  After  penetrating  the 
sarcolemma,  the  axon  or  axis-cylinder 
divides  into  a  number  of  small  branches 
which  appear  to  be  embedded  in  a  rela- 
tively large  mass  of  sarcoplasm  and  nuclei, 
the  whole  forming  the  so-called  "motor 
plate."  Each  muscle-fiber  possesses  one 
such  plate  or  end-organ  in  mammalia, 
several  in  the  frog.  (Fig.  46.) 
In  the  visceral  muscle  the  terminal  nerve-fibers  derived  from  sympathetic 
or  peripheral  neurons  are  primarily  non-medullated.  The  axons  divide  and 
subdivide  and  form  plexuses  which  surround  the  muscle-cell  bundles.  Fine 
fibers  from  the  plexuses  are  given  off  which  ultimately  come  into  relation 
with  each  individual  cell,  on  the  surface  of  which  they  terminate  in  the  form 
of  one  or  more  granular  masses. 

In  the  glands ,  tdi'k.m.g  as  an  illustration  the  parotid  and  mammary  glands, 
the  nerve-fibers,  also  derived  from  sympathetic  or  peripheral  neurons,  pass 
into  the  body  of  the  gland  and  ultimately 
reach  the  acini,  on  the  outer  surface  of 
which  they  ramify  and  form  a  plexus. 
From  this  plexus  fine  fibers  penetrate 
the  acinus  wall  and  end  on  the  gland-cell. 
The  fibers  present  a  varicose  appearance 
(Fig.  47)- 

The  afferent  nerves  as  they  ap- 
proach their  ultimate  terminations  un- 
dergo similar  changes.  The  end-tufts 
become  associated,  in  some  situations, 
with  specialized  end-organs  which  are  extremely  complex. 

In  the  skin  and  mucous  membranes  the  mode  of  termination  varies  con- 
siderably.    The  following  are  some  of  the  principal  modes: 

1.  Free  endings  in  the  epithelium. 

2.  Tactile  cells  of  Merkel. 

3.  Tactile  corpuscles  in  the  papillae  of  the  true  skin. 

4.  Pacinian  corpuscles  found  attached  to  the  nerves  of  the  hands  and 

feet,  to  the  intercostal  nerves,  and  to  nerves  in  other  situations. 

5.  End-bulbs  of  Krause  in  the  conjunctiva,  clitoris,  penis,  etc. 


ta 


Fig.  47. — Terminations  of  Nerve- 
fibers  IN  the  Gland-cells.  A.  Cell 
of  the  parotid  gland  of  a  rabbit.  B. 
Cells  of  the  mammary  gland  of  a  cat  in 
gestation. — (Doyon  and  Moral.) 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE 


97 


Posterear 
Jioot 


Ga/ifflia» 


(A  consideration  of  these  end-organs  will  be  found  in  the  chapters  de- 
voted to  the  organs  of  which  they  form  a  part.) 

In  the  skeletal  muscles  afferent  fibers  become  associated  with  small 
spindle-shaped  structures  known  as  muscle-spindles  or  neuromuscle  end- 
organs.  These  spindles  vary  in  length  from  i  mm.  to  4  mm.  They  consist 
of  a  capsule  of  fibrous  tissue  containing  from  five  to  twenty  muscle-fibers. 
After  penetrating  the  several  layers  of  the  capsule,  the  nerve-fibers  lose  the 
neurilemma  and  myelin  sheaths.  The  axons  or  axis-cylinders  then  divide 
into  several  long  narrow  branches  which  wind  themselves  in  a  spiral  manner 
around  the  contained  muscle-fiber  and  terminate  in  small  oval-shaped  discs. 
Similar  endings  have  been  observed  in  the  tendons  of  muscles. 

Development  and  Nutrition  of  Nerves. — The  efferent  nerve-fibers, 
which  constitute  some  of  the  cranial  nerves  and  all  the  ventral  roots  of  the 
spinal  nerves,  have  their  origin  in  cells  located  in  the  gray  matter  beneath 
the  aqueduct  of  Sylvius,  beneath  the  floor  of  the  fourth  ventricle,  and  in  the 
ventral  horns  of  the  gray  matter  of  the  spinal  cord.  These  cells  are  the 
modified  descendants  of  independent,  oval,  pear-shaped  cells — the  neuro- 
blasts— which  migrate  from  the  medullary  tube.  As  they  approach  the 
surface  of  the  cord  their  axons  are  directed  toward  the  ventral  surface,  which 
eventually  they  pierce.  Emerging 
from  the  cord,  the  axons  continue  to 
grow,  and  become  invested  with  the 
myelin  sheath  and  neurilemma,  thus 
constituting  the  ventral  roots.  (Fig.  48.) 

The  afferent  nerve-fibers,  which 
constitute  some  of  the  cranial  nerv^es 
and  all  the  dorsal  roots  of  the  spinal 
nerves,  develop  outside  of  the  central 
nerve  system  and  only  subsequently 
become  connected  with  it.  (See  Fig, 
48,)  At  the  time  of  the  closure  of 
the  medullary  tube  a  band  or  ridge  of 
epithelial  tissue  develops  near  the  dor- 
sal surface,  which,  becoming  seg- 
mented, moves  outward  and  forms 
the  rudimentary  spinal  ganglia.  The 
cells  in  this  situation  develop  two 
axons,  one  from  each  end  of  the  cell,  roots.- 
which  pass  in  opposite  directions,  one 
toward  the  spinal  cord,  the  other  toward  the  periphery.  In  the  adult 
condition  the  two  axons  shift  their  position,  unite,  and  form  a  T-shaped 
process,  after  which  a  division  into  two  branches  again  takes  place.  In 
the  ganglia  of  all  the  sensori-cranial  and  sensori-spinal  nerves  the  cells 
have  this  histologic  peculiarity. 

The  efferent  fibers  are  therefore  to  be  regarded  as  outgrowths  from  the 
nerve-cells  in  the  ventral  horns  of  the  gray  matter,  and  serve  to  bring  the 
cells  into  anatomic  and  physiologic  relationship  directly  with  the  skeletal 
muscles  and  indirectly,  through  the  intermediation  of  ganglia  (see  sym- 
pathetic nerve  system),  with  visceral  muscles,  blood-vessels,  and  glands.' 

The  afferent*  fibers  are  to  be  regarded  as  outgrowths  from  the  cells  of  the 
■       [7  -  ^- • 


£006 


Fig.  48. — Diagram  Showing  the  Mode 
OF  Origin  of  the  Ventral  and  Dorsal 
-{Edingcr,  after  His ) 


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TEXT-BOOK  OF  PHYSIOLOGY 


dorsal  nerve  ganglia,  and  serve  to  bring  the  skin,  mucous  membrane,  and 
certain  visceral  structures  into  relation  with  specialized  centers  in  the  central 
nerve  system. 

Nerve  Degeneration. — -If  any  one  of  the  cranial  or  spinal  nerves  be  di- 
vided in  any  portion  of  its  course,  the  part  in  connection  with  the  periphery  in 
a  short  time  exhibits  certain  structural  changes,  to  which  the  term  degenera- 
tion is  applied.  The  degenerative  process  begins  simultaneously  through- 
out the  entire  course  of  the  nerve,  and  consists  in  a  disintegration  and  reduc- 
tion of  the  myelin  and  axis-cylinder  into  nuclei,  drops  of  myelin,  and  fat, 
which  in  time  disappear  through  absorption,  leaving  the  neurilemma  intact. 
Coincident  with  these  structural  changes  there  is  a  progressive  alteration  and 
diminution  in  the  excitability  of  the  nerve.  From  these  facts  it  has  been 
assumed  that  the  nerve-cells  exert  over  the  entire  course  of  the  nerve-fibers 
a  nutritive  or  a  trophic  influence.  This  idea  has  been  greatly  strengthened 
since  the  discovery  that  the  axis-cylinder,  or  the  axon,  has  its  origin  in  and  is 
a  direct  outgrowth  of  the  cell.  When  separated  from  the  parent  cell,  the 
fiber  appears  to  be  incapable  in  itself  of  maintaining  its  nutrition. 

It  has  for  a  long  time  been  stated  that  the  portion  of  the  divided  nerve  in 
connection  with  the  brain  or  spinal  cord  retains  its  normal  condition  with  the 
exception  of  a  few  millimeters  at  its  peripheral  end.  This  statement  has 
been  disproved  by  recent  observations  which  show  that  the  cell  body  and 
its  attached  axon  also  in  the  course  of  time  undergo  an  atrophy  which  may 
become  permanent;  also  that  the  chromatin  material  of  the  cell,  Nissl's  gran- 
ules, undergo  dissolution  and  no  longer  stain  with  the  usual  dyes,  a  condition 
which  may  last  for  two  weeks  or  more.  In  some  instances  the  granules 
may  be  restored.  These  degenerative  changes  in  the  nerve  cell  are  probably 
the  result  of  the  cessation  of  its  customary  function. 


Fig.  49. — Degeneration  of  Spinal  Nerves  and  Nerve-roots  after  Section.  A, 
Section  of  nerve-trunk  beyond  the  ganglion.  B,  Section  of  ventral  root.  C,  Section  of  dorsal. 
D.  Excision  of  ganglion,     a.  Ventral  root.     p.  Dorsal  root.     g.  Ganglion. — {Dalton.) 

The  relation  of  the  nerve-cells  to  the  nerve-fibers,  in  reference  to  their 
nutrition,  is  demonstrated  by  the  results  which  follow  section  of  the  ventral 
and  dorsal  roots  of  the  spinal  nerves.  If  the  ventral  root  alone  be  divided 
the  immediate  degenerative  process  is  confined  to  the  peripheral  portion,  the 
central  portion  remaining  for  a  variable  period  normal.  If  the  dorsal  root 
be  divided  on  the  peripheral  side  of  the  ganglion,  degeneration  takes  place 
only  in  the  peripheral  portion  of  the  nerve.  (See  Fig.  49.)  If  the  root  be 
divided  between  the  ganglion  and  the  cord,  degeneration  takes  place  only 
in  the  central  portion  of  the  root.  From  these  facts  it  is  evident  that  the 
trophic  centers  for  the  ventral  and  dorsal  roots  lie  in  the  spinal  cord  and  spinal 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE  99 

nerve  ganglia,  respectively,  or,  in  other  words,  in  the  cells  of  which  they  are 
an  integral  part.  The  structural  changes  which  nerves  undergo  after  separa- 
tion from  their  centers  are  degenerative  in  character,  and  the  process  is 
usually  spoken  of,  after  its  discoverer,  as  the  WaUerian  degeneration. 

Wxhen  the  nerve-cells  from  which  the  nerve-fibers  arise,  whether  efferent 
or  afferent,  undergo  degeneration  from  any  cause  whatever,  the  nen'-e-fiber 
becomes  involved  in  the  degenerative  process  and  when  it  is  completed  the 
structures  to  which  they  are  distributed,  especially  the  muscles,  undergo  an 
atrophic  or  fatty  degeneration,  with  a  change  or  loss  of  their  irritability. 
This  is,  apparently,  not  to  be  attributed  merely  to  inactivity,  but  rather  to  a 
loss  of  nerve  influences,  inasmuch  as  inactivity  merely  leads  to  atrophy  and 
not  to  degeneration. 

Reunion  and  Regeneration. — When  a  nerve-trunk  is  divided  there  is 
a  loss  of  function  of  the  parts  to  which  it  is  distributed,  and  usually  involves 
both  motion  and  sensation.  This,  however,  is  not  necessarily  permanent, 
for  after  a  variable  period  of  time  it  not  infrequently  happens  that  the  func- 
tions are  restored  because  of  a  reunion  of  the  separated  ends  and  a  regenera- 
tion of  the  peripheral  portion.  A  histologic  study  of  the  nerv^e-fibers  after 
separation  from  the  nerve-cells  shows  that  coincidently  with  the  degenerative 
process  there  occurs  a  regenerative  process,  consisting  in  a  multiplication  of 
the  nuclei  lying  just  beneath  the  neurilemma  and  an  accumulation  around 
them  of  a  granular  protoplasm  which  in  due  time  completely  fill  the  neuri- 
lemma. At  this  stage  the  fiber  is  known  as  a  band-fiber.  If  now  the  physical 
conditions  are  such  as  to  permit  of  a  reunion  of  the  nerve,  this  takes  place, 
and  under  the  nutritive  influence  of  the  cell  the  axis-cylinder  grows  into  the 
band-fiber  and  the  protoplasm,  becomes  transformed  into  myelin  as  in  the 
original  fiber.  The  axis-cylinder  continues  to  grow  and  extend  itself 
forward  until  it  reaches  its  ultimate  termination. 

CLASSIFICATION  OF  NERVES 

The  Efferent  Nerves. — The  efl'erent  nerves  may  be  classified,  in  accord- 
ance with  their  distribution  and  the  characteristic  forms  of  activity  to  which 
they  give  rise,  into  several  groups,  as  follows: 

1.  Skeletal-muscle  or  motor  nerves,  those  which  convey  nerve  energy  or  nerve 

impulses  directly  to  skeletal-muscles  and  excite  them  to  activity. 

2.  Gland  or  secretor  nerves,  those  which  convey  nerve  impulses  to  glands  by 

way  of  ganglia  and  influence  in  one  direction  or  another  the  degree  of 
their  activity.  Those  which  cause  the  formation  and  discharge  of  the 
secretion  peculiar  to  the  gland,  are  known  as  secreto-motor,  while  those 
which  decrease  or  inhibit  the  secretion  are  known  as  secreto-inhibitor 
nerves. 

3.  Vascular  or  vaso-motor  nerves,  those  which  convey  nerv^e  impulses  to  the 

muscle-fibers  of  the  blood-vessels  and  change  in  one  direction  or  the 
other  the  degree  of  their  natural  contraction.  Those  which  increase  the 
contraction  are  known  as  vaso-constrictors  or  vaso-augmentors;  those 
which  decrease  the  contraction  are  known  as  vaso-dilatators  or  vaso- 
inhibitors.  The  nerves  which  pass  to  that  specialized  part  of  the 
vascular  apparatus,  the  heart,  transmit  nerve  impulses  which  on  the 
one  hand  accelerate  its  rate  or  augment  its  force,  and  on  the  other  hand 


loo  TEXT-BOOK  OF  PHYSIOLOGY 

inhibit  or  retard  its  rate  and  diminish  its  force.  For  this  reason  they 
are  termed  cardiac  nerves,  one  set  of  which  is  known  as  cardio-accelera- 
tor  and  cardio-augmentor,  the  other  as  cardio-inhibitor  nerves. 

4.  Visceral  or  viscero-motor  nerves,  those  which  transmit  nerve  impulses  to 

the  muscle  walls  of  the  viscera  and  change  in  one  direction  or  another 
the  degree  of  their  contraction.  Those  which  increase  or  augment  the 
contraction  are  known  as  viscero-augmentor,  while  those  which  decrease 
or  inhibit  the  contraction,  are  known  as  viscero-inhibitor  nerves. 

5.  Hair  bulb  or  pilo-motor  nerves,  those  which  transmit  nerve  impulses  to  the 

muscle-fibers  which  cause  an  erection  of  the  hairs. 

Of  the  foregoing  nerves  the  skeletal-muscle  or  motor  nerves  alone  pass 
directly  to  the  muscle.  The  gland,  the  vascular  and  the  visceral  nerves,  all 
terminate  at  a  variable  distance  from  the  peripheral  organ  around  a  local 
sympathetic  ganglion,  which  in  turn  is  connected  with  the  peripheral  organ. 
The  former  are  termed  pre-ganglionic.  The  latter  post-ganglionic  fibers. 
(See  Fig.  13.) 

The  Afferent  Nerves. — The  afferent  nerves  may  also  be  classified,  in  ac- 
cordance with  their  distribution  and  the  character  of  the  sensations  or  other 
modes  of  nerve  activity  to  which  they  give  rise,  into  several  groups,  as  follows: 

1.  Tegumentary  nerves,  comprising  those  distributed  to  skin,  mucous  mem- 

branes and  sense  organs  and  which  transmit  nerve  impulses  from  the 
periphery  to  the  nerve  centers.  They  may  be  divided  into  reflex  and 
sensorifacient  nerves. 

A.  Reflex  nerves,  those  which  transmit  nerve  impulses  to  the  spinal 
cord  and  medulla  oblongata,  where  they  give  rise  to  different 
modes  of  nerve  activity.     They  may  be  divided  into: 

1.  Reflex  excitator  nerves,  which  transmit  nerve  impulses  which 
cause  an  excitation  of  nerve  centers  and  in  consequence  in- 
creased activity  of  peripheral  organs,  e.g.,  skeletal  muscles, 
glands,  blood-vessels  and  viscera. 

2.  Reflex  inhibitor  nerves,  which  transmit  nerve  impulses  which 
cause  an  inhibition  of  nerve  centers  and  in  consequence, 
decreased  activity  of  the  peripheral  organs.  It  is  quite  prob- 
able that  one  and  the  same  nerve  may  subserve  both  sensation 
and  reflex  action,  owing  to  the  collateral  branches  which  are 
given  off  from  the  afferent  roots  as  they  ascend  the  posterior 
column  of  the  cord. 

B.  Sensorifacient  nerves,  those  which  transmit  nerve  impulses  to  the 
brain  where  they  give  rise  to  conscious  sensations.  They  may  be 
subdivided  into: 

1.  Nerves  of  special  sense — e.g.,  olfactory,  optic,  auditory, 
gustatory,  tactile,  thermal,  pain,  pressure — which  give  rise  to 
correspondingly  named  sensations. 

2.  Nerves  of  general  sense — e.g.,  the  visceral  afferent  nerves — 
those  which  give  rise  normally  to  vague  and  scarcely  perceptible 
sensations,  such  as  the  general  sensations  of  well-being  or  dis- 
comfort, hunger,  thirst,  fatigue,  sex,  want  of  air,  etc. 

2.  Muscle  nerves,  comprising  those  distributed  to  muscles  and  tendons  and 

which  transmit  nerve  impulses  from  muscles  and  tendons  to  the  brain 
where  they  give  rise  to  the  so-called  muscle  sensations,  e.g.,  the  direction 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE  loi 

and  the  duration  of  a  movement,  the  resistance  offered  and  the  posture 
of  the  body  or  of  its  individual  parts. 

PHYSIOLOGIC  PROPERTIES  OF  NERVES 

Nerve  Irritability  or  Excitability  and  Conductivity. — These  terms 
are  employed  to  express  that  condition  of  a  nerve  which  enables  it  to  develop 
and  to  conduct  nerve  impulses  from  the  center  to  the  periphery,  or  from 
the  periphery  to  the  center,  in  response  to  the  action  of  stimuli.  A  nerve 
is  said  to  be  excitable  or  irritable  so  long  as  it  possesses  these  capabilities  or 
properties.  For  the  manifestation  of  these  properties  the  nerve  must 
retain  a  state  of  physical  and  chemic  integrity;  it  must  undergo  no  change 
in  structure  or  chemic  composition.  The  irritability  of  an  efferent  nerve  is 
demonstrated  by  the  contraction  of  a  muscle,  by  the  secretion  of  a  gland,  or 
.  by  a  change  in  the  caliber  of  a  blood-vessel,  whenever  a  corresponding  nerve 
is  stimulated.  The  irritability  of  an  afferent  nerve  is  demonstrated  by  the 
production  of  a  sensation  or  a  reflex  action  whenever  it  is  stimulated.  The 
irritability  of  nerves  continues  for  a  certain  period  of  time  after  separation 
from  the  nerve-centers  and  even  after  the  death  of  the  animal,  the  time 
varying  in  different  classes  of  animals.  In  the  warm-blooded  animals,  in 
which  the  nutritive  changes  take  place  with  great  rapidity,  the  irritability 
soon  disappears — a  result  due  to  disintegrative  changes  in  the  nerve,  caused 
by  the  withdrawal  of  the  blood-supply  and  other  non-physiologic  conditions. 
In  cold-blooded  animals,  on  the  contrary,  in  which  the  nutritive  changes 
take  place  relatively  slowly,  the  irritability  lasts,  under  favorable  conditions, 
for  a  considerable  time.  Other  tissues  besides  nerves  possess  irritability, 
that  is,  the  property  of  responding  to  the  action  of  stimuli — e.g.,  glands  and 
muscles,  which  respond  by  the  production  of  a  secretion  or  a  contraction. 

Independence  of  Tissue  Irritability. — -The  irritability  of  nerv^es  is 
distinct  and  independent  of  the  irritability  of  muscles  and  glands,  as  shown 
by  the  fact  that  it  persists  in  each  a  variable  length  of  time  after  their  histo- 
logic connections  have  been  inipaired  or  destroyed  by  the  introduction  of 
various  chemic  agents  into  the  circulation.  Curara,  for  example,  induces 
a  state  of  complete  paralysis  by  modifying  or  depressing  the  conductivity  of 
the  end-organs  of  the  nerves  just  where  they  come  in  contact  with  the  mus- 
cles, without  impairing  the  irritability  of  either  nerve-trunks  or  muscles. 
Atropin  induces  complete  suspension  of  gland  activity  by  impairing  the 
terminal  organs  of  the  secretor  nerves  just  where  they  come  into  relation  with 
the  gland-cells,  without  destroying  the  irritability  of  either  gland-cell  or  nerve. 

Nerve  Stimuli. — Nerves  do  not  possess  the  power  of  spontaneously 
generating  and  propagating  nerve  impulses;  they  can  be  aroused  to  activity 
only  by  the  action  of  an  external  stimulus.  In  the  physiologic  condition  the 
stimuli  capable  of  throwing  the  nerve  into  an  active  condition  act  for  the 
most  part  on  either  the  central  or  peripheral  end  of  the  nerve.  In  the  case 
of  motor  nerves  the  stimulus  to  the  excitation,  originating  in  some  molecular 
disturbance  in  the  nerve-cells,  acts  upon  the  nerve-fibers  in  connection  with 
them.  In  the  case  of  sensor  or  afferent  nerves  the  stimuli  act  upon  the  pecul- 
iar end-organs  with  which  the  sensor  nerves  are  in  connection,  which  in 
turn  excite  the  nerve-fibers.  Experimentally,  it  can  be  demonstrated  that 
nerves  can  be  excited  by  a  sufficiently  powerful  stimulus  applied  in  any 
part  of  their  extent. 


I02  TEXT-BOOK  OF  PHYSIOLOGY 

Nerves  respond  to  stimulation  according  to  their  habitual  function; 
thus,  stimulation  of  a  sensor  nerve,  if  sufficiently  strong,  results  in  the  sensa- 
tion of  pain;  of  the  optic  nerve,  in  the  sensation  of  light;  of  a  motor  nerve, 
in  contraction  of  the  muscle  to  which  it  is  distributed;  of  a  secretor  nerve, 
in  the  activity  of  the  related  gland,  etc.  It  is,  therefore,  evident  that  pecul- 
iarity of  nerve  function  depends  neither  upon  any  special  construction  or 
activity  of  the  nerve  itself  nor  upon  the  nature  of  the  stimulus,  but  entirely 
upon  the  peculiarities  of  its  central  and  peripheral  end-organs. 

Nerve  stimuli  may  be  divided  into — 

1.  General  stimuli,  comprising  those  agents  which  are  capable  of  exciting 

a  nerve  in  any  part  of  its  course. 

2.  Special  stimuli,  comprising   those   agents  which   act   upon   nerves   only 

through  the  intermediation  of  the  end-organs. 

The  end-organs  are  specialized  highly  irritable  structures  placed  between 
the  nerve-fibers  and  the  surface.  They  are  especially  adapted  for  the 
reception  of  special  stimuli  and  for  the  liberation  of  energy,  which  in  turn 
excites  the  nerve-fiber  to  activity. 

General  stimuli: 

1.  Mechanic:  Sharp  taps,  sudden  pressure,  cutting,  etc. 

2.  Thermic:  Sudden  application  of  heated  object. 

3.  Chemic:  Contact  of  various  substances  which  alter  their  chemic  composi- 

tion quickly,  e.g.,  strong  acids  or  alkalies,  sol.  sodium  chlorid  15  per 
cent.,  sugar,  urea,  etc. 

4.  Electric:  Either  the  constant  or  induced  current. 
Special  stimuli: 

For  afferent  nerves — 

1.  Light  or  ethereal  vibrations  acting  upon  the  end-organs  of  the  optic 

nerve  in  the  retina. 

2.  Sound  or  atmospheric  undulations  acting  upon  the  end-organs  of  the 

auditory  nerve. 

3.  Heat  or  vibrations  of  the  air  acting  upon  the  end-organs  in  the  skin. 

4.  Chemic  agencies  acting  upon  the  end-organs  of  the  olfactory  and  gusta- 

tory nerves. 

For  efferent  nerves — 

A  molecular  disturbance  in  the  central  nerve-cells  from  which  they  arise, 
the  nature  of  which  is  unknown. 

Nature  of  the  Nerve  Impulse. — As  to  the  nature  of  the  nerve  impulse 
generated  by  any  of  the  foregoing  stimuli,  either  general  or  special,  but  little 
is  known.  It  has  been  supposed  to  partake  of  the  nature  of  a  molecular 
disturbance,  a  combination  of  physical  and  chemic  processes  attended  by 
the  liberation  of  energy,  which  propagates  itself  from  molecule  to  molecule. 
The  passage  of  the  nerve  impulse  is  accompanied  by  changes  of  electric 
tension,  the  extent  of  which  is  an  indication  of  the  intensity  of  the  molecular 
disturbance.  Judging  from  the  deflections  of  the  galvanometer  needle  it  is 
probable  that  when  the  nerve  impulse  makes  its  appearance  at  any  given 
point  it  is  at  first  feeble,  but  soon  reaches  a  maximum  development,  after 
which  it  speedily  declines  and  disappears.  It  may,  therefore,  be  graphically 
represented  as  a  wave-like  movement  with  a  definite  length  and  time  dura- 
tion. (See  page  106.)  Under  strictly  physiologic  conditions  the  nerve 
impulse  passes  in  one  direction  only;  in  efferent  nerves  from  the  center  to  the 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE  103 

periphery,  in  afferent  nerves  from  the  periphery  to  the  center.  Experimen- 
tally, however,  it  can  be  demonstrated  that  when  a  nerve  impulse  is  aroused 
in  the  course  of  a  nerve  by  an  adequate  stimulus  it  travels  equally  well  in 
both  directions  from  the  point  of  stimulation.  When  once  started,  the 
impulse  is  confined  to  the  single  fiber  and  does  not  diffuse  itself  to  fibers 
adjacent  to  it  in  the  same  nerve-trunk. 

Rapidity  of  Conduction  of  the  Nerve  Impulse. — The  passage  of  a 
nerve  impulse,  either  from  the  brain  to  the  periphery  or  in  the  reverse  direc- 
tion, requires  an  appreciable  period  of  time.  The  velocity  with  which  the 
impulse  travels  in  human  sensory  nerves  has  been  estimated  at  about  50 
meters  a  second,  and  for  motor  nerves  at  from  28  to  33  meters  a  second.  The 
rate  of  movement  is,  however,  somewhat  modified  by  temperature,  cold 
lessening  and  heat  increasing  the  rapidity;  it  is  also  modified  by  electric 
conditions,  by  the  action  of  drugs,  the  strength  of  the  stimulus,  etc.  The 
rate  of  transmission  through  the  spinal  cord  is  considerably  slower  than  in 
nerves,  the  average  velocity  for  voluntary  motor  impulses  being  only  11 
meters  a  second,  for  sensory  impulses  12  meters,  and  for  tactile  impulses  40 
m-eters  a  second. 

Nerve  Fatigue. — Inasmuch  as  nerves  are  parts  of  living  cells,  the  seat 
of  nutritive  changes,  it  might  be  supposed  that  the  passage  of  nerve  impulses 
would  be  attended  by  the  disruption  of  energy-holding  compounds,  the  pro- 
duction of  waste  products,  the  liberation  of  heat,  and  in  time  by  the  phenom- 
ena of  fatigue.  Though  it  is  probable  that  changes  of  this  character  occur, 
yet  no  reliable  experimental  data  have  been  obtained  which  afford  a  clue  as 
to  the  nature  or  extent  of  any  such  changes.  Stimulation  of  motor  nerves 
with  the  induced  electric  current  for  four  hours  appears  to  be  without 
influence  either  on  the  intensity  of  the  nerve  impulse  or  the  rate  of  its 
conduction. 

Identity  of  Efferent  and  Afferent  Nerves  and  Nerve  Impulses. — 
Notwithstanding  the  classification  of  nerve-fibers  based  on  differences  of 
physiologic  actions,  there  are  no  characters,  either  histologic  or  chemic, 
which  serve  to  distinguish  them  from  one  another.  Moreover,  as  the  nerve 
impulse  is  conducted  through  a  nerve-fiber  equally  well  in  both  directions, 
as  determined  by  experiments,  it  is  probable  that  it  does  not  differ  in  char- 
acter in  the  two  classes  of  nerves.  That  the  efferent  fibers  conduct  the 
nerve  impulses  from  the  nerve-centers  to  the  periphery,  and  the  afferent 
nerves  from  the  periphery  to  the  centers,  is  because  of  the  fact  that  they 
receive  their  stimulus  physiologically  only  in  the  centers  or  at  the  periphery. 
The  fundamental  reason  for  difference  of  effects  produced  by  stimulation 
of  different  nerves  is  the  character  of  the  organ  to  which  the  nerv^e  impulse 
is  conducted.  A  nerve  is  merely  the  transmitter  of  the  nerve  impulse, 
which  if  conducted  to  a  muscle  excites  contraction;  to  a  gland,  secretion; 
to  a  blood-vessel,  variation  in  caliber;  to  special  areas  in  the  brain,  sensa- 
tions of  light,  sound,  pain,  etc. 

Electric  Excitation  of  Nerves. — For  the  purpose  of  studying  the 
physiologic  activities  of  nerves  it  has  been  found  convenient  to  employ  the 
nerve-muscle  preparation  (the  gastrocnemius  muscle  and  sciatic  nerve)  and 
to  use  as  a  stimulus  the  induced  electric  current.  (See  Fig.  50.)  When 
kept  moist,  this  preparation  is  extremely  sensitive  to  either  the  galvanic 
or  the  induced  current. 


I04 


TEXT-BOOK  OF  PHYSIOLOGY 


Though  the  development  and  conduction  of  a  nerve  impulse  may  be 
demonstrated  by  the  deflection  of  the  galvanometer  needle  or  the  move- 
ment of  the  mercury  in  the  capillary  electrometer,  it  is  more  conveniently 
demonstrated  by  the  contraction  of  a  muscle,  the  vigor  of  which,  within 
limits,  may  be  taken  as  a  measure  of  the  intensity  of  the  impulse.  The 
preparation  should  be  enclosed  in  a  moist  chamber  and  the  nerve  con- 
nected with  the  inductorium  through  the  intervention 
of  non-polarizable  electrodes.  The  muscle  may  be  at- 
tached to  the  muscle-lever  and  its  contractions  recorded. 
A  single  shock  of  an  induced  current  develops,  it  is 
believed,  a  single  nerve  impulse  followed  by  a  single 
muscle  contraction.  A  minimal  contraction  following  a 
minimal  electric  stimulus  presupposes  the  development 
of  a  nerve  impulse  of  low  intensity.  Within  certain 
limits  a  maximal  contraction  following  a  maximal  elec- 
tric stimulus  presupposes  the  development  of  a  nerve 
impulse  of  high  intensity.  Intermediate  contractions 
indicate  nerve  impulses  of  corresponding  intensity. 

Tetanization  of  a  muscle  indicates  that  the  nerve 
impulses  arrive  at  the  muscle  with  a  frequency  so  great 
that  the  muscle  does  not  succeed  in  relaxing  from  the 
effect  of  one  stimulus  before  the  next  arrives.  Complete 
as  well  as  incomplete  tetanus  may  be  developed  by  grad- 
ually increasing  the  frequency  of  the  stimulus.  The 
character  of  the  contraction  caused  by  indirect  stimu- 
lation— i.e.,  through  the  nerve — does  not  differ  in  any  essential  respect  from 
that  due  to  direct  stimulation. 


Fig.  50. — Nerve> 
MUSCLE  Prepara- 
tion OF  A  Frog.  F. 
Femur.  S.  Sciatic 
nerve.  I.  T  e  n  d  o 
Achillis.  —  {Landois 
and    Stirling.) 


ELECTRIC  PHENOMENA  OF  NERVES 

Electric  Currents  from  Injured  Nerves. — It  was  discovered  by  du- 
Bois  Reymond  that  electric  currents  can  be  obtained  from  nerves  as  well  as 
from  muscles,  and  that  the  electric  properties  of  the  former  correspond  in 
most  respects  to  those  of  the  latter.  The  laws  governing  the  development 
and  mode  of  action  of  the  currents  derived  from  muscles  are  equally 
applicable  to  the  currents  derived  from  nerves. 

A  nerve-cylinder  obtained  by  making  two  transverse  sections  of  any 
given  nerve  presents,  as  in  the  case  of  muscles,  a  natural  and  two  artificial 
transverse  surfaces.  A  line  drawn  around  the  cylinder  at  a  point  lying 
midway  between  the  two  end  surfaces  constitutes  the  equator.  From  such 
a  cylinder  strong  currents  are  obtained  when  the  natural  longitudinal  sur- 
face and  the  transverse  surface  are  connected  with  the  electrodes  of  the 
galvanometer  circuit.  The  strength  of  the  current  thus  obtained  will 
diminish  or  increase  according  as  the  electrode  on  the  longitudinal  surface 
is  removed  from  or  brought  near  to  the  equator.  If  two  symmetric  points 
on  the  longitudinal  surface  equidistant  from  the  equator  are  united,  no 
current  is  obtainable.  When  asymmetric  points  on  the  longitudinal  sur- 
face are  connected,  weak  currents  are  obtained,  in  which  case  the  point 
lying  nearer  the  equator  becomes  positive  to  the  point  more  distant,  which 
becomes  negative.     From  these  facts  it  is  evident  that  all  points  on  the  Ion- 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE 


105 


gitudinal  surface  are  electrically  positive  to  the  transverse  surface  and  that 
the  point  of  greatest  positive  tension  is  situated  near  the  equator  (Fig.  51). 

The  electromotive  force  of  the  nerve  current  varies  in  strength  with  the 
length  and  thickness  of  the  nerve.  The  strongest  current  obtained  from 
the  nerve  of  the  frog  is  equal  to  the  0.002  of  a  Daniell  cell;  that  obtained 
from  the  nerve  of  the  rabbit,  0.026  of  a  Daniell.  The  existence  of  the  nerve 
current,  its  strength,  duration,  etc.,  depend  largely  on  the  maintenance  of 
physiologic  conditions.  All  influences  which 
impair  the  nutrition  of  the  nerve  diminish  the 
current.  With  the  death  of  the  nerve  all 
electric  phenomena  disappear.  [■ 

Negative  Variation  of  the  Nerve  Cur- 
rent.— During  the  passage  of  the  nerve  impulse 
the  resting  nerve  current,  or  the  demarcation 
current,  diminishes  more  or  less  completely  in 
intensity,  undergoes  a  negative  variation,  as 
shown  by  the  return  of  the  galvanometer 
needle,  due  to  a  change  in  its  electromotive 
condition  or  to  a  diminution  of  the  difl'erence 
in  potential  between  the  positive  longitudinal 
and  negative  transverse  sections.  This  nega- 
tive variation  of  the  demarcation  current  is 
observed  equally  well  from  either  the  central 
or  peripheral  end  of  the  nerve.  If  the  two 
ends  of  the  nerve  are  connected  with  galvanom- 
eters and  the  nerve  stimulated  in  the  middle, 
the  demarcation  currents  simultaneously  un- 
dergo a  negative  variation.  This  may  be 
taken  as  a  proof  that  the  excitation  process 
propagates  itself  equally  well  in  both  direc- 
tions. The  negative  variation  is  intimately 
connected  with  changes  in  the  molecular  con- 
dition of  the  nerve  and  is  not  due  to  any  ex- 
traneous electric  or  other  influence.     And  du- 

Bois  Reymond  was  also  enabled  to  obtain  a  negative  variation  of  the  current 
in  the  nerves  of  a  living  frog  which  were  yet  in  connection  with  the  spinal 
cord.  In  this  experiment  the  sciatic  nerve  was  divided  at  the  knee  and 
freed  from  its  connections  up  to  the  spinal  column;  the  transverse  and  longi- 
tudypal  surfaces  were  then  placed  in  connection  with  the  electrodes  of  the 
galvanometer  wires  and  the  current  permitted  to  influence  the  needle. 
The  animal  was  then  subjected  to  the  action  of  strychnin.  Upon  the  ap- 
pearance of  the  muscle  spasms  the  needle  was  observ^ed  to  swing  backward 
toward  the  zero  point  to  the  extent  of  from  i  to  4  degrees,  and  upon  the 
cessation  of  the  spasms  to  return  to  its  previous  position.  In  an  experi- 
ment of  this  nature  it  is  obvious  that  the  negative  variation  was  the  result  of 
a  physiologic  stimulation  of  the  nerve  arising  within  the  spinal  cord. 

The  question  also  here  arises  as  to  whether  the  negative  variation  is  due 
to  a  steady,  continuous  decrease  of  the  natural  current,  or  whether  it  is  due 
to  successive  and  rapidly  following  variations  in  its  intensity,  similar  to  that 
observed  in  muscles.     Though  this  cannot  be  demonstrated  with  the  physio- 


FiG.  51. — DiAGiiAM  TO  Illus- 
trate THE  Currents  in  Nerves. 
The  arrowheads  indicate  the  direc- 
tion; the  thickness  of  the  hncs  in- 
dicates the  strength  of  the  currents. 
— (Landois  and  Stirling.) 


io6  TEXT-BOOK  OF  PHYSIOLOGY 

logic  rheoscope,  as  was  the  case  with  the  muscle,  there  can  be  no  doubt,  both 
from  .experimentation  and  analogy,  that  the  latter  supposition  is  the  correct 
one.  It  has  been  shown  that  when  non-polarizable  electrodes  connected 
with  Siemen's  telephone  are  placed  in  connection  with  the  longitudinal  and 
transverse  sections  of  a  nerve,  low,  sonorous  vibrations  are  perceived  during 
tetanic  stimulation — a  .proof  that  the  active  state  of  the  nerve  is  connected 
with  the  production  of  discontinuous  electric  currents.  The  oscillations 
of  the  mercurial  column  of  the  capillary  electrometer  also  reveal  similar 
electric  changes.  It  was  also  demonstrated  by  Bernstein  with  a  specially 
devised  apparatus,  the  repeating  rheotome,  that  the  negative  variation  is 
composed  of  a  large  number  of  single  variations  which  succeed  each 
other  in  rapid  succession  and  summarize  themselves  in  their  effect  on  the 
needle. 

Electric  Currents  from  Uninjured  Nerves. — The  pre-existence 
of  electric  currents  in  living  and  wholly  uninjured  nerves  while  at  rest  has 
also  been  denied  by  Hermann,  who  regards  all  portions  of  the  nerve  as 
isoelectric,  any  difference  of  potential  being  the  result  of  some  injury  to  its 
surface. 

Action  Currents. — For  reasons  to  be  stated  below,  it  is  very  diffi- 
cult to  determine  the  presence  of  diphasic  action  currents  during  the  pass- 
age of  an  excitatory  impulse  through  the  nerve-fiber.  The  so-called  negative 
variation  of  the  resting  nerve  current — the  demarcation  current — which  is 
occasioned  by  tetanic  stimulation,  Hermann  regards  as  the  expression  of  an 
action  current  which  flows  in  the  nerve  in  a  direction  opposite  to  the  demarca- 
tion current.  The  origin  of  this  action  current  is  to  be  sought  for  in  the 
continuous  negativity  of  that  portion  of  the  longitudinal  surface  of  the  nerve 
in  contact  with  the  diverting  electrode,  while  the  dying  substance  of  the 
transverse  surface  takes  no  part  in  the  excitation.  This  tetanic  action  current, 
or  negative  variation,  was  discovered  by  du-Bois  Reymond,  and  Bernstein 
later  succeeded  in  obtaining  this  action  current  during  the  passage  of  a 
single  excitation  process.  That  the  return  of  the  galvanometer  needle 
toward  the  zero  point  is  not  due  to  an  annulment  of  the  demarcation  current 
itself,  but  to  the  appearance  of  an  action  current,  is  shown  by  the  fact  that 
if  the  former  be  compensated  by  a  battery  current  until  the  needle  rests  on 
the  zer€)  point  the  appearance  of  the  latter  current  will  cause  the  needle 
to  swing  in  a  direction  the  opposite  of  that  caused  by  the  demarcation  current. 
The  negative  variation  and  action  current  may  therefore  be  regarded  as  one 
and  the  same  thing.  It  is  the  expression  of  the  change  the  nerve  is  under- 
going during  the  passage  of  the  nerve  impulse.  The  rapidity  with  which 
the  negative  variation  or  action  current  travels,  the  variation  in  its  intensity 
from  moment  to  moment,  the  time  required  for  it  to  pass  a  given  point, 
would  express  the  change  in  the  nerve  to  which  the  term  nerve  impulse  is 
given.  From  experiments  made  with  the  differential  rheotome,  Bernstein 
calculated  that  the  speed  of  the  negative  variation  is  about  28  meters  a 
second;  that  it  is  at  first  feeble,  soon  rises  to  a  maximum,  and  then  declines; 
that  is  requires  0.0006  to  0.0008  of  a  second  to  pass  a  given  point.  From  these 
data  it  is  evident  that  the  negative  variation  or  action  current  has  a  space 
value  of  0.0006  of  28  meters  or  about  18  mm.  Transferring  these  state- 
ments to  the  nerve  impulse,  it  may  be  said  that  it  is  a  molecular  disturb- 
ance, traveling  at  the  rate  of  about  28  meters  a  second,  is  wave-like  in 


GENER.\L  PHYSIOLOGY  OF  NERYE -TISSUE  107 

character,  the  wave  being  18  millimeters  in  length  and  occupying  from 
0.0006  to  0.0008  of  a  second  in  passing  any  given  point. 

Absence  of  Diphasic  Action  Currents. — When  any  two  points  on 
the  longitudinal  surface  which  do  not  exhibit  a  current  are  connected 
with  the  galvanometer  and  a  single  wave  of  excitation  passes  beneath  the 
electrodes,  it  might  be  expected  that,  as  in  the  case  of  the  muscle,  a  diphasic 
action  current  would  be  obsen^ed,  from  the  fact  that  the  portions  of  the 
nerve  beneath  the  electrodes  become  alternately  negative  with  reference 
to  all  the  rest  of  the  nerve.  This,  however,  is  not  the  case,  the  absence  of 
the  two  opposing  phases  of  the  action  current  being  explained  on  the  suppo- 
sition that  the  negativity  of  the  two  led-off  points  is  of  equal  amount,  and  that, 
owing  to  the  great  rapidity  with  which  the  excitation  wave  travels,  the  two 
phases  fall  together  too  closely  in  time  to  alternately  influence  the  galvan- 
ometer needle.  During  stimulation  of  the  nerve,  when  two  currentless  or 
isoelectric  points  are  connected,  there  is  also  an  absence  of  the  action 
current,  as  was  observed  first  by  du-Bois  Reymond,  and  which  is  to  be  ex 
plained  on  similar  grounds.  It  is  true  that  an  apparent  action  current 
is  sometimes  seen  when  the  stimulating  current  is  very  powerful  or  the  seat 
of  stimulation  too  near  the  diverting  electrodes.  This,  however,  must  be 
attributed  to  an  electrotonic  state  of  the  ner\^e. 

The  Effects  of  a  Galvanic  Current  on  a  Nerve. — When  a  constant 
galvanic  current  of  medium  strength  is  made  to  pass  through  a  portion  of  a 
nerve,  several  distinct  effects  are  produced: 

1.  Tlie  development  of  a  nerve  impulse  at  the  moment  the  current  enters 
and  at  the  moment  the  current  leaves  the  nerve,  i.e.,  at  the  moment  the 
circuit  is  made  and  at  the  moment  it  is  broken.  The  development  of  the 
nerve  impulse  is  made  evident  by  the  contraction  of  the  muscle  if  the  nerve- 
muscle  preparation  be  used.  If  the  current  be  either  very  weak,  or  very 
strong,  the  muscle  contraction  may  not  always  take  place. 

Jj^EY  ^  /7  ^'GALVANOMETER 

'POLARIZING  3 
CURRENT     j 
^ -^        -^- 

anelectrotonic  katelectroton ic 

currents  currents 

Fig.  52. — The  Situation  and  Direction  of  Electrotonic  Currents,  Developed  by  the 
Passage  of  an  Electric  Current  through  a  Portion  of  a  Nerve. 

2,  The  development  of  electric  currents  on  each  side  of  the  positive  pole 
or  anode,  and  the  negative  pole  or  kathode  (see  Fig.  52),  which  can  be  led 
off  by  means  of  wires  into  a  galvanometer  circuit  from  either  the  artificial 
transverse  and  longitudinal  surfaces,  or  from  any  two  points  on  the  longi- 
tudinal surface  as  shown  by  the  deflection  of  the  galvanometer  needle. 
The  direction  of  these  electric  currents  in  the  nerve  coincides  with  that  of 
the  galvanic  or  "polarizing  current."  The  "natural  nerve  currents,"  the 
currents  of  injury  or  demarcation  currents,  as  they  are  variously  termed, 
are  at  the  same  time  increased  and  decreased  at  opposite  extremities  of  the 
nerve  according  to  the  direction  of  the  polarizing  current. 


io8  TEXT-BOOK  OF  PHYSIOLOGY 

To  this  changed  condition  of  the  electromotive  forces  in  a  nerve  the 
term  electrotonus  was  given  (du-Bois  Reymond).  The  currents  them- 
selves are  known  as  electro  tonic  currents;  from  their  relation  to  the  anode 
and  kathode,  they  are  termed  anelectrotonic  and  katelectrotonic  currents. 
The  condition  of  the  nerve  around  the  poles  both  in  the  intra-polar  and 
extra-polar  regions  is  known  as  anelectrotonus  and  katelectrotonus. 

The  electrotonic  currents  vary  considerably  in  strength  and  extent, 
according  to  the  intensity  of  the  polarizing  current,  increasing  steadily 
with  the  intensity  of  the  latter  up  to  the  point  at  which  the  polarizing  cur- 
rent begins  to  destroy  the  physical  and  chemic  integrity  of  the  nerve. 
The  electrotonic  currents  are  strongest  in  the  immediate  neighborhood  of 
the  electrodes,  but  gradually  diminish  in  strength  as  the  distance  between 
the  polarized  and  led-off  portions  is  increased.  The  distance  to  which  the 
electrotonic  currents  extend  along  the  nerve  will  depend  very  largely  upon 
the  strength  of  the  polarizing  current,  though  it  is  conditioned  by  the  phys- 
ical state  of  the  nerve;  for  if  it  be  ligated  or  injured  beyond  the  polarized 
portion,  the  electrotonic  currents  are  abolished.  The  electrotonic  currents 
have  no  necessary  connection  with  the  natural  nerve  currents,  nor  are  they 
to  be  regarded  as  branchings  of  the  galvanic  current.  They  are  in  all 
probability  of  artificial  origin,  due  to  an  inner  positive  and  negative  polari- 
zation of  the  nerve  which  extends  for  a  variable  distance  on  each  side  of 
the  poles,  and  due  to  the  action  of  the  polarizing  or  the  galvanic  current. 

3.  An  alteration  in  the  excitability  and  conductivity  of  the  nerve  in  the 
neighborhood  of  the  poles,  whereby  the  results  of  nerve  stimulation — that 
is,  muscle  contraction,  sensation,  and  inhibition — are  increased  or  decreased 


Fig.  53. — Scheme  of  the  Electrotonic  Excitability. — {Landois  and  Stirling.) 

according  to  the  strength  and  direction  of  the  current.  To  this  condition 
the  term  electrotonus  was  also  given  (Pfliiger).  This  word  has  thus  been 
employed  to  express  two  distinct  series  of  effects  exhibited  by  a  nerve  through 
a  portion  of  which  a  constant  galvanic  current  is  passing.  It  appears  desir- 
able, for  the  sake  of  clearness,  to  limit  the  term  electrotonus  to  the  electric  or 
electrotonic  currents  which  can  be  led  off  from  either  extremity  of  the  nerve, 
and  to  apply  to  the  modifications  of  irritability  which  accompany  electro- 
tonus the  expression,  electrotonic  alteration  of  excitability  and  conductivity. 
During  the  passage  of  the  current  the  excitability  of  the  intra-polar  as 
well  as  the  extra-polar  regions  undergoes  a  change  which,  as  shown  on 
examination,  is  found  to  be  diminished  in  the  neighborhood  of  the  anode  or 
positive  pole  and  increased  in  the  neighborhood  of  the  kathode  or  negative 
pole.  These  alterations  in  the  excitability  are  most  marked  in  the  imme- 
diate vicinity  of  the  electrodes,  though  they  extend  for  some  distance  into 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE  109 

both  the  extra-polar  and  intra-polar  regions,  though  with  gradually  dimin- 
ishing intensity,  until  they  finally  disappear.  Between  the  electrodes 
there  is  a  point  where  the  excitability  is  unchanged  and  known  as  the  neutral 
or  indifferent  point  (Fig.  53).  The  extent  to  which  the  excitability  is  modi- 
fied as  well  as  the  position  of  the  neutral  point  will  depend  largely  on  the 
strength  of  the  polarizing  or  galvanic  current. 


(i- 


\ REGION    OF 

j  INCREASED  EXCITABILITY 


SECONDARY  COIL 

Fig.  54. — Diagram  Showing  the  Region  of  Increased  Excitability  Caused  by  the 
Passage  of  a  Galvanic  Current  Through  a  Portion  of  a  Nerve,  Stimulation  of 
which   Gives   Rise  to   Increased   Contraction. 

The  electrotonic  alterations  of  excitability  and  conductivity  can  be 
experimentally  demonstrated  on  the  muscle-nerve  preparation  in  the  fol- 
lowing manner: 

I.  With  a  descending  current  of  medium  strength.  Previous  to  the  clo- 
sure of  the  polarizing  current,  the  nerve  is  stimulated  first  in  the  extra- 
polar  anodic  region  and  the  extra-polar  kathodic  region  with  an  induc- 


REGION    OF 
DECREASED    EXCITABILITY 


^ i_         ^i^^3 


'  Fig.  55. — Diagram  Showing  the  Region  of  Decreased  Excitability  Caused  by  the 
Passage  of  a  Galvanic  Current  through  a  Portion  of  a  Nerve,  Stimulation  of 
wHiCH^GrvES  Rise  to  Decreased  Contraction.  • 

tion  shock  of  medium  intensity  and  the  height  of  the  contraction  re- 
corded. On  repeating  the  stimulation  after  closure  of  the  polarizing 
current  the  contraction  resulting  from  stimulation  of  the  anodic  region 
will  be  enfeebled  or  may  be  entirely  wanting,  while  the  contraction  from 
stimulation  of  the  kathodic  region  will  be  decidedly  increased.  (See 
Fig.  54-) 


no  TEXT-BOOK  OF  PHYSIOLOGY 

2,      With  an  ascending  current  of  the  same  strength.     After  preliminary 

testing  of  the  excitabihty  and  the  subsequent  closure  of  the  polarizing 

current,  it  will  be  found  that  stimulation  of  the  extra-polar  anodic 

region  will  provoke  a  much  less  energetic  contraction  or  perhaps  none 

at  all.     Stimulation  of  the  extra-kathodic  region,  though  of  increased 

excitability,  as  shown  by  the  previous  experiment,  may  also  fail  to 

provoke  a  contraction,  owing  to  the  diminished  conductivity  of  the 

region  in  the  neighborhood  of  the  anode.     The  impulse  on  reaching 

this  region  is  blocked  in  its  passage.     A  similar  if  not  more  marked 

decrease  in  the  conductivity  may  be  developed  in  the  region  of  the 

kathode  if  the  current  strength  be  very  great.     (See  Fig.  55.) 

The  Law  of  Contraction;  Polar  Stimulation. — It  was  stated  in  a 

previous  paragraph  that  when  a  galvanic  current  of  medium  strength  is 

made  to  enter  a  nerve,  and  when  it  is  withdrawn  from  the  nerve,  there  is  a 

contraction  of  its  related  muscle.     These  are  generally  known  as  the  make 

and  break  effects.     During  the  actual  passage  of  the  current  no  effect  is 

observed  so  long  as  its  strength  remains  uniform.     Any  sudden  variation 

in  the  strength  of  the  current  at  once  arouses  the  nerve  to  activity,  as  shown 

by  a  muscle  contraction. 

The  muscle  response  to  the  make  and  break  of  the  constant  current  is 
more  or  less  variable  unless  the  direction  of  the  current  as  well  as  its  strength 
be  taken  into  consideration.  If  the  current  is  made  to  flow  from  the  central 
toward  the  peripheral  end  of  the  nerve  it  is  termed  a  direct,  descending,  or 
centrifugal  current;  if  it  is  made  to  flow  in  the  reverse  direction,  it  is  termed 
an  indirect,  ascending,  or  centripetal  current.  The  strength  of  the  current  is 
determined  and  regulated  by  means  of  a  rheocord. 

The  make  and  break  of  currents  of  different  but  known  strengths  and 
directions  give  rise  to  contractions  which  occur  with  more  or  less  regularity. 
The  order  in  which  they  occur  under  these  varying  conditions  of  experi- 
mentation has  been  determined  and  tabulated  as  follows  by  Pfliiger,  and  is 
termed  the  law  of  contraction: 


Current  intensity 

Ascending  current 

Descending  ourent 

Make                        Break 

Make 

Break 

Weak 

Contraction              T?e<;t 

Contraction. 
Contraction. 
Contraction. 

Rest 

Medium 

Contraction. 
Rest. 

Contraction. 
Contraction. 

Strong 

Rest  or  weak  con- 
traction. 

The  resiilts  as  above  tabulated  are  sometimes  complicated  on  the  open- 
ing of  the  circuit  by  a  series  of  irregular  pulsations  of  the  muscle,  an  ap- 
parent tetanus,  and  long  known  as  the  opening  tetanus  of  Ritter,  which  is 
attributed  to  rapid  changes  in  the  irritability  of  the  nerve,  in  the  region  of 
the  anode.  A  similar  tetanic  contraction  of  the  muscle  is  sometimes  ob- 
served on  the  closure  of  the  circuit  due  to  continued  excitation  in  the  region 
of  the  kathode.  This  is  known  as  the  closing  tetanus  of  Wundt  or  of  Pfliiger. 
All  the  phenomena  of  the  law  of  contraction  were  explained  by  Pfliiger  on 


GENERAL  PHYSIOLOGY  OF  NERVE -TISSUE  iii 

the  assumption  that  the  current  stimulates  the  nerve  only  at  the  one  electrode, 
at  the  kathode  on  closing,  and  at  the  anode  on  opening;  or,  in  other  words,  by 
the  appearance  of  katelectrotonus  or  by  the  disappearance  of  anelectrotonus, 
both  conditions  being  attended  by  a  rise  of  excitability — not,  however,  by 
the  opposite  changes.  It  is  further  assumed  that  the  appearance  of  kat- 
electrotonus is  more  effective  as  a  stimulus  than  the  disappearance  of  anelec- 
trotonus. For  these  reasons  the  term  polar  stimulation  is  generally  employed 
in  discussing  the  make  and  break  effects  of  the  galvanic  current.  The  law 
of  contraction  may  then  be  explained  as  follows:  Very  feeble  currents, 
either  ascending  or  descending,  produce  contraction  only  upon  the  closure 
of  the  circuit,  the  sudden  increase  of  the  excitability  in  the  katelectroto^iic  area 
being  alone  sufficient  to  generate  an  impulse.  The  contraction  which 
follows  the  closing  of  the  weak  ascending  current  depends  upon  the  fact 
that  the  decrease  of  excitability  and  conductivity  at  the  anode  is  insufficient 
to  interfere  with  the  conduction  of  the  kathodal  stimulus.  Medium  currents, 
either  ascending  or  descending,  produce  contraction  both  on  closing  and 
opening  the  circuit.  The  appearance  of  katelectrotonus  and  the  disap- 
pearance of  anelectrotonus  are  both  sufficiently  powerful  to  generate  an  im- 
pulse without,  however,  seriously  impairing  the  conductivity  of  the  nerve. 

Very  strong  currents  produce  contraction  only  upon  the  opening  of  the 
ascending  and  closure  of  the  descending  currents,  or  upon  the  passage  of  the 
excitability  in  the  former  from  the  marked  anelectrotonic  decrease  to  the 
normal  condition,  and  in  the  latter  from  the  normal  to  that  of  katelectrotonic 
increase.  The  absence  of  contraction  upon  the  closure  of  the  ascending 
current  is  dependent  upon  the  blocking  of  the  kathodal  stimulus  by  the 
decrease  of  the  excitability  and  conductivity  at  the  anode.  With  the  open- 
ing of  the  descending  current  the  disappearance  of  anelectrotonus  should 
also  be  followed  by  contraction,  which  would  indeed  be  the  case  if  the 
stimulus  so  generated  was  not  blocked  by  the  decrease  of  the  conductivity 
at  the  kathode  in  consequence  of  the  fall  of  a  high  state  of  katelectrotonus 
to  the  normal  condition. 

The  order  in  which  the  contractions  occur  may  be  tabulated  as  follows: 

With  Descending  Current. 
K.  C.  C.  — 

K.  C.  C.  A.  O.  C. 

K.  C.  C.  A.  O.  C.(?) 

Polar  Stimulation  of  Human  Nerves. — The  preceding  statements  as 
to  changes  in  the  excitability  caused  by  the  passage  of  a  constant  current, 
as  well  as  to  the  law  of  contraction,  are  based  entirely  on  experiments  made 
with  the  isolated  nerve  of  the  frog.  It  is  probable,  however,  that  the  same 
phenomena  would  have  been  observed  had  the  nerve  of  a  mammal  been 
used  and  its  excitability  been  maintained. 

If  the  electrodes  connected  with  the  wires  of  a  sufficiently  strong  gal- 
vanic battery  be  applied  to  the  skin  over  the  course  of  a  superficially  lying 
nerve,  e.g.,  the  brachial,  it  will  be  found  that  there  occurs  on  the  closure  of 
the  circuit  an  increase  in  the  excitability  in  the  extra-polar  anelectrotonic 
region  and  a  decrease  in  the  excitability  in  the  extra-polar  katelectrotonic 
region,  as  shown  by  stimulating  the  nerve  in  the  extra-polar  regions  with 
the  induced  current — results  which  are  in  apparent  contradiction  to  those 

*K.  C.  C,  kathodal  closing  contraction.  *  A.  O.  C,  anodal  opening  contraction. 


With  Ascending  Current. 

Weak.... 

I.  K.  C.  C.i 

— 

Medium  . 

2.  K.  C.  C. 

A.  O.  C.2 

Strong .  .  . 

3-      — 

A.  O.  C. 

112 


TEXT-BOOK  OF  PHYSIOLOGY 


obtained  with  the  isolated  nerve.  This  want  of  accordance  in  the  results 
of  the  two  classes  of  experiments  arises  from  a  failure  to  recognize  the  fact 
that  the  physiologic  anode  and  kathode  do  not  coincide  with  the  physical 
anode  and  kathode. 

It  has  been  experimentally  demonstrated  that  owing  to  the  large  amount 
of  readily  conducting  tissue  by  which  the  nerve  is  surrounded,  the  current 
density,  though  great  immediately  under  the  electrode,  quickly  decreases 
at  a  short  distance  from  it,  so  that  for  the  nerve  it  becomes  almost  nil.  The 
current,  therefore,  shortly  after  entering,  again  leaves  the  nerve  at  various 
points  which  become  physiologic  kathodes.  Stimulation  of  this  physio- 
logic kathode  with  the  induced  current  gives  rise,  therefore,  to  the  phenome- 
non of  increased  excitability  in  the  region  of  the  anode.  If,  however,  the 
galvanic  and  stimulating  current  be  combined  in  one  circuit  and  both  be 
applied  to  the  same  tract  of  nerve,  results  will  be  obtained  which  harmonize 
with  those  obtained  with  the  frog's  nerve. 

The  changes  in  the  excitability  of  a  nerve  of  a  living  man  and  the  con- 
tractions which  follow  the  closing  and  opening  of  the  constant  current 
have  been  thoroughly  studied  by  Waller  and  de  Watteville.  These  observers 
employed  a  method  similar  to  that  of  Erb,  conjoining  in  one  circuit  the 
testing  and  polarizing  currents.  By  the  graphic  method  they  recorded 
first  the  contraction  produced  by  an  induction  shock  alone;  and,  secondly 


Fig.  56. — Anode  of  Battery. 
Polar  region  of  nerve  is  anodic.  Peri- 
polar  region   of  nerve   is   cathodic. 


Fig.  57. — Cathode  of  Battery. 
Polar  region  of  nerve  is  cathodic.  Peri- 
polar region  of  nerve  is  anodic. — (Waller.) 


the  contraction  produced  by  the  same  stimulus  under  the  influence  of  the 
polarizing  current.  As  a  result  of  many  experiments,  they  also  demonstrated 
an  increase  of  the  excitability  in  the  polar  region  when  it  is  cathodic,  and 
a  decrease  when  it  is  anodic.  Following  the  suggestion  of  Helmholtz,  that  the 
current  density  quickly  decreases  with  the  distance  from  the  electrodes, 
they  recognize,  at  the  point  of  entrance  and  exit  of  the  current  from  the  nerve, 
two  regions — a  polar,  having  the  same  sign  as  the  electrode,  and  a  peripolar, 
having  the  opposite  sign  (Figs.  56  and  57).  The  peripolar  regions  also 
experience  similar  alterations  of  excitability,  though  less  in  degree,  accord- 
ing as  they  are  kathodic  or  anodic. 

As  it  is  impossible  to  confine  the  current  to  the  trunk  of  the  nerve  when 
surrounded  by  living  tissues,  as  is  easily  the  case  when  experimenting  with 
the  frog's  nerves,  it  is  incorrect  to  speak  of  either  ascending  or  descending 
currents.  Waller,^  who  has  thoroughly  studied  the  electrotonic  effects  of 
the  galvanic  current  from  this  point  of  view,  sums  up  his  conclusions  in 
the  following  words:     "We  must  apply  one  electrode  only  to  the  nerve  and 

»  "Human  Physiology,"  p.  363,  1891. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE  113 

attend  to  its  effects  alone,  completing  the  circuit  through  a  second  electrode, 
which  is  applied  according  to  convenience  to  some  other  part  of  the  body. 

"Confining  our  attention  to  the  first  electrode,  let  us  see  what  will 
happen  according  as  it  is  anode  or  kathode  of  a  galvanic  current  (Figs. 
56  and  57).  If  this  electrode  be  the  anode  of  a  current,  the  latter  enters 
the  nerve  by  a  series  of  points  and  leaves  it  by  a  second  series  of  points;  the 
former,  or  proximal  series  of  points,  collectively  constitutes  the  polar  zone 
or  region;  the  latter,  or  distal  series  of  points,  collectively  constitutes  the 
peripolar  zone  or  region.  ■  In  such  case  the  polar  region  is  the  seat  of  entrance 
of  current  into  the  nerve — i.e.,  is  anodic;  the  peripolar  region  is  the  seat 
of  exit  of  current  from  the  nerve — i.e.,  is  kathodic.  If,  on  the  contrary,  the 
electrode  under  observation  be  the  kathode  of  a  current,  the  latter  enters 
the  nerve  by  a  series  of  points  which  collectively  constitute  a  'peripolar' 
region,  and  it  leaves  the  nerve  by  a  series  of  points  which  collectively  con- 
stitute a  'polar'  region.  The  current,  at  its  entrance  into  the  body,  diffuses 
widely,  and  at  its  exit  it  concentrates;  its  'density'  is  greatest  close  to  the 
electrode,  and,  the  greater  the  distance  of  any  point  from  the  electrode,  the 
less  the  current  density  at  that  point;  hence  it  is  obvious  that  the  current 
density  is  greater  in  the  polar  than  in  the  peripolar  region.  These  conditions 
having  been  recognized,  we  may  apply  to  them  the  principles  learned  by 
study  of  frogs'  nerves  under  simpler  conditions. 

"Seeing  that,  with  either  pole  of  the  battery,  whether  anode  or  kath- 
ode, the  nerve  has  in  each  case  points  of  entrance  (constituting  a  collective 
anode)  and  points  of  exit  to  the  current  (constituting  a  collective  kathode), 
and  admitting  as  proved  that  make  excitation  is  kathodic,  break  excitation 
anodic,  we  may,  with  a  sufficiently  strong  current,  expect  to  obtain  a  con- 
traction at  make  and  at  break  with  either  anode  or  kathode  applied  to  the 
nerve;  and  we  do  so,  in  fact.  When  the  kathode  is  applied,  and  the  current 
is  made  and  broken,  we  obtain  a  kathodic  make  contraction  and  a  kathodic 
break  contraction;  when  the  anode  is  applied,  and  the  current  is  made  and 
broken,  we  obtain  an  anodic  make  contraction  and  an  anodic  break  con- 
traction. These  four  contractions  are,  however,  of  very  different  strengths; 
the  kathodic  make  contraction  is  by  far  the  strongest;  the  kathodic  break 
contraction  is  by  far  the  weakest;  the  kathodic  make  contraction  is  stronger 
than  the  anodic  make  contraction;  the  anodic  break  contraction  is  stronger 
than  the  kathodic  break  contraction.  Or,  otherwise  regarded,  if,  instead 
of  comparing  the  contractions  obtained  with  a  sufficiently  strong  current, 
we  observe  the  order  of  their  appearance  with  currents  gradually  increased 
from  weak  to  strong,  we  shall  find  that  the  kathodic  make  contraction  appears 
first,  that  the  kathodic  break  contraction  appears  last,  and  the  formula 
of  contraction  for  man  reads  as  follows: 


"Weak  current 

..   K.  C.  C. 

Medium  current 

....K.  C.  C. 

A.  C.  C. 

A.  0.  C. 

Strong  current 

...K.  C.  C. 

A.  C.  C. 

A.  0.  C. 

K.  0.  C." 

The  constant  or  the  galvanic  current  is  frequently  used  for  therapeutic 
and  diagnostic  purposes.  In  accordance  with  the  statements  above  quoted, 
one  electrode  should  be  applied  to  the  part  to  be  investigated,  the  other 
to  some  indifferent  region.  The  electrode  conveying  the  current  to  or 
from  this  part  should  be  of  a  size  sufficient  to  localize  the  current  and  to 


114  TEXT-BOOK  OF  PHYSIOLOGY 

increase  its  density.  It  was  discovered  by  Duchenne  that  there  are  certain 
points  all  over  the  body  stimulation  of  which  is  more  quickly  followed  by 
muscle  contraction  than  others.  It  was  subsequently  discovered  by  Remak 
that  these  points  coincide  with  the  entrance  of  the  nerve  into  the  muscle. 
It  is  to  these  motor  points  that  the  one  electrode  should  be  applied. 

Reactions  of  Degeneration. — In  consequence  of  the  degeneration 
and  changes  in  irritability  which  occur  in  nerves  when  separated  from  their 
centers  and  in  muscles  when  separated  from  their  related  nerves,  either 
experimentally  or  as  the  result  of  disease,  the  response  of  these  structures  to 
the  induced,  and  the  make  and  break  of  the  constant  current,  differs  from 
that  observed  in  the  physiologic  condition.  The  facts  observed  under  the 
application  of  these  two  forms  of  electricity  are  of  importance  in  the  diagnosis 
and  therapeutics  of  the  precedent  lesions.  The  principal  difference  of 
behavior  is  observed  in  the  muscles,  which  exhibit  diminished  or  abolished 
excitability  to  the  induced  current,  while  at  the  same  time  manifesting 
an  increased  excitability  to  the  constant  current;  so  much  so  is  this  the 
case  that  a  closing  contraction  is  just  as  likely  to  occur  at  the  positive  as 
at  the  negative  pole.  This  peculiarity  of  the  muscle  response  is  termed 
the  reaction  of  degeneration.  The  synchronous  diminished  excitability  of  the 
nerves  is  the  same  for  either  current.  The  term  "partial  reaction  of  degen- 
eration" is  used  when  there  is  a  normal  reaction  of  the  nerves,  with  the  de- 
generative reaction  of  the  muscles.  This  condition  is  observed  in  progressive 
muscular  atrophy. 

Reflex  Action. — Inasmuch  as  many  of  the  muscle  movements  of 
the  body,  as  well  as  the  formation  and  discharge  of  secretions  from  glands, 
variations  in  the  caliber  of  blood-vessels,  inhibition  and  acceleration  in 
the  activity  of  various  organs,  are  the  result  of  stimulations  of  the  terminal 
organs  of  afferent  nerves,  they  are  termed,  for  convenience,  reflex  actions, 
and,  as  they  take  place  for  the  most  part  through  the  spinal  cord  and  medulla 
oblongata  and  independently  of  the  brain  or  of  volitional  influences,  they  are 
also  termed  involuntary  actions.  A  reflex  action  of  skeletal  muscles,  glands, 
or  non-striated  muscles  of  blood-vessels  or  of  viscera,  therefore,  may  be  de- 
fined as  an  action  which  takes  place  independent  of  volition  and  in  response 
to  peripheral  stimulation.  As  many  of  the  processes  to  be  described  in 
succeeding  chapters  are  of  this  character,  requiring  for  their  performance 
the  cooperation  of  several  organs  and  tissues  associated  through  the  inter- 
mediation of  the  nerve  system,  it  seems  advisable  to  consider  briefly,  in 
this  connection,  the  parts  involved  in  a  reflex  action,  as  well  as  their  mode 
of  action.  As  shown  in  Fig.  13,  page  42,  the  necessary  structures  are  as 
follows: 

1.  A  receptive  surface,  skin,  mucous  membrane,  sense-organs,  etc. 

2.  An  afferent  nerve-fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises — 

4.  An  efferent  nerve,  distributed  to  a  responsive  organ,  as 

5.  Skeletal  muscle,  gland,  blood-vessel,  etc. 

Such  a  combination  of  structures  constitutes  a  reflex  mechanism  or  arc, 
the  nerve  portion  of  which,  in  the  case  of  skeletal  muscles,  is  composed  of 
but  two  neurons — an  afferent  and  an  efferent.  In  the  case  of  glands  and 
non-striated  muscles,  whether  of  blood-vessels  or  viscera,  the  efferent  neuron 
instead  of  passing  direct  to  the  responsive  organ,  arborizes  around  the 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE 


115 


nerve-cells  of  a  peripheral  sympathetic  ganglion.  The  reflex  arc  is  then 
continued  by  the  processes  of  the  ganglion  cells.  An  arc  of  this  simplicity 
would  of  necessity  subserve  but  a  simple  movement.  The  majority  of 
reflex  activities,  however,  are  extremely  complex,  and  involve  the  coopera- 
tion and  coordination  of  a  number  of  nerve  centers  situated  at  different 
levels  of  the  spinal  cord  on  the  same  and  opposite  side,  and  of  responsive 
organs  frequently  situated  at  distances  more  or  less  remote  from  one  another. 
This  implies  that  a  number  of  neurons  are 
associated  in  function.  The  transference  of 
nerve  impulses  coming  from  a  localized  area 
of  a  sentient  surface  to  emissive  cells  situated 
at  different  levels  is  accomplished  by  the  inter- 
calation of  a  third  neuron  situated  in  the  gray 
matter  which  is  in  connection,  on  the  one 
hand,  with  the  central  terminals  of  the  afferent 
neuron,  and,  on  the  other  hand,  through  its 
collateral  branches  with  the  dendrites  of  the 
efl'erent  neurons  situated  at  different  levels  of 
the  cord.     (Fig.  58.) 

For  the  excitation  of  a  reflex  action  it  is 
essential  that  the  stimulus  applied  to  the  re- 
ceptive surface  be  of  an  intensity  sufficient  to 
develop  in  the  terminals  of  the  afferent  nerve  a 
series  of  nerve  impulses,  which,  traveling  in- 
ward, will  be  distributed- to  and  received  by 
the  dendrites  of  the  emissive  or  motor  cell. 
With  the  reception  of  these  impulses  there  is 
apparently  a  disturbance  of  the  equilibrium 
of  its  molecules,  a  liberation  of  energy,  and,  in 
consequence,  a  transmission  outward  of  im- 
pulses through  the  efferent  nerve  to  muscle, 
gland,  or  blood-vessel,  separately  or  collectively,  with  the  production  of 
muscle  contraction,  a  secretion,  vascular  dilatation  or  contraction,  etc. 
The  reflex  actions  take  place,  for  the  most  part,  through  the  spinal  cord  and 
medulla  oblongata,  which,  by  virtue  of  their  contained  centers,  coordinate 
the  various  organs  and  tissues  concerned  in  the  performance  of  the  organic 
functions.  The  movements  of  mastication;  the  secretion  of  saliva;  the 
muscle,  gland,  and  vascular  phenomena  of  gastric  and  intestinal  digestion; 
the  vascular  and  respiratory  movements;  the  mechanism  of  micturition, 
etc.,  are  illustrations  of  reflex  activity. 


Fig.  58. — Diagram  Showing 
THE  Relation  of  the  Third 
Neuron  a,  to  the  Afferent 
Neuron  b,  and  to  the  Efferent 
Neurons  c,  c,  c. — {After  Kdlliker.) 


CHAPTER  IX 
FOODS 

The  functional  activity  of  every  organ  and  tissue  of  the  body  is  accom- 
panied by  a  more  or  less  active  disintegration  of  the  living  material,  the  bio- 
plasm, of  which  it  is  composed,  as  well  as  of  the  food  materials  circulating 
in  its  interstices.  The  complex  molecules  of  the  living  material  and  of  the 
non-living  food  materials  are  continually  undergoing  disruption  and  falling 
into  less  complex  and  more  stable  compounds;  these,  through  oxidative 
processes,  are  eventually  reduced  through  a  series  of  descending  chemic 
stages  to  a  small  number  of  simpler  compounds  which,  being  of  no  further 
apparent  value  to  the  organism,  are  eliminated  by  the  various  eliminating 
or  excretory  organs,  the  lungs,  skin,  kidneys,  and  liver.  Among  these 
excreted  compounds  derived  from  tissue  and  from  food  metabolism  the 
most  important  are  urea,  uric  acid,  and  carbon  dioxid.  Many  other  com- 
pounds, organic  as  well  as  inorganic,  are  also  eliminated  from  the  body  in 
the  various  excretions,  though  they  are  present  in  but  small  amounts.  Coin- 
cident with  this  metabolic  process  there  is  a  transformation  of  potential 
into  kinetic  energy,  which  manifests  itself  for  the  most  part  as  heat  and 
mechanic  motion. 

In  order  that  the  organs  and  tissues  may  continue  in  the  performance 
of  their  functions,  it  is  essential  that  they  be  supplied  with  nutritive  mate- 
rials similar  to  those  which  enter  into  their  own  composition:  viz.,  proteins, 
fat,  carbohydrates,  water,  and  inorganic  salts.  These  compounds,  though 
originally  derived  from  the  food,  are  immediately  derived  from  the  blood 
as  it  flows  through  the  capillary  blood-vessels.  The  blood  is  therefore  to  be 
regarded  as  a  reservoir  of  nutritive  material  in  a  condition  to  be  absorbed 
and  transformed  into  utilizable  and  living  material.  Inasmuch  as  the 
materials  which  are  lost  to  the  body  daily,  through  processes  of  disintegra- 
tion and  oxidation,  are  supplied  by  the  blood,  it  is  evident  that  this  fluid 
would  diminish  rapidly  in  volume,  with  a  corresponding  decline  in  func- 
tional activity,  were  it  not  replenished  by  the  introduction  into  the  body  of 
new  material  in  the  food.  This  is  brought  about  by  the  sensations  of  hunger 
and  thirst  which  periodically  arise. 

The  Sensations  of  Hunger  and  Thirst. — For  some  time  it  has  been  sup- 
posed that  hunger  is  a  general  sensation  arising  in  consequence  of  nutritional 
changes  and  referred  to  the  epigastrium.  The  recent  experiments  of  Can- 
non, Carlson  and  others  indicate  that  this  is  not  the  case,  but  that  it  arises 
in  consequence  of  a  contraction  of  the  musculature  of  the  stomach  after  its 
contents  have  been  discharged  into  the  intestine.  By  reason  of  the  dietetic 
habits  these  hunger  contractions  occur  three  or  four  times  a  day  and  when 
once  established  occur  on  an  average  of  about  once  a  minute  and  last  for 
about  half  a  minute.  The  contractions  apparently  stimulate  nerve  endings 
as  a  result  of  which  nerve  impulses  ascend  to  the  cerebrum  and  evoke  the 

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FOODS  117 

sensation  which  is  then  referred  to  the  stomach  region.  With  the  introduc- 
tion of  food  these  contractions  disappear  and  the  sensation  subsides.  The 
characteristic  peristaltic  contractions  occurring  during  digestion  do  not  give 
rise  to  the  hunger  sensation.  The  sensation  of  thirst  arises  when  there  is  a 
deficiency  of  water  in  the  body  and  is  mainly  referred  to  the  mouth  and  fauces. 
With  the  introduction  of  water  into  the  stomach  and  intestine  and  its 
absorption  into  the  blood  and  tissues  the  sensation  speedily  disappears. 
The  nerve  mechanism  by  which  these  various  results  are  brought  about  is 
unknown. 

The  foods  which  are  consumed  daily  in  response  to  sensations  of  hunger 
and  thirst  are  complex  in  composition  and  contain,  though  in  varying 
amounts,  proteins,  fats,  carbohydrates,  water,  and  inorganic  salts,  which, 
in  contradistinction  to  foods,  are  termed  food  principles  or,  as  they  main- 
. tain  the  nutrition,  nutritive  principles.  These  compounds  also  contain  the 
potential  energy  necessary  to  maintain  the  energy  equilibrium  of  the  body 
which  becomes  manifest  as  heat  and  mechanic  motion  in  the  transforma- 
tions of  the  material  used  in  the  nutritive  processes. 

It  has  been  stated  in  a  previous  chapter  that  the  animal  body  may  be 
regarded  as  a  machine  capable  of  performing  each  day  a  certain  amount  of 
work  by  the  expenditure  of  a  definite  amount  of  energy.  In  the  performance 
of  its  work,  whether  it  be  the  raising  of  weights  against  gravity,  or  the  over- 
coming of  friction,  cohesion,  or  elasticity,  the  machine  suffers  disintegration 
and  metabolizes  a  portion  of  the  *food  materials  and  loses  a  portion  of  its 
available  energy.  Unlike  other  machines,  however,  it  possesses  the  power 
within  limits  of  self-renewal,  when  supplied  with  foods  in  proper  quantity 
and  quality. 

QUANTITIES  OF  FOOD  PRINCIPLES  REQUIRED  DAILY 

In  order  that  the  body  may  continue  in  the  performance  of  its  work  and 
yet  retain  a  given  weight,  it  is  essential  that  the  loss  to  the  body  daily  shall 
be  exactly  compensated  by  the  introduction  and  assimilation  of  a  corre- 
sponding amount  of  food  principles.  If  this  condition  is  realized,  the  body 
neither  gains  nor  loses  in  weight,  but  remains  in  a  condition  of  nutritive 
equilibrium.  The  determination  of  the  extent  of  the  metabolism  is  made 
from  an  analysis  of  the  daily  excretions.  If  therefore  these  are  collected 
and  analyzed,  it  will  become  possible  to  determine  from  their  chief  constitu- 
ents the  extent  and  character  of  the  tissue  and  food  metabolized,  and  hence 
to  calculate  the  relative  quantities  of  the  different  food  materials  necessary 
to  replace  the  materials  metabolized.  Thus  of  the  constituents  the  urea  and 
other  nitrogen-holding  compounds  contained  in  the  urine  and  feces  represent 
the  proteins  metabolized;  the  carbon  dioxid  and  water  represent  the  fat  and 
carbohydrates  metabolized.  Therefore  it  becomes  possible  to  determine 
approximately  at  least,  from  the  amounts  of  the  urea  and  carbon  dioxid 
eliminated,  the  dift'erent  amounts  of  the  food  principles  required  to  restore 
the  nutritive  equilibrium  under  any  given  condition.  As  the  activity  of  the 
nutritive  changes  varies  in  accordance  with  age,  weight,  climatic  conditions, 
work  done,  etc.,  and  as  the  excreted  products  vary  in  the  same  ratio,  it  is 
obvious  that  the  required  amounts  of  food  will  vary  in  accordance  with 
these  varying  conditions,  if  nutritive  equilibrium  is  to  be  maintained. 


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TEXT-BOOK  OF  PHYSIOLOGY 


A  Metabolism  Experiment.- — An  experiment  designed  to  determine  the 
character  and  the  extent  of  the  chemic  transformations  which  tissue  and  food 
materials  undergo  in  their  transit  through  the  body  is  termed  a  metabolism 
experiment.  It  consists  primarily  in  placing  an  animal  under  conditions  that 
permit  of  the  collection  of  all  the  excretions  for  purposes  of  analysis,  the 
object  of  which  is  to  deduce  from  the  amounts  of  urea  and  other  nitrogen- 
holding  compounds,  and  of  carbon  dioxid  excreted,  or  better,  the  total  nitro- 
gen and  carbon  they  contain  (i)  the  amount  of  tissue  materials  metabolized, 
during  a  fasting  period  of  longer  or  shorter  duration  and  hence  from  the 
nitrogen  and  carbon  to  calculate  approximately,  at  least,  the  amounts  of  the 
food  principles  and  their  ratio  one  to  another  that  must  be  returned  to  the 
body  if  nutritive  equilibrium  is  to  be  restored;  or  (2)  to  deduce  from  the 
total  nitrogen  and  carbon  excreted,  the  amounts  of  protein,  fat,  and  carbo- 
hydrates metabolized,  that  were  contained  in  the  customary  foods  during  an 


Fig.  60. — A  Respiration  Calorimeter. 


experimental  period.  If  the  outcome  from  the  body  balances  the  income 
the  weight  of  the  animal  remains  stationary  from  which  it  is  assumed  that 
the  amounts  consumed  constitute  a  normal  diet.  Experiments  having  these 
objects  in  view  are  made  possible  by  enclosing  the  animal  or  man  in  a  suit- 
able chamber  (Fig.  60)  in  which  there  is  some  provision  for  collecting  the 
urine  and  feces,  and  to  which,  in  addition,  there  is  attached  an  apparatus 
for  the  absorption  of  water  and  carbon  dioxid,  both  of  which  are  caused  to 
pass  from  the  chamber  through  the  absorption  apparatus  under  the  action 
of  an  aspirating  pump.  Simultaneously  fresh  air  is  introduced  into  the 
chamber  after  passing  through  another  absorption  apparatus  by  which  it 
is  freed  from  water  and  carbon  dioxid.  As  the  apparatus  is  traversed  con- 
stantly by  a  column  of  air  of  normal  composition  and  the  waste  products 
removed  as  rapidly  as  discharged,  the  experiment  can  be  continued  from 
six  to  twenty  hours  or  more  without  detriment  to  the  subject  of  the  ex- 
periment. Previous  to  the  beginning  of  the  experiment  the  animal  and 
the  absorption  apparatus  are  carefully  weighed.  At  the  end  of  the  experi- 
ment the  urine  and  feces  are  collected,  weighed  and  analyzed  for  their  chief 


FOODS 


119 


constituent  and  their  amounts  determined.  The  absorption  apparatus  is 
again  weighed.  The  increase  in  weight  represents  the  amounts  of  water 
and  cairbon  dioxid  eliminated  and  absorbed.  The  animal  is  again  weighed 
and  the  loss  in  weight  noted. 

If  at  the  same  time  it  is  desired  to  collect  the  heat  dissipated  from  the 
body  it  is  necessary  to  surround  the  animal  chamber  with  a  water  jacket,  the 
rise  in  temperature  of  which,  expressed  in  calories,  indicates  the  amount  of 
heat  dissipated  and  collected.  Since  an  apparatus  of  this  character  de- 
termines the  extent  of  the  respiratory  exchange  as  well  as  the  amount  of 
heat  dissipated,  it  is  termed  a  Respiration  Calorimeter. 

For  long-continued  experiments  on  animals  of  large  size  and  on  man, 
one  of  the  larger  and  more  accurately  constructed  forms  of  apparatus,  such 
as  Benedict's  (see  Chapter  on  Animal  Heat)  must  be  employed. 

In  an  experiment  to  determine  the  extent  of  the  metabolism  during  a 
fasting  period  there  was  collected  on  the  first  and  second  days  12.17  ^^d 
12.84  grams  of  nitrogen  and  188.5  ^^d  179.4  grams  of  carbon  respectively. 
From  these  amounts  it  was  calculated  that  76.1  and  80.3  grams  of  protein 
and  206.1  and  19 1.6  grams  of  fat  respectively  were  metabolized.  From  these 
figures  it  is  evident  that  at  least  equal  amounts  of  protein  and  fat  must  be 
consumed.  As  a  matter  of  fact,  however,  these  amounts  would  be  insufficient 
to  maintain  the  energy  equilibrium  of  the  body.  It  has  been  estimated  that 
at  least  from  10  to  14  per  cent,  more  food  must  be  added. 

In  an  experiment  made  by  Vierordt  on  the  customary  diet  the  following 
results,  somewhat  rearranged  were  obtained.  On  the  right,  under  the  term 
outcome,  are  arranged  the  amounts  of  the  substances  eliminated;  on  the  left, 
under  the  term  income,  the  amounts  of  the  food  and  tissue  principles  which 
were  calculated  to  have  been  metabolized.  If  the  body  is  to  retain  its  usual 
weight  it  is  evident  that  equivalent  amounts  of  these  food  principles  must  be 
introduced  into  the  body. 

COMPARISON  OF  THE  INCOME  AND  OUTCOME 


Income                             Grams 

Outcome                               Grams 

Protein 120 

Fat 90 

Carbohydrates 330 

Salts 32 

Water 2818 

Water 3 1 14 .  00 

'  Salts i         26.00 

Urinar}'  Solids    ]  Urea 33  -So 

Extractive 6.00 

Feces 44 .  00 

Carbon  dioxid 910  00 

Oxygen 744 

j     4134 

4134.00 

In  the  foregoing  experiment  the  total  nitrogen  contained  in  the  urine 
which  was  15.8  grams  is  expressed  in  the  table  in  terms  of  urea.  The  chemic 
composition  of  urea,  COH4N2  taken  in  connection  with  the  amount  stated, 
indicates  that  it  is  the  chief  end-product  of  protein  metabolism;  and  as  i 
gram  of  nitrogen  corresponds  to  2.14  grams  of  urea  and  either  to  6.25  grams 
of  protein  it  is  apparent  that  the  urinary  nitrogen  corresponds  to  98.80  grams 
of  protein  metabolized.  The  feces,  however,  contained  3.3  grams  of  nitrogen 
corresponding  to  20.62  grams  of  protein,  making  a  total  approximately  of 
120  grams  of  protein.     From  this  it  is  apparent  that  an  equal  amount  of 


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TEXT-BOOK  OF  PHYSIOLOGY 


protein  would  have  to  be  introduced  into  the  body  in  order  to  restore  protein 
or  nitrogen  equilibrium. 

The  carbon  dioxid  excreted  was  910  grams,  equal  to  248.8  grams  of 
carbon,^  The  chemic  composition  of  carbon  dioxid,  together  with  the 
amount  stated,  indicates  that  it  is  the  chief  end-product  of  the  metabolism 
of  fat  or  carbohydrate,  or  both,  and  hence  from  the  amount  eliminated  it  is 
possible  to  determine  in  terms  of  fat  or  carbohydrate  the  amounts  that 
would  have  to  be  introduced  into  the  body  to  restore  carbon  or  fat  and  carbo- 
hydrate equilibrium.  As  i  gram  of  C  corresponds  to  3.66  grams  of  CO2 
and  either  to  1.3 1  grams  of  fat  it  is  apparent  that  the  carbon,  184  grams, 
or  carbon  dioxid,  676  grams,  eliminated  corresponds  to  242  grams  of  fat 
metabolized;  again  as  i  gram  of  C  or  3.66  grams  of  CO2  corresponds  to 
2.25  grams  of  starch,  it  is  apparent  that  the  carbon  or  carbon  dioxid  elimi- 
nated corresponds  to  416  grams  of  starch.  From  these  figures  it  is  evident 
that  an  equal  amount  of  fac  or  starch  would  have  to  be  introduced  into  the 
body  to  restore  the  carbon  or  the  fat  or  carbohydrate  equilibrium,  an  amount 
which  in  either  case  would  be  larger  than  could  be  readily  disposed  of  by  the 
digestive  apparatus  and  the  assimilative  capacities.  Since  the  carbon  dioxid 
comes  from  both  fat  and  starch  or  sugar,  it  is  difficult  to  determine  the  per- 
centage that  comes  from  the  metabolism  of  the  fat  and  the  percentage  that 
comes  from  the  cairbohydrate.  From  observation  of  the  dietetic  habits  in 
different  countries  and  of  the  results  of  metabolism  experiments,  it  has  been 
deemed  advisable  to  apportion  the  fat  to  the  carbohydrates  in  the  ratio  of  i 
to  3.5  to  I  to  7.  In  the  foregoing  table  Vierordt  regarded  90  grams  of  fat 
and  330  grams  of  starch  sufficient  to  restore  the  losses  sustained. 

The  carbon  dioxid  excreted  also  indicates  the  amount  of  oxygen  utilized 
in  the  oxidation  of  the  carbon  and  the  surplus  hydrogen  of  the  fats  and  hence 
the  amount  of  oxygen  that  must  have  been  absorbed  from  the  air  in  the  lungs. 
The  amount  of  oxygen  stated  in  the  table  is,  however,  mainly  an  inference 
and  determined  by  deducting  the  loss  in  weight  of  the  subject  of  the  experi- 
ment from  the  combined  weight  of  the  CO2  and  the  water  eliminated.  The 
salts  are  balanced  to  a  greater  or  less  degree,  day  by  day,  by  the  salts  intro- 
duced in  the  foods.  The  excess  of  water  discharged,  296  grams,  beyond 
that  taken  into  the  body  arises  from  the  union  of  oxygen  with  the  surplus 
hydrogen  of  the  fats. 

The  following  balance  table,  as  given  by  Ranke,  shows  the  relation 
of  the  nitrogen  to  the  carbon  in  the  average  mixed  diet  and  in  the  excretions 
of  a  man  weighing  70  kilograms,  in  a  condition  of  nutritive  equilibrium : 


Income 

Grams 

N. 

C. 

Outcome 

Grams 

N. 

c. 

Protein 

100 
100 
250 

15-5 

53 -o 
79.0 
93  0 

Urea 

Uric  acid 

Feces 

3i-S\ 
0-5) 

14.4 
I.I 

Fat 

6.16 

Carbohydrates  . . . 

10.84 

208.00 

CO, 

iS-S 

225.0 

iS-S 

225.00 

^  It  must  be  remembered  however  that  of  the  CO2  eHminated  a  portion  arises  from  the  oxidation 
of  the  carbon-holding  residue  of  the  protein,  an  amount  in  this  experiment  of  approximately  64 
grams,  which  would  yield  234  grams  of  CO2.  The  remainder  of  the  CO2,  676  grams,  arose  from 
the  oxidation  of  the  fat  and  carbohydrate  ingested. 


FOODS  121 

From  the  foregoing  considerations  it  is  essential  that  the  constituents  of 
a  normal  diet  should  be  present  in  certain  amounts  and  should  bear  a  certain 
ratio  one  to  another.  Many  attempts  have  been  made  to  construct  a  suit- 
able diet  for  a  man  weighing  70  kilos  while  doing  light  or  moderate  work. 
The  following  are  accepted  estimates: 

Ranke.  Voit.  Moleschott.  Atwater.  Hultgren. 

Grams.  Grams.  Grams.  Grams.  Grams. 

Protein 100             118  130                    125  134 

Fat 100               56  84                    125  79 

Starch 250             500  550                    400  522 

From  the  foregoing  estimates  it  is  assumed  that  for  the  maintenance 
of  nitrogen  equilibrium  an  amount  of  protein,  100  grams  or  more,  or  about 
1.5  to  1.7  grams  for  each  kilogram  of  body  weight  must  be  consumed  each 
day;  and  that  if  the  amount  falls  below  this  minimum  the  tissues  will  be 
called  upon  to  yield  up  a  portion  of  their  protein  and  thereby  undergo 
deterioration  with  a  consequent  loss  of  their  efficiency. 

It  has,  however,  been  established  that  nitrogen  equilibrium  can  be  main- 
tained without  apparent  detriment  to  the  body  or  its  acti\dties,  for  a  variable 
period  of  time,  extending  over  months  and  years,  on  a  diet  much  poorer  in 
its  protein  content  than  in  any  of  the  foregoing  diets.  Chittenden  has  demon- 
strated by  a  long  series  of  carefully  conducted  experiments  on  human 
beings,  that  the  protein  intake  can  be  reduced  to  60  grams  or  0.85  grams 
for  each  kilogram  of  body  weight  without  any  impairment  in  the  working 
capacity  of  the  tissues  or  of  the  individual.  Even  this  amount  is  in  actual 
excess  of  the  tissue  needs  as  the  protein  metabolism  according  to  Chittenden's 
experiments  probably  does  not  amount  to  more  than  0.75  gram  for  each 
kilogram  of  body  weight. 

The  daily  observations  of  some  twenty-four  individuals  who  were  placed 
on  a  diet  in  which  the  protein  content  was  low  for  a  period  varying  from 
five  to  eighteen  months  revealed  the  fact  that  they  not  only  maintained  the 
nitrogen  equilibrium,  but  that  they  gained  in  weight  and  strength  as  shown 
by  their  capacity  to  meet  successfully  various  endurance  tests.  These 
experiments  would  therefore  indicate  that  the  consumption  of  100  or  more 
grams  of  protein  each  day  is  unnecessary  and  that  any  amount  beyond  that 
actually  needed  for  tissue  repair,  approximately  60  grams  or  even  less  for 
an  individual  weighing  70  kilograms  is  undesirable,  for,  as  will  be  stated  in 
subsequent  pages,  all  protein  when  metabolized  yields  a  series  of  nitrogen- 
holding  bodies  which  must  be  subsequently  eliminated  by  the  kidneys  and 
perhaps  the  intestinal  glands  as  well.  This  necessitates  on  the  part  of  the 
kidneys,  the  chief  eliminating  organs,  the  expenditure  of  a  certain  amount 
of  energy.  The  wear  and  tear  of  these  organs  will  be  proportional  to  the 
amount  of  urea  and  other  materials  which  they  are  called  upon  to  excrete 
and  if  the  kidneys  fail  to  excrete  them,  some  may  become  deposited  in  the 
tissues  and  give  rise  to  certain  nutritional  disorders.  Any  unnecessary  con- 
sumption of  proteins  should  therefore  be  avoided. 

It  must  be  remembered,  however,  as  protein  yields  energy  when  me- 
tabolized, that  the  heat  value  of  the  excluded  protein  must  be  balanced  by 
an  increase  in  the  amount  of  either  starch  or  fat  or  both,  an  increase  that 
will  yield  on  oxidation  an  equivalent  amount  of  heat. 

An  advantage  to  the  body  which  a  high  protein  diet  (100  to  125  grams) 


122  TEXT-BOOK  OF  PHYSIOLOGY 

has  over  a  low  jjrotein  diet  (60  to  80  grams)  lies  in  the  fact  that  the  former 
has  a  greater  stimulating  effect  on  the  general  metabolic  process  than  the 
latter  as  shown  by  an  increase  in  the  heat  i)roduced  and  dissiqated  from  the 
body.  This  action  of  protein  is  to  be  attributed,  however,  to  the  action  of 
some  of  the  amino-acids  set  free  during  digestion  and  subsepuently  found 
circulating  in  the  tissue  fluids.  In  this  situation  they  stimulate,  by  reason 
of  their  chcmic  features,  the  activity  of  the  tissue  cells,  whereby  their  power 
to  metabolize  carbohydrates  or  fats  is  considerably  increased.  The  experi- 
ments of  Lusk  have  shown  that  when  amino-acids,  e.g.,  glycocoll,  alanin,  etc., 
are  administered  singly  or  combined  to  an  animal,  there  is  an  increase  in 
heat  production  far  beyond  that  which  would  result  from  oxidation  of  the 
carbonaceous  radical  (glucose)  which  arises  in  the  metabolism  of  the  amino- 
acids  themselves;  from  this  fact  the  inference  is  drawn  that  the  increase  in 
metabolism,  expressed  in  terms  of  heat,  which  follows  the  ingestion  of  meat 
is  to  be  attributed  to  the  mass  action  of  the  absorbed  amino-acids  on  the 
tissue  cells.  For  this  reason  proteins  are  said  to  have  a  specific  dynamic 
action  on  metabolism  presumably  beneficial. 

CLASSIFICATION  OF  FOOD  PRINCIPLES 

Though  the  food  principles  are  grouped  as  proteins,  fats,  carbohydrates, 
etc.,  the  members  of  each  group  differ  somewhat  in  chemic  composition, 
digestibility,  and  nutritive  value.     These  groups  are  as  follows: 

1.  Proteins 

Principle.  Where  found. 

Myosin Flesh  of  animals. 

Albumin,  vitellin White  of  egg,  yolk  of  egg. 

Caseinogen Milk . 

Serum  albumin,  fibrin Blood  contained  in  meat. 

Gliadin  and  glutinin Grain  of  wheat  and  some  other  cereals. 

Vegetable  albumin Soft-growing  vegetables. 

Legumin Peas,  beans,  lentils,  etc. 

2.  Fats 

Animal  fats In  adipose  tissue  of  animals. 

Vegetable  oils In  seeds,  grains,   nuts,  fruits,  and  other 

vegetable  tissues. 

3.  Carbohydrates 

Dextrose  or  grape-sugar 1  j    ,    . 

Levulose  or  fruit-sugar J 

Lactose  or  milk-sugar Milk. 

Saccharose  or  cane-sugar Sugar-cane,  beet  roots. 

Maltose Malt  and  malted  foods. 

Starch  /  Cereals,  tuberous  roots,  and  leguminous 

\      plants. 
Glycogen Liver,  muscles. 


In  nearly  all  animal  and  vegetable  foods. 


4.  Inorganic 

Water 

Sodium  and  potassium  chlorid 

Sodium,  potassium,  and  calcium  phosphates 

and  carbonates 

Iron 

5.  Vegetable  Acids 

Citric,  tartaric,  acetic,  malic In  fniits  and  vegetables. 

6.  Accessory  Foods 

Coffee,  Tea,  Cocoa,  Condiments,  Spices,  Alcohol. 


FOODS  123 

THE  DISPOSITION  OF  THE  FOOD  PRINCIPLES 

The  Proteins. — The  protein  principles  of  the  food  while  in  the  ali- 
mentary canal  undergo  a  series  of  disintegrative  changes  by  virtue  of  which 
they  are  reduced  in  part  to  simple  nitrogen-holding  bodies,  monoamino- 
and  diamino-acids  and  ammonia,  and  in  part  to  their  immediate  antecedents 
peptids  and  polypeptids,  after  which  they  are  absorbed  from  the  intestinal 
contents.  It  has  for  some  time  been  assumed  that  in  the  act  of  absorption, 
the  amino-acids  and  their  immediate  antecedents,  are  combined  and  trans- 
formed into  the  form  of  protein  characteristic  of  blood  and  tissue  fluids, 
viz.,  plasma  albumin.  This  compound  was  looked  upon  as  the  immediate 
source  of  the  protein  necessary  for  tissue  growth  and  repair.  The  manner  in 
which  the  plasma  albumin  was  transformed  into  the  protein  character- 
istic of  each  tissue  has  never  been  satisfactorily  determined.  Recently 
evidence  has  been  adduced  which  makes  it  probable  that  the  amino-acids 
undergo  no  change  in  the  act  of  absorption  but  enter  the  blood  as  such  and 
are  carried  direct  to  the  tissues.  On  reaching  any  given  tissue  the  cells  absorb 
and  synthesize,  perhaps  under  the  influence  of  an  enzyme,  such  amino-acids 
as  they  may  need  for  their  growth  and  repair.  The  surplus  amino-acids, 
i.e.,  those  not  utilized  in  the  synthesis  of  tissue  protein,  may  be  synthesized 
to  plasma-albumin,  or  stored  unchanged  or  be  deaminized,  i.e.,  separated 
perhaps  by  the  action  of  an  enzyme,  into  the  amino-group,  NH2,  and  some 
carbonaceous  radical.  The  amino-group  is  then  combined  with  hydrogen, 
and  subsequently  with  carbon  dioxid,  to  form  ammonium  carbonate  which 
is  then  transformed  into  urea,  a  transformation  that  takes  place  to  some 
extent  in  the  muscles  (Folin);  the  carbonaceous  remainder  may  be  trans- 
formed into  fat  or  sugar,  which  is  subsequently  oxidized  thus  contributing 
to  the  production  of  heat.  In  the  process  of  tissue  metabolism  the  protein 
molecule  undergoes  disintegration  and  gives  rise  to  amino-acids,  the  different 
elements  of  which  may  undergo  changes  similar  to  those  just  stated.  The 
ammonia  absorbed  from  the  intestine  is  changed  to  ammonium  carbonate 
carried  direct  to  the  liver  and  transformed  into  urea. 

The  Fats. — ^The  fat  principles  while  in  the  alimentary  canal  also  undergo 
a  series  of  changes  whereby  they  are  reduced  by  enzymic  action  to  soap  and 
glycerin,  under  which  forms  they  are  absorbed.  During  the  act  of  absorp- 
tion the  soap  and  glycerin  are  synthesized  to  human  fat.  The  fine  particles 
thus  formed  in  the  intestinal  wall  are  carried  by  the  lymph  vessels  to  the 
thoracic  duct,  and  thence  into  the  blood  stream,  from  which  they  rapidly 
disappear.  Though  it  is  possible  that  a  portion  of  the  fat  enters  directly 
into  the  formation  of  the  living  material  in  general,  it  is  generally  believed 
that  it  is  at  once  oxidized  and  reduced  to  carbon  dioxid  and  water  with  the 
liberation  of  energy.  The  natural  supposition  that  a  portion  of  the  synthe- 
sized fat  is  directly  stored  up  in  the  cells  of  the  areolar  connective  tissue,  thus 
gi\dng  rise  to  adipose  tissue,  has  been  a  subject  of  much  controversy,  though 
modern  experimentation  renders  this  very  probable.  The  body-fat,  under 
physiologic  conditions,  is  mainly,  however,  a  product  of  the  transfomation 
of  carbo-hydrates. 

The  question  has  again  been  raised  as  to  whether  the  fine  granules  of 
emulsified  fat  can  be  absorbed  without  undergoing  this  preliminary  cleavage 
into  fat  acids  and  glycerin.  The  evidence  adduced  in  support  of  this  view 
is  conflicting  and  for  its  settlement  further  experimental  work  is  necessary. 


124  TEXT-BOOK  OF  PHYSIOLOGY 

The  Carbohydrates. — Carbohydrate  principles  are  reduced  during  di- 
gestion to  simple  forms  of  sugar,  chiefly  dextrose  and  levulose.  Under 
these  forms  they  are  absorbed  into  the  blood.  These  compounds  are  then 
carried  to  the  liver  and  to  the  muscles  where  they  are  dehydrated  and  stored 
under  the  form  of  starch,  termed  animal  starch  or  glycogen.  Subsequently 
glycogen  is  transformed  by  hydration  to  sugar,  after  which  it  is  oxidized  to 
carbon  dioxid  and  water.  The  intermediate  stages  through  which  sugar 
passes  before  it  is  reduced  to  carbon  dioxid  and  water  are  only  imperfectly 
known.  Though  a  large  part  of  the  carbohydrate  material  is  at  once  oxi- 
dized, it  is  now  well  established  that  another  portion  contributes  to  the  forma- 
tion of,  if  it  is  not  directly  converted  into,  fat.  As  the  carbohydrates  form  a 
large  portion  of  the  food,  they  contribute  materially  to  the  liberation  of  energy. 

The  Inorganic  Principles. — ^The  inorganic  principles,  though  appar- 
ently not  playing  as  active  a  part  in  the  metabolism  of  the  body  as  the  organic, 
are  nevertheless  essential  to  its  physiologic  activity. 

Water  is  promptly  absorbed  after  ingestion  and  becomes  a  part  of  the 
circulating  fluids — blood  and  lymph.  In  the  digestive  apparatus  it  favors 
the  occurrence  of  those  chemic  changes,  in  the  food  necessary  for  their 
absorption,  it  promotes  absorption  of  the  food,  holds  various  constituents 
of  the  blood  and  other  fluids  in  solution,  hastens  the  general  metabolism 
of  the  body,  holds  in  solution  various  products  of  metabolic  activity,  and, 
leaving  the  body  through  the  excretory  organs,  promotes  their  elimination. 

Sodium  chlorid  is  absorbed  into  the  blood  and,  unless  taken  in  excess, 
is  utilized  in  replacing  that  which  is  lost  to  the  organism  daily.  The  exact 
r61e  which  sodium  chlorid  plays  in  the  nutritive  process  is  unknown;  but, 
as  it  is  present  as  a  necessary  constituent  in  all  the  fluids  and  solids  of  the 
body,  and  as  it  is  instinctively  employed  as  a  condiment,  it  may  be  assumed 
to  have  a  more  or  less  important  function. 

When  taken  as  a  condiment,  it  imparts  sapidity  to  the  food  and  excites 
the  flow  of  the  digestive  fluids;  it  ultimately  furnishes  the  chlorin  for  the 
hydrochloric  acid  of  the  gastric  juice.  Judging  from  the  impairment  of  the 
nutrition  as  observed  in  animals  after  deprivation  of  salt  for  a  long  period  of 
time,  it  favorably  influences  the  growth  and  functional  activity  of  all  tissues. 

It  is  well  known  that  herbivorous  animals,  races  of  men  as  well  as 
individuals  who  live  largely  on  vegetable  foods,  require  a  larger  addi- 
tional amount  of  sodium  chlorid  than  carnivorous  animals  or  human 
beings  who  live  largely  on  animal  foods,  even  though  the  two  classes  of 
foods  contain  relatively  the  same  amounts.  The  explanation  is  that  the 
vegetable  foods  contain  potassium  salts  which,  meeting  in  the  blood  with 
sodium  chlorid,  undergo  decomposition  into  potassium  chlorid  and  sodium 
carbonate  or  phosphate,  all  of  which,  when  in  excess,  are  at  once  eliminated 
by  the  kidneys.  The  blood,  therefore,  becomes  poorer  in  sodium  chlorid,  one 
of  its  necessary  constituents. 

Potassium  phosphate  and  carbonate  are  also  essential  to  the  normal 
composition  of  the  solids  and  fluids.  They  impart  a  certain  degree  of 
alkalinity  to  the  blood  and  lymph,  one  of  the  conditions  necessary  to  the 
life  and  activity  of  the  tissue-cells  bathed  by  them.  When  administered 
in  small  doses,  they  increase  the  force  of  the  heart,  raise  the  arterial  pressure 
and  increase  the  activity  of  the  circulation. 

Calcium  phosphate  and   carbonate  are  partly   utilized   in   maintaining 


FOODS  12  5 

the  solidity  of  the  bones  and  teeth,  replacing  the  amount  metabolized 
daily.  Inasmuch  as  the  metabolism  of  these  two  tissues  is  slight,  there  is 
not  much  need  in  the  adult  for  lime  as  an  article  of  food.  In  young  animals 
lime  is  essential  to  the  solidification  and  development  of  bone.  When  de- 
prived of  it,  the  skeleton  undergoes  a  defective  development  similar  to  the 
pathologic  condition  known  as  rickets.  Lime  is  present  in  milk  to  the  extent 
of  0.15  per  cent.,  as  well  as  in  eggs  and  peas  in  relatively  large  quantities. 

Iron  is  contained  in  both  animal  and  vegetable  foods,  not,  however, 
in  the  form  of  inorganic  iron,  nor  in  the  form  of  an  organic  salt,  but  as  a 
compound  with  nuclein,  thus  forming  an  integral  part  of  the  proteid  molecule. 
After  absorption  the  iron  is  utilized  in  the  formation  of  the  coloring-matter 
of  the  blood-corpuscles — hemoglobin.  The  organic  compounds  of  iron 
and  the  nucleins  have  been  termed  hematogens.  The  amount  of  iron  ingested 
has  been  estimated  at  10  to  90  milligrams  daily,  the  larger  part  of  which  is 
eliminated  in  the  feces.  The  relatively  small  part  eliminated  by  the  kidneys 
and  liver  is  usually  taken  as  the  amount  metabolized,  though  it  is  probable 
that  this  is  not  wholly  true,  as  there  is  evidence  that  iron  can  be  retained  in 
the  body  and  utilized  again  in  the  formation  of  new  hemoglobin.  Contrary 
to  what  might  be  expected,  milk  contains  but  a  very  small  quantity  of  iron 
not  more  than  3  or  4  milligrams  in  1000  grams  (human  milk) — an  amount 
insufficient  for  the  development  of  the  necessary  hemoglobin.  This  is 
compensated  for,  however,  by  the  accumulation  of  iron  in  the  liver  during 
intrauterine  life.  According  to  Bunge,  the  liver  of  a  newly  born  rabbit 
contains  as  much  as  18.2  milligrams  per  100  grams  of  body-weight,  while  at 
the  end  of  twenty-four  days  it  contains  only  3.2  milligrams  per  100  grams  of 
body-weight. 

Vegetable  acids  increase  the  secretions  of  the  alimentary  canal,  and 
are  apt,  in  large  amounts,  to  produce  flatulence  and  diarrhea.  After 
entering  into  combination  with  bases  to  form  salts,  they  stimulate  the 
action  of  the  kidneys  and  promote  a  greater  elimination  of  all  the  urinary 
constituents.  In  some  unknown  way  they  influence  nutrition;  when  deprived 
of  these  acids,  the  individual  becomes  scorbutic. 

The  Accessory  Foods. — ^The  accessory  foods — cofi'ee,  tea,  and  cocoa — 
when  taken  in  moderation  have  a  stimulating  influence  on  the  nervous  sys- 
tem, as  shown  by  the  removal  of  both  mental  and  physical  fatigue,  by  an 
increased  capacity  for  sustained  mental  work,  and  by  the  persistent  wake- 
fulness among  those  unaccustomed  to  their  use.  Coffee  more  especially 
increases  the  frequency  and  force  of  the  heart-beat,  raises  the  arterial  pres- 
sure, and  hastens  the  general  blood-flow.  It  has  no  influence  either  in  the 
way  of  increasing  or  decreasing  protein  metabolism. 

Tea  frequently  acts  as  an  astringent  on  the  alimentary  canal  on  account 
of  the  tannin  which  passes  into  the  water  when  the  infusion  is  made.  In- 
asmuch as  tannin  also  coagulates  peptones,  the  excessive  use  of  tea  as  a 
beverage  is  apt  to  derange  the  digestive  organs  and  the  general  process  of 
digestion. 

Cocoa  is  more  nutritive  than  either  coffee  or  tea,  on  account  of  the 
large  amount  of  fat  and  protein  it  contains.     It  is,  however,  less  stimulating. 

The  active  principles  in  coffee,  tea,  and  cocoa,  and  to  which  their  effects 
are  to  be  attributed,  are  caffein,  thein,  and  theohromin  respectively.  These 
alkaloids  are  chemically  closely  related  one  to  the  other  and  to  the  compound 


126  TEXT-BOOK  OF  PHYSIOLOGY 

xanthin.  They  are  present  in  the  coffee  seeds,  the  tea  leaves,  and  the  cocoa 
bean  to  the  extent  of  1.7  per  cent.,  1.4  per  cent.,  and  1.6  per  cent,  respectively. 
When  prepared  as  a  beverage,  however,  there  is  three  times  as  much  caffein 
in  coffee  as  thein  in  tea. 

Alcohol  when  taken  in  small  quantities  stimulates  the  digestive  glands 
to  increased  activity  and  thus  promotes  digestive  power.  Its  absorption  into 
the  blood  is  followed  by  increased  action  of  the  heart,  dilatation  of  the  cutane- 
ous blood-vessels,  a  sensation  of  warmth,  and  an  excitation  of  the  brain. 
In  large  quantities  it  acts  as  a  paralyzant,  depressing  more  especially  the 
vaso-constrictor  nerve-centers  and  certain  areas  of  the  brain,  as  shown  by 
an  impairment  in  the  power  of  sustained  attention,  clearness  of  judgment, 
and  muscle  coordination. 

Alcohol  is  undoubtedly  oxidized  in  the  body,  as  only  about  2  per  cent, 
can  be  obtained  from  the  urine  and  expired  air.  It  thus  contributes  to  the 
store  of  the  body-energy.  Whether  for  this  reason  it  can  be  regarded 
as  a  food — that  is,  whether  it  can  be  substituted  in  part  at  least  for  fat  or 
carbohydrate  material  without  impairing  the  protein  metabolism — is  at 
present  a  subject  of  experimentation  and  discussion.  According  to  some 
investigators,  alcohol  does  not  retard  protein  metabolism,  for  when  it  is 
introduced  into  the  body  in  amounts  equivalent  to  the  carbohydrates  with- 
drawn from  the  food  there  is  at  once  a  rise  in  the  amount  of  nitrogen  excreted. 
Hence  it  cannot  be  regarded  as  a  food.  According  to  other  investigators, 
alcohol  retards  or  protects  protein  metaboHsm  just  as  effectually  as  an 
equivalent  amount  of  starch  or  sugar.  Many  more  experiments  are  required 
to  decide  this  question.  When  taken  habitually  in  large  quantities,  alcohol 
deranges  the  activities  of  the  digestive  organs,  lowers  the  body  temperature, 
impairs  muscle  power,  lessens  the  resistance  to  depressing  external  con- 
ditions, diminishes  the  capacity  for  sustained  mental  work,  and  leads  to  the 
development  of  structural  changes  in  the  connective  tissues  of  the  brain, 
spinal  cord,  and  other  organs.  In  infectious  diseases  and  in  cases  of  depres- 
sion of  the  vital  powers  it  is  most  useful  as  a  restorative  agent. 

THE  ENERGY  OR  HEAT  VALUE  OF  THE  FOOD  PRINCIPLES 

The  food  consumed  not  only  restores  the  material  metabolized  and 
discharged  from  the  body,  but  also  the  energy  which  has  been  expended  as 
heat  and  mechanic  motion.  The  food  principles  are  products  of  the  con- 
structive processes  taking  place  in  the  vegetable  world  during  the  period  of 
growth  and  activity.  At  the  time  of  their  formation  there  is  an  absorption 
and  storing  of  the  sun's  energy  which  then  exists  in  a  potential  condition. 
During  the  metabolism  of  the  animal  body  these  compounds  are  reduced 
through  oxidation  to  relatively  simple  bodies,  such  as  carbon  dioxid,  water, 
urea,  etc.,  with  the  liberation  of  their  contained  energy.  All  of  the  energy 
of  the  body,  whatever  its  manifestations  may  be,  can  be  traced  to  chemic 
changes  going  on  in  the  tissues,  and  more  particularly  to  those  changes 
involved  in  the  oxidation  of  the  food  principles. 

It  becomes,  therefore,  a  matter  of  interest  to  determine  the  heat  loss 
from  the  body  in  twenty-four  hours  for  the  purpose  of  subsequently  deter- 
mining if  the  energy  contained  in  the  foods,  expressed  in  terms  of  heat,  is 
present  in  amounts  sufficient  to  compensate  for  the  loss.  The  total  quan- 
tity of  heat  liberated  in  the  body  and  dissipated  from  it  in  twenty-four  hours 


FOODS  127 

is  determined  by  placing  the  subject  in  a  respiration  chamber  provided  with 
appliances  containing  water,  by  means  of  which  the  heat  can  be  absorbed 
and  measured.  (See  chapter  on  Animal  Heat.)  The  unit  of  heat  measure- 
ment is  the  Calorie,  which  is  defined  as  the  amount  of  heat  necessary  to  raise 
the  temperature  of  one  kilogram  of  water  i°C.  If  therefore  the  volume 
of  the  water  employed  in  the  experiment  expressed  in  kilograms  be  multi- 
plied by  the  number  of  degrees  of  temperature  through  which  it  has  been 
raised,  the  number  of  Calories  will  be  known.  The  average  number  of 
Calories  dissipated  by  a  human  being  in  various  ways,  e.g.,  radiation,  vapori- 
zation of  water  from  lungs  and  skin,  warming  of  foods,  air,  etc.,  has  been 
estimated  at  from  2500  to  3000  each  day.  The  question  then  to  be  deter- 
mined is,  whether  any  given  diet  scale  contains  this  amount  of  energy  and 
whether  it  can  be  liberated  as  heat  on  oxidation  in  the  body. 

The  amount  of  heat  or  energy  which  any  given  food  principle  will  yield 
can  be  determined  by  burning  a  definite  amount  {e.g.,  i  gram)  to  carbon 
dioxid  and  water  and  ascertaining  the  extent  to  which  the  heat  thus  liberated 
will  raise  the  temperature  of  a  given  amount  of  water  {e.g.,  i  kilogram). 
The  amount  of  heat  may  be  expressed  in  gram  or  kilogram  degrees  or 
calories,  a  gram  calorie  or  kilogram  Calorie  being  the  amount  of  heat  required 
to  raise  the  temperature  of  a  gram  or  a  kilogram  (1000  grams)  of  water  1°  C. 
The  apparatus  employed  for  this  purpose  is  termed  a  calorimeter,  and  con- 
sists essentially  of  a  closed  chamber  in  which  the  oxidation  takes  place, 
surrounded  by  a  water  jacket,  the  rise  in  temperature  of  the  water  indicating 
the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calorimeters 
and  different  food  principles  of  the  same  group  vary,  though  within  certain 
limits:  e.g.,  i  gram  of  casein  yields  5.867  kilogram  Calories;  i  gram  of  lean 
beef,  5.656  Calories;  i  gram  of  fat  yields  9.353,  9.423,  9.686  Calories;  i  gram 
of  carbohydrate,  4.182,  4,479,  etc..  Calories.  These  numbers  represent 
the  physical  calorimetric  heat  values  of  these  food  principles. 

In  the  human  body  as  determined  by  calorimetric  methods  the  oxidation 
of  the  food  principles  yields  practically  the  same  amount  of  heat  they  yield 
when  oxidized  outside  the  body,  with  the  exception  of  the  protein,  which 
is  oxidized  only  to  the  stage  of  urea.  As  this  compound  is  capable  of 
further  reduction  in  the  calorimeter  to  carbon  dioxid  and  water  with  the 
liberation  of  heat,  the  quantity  of  heat  it  contains  must  therefore  be  deducted 
from  the  calorimetric  heat  value  of  the  protein.  According  to  Rubner, 
I  gram  of  urea  will  yield  2.523  kilogram  Calories.  As  the  urea  which  results 
from  the  oxidation  of  i  gram  of  protein  is  about  ^  of  a  gram,  the  amount  of 
heat  to  be  deducted  from  the  heat  value  of  the  protein  is  ^  of  2.523,  or 
0.841  Calories.  It  has  also  been  shown  that  some  of  the  ingested  protein 
escapes  in  the  feces,  the  heat  value  of  which  must  also  be  determined  and 
deducted.  This  having  been  done,  the  physiologic  heat  value  becomes 
4.124  Calories. 

The  following  estimates  give  approximately  the  number  of  kilogram 
Calories  produced  when  the  food  is  burned  to  carbon  dioxid,  water,  and 
urea  in  the  body : 

I  gram  of  protein  pelds 4 .  124  Calories. 

1  gram  of  fat  yields 9-353  Calories. 

I  gram  of  carbohydrate  yields : 4. 116  Calories. 


128  TEXT-BOOK  OF  PHYSIOLOGY 

The  total  number  of  kilogram  Calories  or  kilogram  degrees  of  heat 
yielded  by  any  of  the  previously  given  diet  scales  can  be  readily  determined 
by  multiplying  the  quantities  of  food  principles  consumed  by  the  above- 
mentioned  factors.  The  diet  scale  of  Vierordt,  for  example,  yields  the 
following: 

I20  grams  of  protein  yields 494.88  Calories. 

90  grams  of  fat  pelds 841 .77  Calories. 

330  grams  of  starch  yields 1358 . 28  Calories. 

2694.93  Calories 

The  total  Calories  obtained  from  other  diet  scales  would  be  as  follows: 
Ranke,  2335;  Voit,  3387;  Moleschott,  2984;  Atwater,  3331;  Hultgren,  3436. 

Starvation. — The  relation  of  the  different  food  principles  to  the  general 
nutritive  process  becomes  more  apparent  from  an  examination  of  the 
excretions  from  the  body  during  the  process  of  starvation  combined  with  an 
examination  of  the  organs  and  tissues  after  death.  If  an  animal  be  deprived 
entirely  of  food^  a  decline  in  body-weight  at  once  sets  in,  which  continues 
until  about  40  per  cent,  of  the  weight  has  been  lost,  when  death  generally 
ensues.  This  results  from  the  fact  that  the  active  tissue  cells  consume, 
for  the  purpose  of  maintaining  the  normal  temperature  of  the  body,  not 
only  their  own  reserve  food  material,  but  that  of  the  less  active  or  storage 
tissues  as  well;  and,  in  consequence,  there  is  a  progressive  diminution  in 
weight. 

The  phenomena  which  characterize  this  non-physiologic  condition  are 
as  follows:  hunger,  intense  thirst,  gastric  and  intestinal  uneasiness  and 
pain,  diminished  pulse-rate  and  respiration,  muscular  weakness  and  emacia- 
tion, a  lessening  in  the  amount  of  urine  and  its  constituents,  diminished 
expiration  of  carbon  dioxid,  an  exhalation  of  a  fetid  odor  from  the  body, 
vertigo,  stupor,  delirium,  at  times  convulsions,  a  sudden  fall  in  body  tem- 
perature, and  finally  death.  The  duration  of  life  after  complete  depriva- 
tion of  food  varies  from  eight  to  thirteen  days  or  more,  though  this  period 
can  be  prolonged  if  the  animal  be  supplied  with  water,  this  being  more 
essential  under  the  circumstances  than  the  organic  materials  which  can  be 
supplied  by  the  organism  itself.  The  duration  of  the  starvation  period 
will  naturally  vary  in  accordance  with  the  previous  condition  of  the  animal 
and  the  amount  of  reserve  food,  especially  fat,  the  body  contains. 

The  extent  and  the  character  of  the  metabolism  that  the  body  undergoes 
in  starvation  can  be  determined  from  an  examination  of  the  excretions. 
Thus  the  excretion  of  nitrogen  declines  very  rapidly  during  the  first  few  days 
— a  fact  which  has  been  attributed  to  the  consumption  of  the  surplus  protein 
food.  The  amount  of  nitrogen  eliminated  and  hence  the  protein  metabo- 
lized depend,  mainly  on  the  amount  of  protein  consumed  daily  before  the 
starvation  period  was  inaugurated.  At  the  end  of  four  or  five  days  when 
the  surplus  protein  has  been  metabolized  and  the  tissues  begin  to  metabo- 
lize their  own  protein,  the  excretion  remains  fairly  constant,  from  about  13 
to  10  grams  daily,  until  toward  the  close  of  the  starvation  period,  when  the 
amount  eliminated  falls  very  rapidly.  In  several  instances  of  prolonged 
fasting  by  human  beings  the  nitrogen  elimination  fell  as  low  as  4  to  3.5 
grams  daily,  indicating  a  protein  metabolism  only  of  from  25  to  18.75  grams. 
As  proteins  contain  about  16  per  cent,  of  nitrogen,  i  part  of  nitrogen  equals 


FOODS 


129 


6.25  parts  of  protein.  Hence,  for  every  i  gram  of  nitrogen  or  2.14  grams 
urea  excreted,  it  may  be  assumed  that  6.25  grams  of  protein  or,  according 
to  Voit,  30  grams  of  flesh  have  been  metabolized.  The  daily  excretion  of 
urea,  after  the  first  five  days  therefore,  indicates  fairly  accurately  the  extent 
of  the  metabolism  of  the  tissue  protein. 

It  has  been  observed  also  that  there  is  a  steady  diminution  in  the  excre- 
tion of  carbon  dioxid.  As  fat  contains  about  76  per  cent,  of  carbon,  i  part 
of  carbon  equals  3.66  parts  of  carbon  dioxid  or  1.3 1  parts  of  fat.  Hence, 
for  every  i  gram  of  carbon  or  3.66  grams  carbon  dioxid  excreted  it  may  be 
assumed  that  1.3 1  grams  of  fat  have  been  metabolized.  The  daily  excre- 
tion of  carbon,  therefore,  indicates  the  extent  of  fat  metabolism.  The  car- 
bohydrates are  liere  left  out  of  consideration,  as  they  constitute  only  about 
I  per  cent,  of  the  body-weight.  It  must  be  borne  in  mind,  however,  that 
in  the  metabolism  of  protein  a  certain  quantity  of  fat  or  sugar  is  produced, 
which  also  undergoes  oxidation.  The  amount  of  the  carbon  or  the  CO2  that 
the  protein  would  give  rise  to,  as  previously  determined,  must  therefore  be 
subtracted  |rom  that  eliminated  by  the  lungs,  etc.,  in  order  to  determine  the 
amount  of  body-fat  metabolized. 

In  a  fasting  experiment,  voluntarily  undergone  by  a  human  being,  and 
lasting  ten  days,  the  metabolism  of  fat  as  shown  by  the  CO2  excreted 
amiounted  to  136.7  grams  during  the  first  four  days  and  to  132  grams  on 
the  tenth  day.  In  another  similar  experiment  lasting  five  days,  the  fat 
metabolized  amounted  to  206  grams  on  the  first  day  and  181  grams  on  the 
fifth  day.  As  the  starvation  period  lengthens  the  amount  of  fat  metabo- 
lized gradually  declines. 

The  following  table  shows  the  excretion  of  nitrogen  and  carbon  and 
the  calculated  amounts  of  protein  and  fat  metabolized  from  an  experiment 
made  by  Ranke  on  himself  during  a  fast  of  twenty  hours,  beginning  twenty- 
four  hours  after  the  last  meal :    . 


Disintegration  of  Tissue 
(Calculated) 

Expenditure 
(Determined) 

Nitrogen 

Carbon 

Urea,  17  gm 1 

Nitrogen 

Carbon 

7.8            26.1; 

7.2 
0.0 

Protein,  "Co  gm 

Uric  acid,  0.2  gm j 

Carbon  dioxid 

3-4 

Fat,  199.6  gm 

0.0 

157-5 

180  6 

7.8 

184.0 

7.2 

184.0 

Coincidently  with  these  losses  to  the  body  there  is  also  a  gradual  loss  of 
inorganic  salts,  and  toward  the  termination  of  the  period  a  sudden  fall  in 
temperature  of  several  degrees  centigrade,  in  consequence  of  the  final  con- 
sumption of  all  available  foods,  when  death  ensues.  The  immediate  cause 
of  death  is  not,  however,  apparent. 

Post-mortem  Appearances. — It  has  been  experimentally  determined  that 
animals  die  when  the  body-weight  has  declined  to  about  40  per  cent.  Post- 
mortem examination  shows  that  the  loss  of  material,  though  very  generally 
distributed  throughout  the  body,  is  greatest  in  organs  and  tissues  least 
essential  to  life. 


130 


TEXT-BOOK  OF  PHYSIOLOGY 


The  results  of  an  analysis  of  the  organs  and  tissues  of  a  cat  after  a  thirteen- 
day  period  of  starvation,  during  which  the  animal  lost  1017  grams  in  weight, 
are  given  in  the  following  table,  based  on  data  furnished  by  Voit: 

It  will  be  observed  from  this  table  that  the  adipose  tissue  suffers  the 
greatest  loss,  the  entire  amount  disappearing  with  the  exception  of  a  small 
portion  in  the  posterior  part  of  the  orbital  cavity  and  around  the  kidneys. 
The  muscles,  though  losing  only  31  per  cent,  of  their  weight,  yet  furnish 
429  grams  of  presumably  protein  material,  for  nutritive  purposes.  The 
heart  and  nervous  system  experience  but  slight  loss. 


Organ 


Actual  Loss 
of  Tissue 


Adipose  tissue . . 

Spleen 

Liver 

Testes 

Muscles 

Blood 

Kidneys 

Skin  and  hair . . 

Lungs 

Intestines 

Pancreas 

Bones 

Heart 

Nervous  system 


Grams 
267 
6 

49 

I 

429 

37 

7 
89 

3 
21 

I 

55 
o 


Mixed  Diet. — The  chemic  composition  of  the  tissues,  taken  in  con- 
nection with  their  metabolism  during  starvation,  implies  that  no  one  article 
of  food  is  sufficient  for  tissue  repair  and  heat  production;  but  that  all  classes 
of  foods — in  other  words,  a  mixed  diet — are  essential  to  the  maintenance  of 
a  normal  nutrition.  Experimental  investigation  has  also  conclusively 
established  this  fact.  Moreover,  the  amounts  of  nitrogen  and  carbon  elimi- 
nated daily,  and  the  ratio  existing  between  them,  indicate  the  amounts 
of  protein,  fat,  and  carbohydrate  which  are  required  to  cover  the  loss. 

Metabolism  on  a  Purely  Protein  Diet. — Notwithstanding  the  chemic 
composition  of  the  proteins  and  the  possibility  of  their  giving  rise  to  either 
fat  or  a  carbohydrate  during  their  metabolism  it  has  been  found  extremely 
difficult  to  maintain  the  normal  nutrition  for  any  length  of  time  on  a  pure 
protein  or  fat-free  flesh  diet.  This,  however,  has  been  accomplished  with 
dogs.  It  was  found,  however,  that,  in  order  to  maintain  the  nitrogen  equi- 
librium, it  was  necessary  to  increase  the  proteins  from  two  to  three  times 
the  usual  amount.  Thus,  a  dog  weighing  30  to  35  kilograms  required  from 
1500  to  1800  grams  of  flesh  daily  in  order  to  get  the  requisite  amount  of 
carbon  to  prevent  consumption  of  its  own  adipose  tissue.  Under  similar  cir- 
cumstances, a  human  being  weighing  70  kilograms  would  require  more  than 
2000  grams  of  lean  beef — an  amount  which,  from  the  nature  of  the  digestive 
apparatus,  it  would  be  practically  impossible  to  digest  and  assimilate  for  any 
length  of  time.  Even  the  slight  habitual  excess  beyond  the  amount  normally 
required  is  imperfectly  assimilated  and  gives  rise  to  the  production  of 
nitrogen-holding  compounds  which,  on  account  of  the  difficulty  with  which 


FOODS 


131 


they  are  eliminated  by  the  kidneys,  accumulate  within  the  body  and  develop 
the  gouty  diathesis,  with  all  its  protean  manifestations. 

Metabolism  on  a  Fat  and  Carbohydrate  Diet. — As  nitrogen  is  an 
indispensable  constituent  of  the  tissues,  it  is  evident  that  neither  fat  nor 
carbohydrates  can  maintain  nutritive  equilibrium  except  for  very  short 
periods.  On  such  a  diet  the  tissues  consume  their  own  proteins,  as  shown  by 
the  continuous  excretion  of  urea,  though  the  amount  is  less  than  during 
starv^ation.  An  excess  of  fat  retards  the  metabolism  of  proteins.  The  same 
holds  true  for  the  carbohydrates. 

Thus,  in  any  well-arranged  dietary  there  should  be  a  combination 
of  proteins,  fats,  and  carbohydrates  in  amounts  sufhcient  to  maintain  nutritive 
equilibrium;  in  other  words,  to  repair  the  loss  of  tissue  and  to  furnish  the 
requisite  amount  of  energy  in  accordance  with  work  done,  as  well  as  with 
climatic  and  seasonal  variations. 

COMPOSITION  OF  FOODS 

The  food  principles  essential  to  the  maintenance  of  the  nutrition  of 
the  body  are  contained  in  varying  proportions  in  compound  substances 
termed  foods;  e.g.,  meat,  milk,  wheat,  potatoes,  etc.  Their  nutritive  value 
depends  partly  on  the  amounts  of  their  contained  food  principles  and 
partly  on  their  digestibility.  The  dietary  of  civilized  man  embraces  foods 
derived  from  both  the  animal  and  vegetable  worlds. 

The  following  tables  show  the  percentage  composition  of  the  edible 
portions  of  foods  as  well  as  the  amount  of  heat  liberated  per  pound  when 
oxidized  in  the  body,  according  to  Atwater  and  Bryant. 

Composition  of  Animal  Foods. — The  following  table  shows  the  average 
percentage  composition  of  various  kinds  of  meats,  cow's  milk,  and  eggs: 


Kind  of  Food 
Materials 


Water 


Unavail- 
able 
Nutrients 


Proteins       Fat 


Carbo- 
hydrates 


Ash 


Fuel  Value 

Per  Lb. 
453.6  Grams 


Per 

cent. 
Beef: 

Loin,  lean 67  .0 

Loin,  fat 54.7 

Round,  lean 70 

Round,  fat 60 

Veal: 

Cutlets  (round) ....  70.7 

Liver 73  o 

Mutton: 

Leg 62 

Loin 50 

Pork: 

Loin  chops 52 

Ham 53.9 

Fowl:  63.7 

Turkey 55.5 

Mackerel '  73.4 

Halibut 75.4 

Milk :..  87.0 

Eggs,  boiled 73 


Per 

cent. 

1 .2 
1.9 
1 .0 
1.6 

1-3 
0.9 

1-7 
2.4 


i.o 
1.9 

1-3 

1 .1 

0-5 

1 .2 


Per 

cent. 


19, 

17 
20, 


19 
9 


17.9 
15-5 

16. 1 
14.8 
18.7 
20.5 
18. 1 
18.0 
3-2 
12.8 


Per 

cent. 


26 

2 

7 
18 

5 

S 

7 
5 

3 
0 

17. 1 
31-4 


Per 

cent. 


5-0 


Per 

cent. 

1.0 
0.9 
I.I 
1.0 

0.8 


0.8 
0.6 

0.8 
0.6 
0.8 
0.8 
0.9 
0.8 

0-5 
0.6 


Calories 


900 

1470 

735 

"75 

710 
410 

1095 
1660 

1555 
1480 
1040 

853 
650 

570 
310 

755 


133  TEXT-BOOK  OF  PHYSIOLOGY  ; 

Meats. — It  will  be  observed  from  these  analyses  that  the  meats  contain 
from  i8  to  20  per  cent,  of  protein  material.  The  proteins  are  two  in  number 
and  are  known  as  paramyosinogen  or  myosin  and  myosinogen  or  myogen, 
both  of  which  are  in  a  semi-fluid  condition.  The  latter  is  four  or  five  times 
as  abundant  as  the  former.  After  death  these  substances  undergo  coagu- 
lation and  give  rise  to  two  solid  substances  known  as  myosin-fibrin  and 
myogen-fibrin.  After  being  subjected  to  the  cooking  process,  meats  contain 
the  albuminoid  body  gelatin,  a  product  of  the  transformation  of  the  proteins 
of  the  connective  tissue. 

The  percentage  of  fat  contained  within  the  meat  substance  is  very 
small  except  in  mutton  and  pork,  where  it  rises  to  5.4  per  cent,  and  5.8 
per  cent,  respectively.  The  fat-globules  in  these  meats  are  packed  closely 
between  the  muscle-fibers,  and  prevent  the  easy  entrance  of  the  digestive 
fluids,  and  hence  they  are  more  difficult  of  digestion  than  beef.  The  large 
percentage  of  fat  represented  in  the  foregoing  table  is  due  to  the  presence 
in  the  food,  as  eaten,  of  adipose  tissue  which  is  an  addition,  not  a  constituent 
of  meat. 

The  carbohydrates  vary  from  0.5  to  i  per  cent.,  and  are  represented 
by  glycogen.  The  principal  inorganic  salts  are  potassium  phosphate  and 
sodium  chlorid. 

The  composition  of  meat  will  vary  in  composition  in  nutritive  value  and 
in  energy-liberating  capacity  in  accordance  with  the  region  of  the  body 
from  which  the  specimen  is  taken. 

Cooking,  when  properly  done,  not  only  makes  the  meat  more  palatable 
and  appetizing  from  the  development  of  agreeable  flavors,  but  converts 
the  connective  tissue,  which,  in  old  animals  especially,  is  tough  and  resisting, 
into  gelatin,  thus  rendering  it  more  easy  of  mastication  and  digestion.  At 
the  same  time  parasitic  organisms,  such  as  the  embryonic  forms  of  tenia  or 
tapeworm,  and  Trichina  spiralis,  as  well  as  bacterial  growths,  which  fre- 
quently infest  the  bodies  of  animals,  are  destroyed  and  made  harmless. 

Milk  is  the  natural  food  of  the  young  of  all  mammals,  and  is  usually 
regarded  as  typical  on  account  of  the  ratio  existing  among  its  nutritive 
principles.  The  analysis  given  above  is  that  of  cow's  milk.  Examined 
microscopically,  milk  is  seen  to  consist  of  a  clear  fluid,  the  milk  plasma, 
holding  in  suspension  an  enormous  number  of  small,  highly  refractive  oil- 
globules,  which  measure  on  the  average  about  ^  q^qq  of  an  inch  in  diameter. 
Each  globule  is  supposed  by  some  observers  to  be  surrounded  by  a  thin 
albuminous  envelope,  which  enables  it  to  maintain  the  discrete  form. 
Others  deny  the  existence  of  such  a  membrane.  The  chief  protein  con- 
stituent of  milk,  caseinogen,  is  held  in  solution  by  the  presence  of  phosphate 
of  lime.  On  the  addition  of  acetic  acid  or  sodium  chlorid  up  to  the  point  of 
saturation  the  caseinogen  is  precipitated  as  such  and  may  be  collected  by 
appropriate  chemic  methods.  When  taken  into  the  stomach,  caseinogen  is . 
coagulated;  that  is,  it  is  changed  into  casein  or  tyrein.  This  change  is 
brought  about  by  the  presence  in  the  gastric  juice  of  a  special  ferment 
termed  rennin  or  pexin. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures.  It 
is  a  combination  of  olein,  palmitin,  and  stearin,  with  a  small  quantity 
of  butyrin  and  caproin.  When  milk  is  allowed  to  stand  for  some  time, 
the  fat-globules  rise  to  the  surface  and  form  a  thick  layer  known  as  cream. 


FOODS 


133 


When  subjected  to  the  churning  process,  fat-globules  run  together  and 
form  a  coherent  mass — butter. 

Lactose  is  the  particular  form  of  sugar  found  in  milk.  In  the  presence 
of  Bacillus  acidi  lactici,  the  lactose  is  decomposed  into  lactic  acid  and 
carbon  dioxid,  the  former  of  which  not  only  imparts  a  sour  taste  to  the  milk, 
but  causes  a  precipitation  of  the  caseinogen.  The  chief  salt  found  in  milk 
is  phosphate  of  lime,  and  this  is  the  chief  source  of  this  agent  in  the  formation 
of  bones.     Sodium  and  potassium  chlorids  are  also  present. 

Eggs  are  also  to  be  regarded  as  complete  natural  foods,  inasmuch 
as  they  contain  all  the  necessary  food  principles.  The  analysis  given  in 
the  foregoing  table  represents  the  composition  of  the  entire  egg.  The 
white  of  the  egg  contains  12  per  cent,  of  protein  and  2  per  cent,  of  fat. 
The  yolk,  however,  contains  15  per  cent,  of  protein  and  30  per  cent,  of  fat. 

Composition  of  Cereal  Foods. — The  average  composition  of  the 
principal  cereals  is  shown  in  the  following  table: 


Kind  of  Food 
Material 


Water 


Unavail- 
able 
Nutrients 


Proteins       Fat 


Carbo- 
hydrates 


Ash 


Fuel  Value 

Per  Pound 

453.6  Grams 


Per  Per 

cent.  cent. 

Entire  wheat  flour.. . .       11 .4  4.5 

Rye  flour 12.9  I         3.6 

Rice ' 12.3  3.7 

Barley,  pearled n-S  _  4-° 

Buckwheat  flour 13-6  3.5 

Com  meal 12.5  4 

Oat  meal 7 .8  5 

Whole  wheat  bread.. .      38.4  3 

White  bread 35-3  3 

Graham  crackers 5 .4  4 


Calories 

1645 
1610 
1610 
1630 
1600 
1625 

1795 
1125 

"95 
1900 


That  the  cereals  are  most  important  and  useful  articles  of  diet  is  evident 
from  their  composition,  consisting,  as  they  do,  of  proteins  and  carbohydrates 
in  large  proportion.  Owing  to  the  cellulose  or  woody  fiber  which  envelops 
and  penetrates  the  grain,  they  are  somewhat  difficult  of  digestion.  A  section 
of  a  grain  of  wheat  shows  the  external  cellulose  envelope,  the  husk,  beneath 
which  is  a  layer  of  large  cells  containing  the  chief  protein — the  gluten. 
The  interior  of  the  grain  consists  of  small  cavities,  the  walls  of  which  are 
formed  of  cellulose  and  which  contain  the  granules  of  starch,  fat,  small 
quantities  of  protein,  and  inorganic  salts.  All  other  cereals  have  a  similar 
structure. 

In  the  preparation  of  white  flour  from  wheat  it  is  customary  to  remove 
the  husk,  a  process  which  involves  the  removal  also  of  a  portion,  if  not  all, 
of  the  gluten  cells,  so  that  such  flour  contains  less  nitrogenized  material 
than  the  original  grain.  It  is  possible,  however,  in  the  milling  of  wheat, 
to  remove  only  the  husk  and  retain  the  gluten  in  the  flour,  as  in  the  prepa- 
ration of  whole  wheat  flour. 

Bread  is  an  artificially  prepared  food  made  either  of  wheat  or  rye. 
Owing  to  the  fact  that  the  proteins  of  the  other  cereals  do  not  possess  the 
same  adhesive  properties  when  kneaded  with  water,  they  cannot  be  used 


134 


TEXT-BOOK  OF  PHYSIOLOGY 


for  bread-making  purposes.  In  the  making  of  bread,  the  flour  is  kneaded 
with  water  until  a  glutinous  mass — dough — is  formed.  During  this  process, 
salt,  sugar,  and  yeast  are  added.  It  is  then  kept  in  a  temperature  of  about 
ioo°F.  In  the  presence  of  heat  and  moisture  the  natural  ferment  of  the 
flour — diastase — converts  a  portion  of  the  starch  into  sugar,  which  in  turn 
is  split  up  into  carbon  dioxid  and  alcohol  by  the  yeast  plant.  The  sugar  that 
is  added  undergoes  a  similar  change  and  hastens  the  process.  The  bubbles 
of  carbon  dioxid,  becoming  entangled  in  the  dough,  cause  it  to  swell  or  rise 
and  subsequently  give  the  porous  or  spongy  character  to  the  bread.  When 
baked  at  a  temperature  of  4oo°F.,  the  alcohol  is  largely  driven  off;  yeast  cells 
and  other  organisms  are  destroyed;  the  starch,  particularly  that  on  the 
surface,  is  dextrinized.  The  principal  salts  contained  in  wheat  flour  are 
potassium  and  magnesium  phosphate. 

Composition    of    Vegetable    Foods. — The    average    composition    of 
some  of  the  principal  vegetables  is  shown  in  the  following  table: 


Kind  of  Food 
Material 


Water 


Unavail- 
able 
Nutrients 


Fuel  Value 

Per  Pound 

453.6  Grams 


I  Per 
cent. 

Beans,  lima,  dried. ...  10.4 

Beans,  lima,  green. ...  68 . 5 

Beans,  white,  dried. .  .  12  .6 

Beans,  string,  cooked'  95 .3 

Peas,  dried 9.5 

Peas,  green,  cooked'. .  73 .8 

Potatoes,    boiled,  75-5 
cooked. ' 

Potatoes,  sweet 51.9 

Beets,  cooked' 88.6 

Cabbage [  91.5 

Tomatoes 94-3 

Turnips 1  89 .6 

Egg-plant 92 .9 

Spinach,  fresh 92  .3 

Asparagus,  cooked.  . .  91 .6 


Calories 

1565 
525 

1530 
90 

1508 
490 
415 

88s 
170 
140 
100 

17s 
120 
100 
195 


The  vegetable  foods,  as  a  class,  vary  considerably  in  nutritive  value  and 
digestibility,  the  latter  depending  on  the  amount  of  cellulose  they  contain. 
A  section  of  a  vegetable  shows  not  only  the  presence  of  an  external  cellulose 
envelope,  but  also  an  inner  framework  which  penetrates  its  substance  in  all 
directions.  The  nutritive  principles  are  contained  in  small  cavities,  the 
walls  of  which  are  formed  by  the  framework.  Nearly  all  vegetables  require 
cooking  before  being  eaten.  When  subjected  to  heat  and  moisture,  not 
only  is  the  texture  of  the  vegetable  softened  and  disintegrated,  but  the 
starch  grains  are  hydrated  and  partially  prepared  for  conversion  into  dextrin 
and  sugar.  At  the  same  time  various  savory  substances  are  set  free,  which 
make  the  food  more  palatable. 

Beans  and  peas  contain  large  quantities  of  a  protein,  legumin,  and 
starch,  and  hence  are  especially  valuable  as  nutritive  foods.     The  presence 

'  With  butter  etc.,  added. 


FOODS 


135 


of  the  cellulose  envelope,  especially  in  ripe  beans  and  peas,  combined  with 
rather  a  dense  texture,  renders  them  somewhat  difficult  of  digestion.  Pota- 
toes, though  largely  employed  as  food,  are  extremely  poor  in  protein,  2  per 
cent.,  and  carbohydrates,  20  per  cent.  When  sufficiently  cooked  they  are 
easily  digested,  owing  to  the  small  amount  of  cellulose  they  contain. 

Green  vegetables,  e.g.,  lettuce,  spinach,  tomatoes,  asparagus,  onions, 
etc.,  though  containing  food  principles  in  small  amounts,  are,  nevertheless, 
valuable  adjuncts  to  the  dietary,  for  the  reason  that  they  contain  inorganic 
as  well  as  organic  salts,  which  appear  to  be  necessary  to  the  maintenance 
of  the  normal  nutrition.  The  want  of  green  vegetables  has  been  supposed 
to  be  the  cause  of  scurvy. 

Ripe  fruits,  grapes,  cherries,  apples,  pears,  peaches,  strawberries,  lemons, 
oranges,  etc.,  though  consumed  largely,  possess  but  little  nutritive  value. 
.They  consist  largely  of  water,  75  to  85  per  cent.,  proteins  a  trace,  sugar 
from  5  to  13  per  cent.,  organic  acids  (citric,  malic,  tartaric),  pectose,  and 
various  inorganic  salts. 

Relative  Value  of  Animal  and  Vegetable  Foods. — Though  both 
animal  and  vegetable  foods  contain  the  different  classes  of  food  principles, 
it  is  not  a  matter  of  entire  indifference  as  to  which  are  consumed.  It  has 
been  found  by  experiment  that  animal  proteins  are  more  easily  and  com- 
pletely digested  and  absorbed  than  vegetable  proteins;  that  cellulose  is  not 
only  highly  indigestible,  but  by  its  presence  in  large  quantities  retards  the 
digestive  process  and  impairs  the  activity  of  the  entire  digestive  mechanism, 
though  in  moderate  quantity  it  undoubtedly  aids  digestion  indirectly  by 
mechanically  promoting  peristalsis.  The  following  table  shows  the  relative 
digestibility  and  availibility  of  the  two  classes  of  foods: 


Weight  of  Food 

Vegetable 

Animal 

Digested         Undigested 

Digested         Undigested 

Of  100  parts  of  protein 

Of  100  parts  of  fat  .. 

84.00      '         16.00 
90.00               10.00 
97.00                 3.00 

1 

97  3 

■  95                       5 

98  2    ' 

Of  100  parts  of  carbohydrate 

CHAPTER  X 
DIGESTION 

Digestion  is  a  process  partly  physical,  partly  chemic,  by  which  the 
nutritive  principles  of  the  foods  are  prepared  for  absorption.  The  reason 
for  these  changes  lies  in  the  fact  that  the  foods  as  consumed  are  hetero- 
geneous compounds  consisting  of  organic  and  inorganic  nutritive  principles 
associated  with  a  varying  amount  of  non-nutritive  material,  such  as  the 
dense  parts  of  the  connective  tissue  of  the  animal  foods  and  the  woody 
fiber  or  cellulose  of  the  vegetable  foods,  from  which  the  nutritive  prin- 
ciples must  be  freed  before  they  can  be  utilized;  and  in  the  further  fact,  that 
even  when  consumed  in  the  free  state,  the  food  principles  are  seldom  in  a 
condition  to  be  absorbed  into  the  blood  and  subsequently  assimilated  by  the 
tissues.  When  foods  are  consumed  in  their  natural  state  or  after  they  have 
been  subjected  to  the  cooking  process,  they  are  subjected  while  in  the  food 
canal  to  the  solvent  action  of  various  fluids  by  which  they  are  disintegrated 
and  reduced  to  the  liquid  condition.  The  nutritive  principles  freed  from 
their  combinations  are  changed  in  chemic  composition  and  transformed  into 
substances  capable  of  absorption.  To  all  the  physical  and  chemic  changes 
which  foods  undergo  in  the  food  canal  the  term  digestion  has  been  given. 

The  {Digestive  Apparatus. — The  digestive  apparatus  comprises  the 
entire  alimentary  or  food  canal  and  its  various  appendages:  the  lips,  the 
teeth,  the  tongue,  the  salivary  glands,  the  gastric  and  intestinal  glands,  the 
pancreas,  and  the  liver  (Fig.  60). 

The  alimentary  canal  is  a  musculo-membranous  tube  about  eleven  meters 
in  length,  and  extends  from  the  mouth  to  the  anus.  It  may  be  subdivided 
into  several  distinct  portions,  as  mouth,  pharynx,  esophagus,  stomach, 
small  and  large  intestines.  The  mouth  is  provided  (i)  with  teeth,  by  which 
the  food  is  divided,  (2)  with  the  tongue,  and  (3)  with  glands,  by  which  a 
solvent  fluid,  the  saliva,  is  secreted.  The  glands,  though  situated  for  the 
most  part  outside  the  mouth,  are  connected  with  it  by  means  of  ducts. 
Posteriorly  the  mouth  opens  into  the  pharynx  or  throat,  a  somewhat  py- 
ramidal-shaped structure  about  twelve  centimeters  in  length,  which  in  turn  is 
followed  by  the  esophagus  or  gullet,  a  tube. about  twenty-two  centimeters  in 
length.  As  the  esophagus  passes  through  the  diaphragm  it  expands  into  the 
stomach,  a  curved  pyriform  organ,  which  serves  as  a  reservoir  for  the  recep- 
tion and  retention  of  the  food  for  a  varying  length  of  time.  The  small  in- 
testine is  that  portion  of  the  alimentary  canal  extending  from  the  end  of  the 
stomach  to  the  beginning  of  the  large  intestine  in  the  right  iliac  fossa;  owing 
to  its  length,  about  eight  meters,  it  presents  a  very  convoluted  appearance  in 
the  abdominal  cavity.  Embedded  in  its  walls  are  the  intestinal  glands 
which  open  on  its  surface  and  secrete  the  intestinal  fluid.  In  the  upper  por- 
tion of  the  small  intestine,  within  twelve  centimeters  of  the  stomach,  there 
is  an  orifice,  the  outlet  of  a  small  pouch,  the  Ampulla  of  Vater,  into  which 

136 


DIGESTION 


137 


open  the  terminations  of  the  ducts  of  the  Hver  and  pancreas,  organs  which 
secrete  the  bile  and  pancreatic  juice  respectively.  The  large  intestine  is 
from  one  and  three  quarters  to  two  meters  in  length  and  extends  from  the 
end  of  the  small  intestine  to  the  anus.  Its  walls  contain  a  large  number  of 
glands. 

The  general  process  of  digestion  is  largely  accomplished  by  the  chemic 
action  of  the  digestive  fluids  secreted  by  glands,  some  of  which  are  imbedded 


{Nose.  ^<Ji>^'i^~ I  '5'9'"'«'"y  <5't"»tf 


Lerge. 
tnfesri'na 


Vermiform  /Ippemfix  ■ 

Fig.  61.— Diagram  of  the  Alimentary  Caxal.— {Modified  Jroin  Landois.) 


in  the  walls  of  the  canal  while  others  are  situated  outside  of  it  and  com- 
municate with  it  only  by  means  of  ducts.  These  fluids  are  the  saliva,  the 
gastric,  intestinal,  and  pancreatic  juices,  and  the  bile.  Though  taking 
place  throughout  a  large  portion  of  the  food  canal,  the  process  may  be  sub- 
divided into  several  stages:  viz.,  prehension,  mouth  digestion,  deglutition, 
gastric  digestion,  and  intestinal  digestion. 

As  a  result  of  the  action  of  these  fluids  the  nutritive  principles  are  pre- 
pared for  absorption  into  the  blood;  the  non-nutritive  principles,  along 


138  TEXT- BOOK  OF  PHYSIOLOGY 

with  certain  waste  products,  pass  into  the  large  intestine  to  be  finally  ex- 
truded from  the  body. 

FERMENTS;  ENZYMES 

In  a  preceding  chapter  it  was  stated  that  under  favorable  conditions 
the  carbohydrates,  fats,  and  proteins  undergo  reduction  to  simpler  com- 
pounds as  a  result  of  the  action  of  agents  such  as  the  yeast  plant  and  various 
forms  of  bacteria.  To  this  process  of  reduction  the  term  fermentation,  and 
to  the  agent  which  causes  the  fermentation  the  term  ferment,  or  enzyme  has 
been  given.  As  these  compounds  undergo  reduction  to  simpler  substances 
somewhat  different  in  character  in  the  alimentary  canal  during  the  period  of 
digestion  as  a  result  of  the  action  of  ferments,  it  will  be  conducive  to  clearness 
of  ideas  regarding  the  nature  of  the  digestive  process  if  the  nature  and  prop- 
erties of  ferments  in  general  are  briefly  considered  at  this  time. 

A  ferment  or  an  enzyme  may  be  defined  as  an  agent  that  induces  a  change 
of  state,  or  a  change  in  composition  of  an  organic  compound,  for  the  most 
part  hydrolytic,  without  itself  being  utilized  in  the  process  or  appearing  in 
the  end-results  of  the  process. 

Ferments  have  been  divided  for  a  long  time  into  two  classes,  viz.,  organ- 
ized and  unorganized.  Among  the  organized  ferments  may  be  mentioned 
the  yeast  plant  {Saccharomycetes)  and  various  forms  of  bacteria;  among 
the  unorganized  ferments  may  be  mentioned  the  diastase  that  transforms 
the  starch  of  barley,  wheat,  or  other  cereals  into  sugar,  as  well  as  ptyalin, 
pepsin,  steapsin  or  lipase,  and  other  ferments  contained  in  the  digestive 
fluids  that  transform  or  reduce  the  food  principles  to  simpler  compounds. 

It  will  be  recalled  that  if  the  yeast  plant  is  added  to  a  sugar  solution 
containing  in  addition  some  protein  and  various  inorganic  salts  such  as 
phosphates  and  the  solution  kept  at  a  favorable  temperature  the  yeast  cells 
soon  begin  to  grow  and  multiply.  Coincidently  the  sugar  is  reduced  for 
the  most  part  to  carbon  dioxid  and  alcohol.  The  carbon  dioxid  bubbling 
through  the  solution  as  steam  bubbles  through  water  that  is  boiling,  gave  rise 
to  the  expression  fermentation  (from  fervere,  to  boil),  and  as  this  was 
attributed  to  the  life  activities  of  the  yeast  plant  it  was  called  a  ferment. 

Again,  if  dead  protein  matter  is  exposed  to  air  and  moisture  at  a  suitable 
temperature  it  will  be  invaded  by  various  species  of  bacteria,  which  in  a  short 
time  will  begin  to  grow  and  multiply.  Coincidently  the  protein  molecules  are 
reduced  to  simpler  compounds,  such  as  hydrogen  sulphid,  ammonia, 
carbon  dioxid  and  a  number  of  other  compounds,  the  nature  of  which  will 
vary  with  the  character  of  the  protein.  As  this  reduction  is  accompanied  by 
the  bubbling  of  gases  through  the  surrounding  liquid,  it  too  has  received  the 
name  of  fermentation,  and  as  the  reduction  is  attributed  to  the  life  activities 
of  the  bacteria  they  too  have  been  called  ferments.  In  both  instances  the 
ferment  is  a  unicellular  plant  possessing  a  distinct  organization.  For  this 
reason  they  have  been  termed  organized  ferments. 

When  grains  of  barley  or  other  cereals  containing  starch  are  exposed  to 
moisture  and  a  suitable  temperature,  the  starch  is  gradually  changed  to 
sugar,  a  transformation  attributed  to  the  action  of  a  ferment.  When  the 
starches,  fats,  proteins,  and  compound  sugars  are  introduced  into  the 
alimentary  canal  they  are  also  reduced  to  simpler  compounds,  a  reduction 
attributed  to  the  action  of  a  series  of  distinct  and  specific  ferments.     In 


DIGESTION  139 

addition  to  the  changes  that  the  food  principles  undergo  in  the  alimentary 
canal,  the  corresponding  principles  as  well  as  many  other  compounds  undergo 
similar  changes  in  the  body  tissues  as  the  result  of  the  action  of  ferments, 
changes  that  underlie  and  condition  many  if  not  all  the  phenomena  of  the 
nutritive  process.  In  none  of  these  instances,  however,  has  the  ferment  been 
satisfactorily  isolated  or  its  chemic  or  physical  features  determined.  For 
this  reason  these  ferments  have  been  termed  unorganized  ferments. 
Investigations  have  demonstrated,  however,  that  they  are  products  of  the 
metabolism  of  the  cells  of  plant  and  animal  tissues. 

In  recent  years  the  distinction  between  organized  and  unorganized 
ferments  has  become  untenable  owing  to  the  fact  that  chemists  have  succeeded 
in  extracting  from  yeast  cells  as  well  as  from  bacterial  cells,  enzymes  or 
ferments  that  produce  in  sugar  and  protein  the  same  reduction  effects  under 
the  same  conditions  as  in  the  case  of  yeast  cells  and  bacteria  themselves. 
It  is  therefore  probable  that  these  organized  cells  act  not  directly  by  virtue  of 
their  own  activities,  but  indirectly,  by  virtue  of  an  unorganized  ferment 
which  they  secrete  and  discharge  into  the  surrounding  medium.  All  enzymes 
that  produce  their  effects  after  being  discharged  from  cells  are  termed 
extra-cellular  enzymes,  while  those  that  produce  their  effects  in  the  interior 
of  cells  are  termed  intra-cellular  enzymes. 

The  Nature  of  Enzymes. — An  enzyme  is  in  all  probability  organic  in 
character,  though  neither  its  chemic  nature  nor  composition  has  been  de- 
termined. Some  of  them  exhibit  protein,  others  carbohydrate  reactions,  but 
by  reason  of  the  difficulty  in  isolating  enzymes  and  of  freeing  them  absolutely 
from  all  traces  of  protein  and  carbohydrates  it  is  not  possible  to  state 
positively  whether  the  reactions  observed  are  due  to  the  enzyme  or  its 
associated  organic  matter.  The  purer  the  preparation,  however,  the  less  of 
any  chemic  reaction  is  exhibited. 

From  what  is  known  of  their  action,  of  the  effects  produced  and  of  the  condi- 
tions under  which  they  act,  ferments  have  a  resemblance  to  various  inorganic  sub- 
stances or  agents  that  produce  changes  of  composition  and  decomposition 
apparently  by  their  presence  alone,  for,  as  far  as  the  evidence  goes,  they  neither 
enter  into  the  end-products  of  the  reaction  nor  are  they  destroyed.  A  chemic 
change  thus  produced  is  termed  catalysis  and  the  agent  causing  it  is  termed  a 
catalyzer  or  catalyst.  The  substance  on  which  the  catalyst  acts  is  termed  the 
substrate.  In  most,  if  not  in  all  instances  a  catalyst  acts  not  as  an  initiator,  but 
as  an  accelerator  of  a  change  that  would  spontaneously  take  place  with  extreme 
slowness  and  in  some  instances  with  results  so  slight  as  to  be  inappreciable. 
Oxygen  and  hydrogen,  for  example,  spontaneously  combine,  there  are  reasons  for 
believing,  at  room  temperatures  though  at  such  a  slow  rate  that  the  formation  of 
water  cannot  be  detected,  but  if  a  small  quantity  of  finely  divided  platinum  be 
added  the  combination  takes  place  almost  immediately;  carburetted  hydrogen  and 
oxygen  combine  when  they  pass  over  platinum  with  the  formation  of  carbon 
dioxid  and  water;  saccharose  and  water  in  the  presence  of  hydrochloric  acid  will 
combine  and  be  reduced  to  equal  quantities  of  levulose  and  dextrose;  dilute 
peroxid  of  hydiogen  will  slowly  decompose  spontaneously  and  yield  up  oxygen, 
but  if  finely  divided  platinum  or  silver  be  added  the  decomposition  is  greatly  ac- 
celerated. In  all  these  instances,  to  which  many  more  might  be  added,  the 
cata  yst,  simply  by  its  presence  accelerates  a  change  spontaneously  taking  place 
without  itself  appearing  in  the  end-products  of  the  reaction. 

It  has  been  experimentally  demonstrated  that  the  finer  the  catalyst  is  divided 


140  TEXT-BOOK  OF  PHYSIOLOGY 

or  the  greater  the  surface  it  presents  the  more  energetically  it  acts.  Thus,  if  platinum, 
silver,  or  gold  be  changed  to  the  colloidal  state,'  a  state  in  which  the  particles  of  ultra- 
microscopic  size  are  held  in  solution  or  perhaps  suspension  they  become  extremely 
active  catalyzers  even  in  exceedingly  small  quantities. 

From  the  foregoing  facts  and  from  many  others  it  may  be  assumed 
that  the  unorganized  enzymes  exist  in  the  colloidal  state. 

The  Rate  and  Completeness  of  Enzymic  Action. — The  rate  and 
completeness  of  enzymic  action  are  influenced  by  a  variety  of  conditions, 
among  the  more  important  of  which  are  temperature  and  the  rapidity  of 
the  removal  of  the  products  of  their  action. 

Temperature. — All  enzymes  are  sensitive  to  changes  in  temperature. 
At  o°C.  they  appear  to  be  incapable  of  inducing  changes  in  organic  matter. 
As  the  temperature  is  raised  their  reaction  properties  develop  and  increase 
in  velocity,  until  a  temperature  of  40°C.  to  5o°C.  is  reached,  when  they  are 
at  their  maximum.  For  this  reason  this  degree  of  temperature  is  spoken  of  as 
the  optimum  temperature.  Beyond  5o°C.  the  velocity  of  their  action  begins 
to  decrease  and  at  6o°C.  it  comes  to  an  end  for  the  majority  of  enzymes. 
At  ioo°C.  all  reaction  ceases  for  the  reason  that  the  enzymes  are  destroyed, 
especially  if  they  are  moist. 

The  Removal  of  the  Products  of  Enzymic  Action. — The  completeness 
of  enzymic  action  will  depend  on  the  rapidity  with  which  the  products  of 
enzyme  activity  are  removed.  This  is  illustrated  in  the  following:  If  a 
substrate  such  as  fat  and  the  enzyme  lipase  be  mixed  with  water  in  a  dialyzing 
test-tube,  the  fat  will  combine  with  water,  after  which  the  fat  will  undergo 
a  cleavage  into  a  fat  acid  and  glycerin.  If  the  products  of  the  reaction  are 
removed  practically  as  rapidly  as  they  are  formed  the  reaction  will  continue 
until  all  the  fat  is  so  transformed.  Under  such  circumstances  the  action 
will  be  complete.  If,  however,  the  reaction  takes  place  in  a  receptacle  the 
character  of  which  prevents  the  removal  of  the  fat  acid  and  glycerin,  the 
reaction  will  in  time  come  to  an  end,  leaving  apparently  a  percentage  of 
fat  unchanged.  The  explanation  at  one  time  given  for  the  cessation  of  the 
reaction  was  that  the  accumulation  of  the  products  interfered  with  the 
further  action  of  the  enzyme.  It  is,  however,  now  generally  admitted  that 
under  the  circumstances  the  ferment,  shortly  after  the  appearance  of  the 
cleavage  products,  initiates  a  reverse  action,  i.e.,  recombines  the  fat  acid  and 
glycerin  with  the  re-formation  of  the  fat  until  a  condition  of  chemic 
equilibrium  is  established  between  the  opposing  tendencies.  The  dis- 
covery that  many  ferments  are  thus  capable  of  secondarily  reversing  their 
primary  action  has  assisted  in  the  interpretation  of  a  number  of  obscure 
physiologic  processes.  It  must  not  be  overlooked  that  in  this  instance  the 
enzyme  does  not  initiate  the  reverse  action,  but  merely  hastens  what  would 
take  place  by  reason  of  a  want  of  chemic  equilibrium  between  the  substances 
present.^ 

'  The  colloidal  state  may  be  developed  by  passing  an  electric  current  through  electrodes  of 
these  metals  placed  in  distilled  water.  With  the  passage  of  the  current  particles  of  the  metals 
are  discharged  from  one  of  the  electrodes  into  the  water  in  the  form  of  a  cloud. 

2  Reversibility  of  a  chemic  reaction  may  be  defined  as  a  recombination  of  the  products  of  the 
reaction  of  the  original  compound  until  a  condition  of  equilibrium  is  estabUshed  between  the 
analytic  and  the  synthetic  tendencies.  A  classic  illustration  of  the  two  phases  of  a  chemic  reaction 
is  the  following:  If  chemically  equivalent  amounts  of  acetic  acid  and  ethyl  alcohol  are  mixed  at  a 
definite  temperature  a  reaction  occurs  which  eventuates  in  the  formation  of  ethyl  acetate  and 


DIGESTION  141 

The  Specific  Action  of  Enzymes. — The  number  of  enzymes  in  the 
digestive  fluids  and  in  the  various  tissues  of  the  body  has  given  rise  to  the 
idea,  which  has  been  confirmed  by  experiment,  that  an  enzyme  exerts  its 
action  on  but  one  substrate,  in  other  words,  that  its  action  is  specific.  Thus 
an  enzyme  that  would  transform  starch  into  sugar  would  not  be  capable  of 
causing  a  cleavage  of  fat  into  a  fat  acid  and  glycerin;  an  enzyme  that  would 
cause  a  cleavage  of  saccharose  would  not  be  capable  of  causing  a  cleavage  of 
lactose.  So  with  all  other  enzymes.  Each  seems  to  be  specially  adapted  to 
catalyze  but  one  substrate  under  given  conditions.  Various  other  features 
of  enzymes,  their  mode  of  action,  their  origin  from  preexisting  substances, 
the  methods  by  which  they  are  made  active,  the  conditions  under  which 
they  are  most  active,  etc.,  will  be  mentioned  in  connection  with  a  considera- 
tion of  the  fluids  and  tissues  in  which  they  are  present. 


MOUTH  DIGESTION 

The  digestion  of  the  food  as  it  takes  place  in  the  mouth  comprises 
a  series  of  physical  and  chemic  changes,  the  result  of  the  action  of  the 
teeth,  the  tongue,  and  the  saliva.  The  mechanic  division  of  the  food 
and  the  incorporation  of  the  saliva  with  it  are  termed  respectively  mastication 
and  insalivation. 

MASTICATION 

Mastication  is  the  mechanic  division  of  the  food,  and  is  accomplished 
by  the  teeth  and  the  movements  of  the  lower  jaw  under  the  influence  of 
muscle  contractions.  Complete  mechanic  disintegration  of  the  food  is 
important  for  its  subsequent  solution  and  chemic  transformation;  for  when 
finely  divided  it  presents  a  larger  surface  to  the  action  of  the  digestive 
fluids  and  thus  enables  them  to  exert  their  respective  actions  more  effectively 
and  in  a  shorter  period  of  time. 

The  Teeth. — In  man  passing  from  childhood  to  adult  life  two  sets 
of  teeth  make  their  appearance.  The  first  set  constitute  the  temporary, 
deciduous,  or  milk  teeth;  the  second  set  constitute  the  permanent  teeth, 
which  should  last  with  proper  care  through  life  or  to  an  advanced  age. 

The  temporary  teeth,  twenty  in  number,  ten  in  each  jaw,  though  smaller 
than  the  permanent  teeth,  have  the  same  general  conformation.  They 
are  divided  into  four  incisors,  two  cuspids  or  canines,  and  four  molars  for 
each  jaw. 

The  permanent  teeth,  thirty-two  in  number,  sixteen  in  each  jaw,  are 
divided  into  four  incisors,  two  cuspids  or  canines,  four  bicuspids  or  pre- 
molars, and  six  molars  for  each  jaw. 

Each  tooth  may  be  said  to  consist  of  three  portions:  (i)  the  crown, 


water.  In  this  instance  after  a  certain  percentage  of  these  substances  has  thus  united  the  prod- 
ucts of  the  reaction  begin  to  recombine  with  the  formation  of  the  original  compounds  until  the 
opposing  tendencies  are  in  equilibrium,  a  state  in  which  they  remain  so  long  as  the  conditions 
remain  unchanged.  Again,  if  maltose  and  water  be  mixed,  a  reaction  occurs  which  eventuates  in 
the  formation  of  dextrose.  In  time  the  dextrose  molecules  combine  to  form  maltose  and  water 
until  a  condition  of  equiUbrium  is  established.  If  the  yeast  enzyme  be  added  the  reactions  both 
analytic  and  synthetic  are  increased  in  velocity.  The  enzyme,  however,  does  not  initiate,  but 
merely  hastens  a  reaction  already  taking  place. 


142 


TEXT-BOOK  OF  PHYSIOLOGY 


PM 


or  that  portion  which  projects  above  the  gums;  (2)  the  root  or  fang,  that 
portion  embedded  in  the  alveolar  socket;  (3)  the  constricted  portion  or  neck, 
which  is  surrounded  by  the  free  margin  of  the  gum.  The  teeth  are  firmly 
secured  in  their  sockets  by  a  fibrous  membrane,  the  peridental  membrane, 
which  is  attached,  on  the  one  hand,  to  the  alveolar  process,  and,  on  the  other, 
to  the  cementum. 

A  vertical  section  of  a  tooth  shows  that  it  consists  of  three  distinct 
solid  structures,  the  enamel,  the  dentine,  and  the  cementum,  which  have 
the  anatomic  relationship  represented  in  Fig.  62.     In  the  center  of  the 

dentine  there  is  a  cavity  the  general  shape 
of  which  varies  in  different  teeth,  and 
which  is  occupied  during  the  living  condi- 
tion by  the  tooth  pulp. 

Microscopic  examination  of  the  tooth 
reveals  the  presence  of  irregular  stellate 
spaces,  the  interglobular  spaces,  between 
the  dentine  and  the  cementum,  which  are 
occupied  by  connective-tissue  cells.  Clefts 
of  varying  size  are  also  observed  at  the 
junction  of  the  dentine  and  the  enamel, 
and  extending  for  some  distance  into  the 
latter. 

The  enamel  is  composed  of  dense  hard 
cylinders  which,  on  account  of  their  small 
size  and  close  relationship,  appear  to  be 
hexagonal  in  shape.  These  cylinders  are 
held  together  by  cement  substance.  The 
free  border  of  the  enamel  is  covered  by  a 
thin  membrane  known  as  the  cuticle  or 
membrane  of  Nasmyth. 

The  dentine  is  somewhat  less  dense  than 
the  enamel.  It  is  composed  of  connective- 
tissue  fibers  embedded  in  a  ground-sub- 
stance, both  of  which  have  undergone 
calcification  in  the  course  of  development. 
The  dentine  is  penetrated  by  a  series  of  fine 
canals,  the  dentine  canals  or  tubules,  which 
begin  by  open  mouths  on  the  pulp  side.  From  this  point  the  tubules  pass 
outward  to  the  cementum  and  enamel,  where  their  terminal  branches  com- 
municate with  and  terminate  in  the  interglobular  spaces  and  clefts.  In 
their  course  the  tubules  give  off  a  series  of  branches  which  communicate 
freely  with  one  another.  The  dentine  bordering  the  tubule  is  somewhat 
more  dense  than  the  intertubular  portion  and  constitutes  what  is  known  as 
the  dentinal  sheath  or  Neumann's  sheath. 

The  cementum  resembles  bone  because  it  contains  both  lacunae  and 
canaliculi,  though  it  is,  as  a  rule,  devoid  of  Haversian  canals. 

The  pulp  consists  of  a  framework  of  connective  tissue  which  affords 
support  for  blood-vessels  and  nerves,  both  of  which  enter  the  pulp  chamber 
through  a  small  foramen  at  the  apex  of  the  root.  The  outer  surface  of 
the  pulp  is  covered  with  a  layer  of  large  spheric  cells,  the  odontoblasts. 


Fig.  62, — Vertical  Section  of 
Tooth  in  Jaw.  E.  Enamel.  D. 
Dentine.  P.  M.  Periodontal  mem- 
brane. P.C.  Pulp  cavity.  C.  Ce- 
ment. B.  Bone  of  lower  jaw.  V. 
Vein.  a.  Artery.  N.  Nerve. — {Stir- 
ling.) 


DIGESTION  143 

Each  cell  presents  on  its  inner  surface  short  processes  which  pass  into  the 
pulp;  on  its  outer  surface  it  presents  a  long  process  which  enters  a  dentine 
tubule  and  extends  as  far  as  its  ultimate  terminations.  Collectively  these 
processes  are  known  as  the  dentine  fibers.  Inasmuch  as  the  fibers  do  not 
completely  occupy  the  lumen  of  the  tubule,  it  is  probable  that  there  is  a 
free  circulation  of  lymph  from  the  pulp  chamber  through  the  dentine  tubules 
into  the  enamel  clefts,  into  the  interglobular  spaces,  and  possibly  into  the 
lacunae  of  the  cementum. 

The  peridental  membrane  is  composed  of  connective-tissue  fibers  abun- 
dantly supplied  with  blood-vessels  and  nerves. 

Movements  of  the  Lower  Jaw. — The  lower  jaw  is  capable  of  a  down- 
ward and  upward,  an  antero-posterior,  and  a  lateral  movement,  all  depend- 
ent on  the  peculiar  construction  of  the  joint. 

Temporo-maxillary  Articulation. — This  articulation  is  formed  by  the 
anterior  portion  of  the  glenoid  cavity,  the  eminentia  articularis,  and  the 
condyle  of  the  inferior  maxilla,  all  of  which  are  united  by  means  of  liga- 
ments. Situated  between  the  glenoid  cavity  and  the  condyle  is  a  plate  of 
fibro-cartilage  oval  in  shape  and  biconcave.  This  cartilage  divides  the 
joint  into  two  cavities — one  above,  the  other  below — each  of  which  is  pro- 
vided with  a  synovial  membrane.  The  function  of  the  cartilage  is  to  present 
constantly  an  articulating  surface  to  the  condyle  in  the  various  movements 
of  the  lower  jaw,  which  it  is  enabled  to  do  by  virtue  of  its  mobility. 

In  the  downward  movement  of  the  lower  jaw  each  condyle  glides  for- 
ward, carrying  with  it  the  interarticular  fibro-cartilage,  the  upper  concave 
surface  of  which  is  applied  to  the  convex  surface  of  the  eminentia  articularis. 
In  the  upward  movement  of  the  jaw  both  the  condyles  and  the  cartilages  pass 
backward  and  resume  their  normal  position.  The  movements  of  depres- 
sion and  elevation  are  made  possible  by  the  slightly  oblique  direction  of  the 
condyle.  In  the  carnivorous  animals,  whose  food  requires  considerable 
cutting,  these  movements  are  especially  well  developed.  In  these  animals 
the  condyles  are  transversely  arranged  and  at  right  angles  to  the  long  axis 
of  the  jaw.  In  the  antero-posterior  movement  the  jaw  moves  in  a  hori- 
zontal direction  and  the  condyles  and  the  articular  cartilages  glide  forward 
and  backward  in  the  glenoid  fossae.  In  the  rodent  animals  the  long  axis 
of  the  condyle  runs  in  the  antero-posterior  direction,  which  allows  of  a 
considerable  gliding  movement.  When  the  jaw  performs  a  lateral  movement, 
the  condyle  and  cartilage  of  one  side  may  remain  in  their  natural  position 
while  the  opposite  condyle  and  cartilage  glide  forward  in  the  glenoid  fossa, 
directing  the  symphysis  of  the  jaw  to  the  opposite  side  of  the  median  line. 
The  lateral  movements  are  well  exhibited  by  the  herbivorous  animals,  in 
which  they  are  quite  extensive,  and  made  possible  by  the  small  size  of  the 
condyle  and  the  large  extent  of  articulating  surface.  In  man  the  structure 
of  the  joint  is  such  as  to  admit  of  all  these  possibilities,  and  the  lower  jaw 
acquires  in  consequence  great  freedom  of  movement. 

The  Functions  of  the  Muscles  of  Mastication. — The  movements  of 
the  lower  jaw  are  caused  by  the  action  of  numerous  muscles,  which,  having 
fixed  points  of  origin,  are  attached  to  various  points  on  its  surface.  The 
muscles  concerned  in  the  movements  of  mastication  are  presented  in  the 
following  table: 


144 


TEXT-BOOK  OF  PHYSIOLOGY 


Anterior  belly  of  digastric 

Mylohyoid 

Geniohyoid 

Temporal 

Internal  portion  of  masseter 

Internal  pterygoids 

External  pterygoids 

External  portion  of  masseter 

Anterior  fibers  of  temporal 

Posterior  fibers  of  temporal 

Internal  portion  of  masseter 

Digastric,  mylohyoid,  and  geniohyoid 

Internal  pterygoids 

External  pterygoids 

Pterygoids,  external  and  internal 

Temporal 

Masseter 


Depress  the  lower  jaw  and  open  the 
mouth. 

Elevate  the  lower  jaw  and  close  the 
mouth. 

Draw  the  lower  jaw  forward  and  cause 
the  lower  teeth  to  project  beyond 
the  upper. 

Draw  the  lower  jaw  back  to  its  normal 
position. 

Contracting  alternately,  draw  the  jaw 
to  the  opposite  side. 

Produce  grinding  movements  of  the 
lower  jaw. 


The  action  of  the  depressor  muscles  becomes  apparent  when  their  points 
of  origin  and  insertion  are  considered.  The  anterior  belly  of  the  digastric, 
the  mylohyoid,  and  the  geniohyoid  muscles,  agree  in  having  a  similarity  of 
origin — the  hyoid  bone — and  a  common  area  of  insertion,  the  anterior 
portion  of  the  lower  jaw.  Their  anatomic  relation  is  such  that  their 
combined  action  will  depress  the  lower  jaw  and  open  the  mouth. 

The  action  of  the  elevator  muscles  becomes  apparent  when  their  points 
of  origin  and  insertion  are  considered.  The  elevator  muscles  arise  from 
various  points  on  the  side  of  the  head,  and  are  inserted  into  the  coronoid 
process,  ramus,  and  internal  surface  of  the  angle  of  the  lower  jaw.  After  the 
mouth  has  been  opened,  the  simultaneous  contraction  of  these  muscles  will 
elevate  the  jaw  and  closes  the  mouth  with  considerable  force.  The  power 
of  these  muscles,  which  is  very  great,  depends  on  the  shortness  and 
thickness  of  the  muscle-bundles. 

The  action  of  the  rotator  muscles,  the  external  and  internal  pterygoids, 
those  which  give  rise  to  the  lateral  movements  of  the  jaw,  depends  in  like 
manner  on  their  origin  and  insertion.  The  first  arises  from  the  outer 
surface  of  the  external  pterygoid  plate  and  the  great  wing  of  the  sphenoid 
bone;  the  second  arises  mainly  from  the  inner  surface  of  the  external  ptery- 
goid plate;  they  are  inserted  into  the  neck  of  the  condyle  and  angle  of  the 
lower  jaw  respectively.  When  they  contract,  the  condyle  on  the  correspond- 
ing side  is  drawn  forward,  while  the  opposite  condyle  remains  stationary. 
As  a  result,  the  symphysis  of  the  jaw  is  directed  to  the  opposite  side.  The 
grinding  movements  of  the  jaw  are  produced  by  the  coordinated  action  of 
all  the  groups  of  muscles  acting  more  or  less  successively. 

For  the  proper  mastication  of  the  food  it  is  essential  that  it  be  kept 
between  the  opposing  surfaces  of  the  teeth.  This  is  accomplished  by  the 
contraction  of  the  orbicularis  oris  and  buccinator  muscles  from  without  and 
the  tongue  muscles  from  within. 

The  Nerve  Mechanism^  of  Mastication. — Mastication  is  a  complex 
act  and  involves  the  cooperation  of  a  number  of  muscles,  afferent  and  efferent 
nerves,  and  a  central  mechanism  by  which  they  are  excited  to,  and  coordi- 
nated in  their  activity.  The  central  mechanism  is  located  in  the  medulla 
oblongata  in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle. 

During  the  intervals  of  mouth  digestion  the  mouth  is  closed  by  the  con- 

*  By  this  term  is  meant  a  combination  of  nerves,  afferent  and  efferent,  and  nerve  centers 
which  when  excited  to  action  coordinates  the  actions  of  the  organs  with  which  it  is  associated. 


DIGESTION  145 

traction  of  the  elevator  muscles  of  the  lower  jaw.  When  the  occasion  arises 
for  the  introduction  of  food,  the  mouth  is  opened  by  the  depressor  muscles; 
after  the  food  fs  introduced  into  the  mouth  it  is  again  closed  and  that 
combination  of  muscle  contractions  initiated  which  when  continued  results 
in  the  mechanical  division  of  the  food. 

The  nerves  and  nerve  centers  constituting  the  nerve  mechanism  for 
mastication  are  shown  in  the  following  table: 

Afferent  Nerves.  Nerve-center.  Efferent  Nerves. 

1.  Lingual  and  buccal  branches  of  Medulla  oblongata.         i.  Small  root  of   the  trigeminal 
the  trigeminal  nerve.  nerve. 

2.  Glosso-pharyngeal.  2.  Hypoglossal. 

3.   Facial  or  portio  dura. 

The  Efferent  Nerves. — The  efferent  nerves  that  transmit  nerve  im- 
pulses to  the  various  muscles  of  mastication  are  the  small  root  of  the  tri- 
geminal, the  hypoglossal,  and  the  facial. 

The  small  root  of  the  trigeminal  nerve  after  emerging  from  the  cavity 
of  the  cranium  through  the  foramen  ovale  joins  the  inferior  maxillary  divi- 
sion of  the  large  sensor  root.  After  a  short  course  the  efferent  fibers  sepa- 
rate into  two  groups,  an  upper  and  a  lower;  the  upper  group  is  distributed  to 
the  masseter,  temporal,  internal  and  external  pterygoid  muscles,  the  lower 
group  is  distributed  to  the  mylohyoid  and  anterior  belly  of  the  digastric 
muscles.  The  hypoglossal  nerve,  after  emerging  from  the  cranium  through 
the  anterior  condyloid  foramen,  passes  downward  and  forward  to  be  dis- 
tributed to  the  intrinsic  and  extrinsic  muscles  of  the  tongue.  The  facial 
or  portio  dura  after  emerging  from  the  stylo-mastoid  foramen  is  distributed 
to  the  superficial  muscles  of  the  face. 

Stimulation  of  any  one  of  these  nerves  with  induced  electric  currents 
gives  rise  to  convulsive  movements  in  the  muscles  to  which  it  is  distributed 
while  its  division  is  followed  by  paralysis  of  the  muscles. 

The  Central  Mechanism. — The  central  mechanism  that  excites  and 
coordinates  the  action  of  the  nerve-cells  from  which  these  nerves  emerge,  may 
be  excited  to  activity  (i)  by  nerve  impulses  descending  from  the  cerebrum  as 
a  result  of  volitional  efforts;  and  (2)  by  nerve  impulses  transmitted  through 
afferent  nerves  from  the  mouth.  Though  movements  of  mastication  are  pri- 
marily volitional  and  may  so  continue,  nevertheless  when  once  initiated  they 
continue  for  an  indefinite  period,  so  long  in  fact  as  the  nerve  impulses  which 
the  food  develops  in  afferent  nerves  are  received  by  the  central  mechanism, 
thus  falling  into  the  category  of  secondary  or  acquired  reflex  acts.  That 
the  masticatory  movements  are  of  this  reflex  character  is  indicated  by  the 
fact  that  they  will  be  maintained,  even  though  the  volitional  effort  that 
called  them  forth  has  subsided  and  the  attention  has  been  directed  to  some 
entirely  different  subject.  It  would  appear  that  all  that  is  necessary  under 
such  circumstances  is  the  stimulating  action  of  the  food  upon  the  peripheral 
terminations  of  the  afferent  nerves  distributed  to  the  mucous  membrane 
of  the  tongue  and  mouth. 

The  Afferent  Nerves. — The  afferent  nerves,  stimulation  of  which 
excites  the  central  mechanism,  are  the  lingual  and  buccal  branches  of  the 
superior  and  inferior  maxillary  divisions  of  the  trigeminal  nerve,  the  lingual 
branches  of  the  glosso-pharyngeal,  and  in  all  probability  the  gustatory  fibers 
of  the  chorda  tympani.     The  introduction  of  food  into  the  mouth  develops 


146 


TEXT-BOOK  OF  PHYSIOLOGY 


in  the  peripheral  terminations  of  these  nerves,  by  reason  of  its  physical  and 
chemic  properties,  nerve  impulses  which  are  then  transmitted  to  the  central 
mechanism.  If  these  nerves  are  divided,  mastication  is  seriously  impaired. 
When  divided  and  their  central  ends  stimulated  with  induced  electric 
currents,  the  muscle  will  refiexly  be  thrown  into  contraction. 


Secretory- 
tubule. 


Intercalated 
pieces. 


Acini. 


INSALIVATION 

Insalivation  is  the  incorporation  of  the  saliva  with  the  food,  and  takes 
place  for  the  most  part  during  mastication.     The  saliva  ordinarily  present 

in  the  mouth  is  a  complex  fluid  composed 
of  the  secretions  of  the  parotid,  submaxil- 
lary, and  sublingual  glands  and  the  muci- 
parous follicles  of  the  mouth,  which  col- 
lectively constitute  the  salivary  apparatus. 

The  parotid  gland  is  situated  in  front 
of  and  partly  below  the  external  ear,  where 
it  is  held  in  position  by  the  fascia  and  skin. 
From  the  anterior  border  of  the  gland  there 
emerges  a  duct  (Stenson's),  which,  after 
crossing  the  masseter  muscle  to  its  anterior 
border,  turns  inward,  pierces  the  bucci- 
nator muscle  and  opens  on  the  surface  of 
the  cheek  opposite  the  second  upper  molar 
tooth. 

The    submaxillary  gland  is  situated 

Fig.    63. — Scheme   of  the    Human    below  the  jaw  in  the  anterior  part  of  the 

Submaxillary  Gland.— (5/o7^r.)        submaxillary    triangle.     From    the    gland 

there  emerges  a  duct  (Wharton's)  which  opens  into  the  mouth  by  a  minute 

orifice  on  the  surface  of  a  papilla  by  the  side  of  the  tongue. 

The  sublingual  gland  is  situated  just  beneath  the  mucous  membrane 
in  the  anterior  part  of  the  mouth,  where  it  forms  a  projection  between  the 
gums  and  tongue.  The  posterior  part  of  the  gland  gives  origin  to  a  duct 
(the  duct  of  Rivinus,  described  also  by  Bartholin)  which  opens  into  the 
mouth  with  or  very  near  to  the  duct  of  Wharton.  The  anterior  part  of  the 
gland  gives  origin  to  a  varying  number  of  ducts  (the  ducts  of  Walther) 
which  open  separately  along  the  edge  of  the  sublingual  plica  of  the  mucous 
membrane. 

Histologic  Structure  of  the  Salivary  Glands. — In  their  ultimate 
structure  the  salivary  glands  bear  a  close  resemblance  to  one  another. 
They  are  compound  tubulo-alveolar  glands  composed  of  small  irregularly 
shaped  lobules  united  by  areolar  tissue,  and  connected  by  branches  of  the 
salivary  ducts.  Each  lobule  is  made  up  of  a  number  of  small  alveoli  or 
acini  more  or  less  tubular  in  shape  which  are  the  terminal  expansions  of  the 
smallest  ducts.  (See  Fig.  63.)  The  wall  of  the  acinus  is  formed  by  a  . 
reticulated  basement  membrane,  surrounded  externally  by  blood-vessels, 
the  spaces  between  which  constitute  lymph-spaces  or  channels.  The  inner 
surface  of  the  acinus  membrane  supports  a  single  layer  of  irregular  spheric 
or  polygonal  epithehal  cells.  The  cells  do  not  entirely  fill  up  the  cavity 
of  the  acinus,  but  leave  an  intercellular  space,  the  lumen,  which  constitutes 


DIGESTION 


147 


the  beginning  of  the  duct  for  the  transmission  of  the  secretion  to  the  mouth. 
From  each  acinus  there  passes  a  narrow  intercalary  duct  hned  by  a  layer  of 
flattened  cells.  The  common  excretory  duct — formed  by  the  union  of  the 
intralobular  and  interlobular  ducts — consists  also  of  a  basement  membrane, 
lined,  however,  by  tall  columnar  epithelial  cells.  The  salivary  glands  are 
abundantly  supplied  with  blood-vessels  and  nerves  which  are  closely  related 
to  their  activity. 

Based  partly  on  the  character  of  their  secretions  and  partly  on  the  micro- 
scopic appearance  of  their  secreting  cells,  the  salivary  glands  have  been 
divided  by  Heidenhain  into  two  classes:  viz.,  serous  or  albuminous,  and 
mucous  glands.  To  the  first  class  belong  the  parotid,  a  portion  of  the  sub- 
maxillary, and  a  portion  of  the  glands  of  the  tongue.  To  the  second  class 
belong  a  portion  of  the  submaxillary  gland,  the  sublingual,  a  portion  of 
the  glands  of  the  tongue,  the  glands  of  the  cheeks,  lips,  palate,  and  pharynx. 
It  is  possible  that  a  single  alveolus  of  any  gland  may  contain  both  albu- 
minous and  mucous  cells. 


/ 

-t 

■   1 


Fig.  64. — Parotid  Gland  AT  Rest.  1,1, 
Acini;  2,  duct;  -^i,:},,  albuminous  cells  filled 
with  fine  granules;  4,4,  nuclei  almost  con- 
cealed.    (Semi-diagrammatic.) 


Fig.  65. — Submaxillary  ^  Gland  at 
Rest.  1,1,  Acini;  2,  duct;  3,  3,  mucous  cells 
containing  mucin;  4,4,  nuclei,  flattened  and 
dispersed  toward  the  base  of  the  cells;  5,5, 
crescents  of  Giannuzzi. — {After  Vialleton.) 


In  the  serous  glands  the  cells  are  more  or  less  spheric  in  shape,  nucleated, 
and  almost  completely  filled  with  dark  granular  material  (Fig.  64).  In  the 
mucous  glands  the  cells  are  large,  clear  in  appearance,  and  loaded  with  a 
highly  refracting  material  resembling  mucin  (Fig.  65).  Between  the  base- 
ment membrane  and  the  clear  cells  are  to  be  found  in  the  acini  of  the  sub- 
maxillary glands  small  crescentic  shaped  cells  filled  with  granular  material 
which  stains  deeply  with  various  coloring  matters.  These  are  known  as 
the  demilunes  of  Heidenhain.  At  one  time  it  was  supposed  that  they  were 
young  cells  destined  to  take  the  place  of  the  clear  cells  which  were  believed 
to  be  exhausted  and  to  have  undergone  disintegration.  At  the  present  time 
they  are  regarded  as  albuminous  or  serous  cells  which  exhibit  changes 
similar  to  the  cells  of  the  parotid  gland. 

The  glands  embedded  in  the  mucous  membrane  covering  the  tongue, 
lips,  cheek,  palate,  and  pharynx  are  for  the  most  part  lined  with  epithelial 
cells  which  contain  a  highly  refracting  matter  similar  to,  if  not  identical  with, 
that  found  in  the  cells  of  the  submaxillary  and  sublingual  glands. 


148  TEXT-BOOK  OF  PHYSIOLOGY 

Nerve-supply. — Histologic  investigation  has  demonstrated  that  the 
cells  and  blood-vessels  of  the  salivary  glands  are  supplied  with  nerve-fibers 
directly  from  ganglion  cells  situated  in  their  immediate  neighborhood. 
Thus  the  cells  and  blood-vessels  of  the  submaxillary  and  sublingual  glands, 
receive  nerve-fibers  from  the  submaxillary,  sublingual  and  superior  cervical 
ganglia,  while  the  cells  and  blood-vessels  of  the  parotid  gland  receive  nerve- 
fibers  from  the  otic  and  the  superior  cervical  ganglia.  From  their  ultimate 
distribution  it  may  be  inferred  that  some  of  the  ganglion  cells  and  fibers 
influence  the  production  of  the  secretions  (secretor  nerves),  while  others 
influence  the  caliber  of  the  blood-vessels  causing  either  constriction  or  dilata- 
tion (vaso-constrictor  and  vaso-dilatator  nerves).  (Fig.  68.)  The  secretor 
fibers  penetrate  the  basement  membrane  enclosing  the  gland  acinus  and 
finally  terminate  between  and  on  the  surface  of  the  secretor  cells.  The 
vaso-motor  fibers  terminate  between  and  on  the  muscle  cells  in  the  walls  of 
the  blood-vessels. 

The  local  ganglion  cells,  however,  are  in  anatomic  relation  with  fine 
medullated  nerve-fibers  coming  directly  from  the  medulla  oblongata  and  the 
spinal  cord.  As  they  enter  the  ganglia,  their  terminal  branches  arborize 
around  and  closely  invest  the  cells  of  the  ganglia  and  come  into  intimate 
histologic  and  physiologic  connection  with  them.  The  nerve-fibers  coming 
from  the  central  nerve  system  are  known  as  pre-ganglionic  fibers,  while  those 
coming  from  the  ganglia  are  known  as  post-ganglionic  fibers.  Through 
the  intermediation,  therefore,  of  the  ganglion  cells,  the  secretor  cells  of  the 
salivary  glands  and  the  blood-vessels  surrounding  them  are  brought  into 
relation  with  the  central  organs  of  the  nerve  system  and  become  susceptible 
of  being  influenced  by  them. 

The  Parotid  Saliva. — The  parotid  saliva,  as  it  flows  from  the  orifice  of 
Stenson's  duct,  is  clear,  limpid,  free  from  viscidity,  distinctly  alkaline  in 
reaction,  with  a  specific  gravity  of  1.003.  Chemic  analysis  shows  that  it 
consists  of  water,  a  small  quantity  of  protein  matter,  a  trace  of  a  sulpho- 
cyanogen  compound,  and  inorganic  salts.  The  secretion  is  increased  during 
mastication,  and  especially  on  the  side  engaged  in  mastication.  Dry  food 
causes  a  larger  flow  than  moist  food.  The  situation  of  the  orifice  of  the 
parotid  duct  is  such  that  as  the  secretion  is  poured  into  the  mouth  it  is  at 
once  incorporated  with  the  food  by  the  movements  of  the  lower  jaw,  and 
thus  fulfils  the  physical  function  of  softening  and  moistening  it. 

The  Submaxillary  Saliva. — The  submaxillary  saliva  is  clear,  slightly 
viscid,  alkaline  in  reaction,  and  has  a  specific  gravity  of  1.002.  It  consists 
of  water,  protein  matter  (mucin),  and  inorganic  salts. 

The  Sublingual  Saliva. — The  sublingual  saliva  is  clear,  extremely 
viscid,  and  strongly  alkaline  in  reaction.  It  consists  of  water,  protein 
matter  (chiefly  mucin),  and  inorganic  salts. 

The  small  racemose  glands  embedded  in  the  mucous  membrane  on 
the  inner  surface  of  the  cheeks  and  Hps,  on  the  hard  and  soft  palate,  and  on 
the  tongue  and  pharynx,  secrete  a  fluid  which  is  grayish  in  color,  and 
extremely  viscid  and  ropy.     It  contains  a  large  amount  of  mucin. 

Mixed  Saliva. — The  saliva  of  the  mouth  is  a  complex  fluid  composed 
of  ihe  secretions  of  all  the  salivary  glands.  As  obtained  from  the  mouth 
it  is  frothy,  opalescent,  slightly  turbid,  and  somewhat  viscid.  The  specific 
gravity  is  low,  ranging  from  i.ooo  to  1.006.    The  reaction  is  usually  distinctly 


DIGESTION  149 

alkaline.  It  may,  however,  be  neutral  or  even  acid  in  reaction  if  there  is 
any  fermentation  of  food  particles  in  the  mouth  or  in  certain  disorders  of  the 
alimentary  canal.  When  examined  with  the  microscope,  the  saliva  is  seen 
to  contain  epithelial  cells,  salivary  corpuscles  resembling  leukocytes,  particles 
of  food,  and  various  microorganisms,  especially  Leptothrix  buccalis. 

The  chemic  composition  of  the  saliva  is  shown  in  the  following  table: 

COMPOSITION  OF  HUMAN  SALIVA. 

Water 995 -16  994.20 

Epithelium 1.62  2.20 

Soluble  organic  matter i  .34  i  .40 

Potassium  sulphocyanid o  .06  o  .04 

Inorganic  salts i  .82  2 .20 

1000.00  1000.04 

(Jacubowitsch.)     (Hammerbacher.) 

Water  constitutes  the  main  ingredient,  amounting  to  99.5  per  cent. 
The  soluble  organic  matter  is  protein  in  character  and  is  a  mixture  of 
mucin,  globulin,  and  serum-albumin.  The  potassium  sulphocyanid  is 
mainly  derived  from  the  parotid  gland.  Its  presence  can  be  demon- 
strated by  the  addition  of  a  few  minims  of  a  dilute  solution  of  slightly 
acidulated  ferric  chlorid,  when  a  characteristic  red  color  is  developed. 
The  inorganic  constituents  comprise  the  sodium,  calcium,  and  magnesium, 
phosphates,  sodium  carbonate,  and  sodium  and  potassium  chlorids. 

The  relative  amounts  of  the  different  constituents  of  the  saliva  will  depend 
on  the  relative  degree  of  activity  of  the  different  glands,  and  this  in  turn  will 
be  determined  by  the  character  of  the  food.  When  the  food  is  dry,  there 
will  be  an  excess  of  the  parotid  secretion;  when  it  partakes  of  the  consistence 
of  meat,  there  will  be  a  larger  secretion  of  the  submaxillary  saliva.  The 
glands  apparently  adapt  their  activity  to  the  character  of  the  food. 

Quantity  of  Saliva. — The  estimation  of  the  total  quantity  of  mixed 
saliva  secreted  in  twenty-four  hours  is  exceedingly  difl&cult,  and  the  results 
obtained  must  be  only  approximative.  It  is,  of  course,  subject  to  consider- 
able variation,  depending  upon  habit,  the  nature  of  the  food,  etc.  The 
experiments  of  Professor  Dalton  and  the  results  obtained  by  him  are  emi- 
nently trustworthy,  and  in  all  probability  represent  as  nearly  as  possible  the 
exact  amount  secreted.  He  found  that  without  any  artificial  stimulus  he 
was  enabled  to  collect  from  the  mouth  about  36  grams  (540  grains)  of  saliva 
per  hour,  but  that  upon  the  introduction  of  any  stimulating  substance  into 
the  mouth  the  quantity  could  be  greatly  increased.  During  mastication  the 
saliva  was  poured  out  in  greater  abundance,  the  amount  depending  upon 
the  relative  dryness  of  the  food.  He  found  that  wheat  bread  absorbed 
55  per  cent,  of  its  weight,  and  fresh  cooked  meat  48  per  cent.  If,  therefore, 
the  average  quantity  of  bread  and  meat  required  daily  by  a  man  of  ordinary 
physical  development  and  activity  be  assumed  to  be  540  grams  (19  oz.)  of 
the  former  and  450  grams  (16  oz.)  of  the  latter,  these  two  substances  would 
absorb  respectively  297  grams  (4573.8  grains)  and  216  grams  (3326.4  grains), 
making  a  total  of  513  grams  (7900  grains).  If,  therefore,  the  amount 
secreted  and  mixed  with  the  food  during  an  estimated  two  hours  of  mastication 
be  added  to  the  amount  secreted  during  the  remaining  twenty-two  hours,  suppos- 
ing that  it  continues  at  the  rate  of  36  grams  per  hour,  we  have  a  total  amount 
of  513-1-792  grams,  or  1305  grams  (19,780  grains),  or  about  2.8  pounds. 


ISO 


TEXT-BOOK  OF  PHYSIOLOGY 


Histologic  Changes  in  the  Salivary  Glands  during  Secretion. — 

During  and  after  secretion  very  remarkable  changes  take  place  in  the  cells 
lining  the  acini,  which  are  in  some  way  connected  with  the  production  of  the 
essential  constituents  of  the  salivary  fluids.  In  the  case  of  the  parotid  gland, 
which  may  be  regarded  as  the  type  of  a  serous  or  albuminous  gland,  the 
following  changes  have  been  observed  by  Langley  (Fig.  66).  During  the 
period  of  rest  and  just  previous  to  secretor  activity,  the  epithelial  cells  are 
enlarged  and  swollen,  and  encroach  on  the  lumen  of  the  acinus.  The 
protoplasm  of  the  cells  is  so  completely  filled  with  dark  fine  granules  as  not 
only  to  obscure  the  nucleus,  but  almost  to  obliterate  the  line  of  union  of  the 
cells.  As  soon  as  secretion  becomes  active,  however,  the  granules  begin  to 
disappear  from  the  outer  region  of  the  cell  and  move  toward  the  inner  border 
and  into  the  lumen  of  the  acinus.     From  these  observations  it  might  be 


Fig.  66. — Parotid  Gland  After  Pro- 
longed Activity.  i,i,  Acini;  2,  duct;  ^,;^, 
albuminous  cells  almost  free  of  granules;  4, 
nuclei  clear  and  well  defined.  (Semi-diagram- 
matic.) 


Fig.  67. — Submaxillary  Gland  After 
Prolonged  Activity.  1,1,  Acini;  2,  duct; 
3,3,  mucous  cells  free  of  mucin  and  filled 
with  fine  granules;  4,4,  nuclei  rounded  and 
returned  to  the  center  of  the  cell;  5,5,  cells  of 
Giannuzzi,  large  and  distinct.  {After 
Vialleton.) 


inferred  that  during  rest  the  protoplasm  of  the  cells  gives  rise  to  granular 
material,  and  that  during  and  after  secretor  activity  there  is  an  absorption 
of  new  material  from  the  lymph  and  a  reconstruction  of  the  granular  material. 

In  the  submaxillary  gland,  a  portion  of  which  may  be  taken  as  a  type  of  a 
mucous  gland,  similar  changes  have  been  observed  (Fig.  67).  During  rest 
the  epithelial  cells  are  large,  clear  in  appearance,  highly  refractive,  and  loaded 
with  small  globules  resembling  mucin.  The  nucleus,  surrounded  by  a  small 
quantity  of  protoplasm,  lies  near  the  margin  of  the  cell.  That  the  granules 
are  not  protoplasmic  in  character  is  shown  by  the  fact  that  they  do  not  stain 
on  the  addition  of  carmine.  When  treated  with  water  or  dilute  acids,  the 
globules  swell,  coalesce,  and  form  a  uniform  mass.  The  chemic  relations 
of  this  substance  indicate  that  it  is  the  precursor  of  mucin — namely,  mucigen. 
During  secretor  activity  the  cells  discharge  these  mucigen  granules  into  the 
lumen  of  the  acinus  where  they  are  transformed  into  mucin.  Though  the 
appearance  of  the  gland-cell  indicates  it,  there  is  no  evidence  for  the  view 
that  the  cell  itself  undergoes  disintegration  in  the  process. 

The  Physiologic  Actions  of  Saliva. — The  constant  presence  of  salivary 
glands  in  the  different  classes  of  animals  and  the  large  amount  of  secretion 
they  pour  daily  into  the  alimentary  canal  point  to  the  conclusion  that  this 
mixed  fluid  plays  an  important  r61e  in  the  general  digestive  process.     Experi- 


DIGESTION  -  151 

ment  has  demonstrated  that  it  has  a  two-fold  action;  viz.,  physical  and 
chemical. 

Physically,  saliva  softens  and  moistens  the  food,  unites  its  particles  into 
a  consistent  mass  by  means  of  its  contained  mucin,  and  thus  facilitates 
swallowing. 

Chemically  it  converts  starch  into  sugar.  This  action  is  more  marked 
with  boiled  than  with  raw  starch,  a  fact  which  depends  on  the  physical 
structure  of  the  starch  grain.  In  the  natural  condition  each  starch  grain  con- 
sists of  a  cellulose  envelope  or  stroma  in  the  meshes  of  which  is  contained  the 
true  starch  material,  the  granulose.  When  boiled  for  some  minutes,  the 
starch  grain  absorbs  water,  the  granulose  swells  and  ruptures  the  cellulose 
envelope,  after  which  it  passes  into  an  imperfect  opalescent  solution  more  or 
less  viscid,  depending  on  the  relative  amounts  of  water  and  starch.  This  is 
the  change  largely  brought  about  by  the  process  of  cooking.  If  a  portion  of 
this  hydrated  starch  be  kept  in  the  mouth  for  a  few  minutes  it  will  be  con- 
verted into  sugar,  a  fact  made  evident  by  the  sense  of  taste. 

The  chemic  action  of  saliva  in  converting  starch  into  sugar,  as  well  as  the 
intermediate  stages,  can  be  experimentally  shown  in  the  following  manner: 
To  5  volumes  of  a  thin  starch  solution  in  a  test-tube  add  two  volumes  of 
filtered  saliva.  Place  the  mixture  in  a  water-bath  at  a  temperature  of  35°  C. 
In  a  few  minutes  the  starch  passes  into  a  soluble  condition  and  the  fluid 
becomes  clear  and  transparent.  On  testing  the  solution  from  time  to  time 
with  iodin  the  characteristic  blue  reaction  will  be  found  to  disappear,  gradu- 
ally, the  color  passing  from  blue  to  violet,  to  red,  to  yellow.  If  now  the 
solution  be  boiled  with  a  solution  of  cupric  hydroxid  (Fehling's  solution)  a 
copious  red  or  yellow  precipitate  of  cuprous  oxid  is  formed,  which  indicates 
the  presence  of  sugar.  The  polariscope  shows  that  this  sugar  is  dextro- 
rotatory. During  the  conversion  of  the  starch  intermediate  substances 
are  formed  to  which  the  term  dextrin  is  applied.  After  the  starch  has  been 
rendered  soluble  it  undergoes  a  cleavage  into  maltose  and  a  dextrin,  which, 
as  it  gives  rise  to  a  red  color  with  iodin,  is  termed  erythrodextrin.  At  a 
later  stage  this  erythrodextrin  also  undergoes  a  cleavage  into  maltose 
and  a  second  variety  of  dextrin,  which,  as  it  does  not  give  rise  to  any 
color  with  iodin,  is  termed  achroodextrin.  It  is  claimed  by  some  investi- 
gators that  this  form  can  also  in  time  be  transformed  into  sugar.  It  is 
possible  that  a  small  quantity  of  dextrose  is  also  formed. 

The  successive  stages  of  the  conversion  of  starch  into  sugar  may  be 
represented  by  the  following  schema: 

f  T?_  it,     J     *  •        /  Achroodextrin. 
Starch  =  Soluble  Starch  =.  {  mTS  I  ^*^t°^^- 

This  change  consists  in  the  assumption  by  the  starch  of  a  molecule  of  water, 
and  for  this  reason  the  process  is  termed  hydrolysis.  The  nature  of  the 
chemic  change  is  shown  in  the  following  formula : 

3(CeH,oO,)  +  H,0=.C„H,,0,i  +  CeH,oO, 
Starch  +  Water = Maltose  +  Dextrin. 

The  amylolytic\  amyloclastic,  or  starch-changing  action  of  saliva  depends 

'  The  term  amylolytic  has  been  objected  to  on  the  grovind  that  It  does  not  correctly  express 
the  fact,  but  is  analogous  with  electrolytic  and  means  a  transformation  by  means  of  starch. 
Armstrong  has  suggested  the  use  of  the  term  amyloclastic  as  well  as  proteoclastic  and  lipoclastic 
for  the  terms  now  generally  employed. 


152  TEXT-BOOK  OF  PHYSIOLOGY 

on  the  presence  of  an  unorganized  ferment  or  enzyme  known  as  ptyalin  or 
amylase.  This  enzyme  is  present  in  the  secretion  of  each  of  the  salivary 
glands.  The  chemic  character  of  ptyalin  is  unknown,  though  there  are 
reasons  for  believing  that  it  partakes  of  the  nature  of  a  protein.  It  is  a  prod- 
uct in  all  probability  of  the  katabolic  activity  of  the  secretor  cells.  According 
to  Latimer  and  Warren,  ptyalin  is  a  derivative  of  the  zymogen,  ptyalogen. 
This  latter  compound  has  been  shown  to  be  present  in  the  glands  of  the 
dog,  cat,  and  sheep.  Ptyalin  effects  the  transformation  of  starch  merely 
by  its  presence,  and  undergoes  no  perceptible  consumption  in  the  process. 
The  activity  of  this  enzyme  is  very  great,  and  unless  interfered  with  by  an 
excess  of  sugar  and  dextrin,  it  acts  practically  indefinitely. 

The  activity  of  ptyalin  is  modified  by  various  external  conditions,  among 
which  may  be  mentioned  the  chemic  reaction  of  the  medium  in  which  it  is 
placed.  It  is  most  active  when  the  medium  is  moderately  alkaline.  Its 
activity  is  arrested  by  strong  alkalies  or  acids,  though  the  presence  of  a 
small  percentage  of  an  acid  does  not  appear  to  have  any  effect  in  either 
hastening  or  retarding  the  process.  This  fact  has  a  bearing  upon  the  ques- 
tion as  to  whether  the  action  of  the  saliva  is  interfered  with  in  the  stomach 
by  the  presence  of  the  gastric  juice.  At  present  it  is  a  disputed  matter,  but 
the  weight  of  authority  is  in  favor  of  the  view  that  the  transforming  action 
may  continue  for  almost  half  an  hour  during  the  early  stages  of  gastric 
digestion.  The  temperature  also  influences  the  rapidity  with  which  the 
transformation  of  the  starch  is  effected.  At  a  temperature  of  95°  to  io6°F. 
the  ptyalin  acts  most  energetically,  while  its  activity  is  entirely  arrested  by 
reducing  the  temperature  to  the  freezing-point  or  raising  it  to  the  boiling 
point. 

The  Mode  of  Secretion  of  Saliva. — ^The  secretion  of  saliva  is  a  com- 
plex act  and  involves  the  cooperation  of  gland-cells,  blood-vessels,  efferent 
and  aft'erent  nerves  contained  in  different  cranial  nerves,  and  a  central 
mechanism  by  which  they  are  excited  to  and  coordinated  in  activity.  The 
central  mechanism  is  located  in  the  medulla  oblongata  in  the  gray  matter 
beneath  the  floor  of  the  fourth  ventricle  and  subject  to  cerebral  and  surface 
stimulation. 

During  the  intervals  of  mouth  digestion  the  glands  are  practically  at  rest 
as  far  as  the  discharge  of  saliva  is  concerned.  The  cells,  however,  are 
actively  engaged  in  absorbing  from  the  surrounding  lymph-spaces  materials 
derived  from  the  blood,  out  of  which  they  construct  their  characteristic  con- 
stituents. The  blood-vessels  possess  that  degree  of  dilatation  necessary  for 
nutritive  purposes. 

With  the  development  of  the  sensation  of  hunger,  the  sight  and  the  odor  of 
agreeable  foods  develop  visual  and  olfactory  sensations,  which  give  rise  to 
psychic  states  more  or  less  agreeable.  With  their  development  the  entire 
mechanism  is  excited  to  activity  and  a  more  or  less  abundant  discharge  of 
saliva  into  the  mouth  frequently  takes  place.  This  is,  therefore,  known  as  a 
psychic  secretion.  The  fluid  thus  secreted  has  given  rise  to  the  expression 
"watering  of  the  mouth." 

With  the  introduction  of  food  into  the  mouth  and  the  stimulation  of 
afferent  nerves  (nerves  of  general  sensibility  and  nerves  of  taste)  and  the 
onset  of  mastication,  the  blood-vessels  suddenly  dilate,  the  blood-supply  is 
increased,  and  the  gland-cells  begin  to  discharge  water,  inorganic  salts,  and 


DIGESTION  153 

their  organic  constituents  into  the  lumen  of  the  acinus,  materials  that  col- 
lectively constitute  the  saliva  characteristic  of  any  one  of  the  glands.  This 
continues  until  mastication  ceases,  when  all  the  structures  return  to  their 
former  condition  of  relative  inactivity. 

The  Nerve  Mechanism  of  Insalivation. — ^The  nerves  and  nerve-cen- 
ters that  constitute  the  nerve  mechanism  for  the  secretion  of  saliva,  as  de- 
termined by  experimental  investigations  are  shown  in  the  following  table: 

Afferent  Nerves.  Nerve-centers.  Efferent  Nerves. 

1.  Lingual  and  buccal  branches  of     Medulla  oblongata.     The   chorda   tympani   and   its   post- 

the  trigeminal  nerve.  ganglionic  continuations  for  the  sub- 

maxillary and  sublingual  glands; 
the  glosso-pharyngeal  nerve  and  its 
post-ganglionic  continuations  con- 
tained    in    the    auriculo-temporal 

2.  Taste    fibers    in     the     chorda  branch  of  the  trigeminal  nerve,  for 

tympani.  the  parotid  gland. 

3.  Taste    fibers     in     the    glosso-  The   sympathetic  nerve,  both  pre-  and 

pharyngeal.  post-ganglionic    fibers,    for  all    the 

glands. 

The  Efferent  Nerves. — The  efferent  nerve-fibers,  as  stated  in  the  fore- 
going paragraph,  that  transmit  nerve  impulses  to  the  submaxillary,  sub- 
lingual, and  parotid  glands,  as  well  as  to  their  associated  blood-vessels,  belong 
to  the  autonomic  system,  and  are  contained  respectively  in  the  chorda  tym- 
pani and  its  post-ganglionic  continuations,  in  the  glosso-pharyngeal  and  its 
post-ganglionic  continuations  contained  in  the  auriculo-temporal  branch 
of  the  fifth  nerve,  and  in  the  post-ganglionic  branches  of  the  sympa- 
thetic nerve  derived  from  the  superior  cervical  ganglion.  That  these  nerves 
transmit  the  nerve  impulses  to  the  salivary  apparatus  is  shown  by  the  effects 
that  follow  their  division  and  stimulation. 

The  Chorda  Tympani. — ^The  chorda  tympani  nerve  is  apparently  a  branch  of 
the  facial  (though  it  consists  in  part  of  autonomic  nerve-fibers),  the  trunk  of 
which  it  leaves  in  the  aqueduct  of  Fallopius.  It  then  crosses  the  tympanic 
cavity,  emerges  through  the  Glaserian  fissure,  and  joins  the  lingual  branch 
of  the  inferior  maxillary  division  of  the  fifth  nerve.  After  passing  forward 
as  far  as  the  sublingual  gland,  nearly  all  of  the  fibers  leave  the  lingual  nerve 
by  four  or  five  strands  to  become  connected  by  terminal  branches  with  nerve 
ganglion  cells  in  relation  with  the  submaxillary  and  sublingual  glands. 
(See  Fig.  68.) 

The  effects  on  the  secretion  and  flow  of  saliva  from  the  submaxillary 
gland  which  follow  division  and  stimulation  of  the  chorda  tympani  nerve 
are  shown  in  the  following  way:  A  cannula  is  inserted  into  Wharton's  duct 
and  the  rate  of  flow  estimated;  the  nerve  is  then  divided,  after  which  the 
flow  ceases.  The  peripheral  end  of  the  nerve  is  then  stimulated  with 
induced  electric  currents  when  a  copious  secretion  of  a  thin  saliva  takes 
place,  accompanied  by  a  marked  dilatation  of  the  blood-vessels  of  the  gland. 
The  quantity  of  blood  passing  through  the  vessels  is  so  great  as  to  give  to 
the  venous  blood  an  arterial  hue  and  to  the  small  veins  a  distinct  pulsation. 
It  would  appear  from  these  effects  that  the  chorda  contains  two  sets  of  fibers, 
one  of  which  inhibits  the  action  of  a  local  vaso-motor  mechanism  permitting 
the  blood-vessels  to  dilate  (vaso-dilatator  fibers),  the  other  of  which  stimu- 
lates the  secretor  cells  to  activity,  through  the  intermediation  of  local  ganglia. 


154 


TEXT-BOOK  OF  PHYSIOLOGY 


That  local  ganglia  are  involved  is  shown  by  the  effects  which  follow  the 
injection  of  nicotin  into  the  circulation.  After  a  sufficient  dose — lo  milli- 
grams for  the  cat — stimulation  of  the  chorda  has  no  effect.  Stimulation  of 
the  nerve-plexus  beyond  the  ganglion,  the  postganglionic  libers,  however,  is 
at  once  followed  by  vascular  dilatation  and  secretion,  a  fact  that  would 
indicate  that  the  ganglia  are  not  only  stimulated  by  the  chorda  tympani  but 
that  the  conductivity  of  the  nerve  endings  of  the  chorda  around  the  ganglia 
is  impaired.    . 

5'JiMervt 

Glosso-Pnaryn^eal 

Otic  Gantjhon. 
larotid  Gland, 


Jaco6sej?'s  Jverve 


duhMaxillory  Gan^lion^    ,''-'' 


Siih  Maxillary  Gland^ 

uAorda.J^mpani^rve, 


OeroicaX  uanqlian 
Sympathetic  Nerves 


Fig.  68. — Scheme  of  the  Nerves  Involved  in  the  Secretion  of  Saliva. 

It  might  be  inferred  that  the  increase  in  the  flow  of  saliva  is  due  to  filtra- 
tion, the  result  of  the  increased  blood-supply  to  the  gland,  and  not  to  the 
influence  of  any  true  secretor  fibers  stimulating  the  activities  of  the  secretor 
cells.  That  this  is  not  the  case,  however,  can  be  demonstrated  in  several 
ways:  first,  the  pressure  in  the  duct  of  the  submaxillary  gland,  as  shown 
by  the  mercurial  manometer,  rises,  when  the  gland  is  secreting,  considerably 
above  the  pressure  in  the  carotid  artery,  which  could  not  be  the  case  if  it 
were  due  to  a  mere  filtration;  for  if  pressure  alone  were  the  cause,  the  flow 
of  saliva  would  cease  as  soon  as  the  pressure  in  the  tube  equaled  the  pressure 
in  the  blood-vessels.  Second,  even  in  the  absence  of  blood  the  gland  can  be 
made  to  yield  a  secretion,  as  shown  by  stimulating  the  nerve  in  a  recently 
killed  animal.  Third,  after  the  injection  of  atropin  into  the  circulation  the 
secretion  is  abolished,  but  the  local  vaso-motor  mechanism  is  unimpaired, 
for  stimulation  of  the  nerve,  as  in  the  previous  instance,  gives  rise  to  a  dilata- 
tion of  the  vessels  and  an  increased  blood-supply.  There  is  thus  abundant 
proof  that  the  chorda  tympani  contains  two  sets  of  fibers — one  regulating 
the  blood-supply  to  the  gland,  the  other  stimulating  the  secretor  cells. 

The  efferent  fibers,  vaso-motor  and  secretor,  which  constitute  in  part 


DIGESTION  155 

the  chorda  tympani  nerve  have  their  origin  in  cells,  the  nucleus  salivatorius, 
located  beneath  the  floor  of  the  fourth  ventricle,  from  which  they  emerge  in 
the  nerve  of  Wrisberg  or  pars  intermedia,  and  enter  the  trunk  of  the  facial 
nerve  at  the  bottom  of  the  internal  auditory  canal  after  which  they  pursue 
the  course  stated  above. 

The  Glosso-pharyngeal  Nerve.- — ^The  autonomic  nerve-fibers  that  conduct 
nerve  impulses  outward  from  the  medulla  to  the  parotid  gland  are  believed 
to  pass  through  the  glosso-pharyngeal  nerve,  through  the  tympanic  branch  or 
nerve  of  Jacobson,  to  the  otic  ganglion,  with  which  they  become  connected. 
From  this  ganglion  new  nerve-fibers  arise  which  pass  into  the  fifth  nerve  and 
reach  the  secretor  cells  of  the  parotid  gland  through  tJie  auriculo-temporal 
nerve.  The  trunk  of  this  latter  nerve  contains  therefore  post-ganglionic 
fibers  that  bear  the  same  relation  to  the  parotid  gland  and  blood-vessels 
that  the  postganglionic  fibers  from  the  submaxillary  ganglion  bear  to  the 
submaxillary  gland  and  blood-vessels. 

The  influence  of  the  efferent  fibers  in  the  trunk  of  the  glosso-pharyngeal 
on  the  parotid  gland  is  similar  to  the  influence  of  the  chorda  tympani  on  the 
submaxillary  gland;  for  if  the  glosso-pharyngeal  nerve  or  its  post-ganglionic 
continuations  in  the  auriculo-temporal  nerve  be  stimulated  in  any  part  of  its 
course  with  induced  electric  currents  there  follows  a  dilatation  of  the  blood- 
vessels and  an  abundant  discharge  of  a  thin  saliva  rich  in  water  and  salts 
but  poor  in  the  amount  of  organic  matter.  Division  of  the  glosso-pharyngeal 
nerve,  extirpation  of  the  otic  ganglion  or  division  of  the  auriculo-temporal 
nerve  is  followed  by  a  loss  of  reflex  secretion.  Stimulation  of  the  branch 
connecting  the  glosso-pharyngeal  with  the  otic  ganglion  (Jacobson's  nerve) 
gives  rise  to  the  secretion  as  shown  by  Heidenhain.  Division  of  the  nerve 
is  also  followed  by  a  loss  of  reflex  secretion. 

The  Sympathetic  Nerves. — ^The  autonomic  nerve-fibers  which  influence 
the  salivary  secretion  emerge  from  the  spinal  cord  mainly  through  the  second, 
third,  and  fourth  thoracic  nerves.  After  passing  into  the  sympathetic  chain 
they  ascend  to  the  superior  cervical  ganglion,  with  the  cells  of  which  they 
become  connected  through  the  intermediation  of  fine  terminal  branches. 
From  this  ganglion  non-meduUated  nerve-fibers — sympathetic  nerves  proper 
— follow  the  branches  of  the  external  carotid  artery  to  the  different  glands. 
There  is  no  evidence  that  these  fibers  have  any  connection,  anatomic  or 
physiologic,  with  local  ganglia  at  or  near  the  submaxillary,  sublingual,  or 
parotid  glands.  If  the  sympathetic  nerve  in  the  neck,  especially  in  the  dog, 
be  divided  and  the  peripheral  end  stimulated  with  induced  electric  currents, 
there  is  at  once  a  contraction  of  the  smaller  blood-vessels  of  the  submaxillary 
and  sublingual  glands  and  a  diminution  of  the  blood-supply,  a  result  showing 
the  presence  of  vaso-constrictor  fibers.  Nevertheless  both  the  submaxillary 
and  sublingual  glands  pour  out  a  saliva  which  is  different  from  that  poured 
out  when  the  chorda  tympani  is  stimulated.  The  quantity  is  less,  it  is  more 
viscid,  richer  in  organic  matter,  of  a  higher  specific  gravity,  and  more  active  in 
the  transformation  of  starch  into  sugar. 

Stimulation  of  the  sympathetic  fibers  passing  to  the  parotid  gland  is 
followed  by  contraction  of  the  vessels  and  an  abolition  of  the  secretion;  but 
at  the  same  time  there  is  an  increased  activity  of  the  secretor  cells,  for  sub- 
sequent stimulation  of  the  auriculo-temporal  nerve  not  only  causes  an 
increase  in  the  amount  of  water  and  inorganic  salts,  but  an  increase  also  in 


156  TEXT-BOOK  OF  PHYSIOLOGY 

the  amount  of  organic  matter  far  beyond  that  produced  when  the  auriculo- 
temporal alone  has  been  stimulated.  Histologic  examination  shows  that 
the  small  ducts  of  the  gland  are  iGilIed  with  thick  organic  matter  after  stimula- 
tion of  the  cervical  portion  of  the  sympathetic  chain. 

The  foregoing  facts  led  Heidenhain  to  the  conclusion  that  there  are  two 
physiologically  distinct  efferent  nerve-fibers  distributed  to  the  glands,  viz., 
trophic  nerves,  derived  from  the  sympathetic  which  stimulate  the  cells  to  the 
production  of  organic  constituents;  and  secretor  nerves,  derived  from  the 
cranial  nerves,  chorda  tympani  and  glosso-pharyngeal,  which  stimulate 
the  cells  to  the  production  of  water  and  inorganic  salts.  This  view  has, 
however,  been  controverted  by  Langley,  who  regards  the  secretor  fibers  to  the 
glands  as  essentially  the  same,  and  considers  the  differences  in  the  character 
of  the  secretion  to  be  dependent  on  differences  in  the  quantity  of  the  blood- 
supply  induced  by  the  simultaneous  stimulation  of  the  vaso-motor  nerves. 

The  Central  Mechanism. — The  central  mechanism  that  excites  the 
glands  and  blood-vessels  to  activity  through  efferent  nerves  originating  in  its 
cells  maybe  aroused  to  action  (i)  by  nerve  impulses  descending  from  the  cere- 
brum in  consequence  of  psychic  states  induced  by  the  sight  or  the  odor  of 
foods  especially  after  long  abstinence;  (2)  by  nerve  impulses  transmitted 
through  afferent  nerves  from  the  mouth,  developed  by  the  contact  of  the  food 
with  the  peripheral  terminations  of  the  gustatory  or  general  sensor  nerves. 

That  psychic  states,  ideas  and  feelings  aroused  by  the  sight,  odor,  and 
contemplation  of  food  can  give  rise  to  a  stimulation  of  the  cells  of  the  central 
mechanism  in  the  manner  just  stated  is  shown  by  the  flow  of  saliva  which  is 
familiarly  known  as  watering  of  the  mouth.  This  fact  has  been  experimen- 
tally demonstrated  by  Pavlov  on  dogs.  This  investigator  caused  the  ducts 
of  the  glands  to  be  brought  to  the  surface  in  such  a  manner  that  they  healed 
into  the  edges  of  the  skin  wounds.  By  means  of  suitable  receivers  applied 
over  the  orifices  of  the  ducts  the  saliva  could  be  readily  collected.  When  the 
dog  under  such  circumstances  was  tempted  by  the  sight  of  foods  there  was 
at  once  a  free  discharge  of  saliva,  the  quantity  and  quality  of  which  varied 
with  the  character  of  the  foods. 

That  the  central  mechanism  can  be  excited  to  activity  by  nerve  impulses 
reflected  from  the  periphery  can  be  demonstrated  by  the  introduction  of  food 
into  the  mouth,  as  well  as  by  stimulation  of  the  branches  of  the  afferent 
nerves  distributed  to  the  mouth  which  constitute  the  afferent  part  of  this 
mechanism. 

The  Afferent  Nerves. — The  afferent  nerves  that  transmit  nerve  impulses 
from  the  mouth  to  the  central  mechanism,  are  the  taste  fibers  in  the  chorda 
tympani,  the  taste  and  sensor  fibers  of  the  glosso-pharyngeal,  and  the  sensor 
fibers  of  the  lingual  and  buccal  branches  of  the  trigeminal  nerve.  This  is 
shown  by  the  fact  that  if  they  are  transversely  divided  there  is  a  cessation  of 
the  discharge  of  saliva  when  the  peripheral  nerve  endings  in  the  mouth  are 
stimulated  by  the  presence  of  food.  With  these  nerves  intact  the  introduc- 
tion of  food  into  the  mouth  will  invariably  be  followed  by  a  flow  of  saliva. 
Pavlov  has  apparently  demonstrated  that  this  general  fact  must  be  supple- 
mented by  the  further  fact,  that  there  is  a  special  adaptation  between  the 
character  of  food  and  the  different  glands.  Thus,  solid  dry  foods,  cause  a 
large  flow  of  a  thin  saliva  from  the  parotid  glands,  but  a  slight  flow  from  the 
submaxillary;  moist  foods  and  especially  meat  causes  a  large  flow  from  the 


DIGESTION  157 

submaxillary  gland,  but  a  slight  flow  from  the  parotid.  It  is  also  probable 
that  the  glands  respond  by  discharging  a  secretion  of  special  quahty  in 
accordance  with  the  properties  of  the  different  foods. 

Stimulation  of  the  afferent  nerves  with  induced  electric  currents  also  gives 
rise  to  a  discharge  of  saliva.  This  can  be  demonstrated  by  exposing  the 
glands  and  the  afferent  nerves  and  subjecting  them  to  experiment.  Under 
such  circumstances,  if  a  cannula  be  placed  in  the  duct  of  the  submaxillary 
gland,  and  the  lingual  nen^e  stimulated  by  induced  electric  currents  of 
moderate  strength,  a  copious  flow  of  saliva  at  once  takes  place.  If  now  the 
glosso-pharyngeal  nerve  or  the  central  end  of  the  divided  chorda  tympani 
nerve  be  stimulated  in  a  similar  manner,  the  effect  on  the  secretion  will  be 
the  same.  Division  of  these  nen^es  in  an  animal,  in  such  a  way  as  to  prevent 
the  nerve  impulses  from  reaching  the  medulla  oblongata,  is  followed  by  a 
marked  diminution  in  the  amount  of  saliva  secreted.  The  reflex  centers, 
however,  may  receive  impulses  and  be  excited  to  activity  by  impulses  coming 
through  other  nerves — e.g.,  the  pneumogastric,  when  the  mucous  membrane 
of  the  stomach  is  stimulated;  the  sciatic,  when  after  division,  its  central  end 
is  stimulated. 

Resrnne  of  the  Factors  Involved  in  the  Secretion  of  Saliva. — From 
the  foregoing  statements  it  is  apparent  that  the  secretion  of  saliva  is  a  complex 
act  involving  the  cooperation  of  several  different  factors.  As  the  mechanism 
for  the  elaboration  of  this  secretion  is  typical  of  that  for  many  secretions  it 
will  be  of  advantage  to  summarize  these  factors  and  their  specific  functions. 
These  are  as  follows: 

1.  Epithelial  cells,  the  physiologic  actions  of  which    are    the    production 

of  the  specific  characteristic  constituents  of  the  saliva,  e.g.,  mucin, 
albumin,  the  enzyme  ptyalin,  as  well  as  the  absorption  and  discharge  of 
water  and  inorganic  salts. 

2.  Lymph,  which  contains  the  nutritive  material  necessary  for  the  growth, 

repair,  and  metabolic  activities  of  the  secreting  cells. 

3.  Capillary  blood-vessels,  which  permit  the  passage  of  those  constituents  of 

the  blood  that  collectively  constitute  lymph. 

4.  V aso-motor  nerves,  some  of  which  at  the  beginning  of  secretor  activity 

dilate  the  blood-vessels  and  thus  increase  the  blood-supply  and  the 
production  of  lymph  (vaso-dilatator  nerves);  others  of  which  at  the 
end  of  secretor  activity  perhaps  actively  contract  the  blood-vessels 
and  thus  decrease  the  blood-supply  to  the  previous  condition  (vaso- 
constrictor nerves). 

5.  Secretor  nerves,  which  stimulate  the  epithelial  cells  to  increased  activity 

causing  them  to  discharge  their  specific  metabolic  constituents  along 

with  water  and  inorganic  salt  in  characteristic  proportions  from  the 

orifices  of  the  gland  ducts  (secreto-motor  nerves). 

The  central  mechanism  is  excited  to  coordinate  activity,  primarily,  by 
nerve  impulses  descending  from  the  cerebrum  as  a  result  of  psychic  states 
developed  by  the  sight  and  odor  of  food,  and  secondarily,  by  nerve  impulses, 
transmitted  by  the  nerves  of  gustation  and  general  sensibility  and  developed 
by  the  contact  of  food  on  their  peripheral  terminations  during  the  act 
of  mastication. 

Modifications  of  the  Nerve  Mechanism  of  Insalivation  due  to  the 
Physiologic  Action  of  Drugs. — The  functions  of  different  portions  of 


158  TEXT-BOOK  OF  PHYSIOLOGY 

the  nerve  mechanism  of  insalivation  may  be  made  apparent  by  an  analysis  of 
the  effects  that  follow  the  administration  of  physiologic  or  slightly  toxic 
doses  of  the  alkaloids  of  various  drugs.  The  effects  can  be  shown  to  be  due 
to  a  depression  or  stimulation  of  the  normal  activity  of  one  or  more  portions 
of  the  mechanism.  As  a  result  the  secretion  may  be  decreased  or  increased 
in  volume.  The  following  examples  will  illustrate  the  action  of  alkaloids 
in  general. 

Nicotin. — When  nicotin  in  sufficiently  large  doses  is  given  to  an  animal 
hypodcrmatically,  the  secretion  of  saliva  after  a  variable  period  of  time  ceases 
and  the  mouth  becomes  dry.  If  the  chorda  tympani  nerve,  i.e.,  the  pre- 
ganglionic portion,  be  then  stimulated  with  induced  electric  currents  the 
usual  phenomenon,  viz.,  a  free  flow  of  saliva,  fails  to  occur.  If,  however, 
the  nerve  branches  emerging  from  the  submaxillary  ganglion,  i.e.,  the  post- 
ganglionic portion,  be  stimulated  with  electric  currents,  the  saliva  will  be 
discharged  as  usual.  The  inference  is  that  the  conductivity  of  the  peripheral 
terminations  of  the  preganglionic  chorda  fibers  is  depressed  so  that  the  nerve 
impulses  discharged  by  the  central  mechanism  fail  to  reach,  and  therefore  to 
stimulate,  the  submaxillary  ganglion  cells.  The  inference  as  to  the  seat  of 
action  of  nicotin  is  supported  by  the  fact  that  painting  the  surface  of  the 
superior  cervical  sympathetic  ganglion  with  nicotin  will  impair  the  conductiv- 
ity of  the  terminal  branches  of  the  pre-ganglionic  fibers  emerging  from  the 
cord  so  that  stimulation  of  these  fibers  fails  to  produce  beyond  the  ganglion 
the  usual  secretor  effects.  It  is  probable  that  nicotin  has  a  similar  action 
on  the  peripheral  terminations  of  Jacobson's  nerve  which  arborize  around 
the  nerve  cells  of  the  otic  ganglion. 

Atropin. — Atropin  in  doses  of  i  milligram  also  causes  a  complete  cessa- 
tion in  the  flow  of  saliva  and  consequently  an  extreme  dryness  of  the  mouth. 
After  the  occurrence  of  this  condition  neither  stimulation  of  the  pre-ganglionic 
chorda  tympani  fibers  nor  of  the  post  ganglionic  fibers,  will  cause  the  glands  to 
secrete.  But  as  stimulation  of  the  sympathetic  nerve  in  the  cervical  region 
will  excite  a  secretion  the  inference  is  that  the  atropin  exerts  a  depressing 
effect  on  the  conductivity  of  the  nerve  endings  in  contact  with  the  gland  cells 
thus  interfering  with  the  transmission  of  nerve  impulses,  rather  than  on  the 
gland  cells  themselves.  The  same  holds  true  for  the  nerve  terminations  in  the 
post-ganglionic  fibers  distributed  to  the  parotid  gland.  The  action  of  atropin 
is  not  limited,  however,  to  the  nerve  terminations  in  connection  with  salivary 
glands  but  extends  to  the  nerve  terminations  in  connection  with  many  other 
glands  in  the  alimentary  canal  and  skin.  Even  though  the  dose  of  atropin 
be  large,  10  to  15  milligrams  for  a  dog,  its  action  is  confined  to  the  terminal 
nerve-fibers  in  connection  with  the  gland  cells,  for  when  the  chorda  tympani 
is  stimulated  the  blood-vessels  around  the  gland  dilate  as  usual,  a  fact  which 
indicates  that  the  submaxillary  ganglion  gives  off  fibers  of  a  vaso-dilatator  as 
well  as  a  secretor  character.  Unless  the  dose  of  atropin  be  largely  increased, 
e.g.,  100  milligrams,  it  fails  to  depress  the  conductivity  of  the  terminals  of 
the  sympathetic  nerve-fibers. 

Pilocarpin. — Pilocarpin  in  small  doses,  from  2  to  5  milligrams,  hypo- 
dcrmatically causes  in  the  cat  a  free  flow  of  saliva  which  may  amount  to  half 
a  liter  or  more  in  the  course  of  several  hours.  In  human  beings  its  effect  on 
the  flow  of  saliva  is  equally  marked.  Ringer  reports  that  in  two  patients, 
after  taking  a  medicinal  dose,  the  amount  of  saliva  discharged  was  622  c.c. 


DIGESTION  159 

and  764  c.c.  respectively.  Since  division  of  the  nerves  both  pre-  and  post- 
ganglionic, does  not  diminish  or  abolish  the  secretion,  the  inference  is  that 
the  pilocarpin  exerts  a  stimulating  action  on  the  nerve  endings  in  connection 
with  the  gland  cells.  This  inference  is  strengthened  by  the  fact  that  the 
pilocarpin  effect  is  antagonized  and  the  secretion  checked  by  a  suitable  dose 
of  atropin,  the  seat  of  the  action  of  which  is  known.  The  two  alkaloids  thus 
appear  to  be  in  physiologic  antagonism  in  their  action  on  these  nerve  termi- 
nations. The  action  of  pilocarpin  is  not  limited  to  the  salivary  glands,  but 
extends  to  glands  found  in  the  alimentary  canal,  respiratory  passages,  and 
skin. 

.     DEGLUTITION 

Deglutition  is  that  part  of  the  digestive  process  which  is  concerned  in 
the  transference  of  the  food  from  the  mouth  through  the  pharynx  and 
esophagus  into  the  stomach.  This  is  an  extremely  complex  act  and  involves 
the  action  of  a  large  number  of  structures,  all  of  which  are  made  to  act  in 
proper  sequence  under  the  coordinating  influence  of  the  nerve  system.  The 
deglutitory  canal  consists  of  the  mouth,  pharynx,  and  esophagus,  each  of 
which  presents  certain  anatomic  features  on  which  its  physiologic  action 
depends. 

The  Mouth. — ^The  cavity  of  the  mouth  is  bounded  in  front  and  on  the 
sides  by  the  alveolar  arches  and  the  teeth;  the  roof  Is  formed  by  the  hard 
palate  and  the  floor  by  the  tongue.  Anteriorily,  the  mouth  communicates 
with  the  exterior  by  a  transverse  opening  the  buccal  orifice,  and  posteriorly 
with  the  pharynx  by  an  opening,  the  isthmus  of  the  fauces.  The  cavity  of 
the  mouth  is  lined  by  mucous  membrane  in  which  are  Imbedded  numerous 
mucous  glands.  The  Interior  of  the  mouth  Is  separated  In  part  from  the 
interior  of  the  pharynx  by  an  Incomplete  septum  formed  above  by  the  soft 
palate,  laterally  by  the  anterior  and  posterior  palatal  arches  and  below  by 
the  tongue. 

The  Pharynx. — ^The  pharynx  is  an  oval-shaped  cavity  extending  from 
the  base  of  the  skull  to  the  lower  border  of  the  cricoid  cartilage,  a  distance 
of  about  12  centimeters.  Its  walls  are  formed  mainly  by  three  pairs  of 
muscles — the  superior,  middle,  and  Inferior  constrictors — each  consisting 
of  red,  striated  muscle-fibers,  and  hence  capable  of  rapid  and  energetic  con- 
tractions. Superiorly  the  pharynx  Is  attached  to  and  supported  by  the 
basilar  process  of  the  occipital  bone;  Inferiorly  it  becomes  continuous  with 
the  esophagus.  The  anterior  wall  of  the  pharynx  is  Imperfect  and  presents 
openings  which  communicate  with  the  nasal  chambers,  the  mouth,  and  the 
larynx.  The  lateral  w^all  on  either  side  presents  the  opening  of  the  Eustachian 
tube  which  leads  directly  Into  the  cavity  of  the  middle  ear.  The  Interior  of 
the  pharynx  Is  lined  by  mucous  membrane.  The  pharynx  is  partially  sepa- 
rated from  the  mouth  by  the  velum  pendulum  palati,  a  muscular  structure 
attached  above  to  the  hard  palate;.  Its  lower  edge  or  border  Is  directed 
downward  and  backward  and  presents  In  the  middle  line  a  conical  process, 
the  uvula.  On  either  side  the  palate  presents  two  curved  arches,  the  anterior 
and  posterior,  formed  respectively  by  the  palato-glossel  and  palato-pharyngei 
muscles.  The  superior  laryngeal  aperture  Is  placed  just  beneath  the  base 
of  the  tongue.  It  Is  triangular  in  shape,  wide  in  front,  narrow  behind,  and 
directed  downward  and  backward.     It  is  bounded  above  by  a  thin  plate  of 


i6o  TEXT-BOOK  OF  PHYSIOLOGY 

cartilage,  the  epiglottis,  placed  just  behind  the  tongue  and  so  arranged  that 
it  can  easily  be  depressed  and  elevated. 

The  Esophagus. — ^The  esophagus,  the  continuation  of  the  deglutitory 
canal,  extends  downward  from  the  lower  border  of  the  cricoid  cartilage  for  a 
distance  of  from  22  to  25  centimeters,  to  a  point  opposite  the  ninth  thoracic 
vertebra,  where  it  expands  into  the  stomach.  Its  walls  are  composed  of  an 
internal  or  mucous  and  an  external  or  muscle  coat,  united  by  areolar  tissue. 
The  muscle  coat  consists  of  an  external  layer  of  longitudinal  fibers  arranged 
in  three  bands  and  of  an  internal  layer  composed  of  fibers  arranged  circularly 
in  the  upper  part  and  obliquely  in  the  lower  part  of  the  esophagus.  In  the 
upper  third  the  fibers  are  striated;  in  the  middle  third  they  are  a  mixture  of 
both  striated  and  non-striated;  in  the  lower  third  they  are  entirely  non- 
striated. 

The  muscle-fibers  surrounding  the  esophago-gastric  orifice  are  arranged 
in  the  form  of  and  play  the  part  of  a  sphincter  muscle,  and  for  this  reason 
may  be  termed  the  sphincter  cardicB  muscle.  By  its  action  it  prevents  a 
return  under  normal  conditions  of  food  into  the  esophagus. 

The  deglutitory  act  may  be  for  convenience  divided  into  three  stages,  viz. : 

1.  The  passage  of  the  food  from  the  mouth  into  the  pharynx. 

2.  The  passage  of  the  food  through  the  pharynx  into  the  esophagus. 

3.  The  passage  of  the  food  through  the  esophagus  into  the  stomach. 

In  the  first  stage  the  bolus  of  food  is  placed  on  the  superior  surface  of  the 
tongue.  The  mouth  is  then  closed  and  respiration  is  momentarily  sus- 
pended. The  tip  of  the  tongue  is  placed  against  the  posterior  surfaces  of  the 
teeth.  The  tongue,  by  reason  of  its  intrinsic  musculature,  then  arches  from 
before  backward  against  the  roof  of  the  mouth  and  by  the  contraction  of  the 
hyoglossus  muscle  pushes  the  bolus  of  food  through  the  isthmus  of  the  fauces 
into  the  pharynx.  This  completes  the  first  stage.  It  is  a  voluntary  effort 
and  accomplished  partly  by  the  tongue,  though,  as  shown  by  Meltzer,  mainly 
by  the  mylohyoid  muscles. 

The  second  and  third  stages,  or  the  passage  of  the  food  through  the 
pharynx  and  esophagus  into  the  stomach,  have  been  attributed  until  quite 
recently  entirely  to  peristaltic  movements  of  their  musculature.^  It  has  been 
stated  that  with  the  passage  of  the  food  through  the  isthmus  of  the  fauces  the 
posterior  wall  of  the  pharynx  advances  and  seizes  the  food,  which,  in 
consequence  of  a  rapid  peristaltic  movement  running  through  the  constrictor 
muscles  from  above  downward  is  transferred  to  the  esophagus;  that  with  the 
entrance  of  the  food  into  the  esophagus  a  similar  peristalsis,  varying  in  rapidity 
in  different  sections  in  consequence  of  a  change  in  the  character  of  its 
musculature,  gradually  transfers  the  food  into  the  stomach.  There  can  be 
but  slight  doubt  that  by  this  method  the  bolus  of  food,  especially  if  it  is  of  firm 
consistence  and  of  a  size  sufficient  to  distend  the  esophagus,  is  transferred 
into  the  stomach,  but  that  it  is  not  the  case  with  liquids  has  been  demon- 
strated by  Kronecker,  Falk,  and  Meltzer. 

In  1880  the  first  of  these  experimenters  made  the  observation  that  the 
sensation  in  the  stomach  following  the  swallowing  of  a  mouthful  of  cold  water 
occurred  too  quickly  to  be  explained  by  the  prevalent  belief  that  its  transfer- 

*  Peristalsis  may  be  defined  as  a  progressive  wave-like  movement  which  passes  over  different 
portions  of  the  walls  of  the  aUmentary  canal.  Its  effect  physiologically  is  the  propulsion  of  its 
solid  and  semisoUd  contents.  It  is  characterized  by  a  contraction  of  the  muscle-fibers  behind 
the  object  and  an  inhibition  or  relaxation  of  the  muscle-fibers  in  front  of  it.     (Bayhss  and  StarUng.) 


DIGESTION 


i6i 


ence  was  caused  by  ordinary  peristalsis,  the  rate  of  progression  of  which  was 
known  to  be  slow.  Falk  then  discovered  the  fact,  by  introducing  through  the 
mouth  into  the  pharynx  a  tube  connectedexternally  with  a  water  manome- 
ter, that  during  the  act  of  swallowing  there  is  a  sudden  rise  of  pressure 
equal  to  about  twenty  centimeters  of  water. 

These  experiments  demonstrated  that  at  the  beginning  of  deglutition 
there  is  a  sudden  rise  of  pressure,  the  result  of  a  quickly  acting  force  resident 
in  the  mouth  or  pharynx,  in  consequence  of  which  the  liquid  foods  are 
rapidly  thrown  down  toward  the  end  of  the  esophagus,  peristalsis  playing 
no  part  in  the  process.  The  proof,  however,  of  these  statements  was  fur- 
nished by  Meltzer.  This  observer  introduced  into  the  pharynx  and  esophagus 
rubber  tubes,  the  ends  of  which  were  provided  with  thin-walled  rubber  bal- 
loons which  could  be  distended  with  air.  The  outer  ends  of  the  tubes  were 
connected  with  Marey's  recording  tambours.  Any  compression  of  the  bal- 
loon would  be  followed  by  the  passage  of  the  air  into  the  tambour  and  an 
elevation  of  the  lever.  With  one  balloon  in  the  pharynx  and  the  other  in 
the  esophagus  at  varying  depths,  and  the  recording  levers  of  the  tambours 


■           -     -V 

/-— '         ■       :. 

o    ,    ■                                                                          ... 

.3 -^ 

L_l                '_;           1^               LJ             1 •             L_l             !_!               L_J 

Fig.  6g. — Tracing  of  the  Act  of  Deglutition,  i.  A  indicates  the  compression  of  the 
elastic  bag  caused  by  the  bolus  projected  by  the  contraction  of  the  mylohyoid  muscles.  B.  Con- 
traction of  the  pharjTix.  2.  Line  marking  seconds.  3.  Tracing  of  the  bag  in  the  esophagus 
12  cm.  from  the  teeth.  C.  Compression  of  the  bag  by  the  bolus  corresponding  to  A.  D.  Com- 
pression by  the  residues  of  the  bolus  carried  on  by  the  contraction  of  the  pharynx,  B.  E.  Contrac- 
tion of  the  esophagus. — (Landois  and  Stirling.) 

applied  against  the  surface  of  a  revolving  cylinder,  it  became  possible,  with 
the  addition  of  a  chronograph,  to  obtain  a  graphic  representation  of  the  time 
relations  of  simultaneous  and  successive  compressions  of  the  two  balloons. 

It  was  found  as  the  result  of  many  experiments  that  no  matter  how  deep 
the  position  of  the  esophageal  balloon,  it  was  compressed  almost  simultane- 
ously with  the  pharyngeal  balloon,  as  shown  by  the  rise  of  the  levers  on 
swallowing  a  mouthful  of  water.  The  interval  of  time  between  the  rise  of 
the  two  levers  did  not  amount  to  more  than  the  tenth  of  a  second.  The 
inference  was  that  the  water  was  projected  or  shot  down  the  pharynx  and 
esophagus  in  this  period  of  time,  and  in  its  passage  compressed  both  balloons 
practically  at  the  same  instant.  The  same  was  found  to  be  true  when  small 
masses  of  more  consistent  food  were  swallowed. 

The  curves  of  the  entire  deglutitive  act  recorded  by  the  two  levers  are, 
however,  different  in  form.  (See  Fig.  69.)  The  pharyngeal  curve,  i, 
presents  two  crests,  the  first.  A,  being  due  to  the  compression  caused  by  the 
passage  of  the  bolus,  the  second,  B,  due  to  the  compression  exerted  by  the 
contraction  of  the  pharyngeal  muscles.     The  interv^al  of  time  between  these 


1 62  TEXT-BOOK  OF  PHYSIOLOGY 

two  crests  amounts  to  not  more  than  0.3  second.  In  the  esophageal  curve, 
3,  the  elevation,  C,  corresponds  to  the  elevation.  A,  and  is  likewise  due  to 
the  compression  exerted  by  the  bolus.  The  interval  of  time  between  the 
beginning  of  the  first  and  second  curves  was  not  more  than  o.i  second, 
regardless  of  the  depth  to  which  the  esophageal  balloon  was  plunged.  At  a 
later  period  a  second  rise  of  the  lever  was  recorded;  the  time  of  its  appear- 
ance, height,  duration,  etc.,  were  found  to  increase  with  the  depth  of  the 
balloon. 

These  facts  demonstrate  that  deglutition  consists  of  two  phases:  (i)  a 
rapid  rise  of  pressure  in  the  pharynx,  as  a  result  of  which  liquid  or  semi- 
liquid  foods  are  suddenly  shot  down  to  the  lower  end  of  the  esophagus;  (2) 
a  peristaltic  contraction  of  the  musculature  of  the  canal,  which,  acting  as  a 
supplementary  force,  carries  onward  any  particles  of  food  in  the  canal 
and  forces  the  bolus  through  the  closed  sphincter  cardicE  at  the  end  of  the 
esophagus. 

The  immediate  cause  of  the  sudden  rise  of  pressure  was  shown  by 
Meltzer  to  be  the  contraction  of  the  mylohyoid  muscles.  When  the  nerves 
going  to  these  muscles  were  divided  in  a  dog,  deglutition  was  practically 
abolished.  These  muscles  are  probably  assisted  in  their  action  by  the 
contraction  of  the  hyoglossus  muscles  as  well  as  the  tongue  itself. 

The  time  required  for  a  mouthful  of  liquid  food  to  pass  to  the  lower  end 
of  the  esophagus  is  approximately  about  o.i  second.  If  the  cardiac  orifice 
is  normally  closed,  a  period  of  about  6  or  7  seconds  may  elapse  before  the  on- 
coming peristaltic  wave  reaches  the  end  of  the  esophagus  and  forces  the 
fluid  into  the  stomach.  If,  however,  a  series  of  deglutitory  acts  follow  one 
another  in  quick  succession  there  is  an  inhibition  of  the  cardiac  sphincter  and 
the  peristaltic  wave  until  after  the  last  swallow.  The  time  required  for  the 
food  to  pass  down  the  esophagus  and  into  the  stomach  may  vary  in  different 
animals  and  in  different  human  beings. 

An  examination  of  the  action  of  the  esophagus  during  deglutition,  made 
by  Cannon  and  Moser  with  x-rays  and  the  fluoroscope,  disclosed  the  fact 
that  the  method  of  food  transmission  varied  in  different  animals.  In  the 
cat  and  dog  the  transmission  was  effected  by  peristalsis  alone.  The  time 
required  for  the  food  to  reach  the  stomach  varied  in  the  cat  from  nine  to 
twelve  seconds  and  in  the  dog  from  four  to  five  seconds.  The  descent  of 
the  bolus  was  more  rapid  in  the  upper  than  in  the  lower  part  of  the  esophagus. 
In  man,  liquids  descended  rapidly,  at  the  rate  of  several  feet  a  second,  in 
consequence  of  the  rapid  and  energetic  contraction  of  the  mylohyoid  muscles. 
A  peristaltic  contraction,  passing  over  the  entire  esophagus,  was  necessary 
to  the  passage  of  solid  and  semisolid  food  through  it. 

Closure  of  the  Posterior  Nares  and  Larynx. — Because  of  the  rapid 
rise  of  pressure  in  the  deglutitory  canal  during  the  act  of  swallowing  it  is 
essential  that  the  openings  into  the  nasal  and  laryngeal  cavities  be  closed  to 
prevent  the  entrance  of  food  into  them,  which  would  otherwise  take  place. 
Under  normal  circumstances  this  is  done  so  effectually  that  it  is  seldom  that 
any  portion  of  the  food,  liquid  or  solid,  ever  enters  the  nasal  chambers 
or  the  cavity  of  the  larynx.  The  mechanism  by  which  these  openings  are 
closed  is  as  follows: 

At  the  moment  the  food  passes  into  the  pharynx  the  posterior  nasal  open- 
ings are  closed  against  the  entrance  of  the  food  by  a  septum  formed  by  the 


DIGESTION 


163 


pendulous  veil  of  the  palate  and  the  posterior  half  arches.  The  palate  is 
drawn  upward  and  backward  by  the  levator  palati  muscles,  until  it  meets 
the  posterior  wall  of  the  pharynx,  which  at  this  moment  advances.  At  the 
same  time  it  is  made  tense,  by  the  action  of  the  tensor  palati  muscles.  (Fig. 
70).  This  septum  is  completed  by  the  advance  toward  the  middle  line  of  the 
posterior  half  arches  caused  by  the  contraction  of  the  muscles,  the  palato- 
pharyngei,  which  compose  them.  When  these  structures  are  impaired  in 
their  functional  activity,  as  in  diphtheritic  paralysis  and  ulcerations,  there 
is  not  infrequently  a  regurgitation  of  food,  especially  liquids,  into  the 
nose. 

The  larynx  is  equally  protected  against  the  entrance  of  food  during 
deglutition  under  normal  circumstances.  That  this  accident  occasionally 
happens,  giving  rise  to  severe  spasmodic  coughing,  and  even  in  extreme 
cases  to  suffocation,  is  abundantly  shown  by  the  records  of  clinical  medicine. 
Usually  it  does  not  occur,  for  the  following  reasons:  just  preceding  and  dur- 
ing the  act  of  deglutition  there  is  a  complete  suspension  of  the  act  of  inspira- 
tion, by  which  particles  of  food  might  otherwise  be  drawn  into  the  larynx; 
at  the  same  time  the  larynx  is'  always  drawn  well  up  under  the  base  of  the 
tongue  and  its  entrance  closed  by  the 
downward  and  backward  movement 
of  the  epiglottis,  the  result  of  the  con- 
traction of  the  arj'teno-epiglottidean 
muscle. 

The  action  here  attributed  to  the 
epiglottis  has  been  denied  by  Stuart 
and  McCormick.  These  observers 
had  the  opportunity  of  looking  into 
a  naso-pharynx  which  had  been  laid 
open  by  a  surgical  operation  for  the 
removal  of  a  morbid  growth.  In  this 
patient,  the  epiglottis,  at  the  time  of 
deglutition,  was  always  more  or  less 
erect  and  closely  applied  to  the  base 
of  the  tongue.  So  complete  was  this 
that  the  food  passed  over  its  posterior 
or  inferior  surface  for  a  certain  dis- 
tance. In  no  instance  was  it  ever  ob- 
served to  fold  backward  like  a  lid. 

Because  of  the  possibility  that  this 
position  of  the  epiglottis  was  due  to 
pathologic  causes,  Kanthack  and  An- 
derson instituted  a  new  series  of  experiments  with  a  view  of  determining 
the  action  of  the  epiglottis.  As  a  result  of  many  experiments  on  animals 
and  of  observations  on  themselves,  these  observers  reaffirm  the  gener- 
ally accepted  view,  that  under  normal  conditions,  the  entrance  of  the  larynx 
is  always  closed  by  the  epiglottis  after  the  manner  of  a  lid. 

In  addition  to  the  downward  and  backward  movement  of  the  epiglottis 
and  the  ascent  of  the  larynx  under  the  base  of  the  tongue,  it  is  also  certain 
from  the  observations  of  Meltzer  that  the  larynx  is  protected  from  the  en- 
trance of  food,  in  the  rabbit  at  least,  by  the  closure  of  the  glottis  itself.     This 


Trojchea/ 


Fig.  70. 


-Diagram  Showing  the  Man- 
ner OF  Closure  of  the  Posterior  Nares 
AND  Larynx  during  Deglutition. —  {Lan- 
dois  and  Stirling.) 


1 64  TEXT-BOOK  OF  PHYSIOLOGY 

experimenter  noticed,  while  observing  the  interior  of  the  larynx,  both  from 
above,  through  an  opening  in  the  hyothyreold  membrane,  and  from  below, 
through  an  opening  in  the  trachea,  that  when  an  act  of  deglutition  was  excited 
by  touching  the  soft  palate  with  a  sound,  there  was  simultaneously  with  the 
contraction  of  the  mylohyoid  muscles,  a  firm  closure  of  the  glottis.  This 
was  accomplished  by  an  approximation  of  the  true  vocal  bands,  a  close 
approximation  and  a  downward  and  forward  movement  of  the  arytenoid 
cartilages,  until  they  almost  touched  the  anterior  wall  of  the  thyroid  carti- 
lage. This  movement  preceded  the  ascent  of  the  larynx.  When  the  larynx 
was  separated  from  all  surrounding  structures  with  the  exception  of  the 
laryngeal  nerves,  a  touch  of  the  palate  excited  the  same  phenomenon.  Under 
such  circumstances  the  closure  of  the  glottis  must  have  been  due  to  the  con- 
traction of  its  intrinsic  muscles  and  in  consequence  of  a  reflex  action  through 
the  inferior  laryngeal  nerves. 

The  Nerve  Mechanism  of  Deglutition. — Deglutition  is  almost  exclu- 
sively a  reflex  act  throughout  its  entire  extent,  and  requires  for  its  inaugura- 
tion merely  a  stimulus  to  some  portion  of  the  mucous  membrane  in  the 
anterior  part  of  the  deglutitory  canal.  The  first  stage  is  primarily  voluntary, 
but  from  inattention  to  the  process  may  become  secondarily  reflex.  The 
origin  and  course  of  the  aft'erent  nerves,  stimulation  of  which  excite  reflexly 
the  movements  of  the  pharynx  and  esophagus,  however,  are  practically  un- 
known. In  the  rabbit  deglutition  can  be  excited  by  stimulating  the  anterior 
central  part  of  the  soft  palate;  in  man  it  has  not  yet  been  possible  to  locate 
an  area  stimulation  of  which  will  give  rise  to  a  reflex  deglutitory  act.  Though 
electric  stimulation  of  the  superior  laryngeal  nerve  will  cause  reflex  degluti- 
tory movements,  it  is  obvious  that  the  terminals  of  this  nerve  cannot  be  the 
source  of  the  natural  afferent  impulses.  Stimulation  of  the  glosso-pharyn- 
geal  nerve  causes  an  inhibition  of  the  movements. 

The  center  from  which  emanate  nerve  impulses  which  excite  the  various 
muscles  to  action  has  been  located  experimentally  in  the  medulla  oblongata 
just  above  the  alas  cinereae.  The  efferent  nerves  comprise  branches  of  the 
facial,  hypoglossal,  motor  filaments  of  the  third  division  of  the  fifth  nerve, 
motor  filaments  of  the  glosso-pharyngeal  and  vagus  nerves  derived  in  all 
probability  directly  from  the  medulla  oblongata.  Inasmuch  as  the  different 
mechanisms  of  this  reflex,  act  not  only  in  a  coordinate  but  sequential  manner, 
it  would  appear  as  if  the  deglutition  center  sent  out,  in  response  to  the  nerve 
impulses  coming  from  a  single  peripheral  area,  a  series  of  nerve  impulses 
successively  to  successive  portions  of  the  canal,  through  the  groups  of  nerve- 
cells  corresponding  to  the  origins  of  the  efferent  nerves.  That  this  orderly  and 
progressive  peristalsis  usually  observed  is  due  to  a  sequence  of  changes  in  the 
central  nerv^e  system  is  shown  by  the  fact,  that  if  the  esophagus  is  divided  or 
a  ring  of  it  excised,  the  extremity  in  connection  with  the  stomach  will  exhibit 
a  well-marked  peristalsis  after  a  short  interval,  when  an  act  of  deglutition  is 
excited  in  the  customary  manner.  The  efferent  nerve-fibers,  which  stimulate 
the  esophageal  muscles  to  action  are  contained  in  the  trunk  of  the  vagi  nerves 
for  after  their  division  the  peristalsis  is  abolished. 

In  addition  to  this  primary  reflex  mechanism,  the  esophagus  appears  to 
possess  a  secondary  reflex  mechanism  consisting  of  a  series  of  reflex  arcs, 
whose  afferent  and  efferent  paths  are  found  in  the  trunk  of  the  vagus  and 
both  connected  with  successive  portions  of  the  esophagus.     The  first  mechan- 


DIGESTION 


165 


ism  is  temporarily  suspended  during  deep  anesthesia  while  the  second  per- 
sists.    (Meltzer.) 

Though  the  peristalsis  of  the  esophagus  is  excited  by  nerv^e  impulses 
coming  through  the  vagus  nerves  and  is  abolished  by  their  division,  Cannon 
has  shown  by  means  of  the  Rontgen  rays  that  this  effect  for  the  lower  portion 
of  the  esophagus,  at  least  in  the  cat  and  monkey,  is  of  a  temporary  duration 
only,  lasting  from  one  to  several  days,  after  which  a  peristalsis  again  develops 
with  sufficient  vigor  to  force  food  through  the  cardiac  orifice  into  the  stom- 
ach. The  muscle  coat  of  this  portion  of  the  esophagus  is  composed  of  non- 
striated  muscle-fibers,  is  supplied  with  a  myenteric  nerve  plexus  and  resem- 
bles lower  portions  of  the  alimentary  canal.  It  is  capable  of  developing  a 
peristalsis  merely  in  response  to  the  pressure  of  food  within  and  independent 
of  extrinsic  nerves. 

GASTRIC  DIGESTION 

After  the  food  has  passed  through  the  esophagus  it  is  received  by  the 
stomach,  where  it  is  retained  for  a  variable  length  of  time,  during  which 
important  changes  are  induced  in  its  physical  and  chemic  composition. 
The  disintegration  of  the  food  inaugurated  by  mastication  and  insalivation 


EsopJ/a^o-^r/sf//(  o7/f7ff 


Incisuraan^iik 


f 


■■0M  ^^^fiti         Wi u  it-  /v 


Tl 


bastro-daodcrial  mtsMe/io7i  V'iabiile      ~' 

Fig.  71. — Anatomic  Featxires  of  the  Stomach. 

is  Still  further  carried  on  in  the  stomach  by  the  solvent  action  of  the  acid  fluid 
there  present,  until  the  entire  mass  is  reduced  to  a  liquid  or  semi-liquid 
condition. 

The  Stomach.- — ^The  stomach  is  the  dilated  and  highly  specialized  por- 
tion of  the  alimentary  canal  intervening  between  the  esophagus  and  small 
intestine.  When  moderately  distended  with  food,  it  is  somewhat  conical  or 
pyriform  in  shape  and  slightly  curved  on  itself.  It  is  situated  obliquely  and 
in  some  individuals  almost  vertically  in  the  upper  part  of  the  abdominal 
ca^dty,  extending  from  the  left  hypochondrium  to  the  right  of  the  epigastrium. 
The  dimensions  and  capacity  of  the  stomach  undergo  considerable  periodic 
variation  according  to  the  extent  to  which  it  is  distended  by  food.  In  the 
average  condition  it  measures  in  its  long  diameter  from  25  to  35  centimeters, 
in  its  vertical  diameter  at  the  cardia  15  centimeters,  in  its  antero-posterior 


i66  TEXT-BOOK  OF  PHYSIOLOGY 

diameter  from  ii  to  12  centimeters.  The  capacity  of  the  stomach  varies 
from  1500  to  1700  c.c.  In  the  empty  condition  its  walls  are  contracted  and 
partly  in  contact,  and  the  entire  organ  is  drawn  up  into  the  upper  part  of 
the  abdominal  cavity.  The  stomach  being  a  contractile  and  distensible  organ 
adapts  itself  to  varying  amounts  of  food  and  hence  continually  varies  in 
size.  The  opening  through  which  the  food  passes  into  the  stomach  is  known 
as  the  esophago-gastric  or  the  cardiac  orifice.  The  opening  through  which 
it  passes  into  the  intestine  is  known  as  pylorus,  the  pyloric,  or  gastro-duodenal 
orifice,  indicated  on  the  outer  surface  by  a  slight  constriction  known  as  the 
gastro-duodenal  constriction.  Between  these  two  orifices,  the  stomach, 
along  its  upper  border,  presents  a  curve  and  along  its  lower  border  a  much 
larger  curve,  known  as  the  lesser  and  greater  curvatures  respectively.  Along 
the  lesser  curvature  there  is  a  slight  indentation  due  to  a  change  in  direction 
of  the  right  end  of  the  stomach,  known  as  the  incisura  angularis.  If  this 
indentation  is  connected  by  a  line  with  a  point  or  angle  almost  opposite  on 
the  greater  curvature  the  stomach  will  be  divided  into  a  pyloric  portion  to  the 
right  and  a  cardiac  portion  to  the  left.  If  a  line  be  drawn  transversely  across 
the  stomach  from  the  esophago-gastric  or  cardiac  orifice  to  the  greater  curva- 
ture, the  cardiac  portion  will  be  subdivided  into  the  fund/us,  to  the  extreme 
left,  and  the  cardiac  portion  proper  or  the  body  included  between  the  fundus 
and  the  pyloric  portion. 

If  a  vertical  section  of  the  stomach  is  made,  it  will  be  observed  that  the 
walls  are  extremely  thin  to  the  left  but  gradually  increase  in  thickness  in 
passing  toward  the  pyloric  orifice.  The  cavity  of  the  pyloric  portion,  is, 
when  the  stomach  is  empty,  subdivided  into  a  vestibule  and  a  pyloric  canal 
which  latter  is  said  to  begin  at  a  point  opposite  the  sulcus  intermedius,  and 
to  extend  to  the  pyloric  orifice.  The  pyloric  canal  is  narrow  and  cylindric 
and  measures  from  2.5  to  3.5  centimeters.  The  vestibule  and  pyloric  canal 
when  distended  with  food  form  a  tubular  region  which  has  been  termed  the 
antrum  or  the  gastric  canal. 

The  Walls  of  the  Stomach. — The  walls  of  the  stomach  are  formed  by 
four  distinct  coats  united  by  areolar  tissue  and  named,  from  without  inward, 
as  the  serous,  muscle,  submucous,  and  mucous. 

The  external  or  serous  coat  is  thin  and  transparent  and  formed  by  a 
reduplication  of  the  general  peritoneal  membrane. 

The  middle  or  muscle  coat  consists  of  three  layers  of  non-striated  muscle- 
fibers,  named  from  their  direction  the  longitudinal,  circular,  and  oblique. 
The  longitudinal  fibers  are  most  abundant  along  the  lesser  curvature 
and  are  a  continuation  of  those  of  the  esophagus;  over  the  remainder  of 
the  stomach  they  are  thinly  scattered,  but  toward  the  pyloric  orifice  they 
are  more  numerous  and  form  a  tolerably  thick  layer  which  becomes  con- 
tinuous with  the  fibers  of  the  small  intestine.  The  circular  fibers  form  a 
complete  layer  encircling  the  entire  organ,  with  the  exception,  perhaps,  of  a 
portion  of  the  fundus.  The  fibers  of  this  coat  cross  the  longitudinal  fibers 
at  right  angles.  At  the  lower  end  of  the  esophagus  and  surrounding  the 
cardia  the  circular  muscle-fibers  form  a  true  sphincter  which  is  known  as 
the  sphincter  cardice.  At  the  junction  of  the  pyloric  and  cardiac  regions,  x-x, 
Fig.  71,  the  circular  fibers  form  a  well-defined  bundle — the  transverse  band — 
which  was  supposed  to  contract  periodically  during  digestion  and  thus  par- 
tially di\dde  the  stomach  into  two  portions,  the  portion  to  the  right  of  which 


DIGESTION  167 

was  termed  the  antrum,  and  hence  this  transverse  band  has  been  termed  the 
sphincter  antri  pylorici.     This  function,  however,  has  been  questioned. 

In  the  pyloric  region  the  circular  fibers  are  more  closely  arranged,  form- 
ing thick  well-defined  rings  termed  the  antral  muscles.  At  the  pyloric  open- 
ing the  circular  fibers  are  crowded  together  and  form  a  distinct  muscle  band 
known  as  the  sphincter  pylori  which  projects  for  some  distance  into  the  inte- 
rior of  the  canal.  It  has  been  stated  by  Riidinger  that  the  inner  fibers  of 
the  longitudinal  coat  become  connected  with  this  circular  band  and  consti- 
tute a  distinct  muscle,  the  dilatator  pylori.  The  oblique  fibers  are  most  dis- 
tinct over  the  cardiac  portion  of  the  stomach,  but  extend  from  left  to  right 
as  far  as  the  junction  of  the  middle  and  last  thirds  of  the  stomach.  They 
are  continuations  of  the  circular  fibers  of  the  esophagus. 

The  submucous  coat  consists  of  loose  areolar  tissue  carrying  blood-vessels, 
nerves,  and  lymphatics.  It  serves  to  unite  the  muscle  to  the  mucous 
coat.  Its  inner  surface  bears  a  thin  layer  of  muscle  tissue,  the  muscularis 
mucosa,  which  supports  the  mucous  membrane. 

The  internal  or  mucous  coat  is  loosely  attached  to  the  muscle  coat.  In 
the  empty  and  contracted  state  of  the  stomach  it  is  thrown  into  longitudinal 
folds,  or  rugae,  which  are,  however,  obliterated  when  the  organ  is  distended 
with  food.  The  mucous  membrane  in  adult  life  is  smooth  and  velvety  in 
appearance,  gray  in  color,  and  covered  with  a  layer  of  mucus.  Its  average 
thickness  is  about  one  millimeter.  The  surface  of  the  membrane  is  covered 
with  a  layer  of  columnar  epithelial  cells.  In  passing  from  the  cardiac 
toward  and  into  the  pyloric  region  the  mucous  membrane  becomes  thicker 
and  forms  the  inner  wall  of  the  vestibule  and  pyloric  canal.  It  finally  be- 
comes continuous  with  the  mucous  membrane  of  the  intestine.  At  the  pyloric 
orifice  there  is  a  circular  involution  of  the  mucous  membrane  which  is  known 
as  the  pyloric  valve.  This  is  strengthened  by  fibrous  tissue  and  embraced 
by  the  sphincter  muscle  previously  described. 

Gastric  Glands. — The  surface  of  the  mucous  membrane  when  examined 
with  a  low  magnifying  power  presents  throughout  innumerable  depressions, 
polygonal  in  shape  and  separated  by  slightly  elevated  ridges.  At  the  bottom 
of  these  spaces  are  to  be  seen  small  orifices,  which  are  the  mouths  of  the 
glands  embedded  in  the  mucous  membrane.  A  vertical  section  of  the 
gastric  walls  shows  not  only  the  position  and  the  appearance  of  the 
glands,  but  the  relation  of  the  various  tissues  which  enter  into  the  formation 
of  these  walls.  An  examination  of  the  mucous  membrane  in  different 
regions  of  the  stomach  reveals  the  presence  of  two  distinct  types  of  glands, 
which  from  their  situation  are  termed  cardiac,  and  pyloric,  which  differ 
not  only  in  histologic  structure,  but  also  in  function.  Both  types  extend 
through  the  entire  thickness  of  the  mucosa. 

The  cardiac  glands  are  formed  by  an  involution  of  the  basement  mem- 
brane of  the  mucosa  and  lined  by  epithelial  cells.  Each  gland  may  be 
said  to  consist  of  a  short  duct,  or  neck,  and  a  body,  or  fundus  (Fig.  72). 
The  latter  portion  is  wavy  or  tortuous  and  frequently  subdivided  into  as 
many  as  four  distinct  and  separate  tubules.  The  duct  is  lined  by  columnar 
epithelial  cells  similar  to  that  covering  the  surface  of  the  mucosa.  The 
lumen  of  the  gland  is  bordered  by  epithelial  cells,  cuboid  in  shape,  and  con- 
sisting of  a  granular  protoplasm  containing  a  distinct  spherical  nucleus. 
These  cells  are  generally  spoken  of  as  the  chief  or  central  cells.     In  addi- 


i68 


TEXT-BOOK  OF  PHYSIOLOGY 


tion  to  the  chief  cells,  the  cardiac  glands  contain  a  second  variety  of  cell, 
which  is  of  a  larger  size,  of  a  triangular  or  oval  shape,  and  consisting  of  a 
finely  granular  protoplasm.  From  their  situation  in  and  just  beneath  the 
gland  wall  they  have  been  termed  parietal  or  border  cells.  Each  parietal 
cell  appears  to  be  surrounded  and  penetrated  by  a  system  of  passages  which 
open  into  the  lumen  of  the  gland  by  means  of  a  delicate  cleft  or  canaliculus 
(Fig.  73).  Glands  with  these  histologic  features  are  most  abundant  in  the 
middle  zone  of  the  stomach. 

The  pyloric  glands  are  also  formed  by  an  involution  of  the  mucous  mem- 
brane and  Hned  by  epithelial  cells  (Fig.  74).  The  ducts  are  much  longer 
than  the  ducts  of  the  cardiac  glands.  At  its  ex- 
tremity each  duct  becomes  branched,  giving  rise  to  a 
number,  from  2  to  10,  of  short  tubes,  each  of  which 
has  a  large  lumen  and  communicates  with  the  duct  by 
a  narrow  short  neck.  The  ducts  are  lined  through- 
out by  columnar  epithelium.  According  to  Mall,  the 
total  number  of  openings  on  the  surface  of  the  mucous 
membrane  of  the  dog's  stomach  is  somewhat  over 
1,000,000,  and  the  total  number  of  blind  tubes  opposite 
the  muscularis  mucosae  exceeds  16,500,000.  Accord- 
ing to  Sappey,  the  surface  of  the  mucous  membrane 
of  the  human  stomach  presents  over  5,000,000  orifices 
of  gastric  glands. 


Fig.  72. — Cardiac  Gland. 
m  Mouth  of  the  duct;  n,  neck; 
/.  fundus;  c,  central  cells; 
p,  parietal  cells.  {Landois 
and  Stirling.) 


Fig.  73. — Section  of  Cardiac  Gland 
OF  Mouse.  Left  upper  half  drawn  after 
an  alcohol  preparation,  right  upper  half 
after  a  Golgi  preparation.  The  entire 
lower  portion  is  a  diagrammatic  combi- 
nation of  both  preparations.     (Stdhr.) 


Blood-vessels  and  Nerves. — ^The  blood-vessels  of  the  stomach  after 
entering  the  mucosa  break  up  into  a  number  of  branches  which  are  dis- 
tributed to  the  muscle  and  mucous  coats.  The  branches  to  the  latter  soon 
form  a  capillary  network  with  oblong  meshes  which  not  only  surround  the 
tubules  but  form  a  network  just  beneath  the  surface  of  the  mucosa.  Veins 
gradually  arise  from  the  capillaries  which  empty  into  the  larger  veins  of  the 
mucosa.  The  glands  are  also  supported  by  processes  of  smooth  muscle- 
fibers  passing  up  from  the  muscularis  mucosae. 

The  nerve  elements  are  partly  intrinsic  and  partly  extrinsic.     The  in- 


DIGESTION 


169 


trinsic  nerve  elements  consist  of  two  nerve  plexuses,  one  which  lies  between 
the  muscle  coats,  another  which  Hes  between  the  muscle  and  the  mucous 
coats.  They  have  been  named  after  their  discoverers,  Auerbach's  and 
Meissner's  plexuses  respectively.  Each  plexus  consists  of  microscopic 
ganglia  and  nerve  processes  which  interlace  with  one  another  to  form  a 
very  complicated  network.  The  nerve  processes  are  distributed  mainly  to 
muscle-fibers.  As  these  plexuses  are.  associated  with  a  large  part  of  the 
alimentary  canal  they  together  have  been  termed  the  myenteric  plexus. 
The  extrinsic  nerves,  which  associate  the  central  nerve  system  with  the 
gastric  structures  are  the  vagus  and  the  great  splanchnic.  The  vagus  nerve, 
through  its  efferent  nerve-fibers  is  in  physiologic  relation  with  muscle-fibers 
and  gland-cells,  through  the  intermediation  of  gang- 
lionic cells.  The  great  splanchnic  through  its  efferent 
nerve-fibers  is  in  physiologic  relation  with  the  muscle- 
fibers  and  blood-vessels  through  the  intermediation 
of  the  post-ganglionic  fibers  derived  from  the  cells  of 
the  semilunar  ganglion. 

Gastric  Fistulae. — The  general  process  of  diges- 
tion, as  it  takes  place  in  the  stomach,  has  been 
studied  in  human  beings  and  animals  with  a  fistula 
in  the  walls  of  the  stomach  and  abdomen,  the  result 
either  of  accident,  of  necessary  surgical  or  of  experi- 
mental procedures. 

The  earliest  observations  on  gastric  digestion 
were  made  by  Dr.  Beaumont  on  Alexis  St.  Martin, 
who,  as  the  result  of  a  gunshot  wound,  was  left  with 
a  permanent  fistulous  opening  into  the  fundus  of  the 
stomach.  This  opening  two  years  after  the  acci- 
dent was  about  two  and  a  half  inches  in  circum- 
ference and  usually  closed  from  within  by  a  fold  of 
mucous  membrane  which  prevented  the  escape  of 
the  food.  This  valve  could  be  readily  displaced  by 
the  finger  and  the  interior  of  the  stomach  exposed 
to  view.  After  the  complete  recovery  of  St.  Martin, 
Dr.  Beaumont  during  the  years  between  1825  and 
183 1  at  intervals  made  numerous  experiments  on 
the  nature  of  gastric  digestion.  As  the  result  of  an 
admirable  series  of  investigations  it  was  established 
that  the  digestion  of  the  food  is  largely  a  chemic 
act,  due  ta  the  presence  of  an  acid  fluid  secreted  by 
the  mucous  membrane;  that  this  fluid  is  secreted  most  abundantly  after  the 
introduction  of  food  into  the  stomach;  that  different  articles  of  food  possess 
varying  degrees  of  digestibility;  that  the  duration  of  digestion  varies  ac- 
cording to  the  nature  of  the  food,  exercise,  mental  states,  etc.,  and  that  the 
process  is  aided  by  continuous  movements  of  the  muscle  walls. 

Since  Dr.  Beaumont's  time  the  establishing  of  a  gastric  fistula  in  human 
beings  has  been  necessitated  by  pathologic  conditions  of  the  esophagus. 
After  recovery  these  cases  offered  fair  facilities  for  the  study  of  the  process 
when  the  food  was  introduced  through  the  opening.  Similar  fistulae  have 
been  established  in  both  carnivorous  and  herbivorous  animals  with  a  view 


Fig.  74. 
Gland    of  the 
m.  Mouth  of  duct; 
(Landois.) 


lyo 


TEXT-BOOK  OF  PHYSIOLOGY 


of  studying  the  process  as  it  takes  place  in  them.  The  results  obtained  in 
these  instances  in  many  respects  corroborate  those  obtained  by  Dr.  Beau- 
mont, though  many  new  facts,  unobserved  by  him,  have  been  brought  to 
light. 

Much  additional  information  as  to  the  mode  of  secretion  and  the  char- 
acteristics of  the  gastric  juice  has  been  obtained,  since  the  introduction  of  two 
new  procedures  by  Pavlov.  The  first  consists  in  establishing  a  gastric 
fistula  and  subsequently  dividing  the  esophagus  in  the  neck,  and  then  so 
adjusting  the  divided  ends  that  they  heal  separately  into  an  angle  of  the  skin 
incision.  The  second  procedure  consists  in  forming  a  diverticulum  or  pouch 
out  of  the  cardiac  end  of  the  stomach  which  opens  on  the  surface  of  the  ab- 
domen but  is  separated  from  the  rest  of  the  stomach  by  a  thin  septum  formed 
of  two  layers  of  mucous  membrane.  (Fig.  75.)  The  serous  and  muscle-coats 
of  this  pouch  are  in  direct  continuity  with  the  large  stomach  and  all  possess 

the  same  vascular  and  nerve  connections. 
Because  of  this  fact  this  miniature  stomach, 
about  one-tenth  the  size  of  the  natural 
stomach,  exhibits  the  same  phenomena,  so 
far  as  the  secretion  of  the  gastric  juice  is  con- 
cerned, as  the  large  stomach  does.  The  phe- 
nomena which  are  observed  in  it  may  be 
taken  as  an  indication  as  to  the  phenomena 
which  are  taking  place  in  the  natural  stomach. 
By  the  first  procedure  it  is  possible  to  feed 
an  animal  with  different  kinds  of  food  and  to 
observe  the  effects  of  psychic  states  on  the 
secretion  of  gastric  juice.  As  the  swallowed 
food  is  discharged  from  the  lower  end  of  the 
divided  esophagus  the  appetite  continues,  and 
hence  the  animal  will  eat  for  several  hours. 
By  the  second  procedure  it  is  possible  to  col- 
lect gastric  juice  from  the  miniature  stomach 
and  to  study  the  effects  on  its  quantity  and 
quality  produced  by  psychic  states,  mastica- 
tion, different  articles  of  food,  and  by  the 
process  of  digestion  itself  as  it  goes  on  in  the  large  stomach.  In  both  in- 
stances the  juice  is  obtained  free  from  admixture  with  saUva  or  food. 

Gastric  Juice. — The  gastric  juice  obtained  from  the  human  stomach 
free  from  mucus  and  other  impurities  is  a  clear,  colorless  fluid  with  a  con- 
stant acid  reaction,  a  slightly  saline  and  acid  taste,  and  a  specific  gravity 
varying  from  1.002  to  1.005.  The  juice  obtained  from  the  dog's  stomach 
possesses  essentially  the  same  characteristics,  though  its  acidity  as  well  as  its 
specific  gravity  are  slightly  greater.  When  kept  from  atmospheric  influences, 
it  resists  putrefactive  change  for  a  long  period  of  time,  undergoes  no  apparent 
change  in  composition,  and  loses  none  of  its  digestive  power.  It  will  also 
prevent  and  even  arrest  putrefactive  change  in  organic  matter.  The  chemic 
composition  of  the  gastric  juice  has  never  been  satisfactorily  determined, 
owing  to  the  fact  that  the  secretion  as  obtained  from  fistulous  openings  has 
not  been  absolutely  normal.     It  may  however  be  said  to  consist  of  water, 


Fig.  75. — Diagram  Showing 
THE  Relation  of  the  Natural 
Stomach  to  the  Miniature 
Stomach  or  Pouch  made  Ac- 
cording TO  the  Procedure  De- 
vised BY  Pavlov.  V.  The  nat- 
ural stomach.  5.  The  miniature 
stomach,  e,  e.  The  septum  formed 
by  the  mucous  membrane.  A,  A. 
The  abdominal  walls. 


DIGESTION  171 

organic  matter,  hydrochloric  acid  and  various  inorganic  salts.  The  quan- 
titative composition  of  the  juice  varies  somewhat  in  different  animals. 

The  organic  matter  present  in  gastric  juice  is  a  mixture  of  mucin  and  a 
protein,  products  of  the  metabolic  activity  of  the  epitheHal  cells  on  the  sur- 
face of  the  mucous  membrane  and  of  the  chief  or  central  cells  of  the  gastric 
glands  respectively.  Associated  with  the  protein  material  are  two  possibly 
three  ferment  or  enzyme  bodies,  termed  pepsin,  rennin  and  lipase.  As  is 
the  case  with  other  enzymes,  their  true  chemic  nature  is  practically  un- 
Toiown. 

Pepsin. — ^Pepsin,  though  present  in  gastric  juice,  is  not  present  as  such  in 
the  chief  cells  of  the  glands,  but  is  derived  from  a  zymogen,  propepsin  or 
pepsinogen,  when  the  latter  is  treated  with  hydrochloric  acid.  This  ante- 
cedent compound  is  related  to  the  granules  observed  in  and  produced  by  the 
cell  protoplasm  during  the  period  of  rest.  Though  pepsin  is  largely  produced 
by  the  central  cells  of  the  cardiac  glands,  it  is  also  produced,  though  in  less 
amount,  by  the  cells  of  the  pyloric  glands.  Pepsin  is  the  chief  proteolytic 
or  proteoclastic  agent  of  the  gastric  juice  and  exerts  its  influence  most  ener- 
getically in  the  presence  of  hydrochloric  acid  and  at  a  temperature  of  about 
40°C.  Other  acids — e.g.,  phosphoric,  nitric,  lactic,  etc. — are  also  capable 
of  exciting  it  to  activity,  though  with  less  intensity. 

Rennin. — ^Rennin  or  pexin  is  present  in  the  gastric  juice  not  only  of  man 
and  all  of  the  mammalia,  but  also  of  birds  and  even  fish.  In  its  origin  from 
a  zymogen  substance;  in  its  relation  to  an  acid  medium  and  an  optimum 
temperature  it  bears  a  close  resemblance  to  pepsin.  Its  specific  action  is 
the  coagulation  of  milk,  a  condition  due  to  a  transformation  of  soluble  case- 
inogen  into  a  solid  flaky  body,  casein. 

Lipase. — Lipase,  an  enzyme  found  in  pancreatic  juice,  has  also  been 
shown  to  be  present  in  gastric  juice,  the  specific  function  of  which  appears 
to  be  the  digestion  or  hydrolysis  of  finely  emulsified  fat  such  as  is  found  in 
milk. 

Hydrochloric  Acid. — Hydrochloric  acid  is  the  agent  which  gives  to  the 
gastric  juice  its  normal  acidity.  Though  the  juice  frequently  contains  lactic, 
acetic,  and  even  phosphoric  acids,  it  its  generally  believed  that  they  are  the 
result  of  fermentation  changes  occurring  in  the  food,  the  result  of  bacterial 
action.  The  percentage  of  hydrochloric  acid  has  been  the  subject  of  much 
discussion.  The  most  recent  investigations  show  that  the  initial  acidity  of 
the  freshly  secreted  human  gastric  juice  is  between  0.32  and  0.48  per  cent. 
HCl.  This  initial  acidity  is  reduced  by  combination  with  food,  admixture 
with  saliva  and  gastric  mucus,  and  by  regurgitation  of  alkaline  duodenal 
contents,  to  0.15  or  0.2  per  cent.  HCl,  the  optimum  acidity  for  the  proteolytic 
activity  of  pepsin.  As  observed  clinically,  following  various  test  meals, 
the  acidity  of  the  gastric  contents  is  seen  to  rise  to  a  maximum  as  diges- 
tion progresses,  after  which  it  falls  to  the  optimum  point  of  about  0.2  per 
cent.  HCl. 

The  immediate  origin  of  the  hydrochloric  acid  is  difficult  of  explanation. 
That  it  is  derived,  however,  primarily  from  the  chlorids  of  the  food  and 
secondarily  from  the  chlorids  of  the  blood-plasma  has  been  established  by 
direct  experiment.  If  all  the  chlorids  be  removed  from  the  food  and  all 
the  chlorids  be  withdrawn  from  the  animal  tissues  by  the  administration  of 
various  diuretics — e.g.,  potassium  nitrate — there  will  be  a  total  disappearance 


172  TEXT-BOOK  OF  PHYSIOLOGY 

of  hydrochloric  acid  from  the  stomach.  On  the  addition  of  sodium  or  potas- 
sium chlorids  to  the  food,  there  is  at  once  a  reappearance  of  the  acid. 

As  to  the  nature  of  the  process  by  which  the  acid  is  formed,  nothing 
definite  is  known.  Various  theories  of  a  chemic  and  physical  character 
have  been  offered,  all  of  which  are  more  or  less  unsatisfactory.  As  no  hydro- 
chloric acid  is  found  either  in  the  blood  or  lymph,  the  most  plausible  \iew  as 
to  its  origin  is  that  which  regards  it  as  one  of  the  products  of  the  metabohsm 
of  the  gland-cells,  and  more  particularly  of  the  parietal  or  border  cells,  and 
which  for  this  reason  have  been  termed  acid-producing  or  oxyntic  cells. 
From  the  chlorids  furnished  by  the  blood  the  chlorin  is  derived,  which, 
uniting  with  hydrogen,  forms  the  HCl.  The  base  set  free  returns  to  the 
blood,  which  in  part  accounts  for  its  increased  alkalinity  during  digestion  as 
well  as  the  diminished  acidity  of  the  urine.  The  acid  thus  formed  passes 
through  the  canaHculi,  which  penetrate  and  surround  the  cells,  into  the 
lumen  of  the  gland. 

Hydrochloric  acid  exerts  its  influence  in  a  variety  of  ways.  It  is  the 
main  agent  in  the  derivation  of  pepsin  and  rennin  or  pexin  from  their  ante- 
cedent zymogen  compounds,  pepsinogen  and  pexinogen  (Warren) ;  it  imparts 
activity  to  these  ferments;  it  prevents  and  even  arrests  fermentative  and 
putrefactive  changes  in  the  food  by  destroying  microorganisms;  it  softens 
connective  tissue,  it  dissolves  and  acidifies  the  proteins,  thus  making  possible 
the  subsequent  action  of  pepsin. 

The  inorganic  salts  of  the  gastric  juice  are  probably  only  incidental  and 
play  no  part  in  the  digestive  process. 

Mode  of  Secretion. — The  observations  of  Dr.  Beaumont  and  the  experi- 
ments of  many  physiologists  have  made  it  certain  that  the  secretion  of  the 
gastric  juice  is  intermittent  and  not  continuous,  that  it  is  only  on  the  intro- 
duction and  digestion  of  the  food  that  the  normal  amount  is  poured  out. 
During  the  inter\'als  of  digestive  activity  the  stomach  is  practically  free  from 
all  traces  of  the  juice.  The  mucous  membrane  is  pale  and  covered  with  a 
layer  of  mucus  having  an  alkaline  or  neutral  reaction.  The  introduction, 
however,  of  small  portions  of  food  or  irritation  with  a  glass  rod  causes  a 
change  in  the  appearance  of  the  mucous  membrane.  At  the  points  of 
irritation  the  membrane  becomes  red  and  vascular  and  in  a  few  minutes 
small  drops  of  a  secretion  make  their  appearance;  these  coalesce  and  run 
down  the  sides  of  the  stomach. 

The  statements  of  Beaumont  and  many  subsequent  investigators  that 
the  secretion  thus  obtained  is  gastric  juice  have  been  apparently  disproved 
by  Pavlov,  who  asserts  that  it  is  only  an  alkaline  mucous  the  function  of 
which  is  protective  in  character.  According  to  this  investigator,  mechanic 
stimulation  is  incapable  of  exciting  the  secretion.  The  results  of  modem 
methods  of  investigation  make  it  apparent  that  the  production  and  discharge 
of  the  secretion  is  the  result  of  the  action  of  two  different  stimuli,  a  primary 
and  a  secondary. 

The  primary  stimulus  to  gastric  secretion,  according  to  Pavlov,  is  a 
psychic  state  induced,  on  the  one  hand,  by  the  sight  or  the  odor  of  food 
especially  if  the  animal  is  hungry  and  the  food  appetizing;  and  on  the  other 
hand  by  the  mastication  of  food  which  is  agreeable  to  the  animal.  Thus 
when  a  dog  was  tempted  by  the  sight  of  food,  the  secretion  made  its  appear- 
ance at  the  end  of  six  minutes  and  during  the  time  of  the  experiment,  which 


DIGESTION  173 

lasted  for  an  hour  and  a  half,  80  cubic  centimeters  of  the  juice  were  obtained. 
This  is  known  as  psychic  or  appetite  juice.  The  character  of  a  psychic 
state,  however,  greatly  influences  the  amount  of  the  juice  secreted.  Agree- 
able emotions  increase,  depressing  emotions  inhibit  it.  Again  when  a  dog 
with  a  divided  esophagus  and  a  gastric  fistula  was  subjected  to  sham  feeding, 
mastication  continued  for  five  or  six  hours  during  which  time  700  cubic 
centimeters  of  juice  were  obtained  from  the  stomach.  Similar  results  have 
been  obtained  in  human  beings  with  an  occluded  esophagus  and  a  gastric 
fistula.  It  is  evident  from  these  facts  that  the  secretion  of  gastric  juice  is 
favorably  influenced  by  the  sight  and  odor  of  appetizing  food,  by  exhilarating 
emotional  states  and  thorough  mastication. 

As  a  result  of  the  psychic  states  induced  by  the  sight  and  odor  of  food  and 
of  the  taste  of  food  during  mastication,  nerve  impulses  not  only  descend  from 
the  brain  but  are  also  transmitted  from  the  mouth  through  afferent  nerves, 
to  some  central  mechanism;  and  that  from  this  mechanism,  nerve  impulses 
must  in  turn  be  discharged  to  be  transmitted  through  efferent  nerve-fibers 
which  are  distributed  to  the  epithelium  of  the  gastric  glands.  Experimental 
investigations  render  it  probable  that  the  central  mechanism  is  located  in  the 
medulla  oblongata  and  that  the  efferent  path  for  the  secretor  fibers  lies  in  the 
trunk  of  the  vagus  nerve.  Though  this  ners^e  has  been  the  subject  of  much 
experimentation,  the  results  which  have  been  obtained  have  not  been  uni- 
form. The  investigations  of  Pavlov  seem  to  be  the  most  reliable.  He 
found  that  after  di\'ision  of  the  nerve,  secretion  was  arrested,  and  that  stimu- 
lation of  the  peripheral  ends  with  induced  electric  currents  at  the  rate  of  one 
or  two  per  second,  caused  after  a  latent  period  of  several  minutes'  duration 
a  flow  of  gastric  juice.  Coincidently  with  the  development  of  the  psychic 
secretion  there  is  a  dilatation  of  the  gastric  blood-vessels  and  an  increase  in 
the  supply  of  blood  to  the  gastric  glands.  Whether  this  is  due  to  the  action 
of  vaso-dilatator  fibers  or  to  an  inhibition  of  the  action  of  vaso-constrictor 
fibers  is  uncertain. 

Though  the  secretion  of  the  gastric  juice  can  be  initiated  by  these  means, 
the  amount  secreted  is  but  small  compared  with  the  quantity  secreted  after 
digestion  has  begun.  Then  it  is  that  the  blood-vessels  dilate  to  their  full 
capacity  and  furnish  for  several  hours  the  requisite  materials  for  the  pro- 
duction of  the  juice  on  a  relatively  large  scale.  That  some  factor  is  active 
in  keeping  up  the  secretion  in  the  stomach,  is  apparent  from  the  in- 
crease in  the  quantity  and  the  change  in  the  quality  of  the  juice  secreted  by 
the  miniature  stomach. 

The  secondary  stimulus  to  the  gastric  secretion  is  in  all  probability 
chemic  in  character  and  developed  in  the  stomach  or  in  its  walls  during 
digestive  activity,  inasmuch  as  the  secretion  takes  place  independent  of 
nerve  influences  and  after  division  of  all  afferent  and  efferent  nerv^es  that 
pass  from  and  to  the  stomach.  On  the  assumption  that  this  factor  might 
be  developed  in  the  walls  of  the  stomach  itself,  Edkins  conducted  a  series  of 
experiments,  the  results  of  which  lead  to  the  inference  that  there  is  developed 
in  the  mucous  membrane  of  the  pyloric  region,  by  the  action  of  certain 
articles  of  food,  e.^.,  dextrin,  meat  broths,  soups,  etc.,  or  by  the  first  products 
\  of  digestive  activity,  a  chemic  agent,  which  is  absorbed  by  the  blood  and  is 
carried  to  the  glands  throughout  the  stomach  and  which,  on  reaching  the 
glands,  stimulates  their  cells  in  a  specific  manner.     For  this  reason  it  has 


174  TEXT-BOOK  OF  PHYSIOLOGY 

been  called  the  gastric  hormone^  or  the  gastric  secretin.  Whatever  the  agent 
or  the  mechanism  may  be,  there  is  not  only  an  increase  in  the  quantity  but 
a  change  in  the  quality  of  the  juice  in  accordance  with  the  character  of  the 
food;  in  other  words,  there  is  an  adaptation  of  the  juice  to  the  kind  of  food 
to  be  digested.  Thus  the  protein  of  bread  causes  a  secretion  of  five  times  more 
pepsin  than  the  same  amount  of  the  protein  of  milk,  while  the  protein  of 
meat  causes  a  secretion  of  25  per  cent,  more  pepsin  than  milk.  Meat  extract 
and  bouillon  have  a  very  stimulating  effect  on  the  quantity  of  juice  produced, 
while  alkalies  have  an  inhibitor  effect. 

Histologic  Changes  in  the  Gastric  Cells  during  Secretion. — During 
the  periods  of  rest  and  secretor  activity  the  cells  of  the  gastric  glands  undergo 
changes  in  histologic  structure  which  are  believed  to  be  connected  with  the 
production  of  the  enzymes,  pepsin  and  rennin,  and  the  acid.  In  the  resting 
period  the  protoplasm  of  the  chief  or  central  cells  of  the  cardiac  glands  be- 
comes crowded  with  large  and  well-defined  granules,  which  during  the  period 
of  secretory  activity  largely  disappear,  so  much  so  that  only  the  luminal 
border  of  the  cell  is  occupied  by  them,  the  outer  border  being  clear  and  hya- 
line in  appearance.  The  parietal  cells  during  rest  are  large  and  finely 
granular,  but  after  secretion  they  are  smaller  in  size  though  still  granular. 

The  cells  of  the  pyloric  glands,  though  containing  granules,  do  not  show 
any  marked  difiference  between  the  resting  and  active  conditions.  According 
to  some  observers  they  contain  pepsinogen;  according  to  others,  mucin. 
The  epithehal  cells  lining  the  ducts  of  the  pylorus  and  cardiac  glands,  if  not 
identical  with  the  epithelial  cells  on  the  surface  of  the  mucous  membrane, 
pass  by  transitional  forms  into  them.  Among  these  cells  are  found  many 
goblet  cells  which  secrete  a  portion  of  the  mucin  found  in  the  stomach  and 
gastric  juice.  In  the  period  of  rest  the  protoplasm  of  the  epithelial  cells 
absorbs  and  assimilates  from  the  surrounding  lymph-spaces  material  which 
eventually  makes  its  reappearance  as  a  product  of  metabolism  in  the  form  of 
granules  and  hydrochloric  acid.  With  the  onset  of  digestive  activity  there 
is  a  dilatation  of  the  blood-vessels,  an  increase  in  the  blood-supply,  a  stimu- 
lation through  the  nerve-supply  of  the  cells,  and  an  output  of  a  fluid  to  which 
the  name  gastric  juice  is  given. 

The  Physiologic  Action  of  Gastric  Juice. — In  the  study  of  the  physi- 
ology of  gastric  digestion  as  it  takes  place  under  normal  conditions  it  is 
important  to  bear  in  mind  that  the  foods  introduced  into  the  stomach  are 
heterogeneous  compounds  consisting  of  both  nutritive  and  non-nutritive 
materials,  and  that  before  the  former  can  be  digested  and  utilized  for  nutri- 
tive purposes  they  must  be  freed  from  their  combinations  with  the  latter. 
This  is  accomphshed  by  the  solvent  action  of  the  gastric  juice,  which  in 
virtue  of  the  chemic  activity  of  its  constituents  on  proteins,  gradually  disinte- 
grates the  food  and  reduces  it  to  the  liquid  or  semiliquid  condition. 

The  nature  of  this  change  and  the  respective  influence  which  the  acid 
and  pepsin  exert  can  be  studied  with  almost  any  form  of  protein.  A  most 
convenient  form,  however,  is  fibrin  obtained  from  blood  by  whipping  and 
thoroughly  freed  from  corpuscles  by  washing  under  a  stream  of  water.     The 

'A  hormone  (from  oppiaco,  "I  excite")  may  be  defined  as  an  agent  of  known  or  unknown 
composition  which  is  secreted  by  some  one  organ,  is  discharged  into  and  carried  by  the  blood  stream 
to  some  correlated  organ  near  or  remote,  on  the  functional  activities  of  which  it  exerts  an  excitator 
or  stimulating  influence.     (See  Chapter  on  Internal  Secretion.) 


DIGESTION  175 

chemic  features  of  proteins,  as  well  as  the  typical  forms  contained  in  the 
different  articles  of  food,  have  been  considered  in  connection  with  the  chemic 
composition  of  the  body  and  the  composition  of  foods  (see  pages  15  and  119). 
For  purposes  of  experimentation  artificial  gastric  juice  may  be  employed. 
This  is  as  effective  as  the  normal  secretion  and  in  no  essential  respect  differs 
from  it.  A  glycerin  extract  of  the  mucous  membrane  acidulated  with  0.2 
per  cent,  hydrochloric  acid  is  probably  the  best. 

If  the  small  pieces  of  fibrin  be  suspended  in  clear  gastric  juice  and  kept 
at  a  temperature  of  io4°F.  (4o°C.)  for  an  hour  or  two,  they  will  be  dissolved 
and  will  entirely  disappear,  giving  rise  to  a  slightly  opalescent  mixture.  In 
the  early  stages  of  the  process  the  fibrin  becomes  swollen  and  transparent 
and  partly  dissolved.  If  at  this  time  the  solution  be  carefully  neutralized, 
the  dissolved  portion  can  be  regained  in  the  form  of  acid  fibrin — a  fact  which 
indicates  that  the  first  effect  of  the  gastric  juice  is  the  acidification  of  the 
protein.  This  having  been  accomplished,  the  pepsin  becomes  operative, 
and  in  a  varying  length  of  time  transforms  the  acid-protein  into  a  new  form 
of  protein,  termed  peptone  which  differs  from  all  other  forms  of  protein 
in  being  soluble  in  both  acids  and  alkalies  and  non-coagulable  by  heat. 
In  the  transformation  of  acid-protein  into  peptone  it  is  possible  to  isolate 
by  the  addition  of  magnesium  sulphate  and  ammonium  sulphate  inter- 
mediate bodies  to  which  the  term  proteoses  has  been  given,  and  which 
differ  somewhat  in  their  solubility.  The  proteoses  are  termed,  from  the 
order  in  which  they  make  their  appearance,  primary  and  secondary.  The 
primary  proteoses  are  precipitated  by  magnesium  sulphate,  the  secondary 
by  ammonium  sulphate.  This  supposed  change  produced  by  gastric  juice 
is  represented  by  the  following  scheme: 

Protein — Acid-protein— Proteose — Proteose — ^Peptone 
(Primary)     (Secondary) 

From  the  fact  that  when  peptones  are  subjected  to  the  prolonged  action 
of  pancreatic  juice  there  arise  compounds  such  as  leucin,  tyrosin,  aspartic 
acid,  arginin,  etc.,  it  was  believed  that  two  kinds  of  peptones  were  formed 
out  of  a  simple  protein  one  of  which  succumbed  to  the  destructive  action  of 
pancreatic  juice,  while  the  other  resisted  it;  for  this  reason  the  latter  was 
termed  anti-  and  the  former  hemi-peptone.  The  two  were  included  under 
the  term  ampho-peptone.  It  is  generally  admitted  now,  however,  that  there 
is  but  one  kind  of  peptone  formed  from  any  given  protein,  which  under  the 
influence  of  pancreatic,  and  intestinal  juice  as  well,  is  reduced  by  hydrolysis, 
through  successive  stages  to  amino-acids  or  perhaps  only  to  the  antecedent 
stage,  in  which  two  or  more  amino-acids  yet  remain  united  forming  sub- 
stances known  as  peptids. 

Nearly  all  forms  of  protein  are  in  a  similar  manner  transformed  into 
peptones  by  gastric  juice.  Beyond  this  stage,  however,  there  does  not  seem 
to  be  any  further  change,  peptones  apparently  being  the  final  products  of 
gastric  digestion.  The  intimate  nature  of  this  change  is  practically  un- 
known, but  there  are  reasons  for  thinking  that  it  is  a  process  of  hydra- 
tion, attended  by  cleavage,  with  increasing  solubihty  of  the  resulting 
products. 

Characters  of  Peptones. — The  peptones  resulting  from  the  digestion  of 
different  proteins,  though  resembling  each  other  in  many  respects,  yet  possess 


176  TEXT-BOOK  OF  PHYSIOLOGY 

different  chemic  characteristics,  as  shown  by  their  reaction  to  various  chemic 
reagents.  Though  having  some  resemblance  to  the  proteins  from  which 
they  are  derived,  they  are  to  be  distinguished  from  them  by  the  following 
general  characteristics: 

1.  They  are  not  coagulable  either  by  heat  or  by  nitric  acid. 

2.  They  are  soluble  in  water,  either  hot  or  cold,  and  in  acid  and  alkaline 

solutions. 

3.  They   are  diffusible,   passing   through  animal  membranes  with  great 

rapidity.  It  has  been  demonstrated  that  peptones  diffuse  about  twelve 
times  as  rapidly  as  the  proteins  from  which  they  are  derived. 

From  the  foregoing  facts  it  may  be  inferred  that  in  the  digestion  of  pro- 
teins there  is  a  progressive  diminution  in  the  size  of  the  molecules  through  a 
series  of  hydrolytic  changes.  The  molecules  of  the  proteins,  which  from 
various  causes  are  coagulated,  are  transformed  into  smaller  molecules  which 
are  non-coagulable,  soluble,  and  diffusible. 

On  liquid  fat  and  hydrated  starch  gastric  juice  has  no  appreciable  action. 
It  has  apparently  been  demonstrated,  however,  that  when  fat  in  the  emulsi- 
fied state,  the  state  in  which  it  exists  in  milk,  is  introduced  into  the  stomach 
it  undergoes  a  cleavage  into  fat  acids  and  glycerin,  in  a  manner  similar  to 
that  which  fat  undergoes  in  the  intestine  under  the  action  of  pancreatic 
juice,  as  will  be  stated  in  a  future  paragraph.  This  presupposes  the 
existence  of  a  ferment  to  which  the  name  lipase  has  been  given.  Though 
the  action  of  saliva  on  starch  is  interfered  with  and  even  checked  by  a  small 
percentage  of  hydrochloric  acid  it  is  certain  from  the  results  of  recent  experi- 
ments, that  starch  digestion  continues  for  from  twenty  minutes  to  a  half 
hour  or  longer,  for  the  reason  that  the  acid  as  fast  as  it  is  secreted  combines 
with  the  proteins  and  is  thus  rendered  inoperative  and  for  the  reason  also 
that  the  food  is  largely  retained  in  the  extreme  fundic  end  of  the  stomach 
where  the  gastric  juice  is  not  abundant.  After  the  above-mentioned  period, 
free  acid  makes  its  appearance  when  salivary  digestion  ceases. 

Notwithstanding  the  fact  that  dilute  solutions  of  hydrochloric  acid  (0.3 
per  cent.)  will  promptly  invert  cane-sugar  to  dextrose  and  levulose,  and  that 
gastric  juice  will  accomplish  the  same  result  in  test-tubes,  there  is  no  strong 
evidence  for  the  belief  that  the  inversion  of  cane-sugar  takes  place  to  any 
marked  extent  in  the  stomach  under  normal  conditions. 

Action  of  Gastric  Juice  on  Foods. — The  action  of  gastric  juice  on 
proteins  affords  a  key  to  its  action  in  the  reduction  of  foods  to  the  liquid  or 
semiliquid  condition.  It  is  evident  that  it  will  be  most  active  in  the  digestion 
of  food  consisting  largely  of  protein  materials,  such  as  meat,  eggs,  milk,  etc. 
Meat  is  disintegrated  first  by  the  conversion  of  the  proteins  of  the  connective 
tissue,  which  have  been  more  or  less  gelatinized  by  cooking,  into  peptones. 
The  sarcolemma  of  the  muscle-fibers  which  have  been  thus  separated  is  in  a 
similar  manner  attacked  and  converted  into  peptones.  The  true  muscle 
or  sarcous  substance,  consisting  largely  of  myosin,  undergoes  a  corresponding 
change.  If  the  quantity  of  meat  be  not  too  large  and  the  gastric  juice  be 
secreted  in  proper  amount,  it  is  possible  that  all  the  meat  will  be  digested  in 
the  stomach.  It  is  quite  probable,  however,  that  this  is  not  the  case  and  that 
a  portion  of  the  semidigested  meat  passes  into  the  intestine,  where  its  final 
solution  is  effected. 

The  white  of  egg,  especially  when  slightly  boiled,  is  much  more  readily 


DIGESTION  177 

digested  than  when  raw  or  firmly  coagulated  by  prolonged  boiling.  In  either 
condition,  however,  the  supporting  tissue  is  dissolved  and  peptonized,  after 
which  the  native  albumin  undergoes  the  same  change.  The  yolk  of  the  egg 
consists  largely  of  fat  held  in  suspension  by  a  protein  substance,  vitellin, 
which  is  also  capable  of  transformation  into  peptone. 

Adipose  tissue  is  similarly  reduced.  The  protein  of  the  connective  tissue 
and  of  the  fat  vesicles  is  dissolved  and  peptonized  and  the  fat-drops  set 
free. 

Milk  undergoes  a  peculiar  change  in  composition  before  its  chief  protein 
constituent,  caseinogen,  can  be  transformed  into  peptone.  The  caseinogen 
in  the  presence  of  calcium  salts  is  always  in  the  soluble  state.  When  acted 
on  by  the  gastric  juice,  the  caseinogen  undergoes  a  chemic  change  by  reason 
of  which  it  combines  with  calcium  salts  and  is  then  transformed  into  a  solid 
compound  casein.  This  change  is  due  to  the  presence  and  activity  of  the 
enzyme,  rennin.  The  necessity  for  this  change  in  the  process  of  digestion, 
however,  is  not  apparent.  The  coagulated  casein  presents  itself  in  the  form 
of  a  fiocculent  curd,  which  is  finer  in  human  than  in  cow's  milk,  and  hence 
more  easily  digestible.  After  its  production,  the  casein  is  acidified  by  the 
hydrochloric  acid  and  then  converted  by  the  pepsin  into  peptone. 

Vegetables,  though  consisting  of  a  woody  or  cellulose  framework,  undergo 
a  partial  disintegration  in  the  stomach.  When  they  are  boiled  and  dis- 
integrated by  the  teeth,  the  gastric  juice  is  enabled  to  penetrate  the  frame- 
work and  dissolve  and  peptonize  the  various  protein  constituents.  As  a 
general  rule,  the  vegetable  proteins  are  more  diflScult  of  digestion  than  the 
animal  proteins. 

Duration  of  Gastric  Digestion. — The  length  of  time  the  food  remains 
in  the  stomach  and  the  relative  digestibility  of  different  articles  of  food  were 
carefully  studied  by  Dr.  Beaumont  on  St.  Martin,  and  though  the  results 
obtained  by  him  may  not  be  absolutely  correct,  viewed  in  the  light  of  recent 
knowledge  of  the  digestive  process,  yet  in  the  main  they  have  been  corrobo- 
rated in  various  ways.  As  a  result  of  many  observations  Dr.  Beaumont 
came  to  the  conclusion  that  the  average  length  of  time  an  ordinary  meal 
consisting  of  meat,  bread,  potatoes,  etc.,  remained  in  the  stomach  under- 
going digestion  was  about  three  and  a  half  hours,  the  duration  of  the  proc- 
ess, however,  being  increased  when  an  excessive  quantity  of  food  was 
taken  or  the  quantity  and  quality  of  the  gastric  juice  impaired  by  abnormal 
conditions  of  the  system.  As  soon  as  the  food  is  liquefied  by  the  gastric 
juice  that  portion  not  absorbed  by  the  gastric  vessels  passes  into  the  intes- 
tines, this  continuing  for  two  to  three  hours  until  the  stomach  is  completely 
emptied.  The  relative  digestibility  of  the  different  foods  was  also  made  the 
subject  of  many  experiments  by  Dr.  Beaumont.  After  repeating  and 
verifying  his  observations  made  under  varying  conditions,  he  summed  up 
his  results  in  a  table,  of  which  the  following  is  an  abstract,  in  which  the 
mode  of  preparation  and  the  time  required  for  the  digestion  of  different 
foods  are  exhibited : 

The  time  required  for  the  stomach  to  discharge  any  given  article  of 
food  has  been  shown  by  Cannon  to  depend  partly  on  its  chemic  composition 
and  partly  on  its  capacity  for  absorbing  hydrochloric  acid.  From  an 
examination  of  the  stomach  and  duodenum  of  the  cat  by  means  of  Rontgen 
rays  and  the  fluoroscopic  screen,  after  the  administration  of  equal  quantities. 


178  TEXT-BOOK  OF  PHYSIOLOGY 

25  c.c.,of  pure  protein,  fat,  and  carbohydrate,  mixed  with  5  grams  of  bismuth, 
it  became  possible  to  determine  the  rate  at  which  they  left  the  stomach  from 
the  length  of  the  food  masses  in  the  duodenum  and  small  intestine  as  indi- 
cated by  the  shadows  on  the  screen,  at  intervals  of  half  an  hour  or  longer. 
The  duration  of  the  observations  extended  over  a  period  of  seven  hours. 

TABLE  SHOWING  THE  DIGESTIBILITY  OF  VARIOUS  ARTICLES  OF  FOOD 

Hours.  Minutes.                                                                  Hours.  Minutes. 

Eggs,  whipped i  20  Soup,  barley,  boiled i  30 

Eggs,  soft  boiled 3  .  .  Soup,  bean,  boiled 3 

Eggs,  hard  boiled 3  30  Soup,  chicken,  boiled 3 

Oysters,  raw 2  55  Soup,  mutton,  boiled 3  30 

Oysters,  stewed 3  30  Sausage 3  20 

Lamb,  broiled 2  30  Green  com,  boiled. 3  45 

Veal,  broiled 4  . .  Beans,  boiled 2  30 

Pork,  roasted 5  15  Potatoes,  roasted 2  30 

Beefsteak,  broiled 3  . .  Potatoes,  boiled 3  30 

Turkey,  roasted 2  25  Cabbage,  boiled 4  30 

Chicken,  boiled 4  .  .  Turnips,  boiled 3  30 

Chicken,  fricasseed 2  45    •        Beets,  boiled 3  45 

Duck,  roasted 4  . .  Parsnips,  boiled 2  30 

When  a  pure  protein,  e.g.,  boiled  beef  free  from  fat,  boiled  haddock,  or 
the  white  meat  of  fowls  is  administered,  foods  which  not  only  excite  the 
flow  of  gastric  juice  but  readily  absorb  hydrochloric  acid,  the  pylorus  remains 
closed  for  some  time,  scarcely  any  protein  leaving  the  stomach  during  the  first 
half  hour.  Shortly  after  this  when  free  hydrochloric  acid  makes  its  appear- 
ance, the  signal  for  the  relaxation  of  the  sphincter,  the  pylorus  opens  from 
time  to  time  and  the  passage  of  the  protein  into  the  duodenum  begins  and 
gradually  increases  in  rapidity  until  the  maximum  speed  is  attained,  about 
two  hours  after  ingestion ;  from  this  time  on,  the  speed  of  discharge  gradually 
diminishes  until  the  end  of  the  observation  period. 

When  fat,  e.g.,  beef,  mutton,  or  pork  fat,  is  administered,  they  remain 
in  the  stomach  for  some  time  and  when  they  begin  to  leave,  the  rate  of  dis- 
charge is  so  slow  that  they  are  digested  and  absorbed  almost  as  fast  as 
discharged  and  hence  seldom  accumulate  in  the  small  intestine.  These 
compounds  delay  the  secretion  of  gastric  juice  and  therefore  free  hydrochloric 
acid,  the  presence  of  which  appears  to  be  necessary  for  the  relaxation  of  the 
pyloric  sphincter. 

When  carbohydrates,  e.^.,  starch  paste,  boiled  rice,  boiled  mashed  pota- 
toes are  administered,  their  discharge  begins  shortly  after  their  entrance  into 
the  stomach;  they  pass  out  rapidly,  the  velocity  of  discharge  reaching  its 
maximum  at  the  end  of  two  hours,  after  which  the  speed  declines  to  the  end 
of  the  observation  period.  The  reason  for  the  early  and  rapid  discharge  is 
to  be  found  in  the  fact  that  while  the  carbohydrates  excite  the  secretion  of 
gastric  juice  they  do  not  absorb  the  hydrochloric  acid  to  any  appreciable 
extent.  A  combination  of  equal  quantities  of  protein  and  carbohydrate 
varies  the  rate  of  discharge  of  each  separately.  Thus  under  these  circum- 
stances the  carbohydrates  are  not  discharged  so  rapidly  nor  are  the  proteins 
detained  so  long  as  usual;  a  combination  of  fat  with  either  protein  or  carbo- 
hydrate delays  the  time  of  discharge  of  both.  From  these  facts  it  may  be 
inferred  that  the  time  any  given  food  remains  in  the  stomach  will  depend  on  its 
chemic  composition  or  the  relative  amounts  of  its  contained  protein,  fat, 
and  carbohydrate  principles. 


DIGESTION 


179 


Movements  of  the  Stomach.— During  the  period  of  gastric  digestion 
the  muscle  walls  of  the  stomach  become  the  seat  of  a  series  of  movements, 
usually  described  as  peristaltic  in  character,  which  not  only  incorporate  the 
gastric  juice  with  the  food,  but  also  serve  to  eject  the  liquefied  portions  of 
the  food  into  the  small  intestine. 

The  movements  of  the  human  stomach  as 
described  by  Beaumont,  as  well  as  the  movements 
of  the  dog's  stomach  as  stated  by  different  ob- 
servers are  not  in  agreement  in  all  respects,  and 
are,  moreover,  open  to  question  for  the  reason 
that  they  were  not  observed  under  strictly  physio- 
logic conditions.  The  more  recent  investigations 
of  Cannon  have  thrown  new  light  on  this  sub- 
ject. By  means  of  the  Rontgen  rays  he  has  been 
enabled  to  study  the  movements  in  the  living 
animal  and  under  normal  conditions.  The 
animal  (the  cat)  was  fed  with  bread  and  milk, 
to    which    was    added    subnitrate    of   bismuth. 


Right 


Left 


Fig.  76- — Shadow  Sketches 
OF  THE  Outlines  of  the 
Stomach  of  a  Cat  Immedi- 
ately AFTER  A  Meal  (ii.o), 

AND    AT     VaKIOUS     INTERVALS 

Afterward   (at   12.0.  at  2.0, 
3.30,  4.30). — (W.  B.  Cannon.) 


Post 


Fig.  77. — The  cardiac  portion  is  all  that 
part  to  the  left,  as  the  stomach  lies  in  the 
body,  of  WX.  The  cardia  is  at  C.  The 
pylorus  is  at  P,  and  the  pyloric  portion 
is  the  part  between  P  and  WX.  This 
has  two  divisions:  the  antrum,  between 
P  and  YZ,  and  the  pre-antral  part,  between 
WX  and  YZ.  The  lesser  curvature  is  on  the 
top  of  the  outline  between  C  and  P,  and  the 
greater  curvature  between  the  same  points 
along  the  lower  border. — {Amer.  Jour,  oj 
Physiology,  Cannon.) 


This  substance,  being  opaque,  rendered  the  movements  of  the  stomach  walls 
visible  on  the  fluorescent  screen.  With  paper  placed  over  the  screen  it 
was  possible  to  sketch  the  change  in  shape  that  the  stomach  undergoes  at 
different  periods  of  the  digestive  act.  The  results  of  these  investigations 
will  be  referred  to  in  following  paragraphs. 

As  a  result  of  many  methods  of  investigation  it  has  become  apparent  that 
the  activities  of  different  portions  of  the  stomach,  whereby  food  is  admitted 
into  it,  retained  there,  triturated  and  mixed  with  the  gastric  juice  and  finally 
discharged  into  the  duodenum,  are  due  (i)  to  causes  resident  in  the  stomach 
walls  and  the  stomxach  contents  and  (2)  to  nerve  impulses  descending  from 
the  central  nerve  system  through  the  vagi  and  splanchnic  nerves. 

At  the  end  of  a  digestive  period  the  walls  of  the  stomach  contract  and 
almost  obliterate  its  ca^^ty.     The  sphincter  cardiac  and  sphincter  pylori 


i8o  TEXT-BOOK  OF  PHYSIOLOGY 

are  also  contracted  and  the  orifices  they  surround  are  more  or  less  tightly 
closed. 

The  Movements  of  the  Sphincter  CardicB. — ^The  sphincter  car  dice  muscle 
surrounding  the  esophago-gastric  orifice  is  always,  under  normal  conditions, 
tonically  contracted  and  the  orifice  closed.  This  contraction  is  partly  due 
to  inherent  causes  as  shown  by  the  fact  that  it  persists  for  from  24  hours  to 
several  days  after  division  of  all  nerves  distributed  to  it.  The  contraction 
may  be  so  pronounced  as  to  offer  considerable  resistance  not  only  to  the  pass- 
age of  food  but  even  to  the  introduction  of  a  sound  into  the  stomach.  (Can- 
non.) That  the  normal  contraction  is  under  the  influence  of  the  central 
nerve  system  is  shown  by  the  effects  which  follow  division  and  stimulation 
of  the  peripheral  end  of  the  vagus.  If  it  is  stimulated  with  weak  induced 
currents,  the  contraction  of  the  sphincter  is  somewhat  inhibited  and  the 
orifice  enlarged;  if  it  is  stimulated  with  strong  currents  the  contraction  is 
markedly  increased  and  the  orifice  diminished.  Apparently  there  are  in  the 
vagus  two  sets  of  efferent  nerve-fibers,  one  of  which  inhibits  while  the  other 
augments  the  contraction,  and  corresponding  to  the  nerves  there  must  be 
in  the  medulla  oblongata  two  centers  from  which  they  arise,  an  augmentor 
and  an  inhibitor. 

Observation  has  also  shown  that  at  the  beginning  of  each  act  of  degluti- 
tion, there  is  an  inhibition  of  the  sphincter  muscle,  and  if  the  acts  follow  each 
other  in  quick  succession,  the  inhibition  and  relaxation  are  increased. 
(Meltzer.)  With  the  passage  of  food  into  the  stomach  the  tonic  contrac- 
tion again  supervenes.  These  effects  also  follow  stimulation  of  the  glosso- 
pharyngeal nerve.  Whether  the  sphincter  inhibition  is  the  result  of  an 
inhibition  of  the  center  which  maintains  the  tonus,  or  a  stimulation  of  an 
inhibitor  center,  is  uncertain. 

Though  the  tonus  of  the  sphincter  cardiae  is  capable  of  being  inhibited 
and  augmented  by  the  central  nerve  system,  the  most  frequent  cause  under 
physiological  conditions  for  an  augmentation  of  the  tonus,  is  the  presence  of 
hydrochloric  acid  on  the  gastric  side  of  the  cardia.  In  the  early  stages  of 
digestion  when  the  percentage  of  the  acid  is  below  the  normal,  the  orifice 
frequently  opens  and  food  is  regurgitated  for  a  short  distance  into  the  esopha- 
gus. The  ascent  of  the  food  far  into  the  esophagus  is  prevented  by  a  con- 
traction and  peristalsis  of  its  muscle  walls,  by  which  the  food  is  returned  to 
the  stomach.  As  the  free  acid  accumulates  the  sphincter  tightly  closes  the 
orifice.  As  this  takes  place  after  division  of  the  nerves  passing  to  this  region 
of  the  esophagus  the  inference  is  that  the  contraction  of  the  sphincter  is 
brought  about  through  the  intermediation  of  a  local  reflex  nerve  mechanism 
in  the  walls  of  the  esophagus. 

It  has  recently  been  reported  by  Cannon  that  a  similar  inhibition  or 
relaxation  of  the  musculature  of  the  cardiac  end  of  the  stomach  is  occasioned 
by  each  act  of  deglutition  and  that  it  continues  and  increases  if  the  acts 
follow  each  other  in  quick  succession.  As  the  bolus  descends  the  esophagus 
and  before  it  reaches  its  termination  there  is  a  relaxation  of  the  musculature  of 
the  cardiac  end,  a  fall  of  intragastric  pressure,  an  enlargement  of  the  stomach 
capacity  and  hence  a  readier  receptivity  of  the  bolus.  That  this  inhibition 
is  caused  by  impulses  descending  the  vagus  is  shown  by  the  effects  which 
follow  a  moderate  stimulation  of  the  vagus  nerve  and  by  the  fact  that  it 
does  not  take  place  if  the  vagus  nerves  are  divided.     To  this  inhibition  and 


DIGESTION  i8i 

enlargement  of  the  cardiac  end  of  the  stomach  the  term  receptive  relaxa- 
tion has  been  given. 

The  investigations  of  Cannon  previously  referred  to,  show  that  not  only 
the  sphincter  cadiae,  but  other  portions  of  the  stomach  walls  exhibit  differ- 
ent forms  of  activity  which  for  convenience  of  description  are  separately 
described  by  him  as  follows: 

1.  The  Movements  of  the  Pyloric  Portion. — ^The  movements  of  this  region 
of  the  stomach  were  first  studied  and  sketched  by  means  of  the  Rontgen 
rays  by  Cannon.  Some  of  the  changes  in  shape  due  to  these  movements 
are  shown  in  Fig.  76.  Within  five  minutes  after  a  cat  has  finished  a  meal 
of  bread  there  is  visible  near  the  duodenal  end  of  the  antrum  or  vestibule 
a  slight  annular  contraction  which  moves  peristaltically  to  the  pyloric  orifice; 
this  is  followed  by  several  waves  recurring  at  regular  intervals.     Two  or 

.  three  minutes  after  the  first  movement  is  seen,  very  slight  constrictions  ap- 
pear near  the  middle  of  the  stomach,  and,  pressing  deeper  into  the  greater 
curvature,  course  slowly  toward  the  pyloric  end.  As  new  regions  enter  into 
constriction,  the  fibers  just  previously  contracted  become  relaxed,  so  that 
there  is  a  true  moving  wave,  with  a  trough  between  two  crests.  When  a 
wave  swings  round  the  bend  in  the  pyloric  part,  the  indentation  made  by  it 
deepens;  and  as  digestion  goes  on  the  antrum  or  vestibule  elongates  and  the 
constrictions  running  over  it  grow  stronger,  but,  until  the  stomach  is  nearly 
empty,  they  do  not  entirely  divide  the  cavity.  After  the  pyloric  region  has 
lengthened,  a  wave  takes  about  thirty-six  seconds  to  move  from  the  middle  of 
the  stomach  to  the  pyloric  orifice.  At  all  periods  of  digestion  the  waves 
recur  at  intervals  of  almost  exactly  ten  seconds.  It  results  from  this  rhythm 
that  when  one  w^ave  is  just  beginning  several  others  are  already  running  in 
order  before  it.  Between  the  rings  of  constriction  the  stomach  is  bulged 
out,  as  shown  in  the  various  outlines  in  Fig.  77. 

2.  The  Movements  of  the  Sphincter  Pylori. — ^During  the  first  ten  or  fifteen 
minutes  after  the  introduction  of  food  the  pyloric  orifice  is  more  or  less 
tightly  closed.  After  this  period  it  opens  at  irregular  intervals  to  permit  the 
passage  of  liquefied  food  which  is  ejected  by  peristaltic  waves  for  a  distance 
of  two  or  three  centimeters  into  the  duodenum.  The  frequency  with  which 
the  pyloric  orifice  opens  depends  apparently  on  the  degree  to  which  the  food 
is  softened.  When  the  food  is  hard,  the  orifice  is  closed  more  tightly  and 
remains  closed  a  longer  period  than  when  it  is  soft. 

The  physiologic  cause  for  the  relaxation  or  inhibition  of  the  sphincter 
pylori  appears  to  be  the  presence  of  free  acid  at  the  pylorus;  its  contraction, 
the  presence  of  free  acid  in  the  duodenum.  With  the  neutralization  of  the 
acid  in  the  duodenum,  its  influence  on  the  sphincter  muscle  is  weakened,  after 
which  the  muscle  again  becomes  susceptible  to  the  inhibitor  influence  of  the 
acid  within  the  stomach.  It  is  probably  for  this  reason  that  carbohydrates, 
which  do  not  absorb  the  acid,  are  discharged  from  the  stomach  early;  that 
the  proteins,  which  postpone  the  appearance  of  free  acid,  are  retained  longer 
and  that  fats,  which  check  the  secretion  of  gastric  juice  are  discharged 
slowly  (Cannon).  It  should  be  emphasized,  however,  that  the  relaxation 
and  contraction  of  the  pyloric  sphincter,  due  to  the  action  of  free  acid  on 
the  gastric  and  duodenal  sides,  respectively,  can  take  place  independently 
of  the  central  nerve  system,  and  through  the  intermediation  of  a  local  reflex 
nerve  mechanism,  the  myenteric  plexus,  in  the  walls  of  the  pylorus. 


1 82  TEXT-BOOK  OF  PHYSIOLOGY 

This  explanation  of  the  opening  of  the  pylorus  is  believed  by  its  advocates 
to  supersede  a  former  view,  yet  widely  held,  however,  that  the  pylorus  re- 
mains open  until  closed  refiexly  from  the  duodenum  by  the  passage  into  it 
of  acid,  coarse  food  particles  or  other  irritants  from  the  stomach.  When 
these  irritants  are  neutralized  or  removed,  the  pylorus  relaxes,  to  be  closed 
again  by  another  duodenal  reflex.  This  intermittent  opening  and  closing 
continues  until  the  stomach  empties.  Recent  investigations  apparently 
suggest  this  interpretation  of  the  mechanism  to  be  the  one  that  obtains  for 
the  human  pylorus.  In  these  experiments  the  stomach  was  found  to  empty 
more  quickly  when  its  contents  were  of  a  weak  alkaline  reaction  than 
when  acid  was  present  in  the  gastric  contents. 

If  the  pylorus  is  more  or  less  open  during  the  early  stages  of  digestion  it 
is  easier  to  understand  how  the  intestinal  juices  can  be  regurgitated  into  the 
stomach,  as  appears  to  be  experimentally  proven  by  the  investigations  of 
Boldyreff,  Spencer  and  his  co-workers  with  the  result  of  neutralizing  the 
superfluous  acidity  of  the  gastric  juice.  The  initial  high  acidity  of  this  fluid, 
viz. :  0.32  to  0.48  causes,  on  its  passage  into  the  duodenum,  a  secretion  of 
pancreatic  juice  and  bile  and  the  development  of  an  anti-peristaltic  wave  or 
rhythmic  pulsations  which  drives  these  alkaline  fluids  into  the  stomach  until 
the  acidity  is  brought  down  to  that  point  where  the  juice  is  no  longer 
irritating  to  the  duodenal  mucous  membrane. 

3.  The  Movements  of  the  Cardiac  Portion. — ^As  digestion  proceeds,  the  pre- 
vestibular  or  cardiac  portion  of  the  stomach  elongates  and  assumes  the  shape 
of  a  tube,  which  becomes  the  seat  also  of  peristaltic  constriction  waves.  As 
a  result,  some  of  the  food  is  gradually  forced  into  the  pyloric  region  to  succeed 
that  which  has  been  prepared  and  ejected  into  the  duodenum.  As  the  pre- 
vestibular  tube  is  emptied  of  its  contents  the  longitudinal  and  circular  fibers 
of  the  fundus  steadily  contract  and  gradually  force  its  contents  into  the 
tubular  portion.  This  continues  until  the  fundus  is  completely  emptied. 
The  changes  in  shape  which  the  cardiac  portion  undergoes  during  digestion 
are  represented  in  Fig.  76.  The  fundus  acts  as  a  reservoir  for  the  food  and 
forces  out  its  contents  a  little  at  a  time  as  the  vestibular  mechanism  is  ready 
to  receive  them.  Since  peristaltic  movements  are  absent  from  the  fundus 
portion,  the  food  is  not  mixed  with  gastric  juice,  and  therefore  salivary  diges- 
tion can  continue  for  a  considerable  period.  There  is  no  evidence  of  a  circu- 
lation of  food  in  the  stomach  as  sometimes  described.  On  the  contrary,  the 
movement  through  the  elongated  tube  is  in  general  a  progressive  though  an 
oscillating  one.  As  the  constriction  waves  rapidly  pass  over  the  food  it  is 
advanced  toward  the  pyloric  orifice,  but  as  this  is  closed  the  food  is  forced 
backward  through  the  advancing  constricted  ring  for  a  variable  distance. 

The  effect  of  the  constriction  waves  is  to  mLx  the  food  with  the  gastric 
juice,  triturate  and  soften  it.  So  soon  as  this  is  effected,  the  pyloric  orifice 
opens,  when  the  advancing  constriction  wave  expels  it  into  the  intestine. 
With  its  expulsion,  room  is  afforded  for  an  additional  quantity  of  food,  and 
hence  there  is  a  general  advance  of  the  food  mass  toward  the  pylorus. 

Though  these  observations  were  made  on  the  cat,  evidence  is  accumu- 
lating which  goes  to  show  that  in  human  beings  the  walls  of  the  stomach 
exhibit  constriction  waves  which  are  similar  in  all  respects  to  those  above 
described.  Evidence  of  this  character  has  been  furnished  by  experiments 
conducted  in  the  author's  laboratory  by  Spencer  and  Meyer. 


DIGESTION  183 

Fig.  78  is  a  record  of  the  gastric  contractions  in  man  as  recorded  by  the 
lever  of  a  Marey  tambour  in  connection  with  a  thin  rubber  sausage-shaped 
balloon.  The  balloon  was  swallowed  in  the  collapsed  state  and  then  in- 
flated under  a  pressure  of  about  eight  millimeters  of  mercury.  During  the 
experiment,  which  lasted  from  one  to  an  hour  and  a  half,  the  subject  was 
placed  in  the  recumbent  position  and  frequently  fell  asleep.  The  abdomen 
was  auscultated  for  sounds  of  peristalsis  in  the  intestine.  The  passage  of 
food  from  the  stomach  into  the  duodenum  was  easily  heard  and  coincided 
with  a  fall  of  intragastric  pressure  as  recorded  in  the  tracing. 

The  conditions  necessary  for  the  development  of  the  gastric  peristalsis 
are  (i)  a  condition  of  tonicity  of  the  musculature,  i.e.,  a  slight  degree  of  con- 
traction whereby  the  muscle  is  shortened;  (2)  intragastric  pressure.  When 
these  two  conditions  are  mutually  adapted  the  musculature  acquires  a  cer- 
tain degree  of  tension  whereupon  the  peristalsis  arises.     An  excess  or  de- 


FiG.  78. — Gastric  Contractions  during  Digestion.  (Two  hours  after  eating.) 
The  upper  tracing  shows  respiratory  excursions,  the  variation  in  the  bases  of  which  records  the 
changes  in  intragastric  pressure.  The  heavier  markings  at  the  bases  of  the  respiratory  tracings 
are  heart-beats.  Time  marked  in  minutes.  Analysis  of  the  gastric  contractions  divides  them  into 
long  tonus  waves  of  about  60  to  90  seconds  duration,  superposed  upon  which  are  shorter  peristaltic 
waves  about  20  seconds  in  duration. 

The  lower  tracing  shows  gastric  digestive  contractions  with  heart-beats,  taken  while  holding  the 
breath  at  the  end  of  an  expiration.  The  intervals  represent  periods  of  respiration  during  which  the 
pen  was  removed  from  the  drum.  Were  the  intervals  connected,  a  regular  sequence  of  gastric  tonus 
waves  with  peristaltic  waves  superposed  would  appear. 

ficiency  of  internal  pressure  as  well  as  a  loss  of  tonicity  prevents  peristalsis. 
The  peristalsis  has  no  necessary  fixed  point  of  origin  but  arises  at  that 
portion  of  the  stomach  in  v/hich  the  two  factors  previously  mentioned  bear 
a  certain  relation  one  to  the  other.  From  their  origin  the  peristaltic  waves 
pass  toward  the  pylorus  as  a  result  of  increased  internal  pressure.  The 
necessary  degree  of  the  preliminary  tonus  is  imparted  to  the  musculature 
by  nerve  impulses  descending  the  vagi.  If  these  nerves  are  cut,  the  tonus  is 
impaired  and  peristalsis  fails  to  develop.  After  a  variable  period  the  neuro- 
muscular mechanism  develops  a  tonus  like  that  given  to  it  by  the  vagi,  after 
which  the  usual  peristalsis  returns.  When  once  the  peristalsis  is  well  devel- 
oped in  digestion,  division  of  the  vagi  has  no  efifect.     (Cannon.) 

The  Nerve  Mechanism  of  the  Stomach. — In  preceding  paragraphs 
it  was  stated  that  during  the  period  of  gastric  digestion  the  food  is  retained 


1 84  TEXT-BOOK  OF  PHYSIOLOGY 

in  the  stomach  because  of  the  closure  of  the  cardia  (the  esophago-gastric 
orifice)  and  of  the  pylorus  (the  gastro-duodenal  orifice)  both  orifices  being 
tightly  closed  by  the  tonic  contraction  of  sphincter  muscles;  that  both  sphinc- 
ters relax  from  time  to  time,  the  one  to  permit  the  entrance  of  food  into  the 
stomach  for  further  digestion,  the  other  to  permit  the  exit  of  food  into 
the  intestine  after  its  more  or  less  complete  digestion,  after  which  in  both 
instances  the  sphincters  again  contract  and  close  the  orifices;  that  the  fundus 
muscles  are  tonically  contracted  and  steadily  pressing  the  food  into  and 
through  the  cardiac  region  to  the  vestibule;  that  the  pyloric  or  vestibular 
muscles  are  vigorously  active  throughout  the  digestive  period,  triturating 
the  food,  mixing  it  with  gastric  juice,  and  finally  driving  it  through  the 
temporarily  open  pylorus  into  the  intestine. 

These  separate  but  related  groups  of  muscle-fibers,  by  reason  of  their 
physiologic  endowments,  and  possibly  by  virtue  of  the  presence  of  local  nerve 
mechanisms,  exhibit  activities  which  are  independent  of  the  central  nerve 
system.  Thus  the  isolated  stomach  of  the  dog  and  of  other  animals  as  well, 
if  kept  warm  and  moist,  will  exhibit  rhythmic  movements  for  a  period  of 
time  varying  from  an  hour  to  an  hour  and  a  half.  Though  nerve-cells  and 
nerve-fibers  (Auerbach's  plexus)  are  present  in  the  walls  of  the  stomach  be- 
tween the  layers  of  muscle-fibers,  it  is  not  believed  that  they  are  the  immedi- 
ate sources  of  the  stimulus  to  the  contraction,  though  they  may  act  as  a  coordi- 
nating mechanism. 

Though  the  activities  of  the  pyloric  muscles  and  of  the  pyloric  sphincter, 
if  not  the  entire  gastric  musculature,  are  due  in  large  measure  to  conditions 
referred  to  in  foregoing  paragraphs,  nevertheless  the  activities  of  all  portions 
of  the  musculature  are  susceptible  to  modifications  by  the  central  nerve  sys- 
tem either  in  the  way  of  augmentation  or  inhibition  and  in  response  to  intra- 
gastric stimulation.  The  nerves  more  especially  concerned  in  the  main- 
tenance and  regulation  of  the  gastric  tonus  and  contractions  are  the  vagi  and 
the  splanchnics.  The  afferent  fibers  through  which  nerve  impulses  pass  to 
the  nerve  centers  are  in  all  probability  contained  in  the  trunk  of  the  vagus 
nerve;  the  efferent  fibers  through  which  nerve  impulses  from  the  centers 
reach  the  stomach,  are  contained  partly  in  the  trunk  of  the  vagus  and  partly 
in  the  trunk  of  the  splanchnic  nerve. 

The  Vagi. — If  the  vagus  nerves  are  divided  in  the  neck,  there  is  a  loss  of 
muscle  tonus  though  the  contractions  do  not  wholly  disappear.  Stimulation 
of  the  peripheral  end  of  one  divided  vagus  is  followed  by  an  augmentation 
in  the  vigor  of  the  contraction  of  the  antral  muscles,  an  increase  in  the  tone  of 
the  fundus  muscles,  as  well  as  an  increase  in  the  contraction  of  the  sphincter 
pylori  and  sphincter  cardise.  Though  this  is  the  usual  result  there  may  be 
a  primary  relaxation  or  inhibition  of  short  duration  of  one  or  all  of  these 
structures  before  the  augmentation  occurs.  May  states  that  this  was  always 
the  case  in  his  experiments.  A  similar  inhibition  may  be  brought  about 
reflexly  by  stimulation  of  the  central  end  of  a  divided  vagus.  This  result 
will  not  be  produced  if  the  opposite  vagus  has  previously  been  divided.  The 
vagi,  therefore,  apparently  contain  both  inhibitor  and  augmentor  nerve- 
fibers  for  the  gastric  musculature. 

The  Splanchnics. — If  the  splanchnic  nerves  are  divided  and  the  peripheral 
end  stimulated  with  induced  electric  currents  there  follows  an  inhibition  of 
the  peristalsis  and  a  loss  of  tone.     Morat,  however,  has  observed  a  primary 


DIGESTION  185 

opposite  effect.  The  splanchnic  nerves  therefore  apparently  contain  both 
inhibitor  and  augmentor  fibers  for  the  gastric  musculature  though  the  inhibi- 
tor fibers  largely  predominate.  From  these  facts  it  would  appear  that  the 
gastric  muscles  receive  both  inhibitor  and  augmentor  fibers  from  two  differ- 
ent sources. 

INTESTINAL  DIGESTION 

The  physical  and  chemic  changes  which  the  food  principles  undergo  in 
the  small  intestine,  and  which  collectively  constitute  intestinal  digestion, 
are  complex  and  probably  more  important  than  those  taking  place  in  the 
stomach,  for  the  food  is,  in  this  situation,  subjected  to  the  solvent  action  of 
the  pancreatic  and  intestinal  juices,  as  well  as  to  the  action  of  the  bile,  each 
of  which  exerts  a  transforming  influence  on  one  or  more  substances  and 
still  further  prepares  them  for  absorption  into  the  blood. 

To  rightly  appreciate  the  physiologic  actions  of  the  digestive  juices 
poured  into  the  intestine,  the  nature  of  the  partially  digested  food  as  it 
comes  from  the  stomach  must  be  kept  in  mind.  This  consists  of  water, 
inorganic  salts,  acidified  proteins,  proteoses,  peptones,  starch,  maltose, 
liquefied  fat,  saccharose,  lactose,  dextrose,  cellulose,  and  the  indigestible 
portion  of  meats,  cereals,  and  fruits.  Collectively  they  are  known  as  chyme. 
As  this  acidified  mass  passes  through  the  duodenum  its  contained  acids 
excite  a  secretion  and  discharge  of  the  intestinal  fluids:  e.g.,  pancreatic 
juice,  bile,  and  intestinal  juice.  Inasmuch  as  these  fluids  are  alkaline  in 
reaction  they  exert  a  neutralizing  and  precipitating  influence  on  various 
constituents  of  the  chyme.  As  soon  as  this  has  taken  place,  gastric  diges- 
tion ceases  and  those  chemic  changes  are  inaugurated  which  eventuate  in 
the  transformation  of  all  the  remaining  undigested  nutritive  materials  into 
absorbable  and  assimilable  compounds  which  collectively  constitute  intes- 
tinal digestion. 

THE  SMALL  INTESTINE 

The  Small  Intestine. — This  portion  of  the  alimentary  canal  is  a  convo- 
luted tube,  measuring  about  seven  meters  in  length  and  3.5  cm.  in  diameter, 
and  extending  from  the  pyloric  orifice  of  the  stomach  to  the  beginning  of 
the  large  intestine. 

The  Walls  of  the  Small  Intestine. — The  walls  of  the  intestine  consist 
of  four  coats:  viz.,  serous,  muscle,  submucous,  and  mucous. 

The  seraus  coat  is  the  most  external  and  is  formed  by  a  reflection  of  the  general 
peritoneal  membrane.     It  is,  however,  wanting  in  the  duodenal  portion. 

The  muscle  coat,  situated  just  beneath  the  former,  surrounds  the  entire  intestine. 
It  is  composed  of  non-striated  fibers,  which  are  more  abundant  and  better  de- 
veloped in  the  upper  than  in  the  lower  portions  of  the  intestine.  The  muscle  coat 
consists  of  two  layers  of  fibers:  (i)  an  external  or  longitudinal,  and  (2)  an  internal 
or  circular  layer.  The  longitudinal  fibers  are  most  marked  at  that  border  of 
the  intestine  free  from  peritoneal  attachment,  though  they  form  a  thin  layer  all 
over  the  intestine.  The  circular  fibers  are  much  more  numerous,  and  completely 
encircle  the  intestine  throughout  its  entire  extent.  It  has  been  demonstrated 
that  at  the  junction  of  the  ileum  and  colon,  and  surrounding  the  orifice,  the  ileo- 
colic, common  to  both,  the  muscle-fibers  are  arranged  in  the  form  of,  and  play  the 
part  of,  a  sphincter  muscle,  which  has  been  termed  the  ileo-colic  sphincter.  It  is 
usually  in  a  state  of  tonic  contraction  and  regulates  the  passage  of  materials  from 


i86  TEXT-BOOK  OF  PHYSIOLOGY 

the  small  into  the  large  intestine,  and  possibly  also  in  the  reverse  direction  under 
special  circumstances. 

The  submucous  coat  consists  of  areolar  tissue  and  serves  to  unite  the  muscle 
with  the  mucous  coat.  A  thin  layer  of  muscle-fibers,  the  muscularis  mucosa, 
is  placed  on  its  inner  surface. 

The  mucous  coat  is  soft  and  velvety  in  appearance  and  covered  by  a  single 
layer  of  columnar  epithelium.  Its  entire  surface  is  covered  with  small  conical 
projections  termed  villi.  Throughout  its  entire  extent,  with  the  exception  of  the 
lower  portion  of  the  ileum  and  the  duodenum,  the  mucous  membrane  presents  a 
series  of  transverse  folds — the  valvules  conniventes,  or  valves  of  Kirkring.  These 
folds  vary  from  one-fourth  to  half  an  inch  in  width  and  extend  one-half  to  two- 
thirds  of  the  distance  around  the  interior  of  the  bowel.  Each  valve  consists  of  two 
layers  of  the  mucous  membrane  permanently  united  by  fibrous  tissue.  It  is 
believed  that  the  valves  retard  to  some  extent  the  passage  of  the  food  through  the 
intestine  and  present  a  greater  surface  for  absorption. 

Blood-vessels,  Nerves,  and  Lymphatics. — The  blood-vessels  of  the  small 
intestine,  which  are  very  numerous,  are  derived  mainly  from  the  superior  mesenteric 
artery.  After  penetrating  the  intestinal  walls  the  smaller  vessels  ramify  in  the 
submucous  coat  and  send  branches  to  the  muscle  and  mucous  coats,  supplying  all 
their  structures  with  blood.  After  circulating  through  the  capillary  vessels  the 
blood  is  returned  by  small  veins  which  subsequently  unite  to  form  the  superior 
mesenteric  vein,  which,  uniting  with  the  splenic  and  gastric  veins,  forms  the  portal 
vein.  The  nerve  elements  in  the  intestinal  wall  consist  of  two  plexuses,  one 
(Auerbach's)  lying  between  the  muscle  coats,  the  other  (Meissner's)  lying  in  the 
submucous  coat.  To  this  nerve  net,  composed  of  nerve  cells  and  nerve  processes, 
found  in  connection  with  the  muscle  coats  of  the  stomach,  of  the  small  and  of  the 
large  intestine  as  well,  the  term  myenteric  plexus  has  been  given.  The  lymphatics, 
which  originate  in  the  mucous  and  muscle  coats,  are  very  abundant.  They  unite 
to  form  those  vessels  seen  in  the  mesentery  and  empty  into  the  thoracic  duct. 

Intestinal  Glands. — The  gland  apparatus  of  the  intestine  by  which  the 
intestinal  juice  is  secreted  consists  of  the  duodenal  (Brunner's)  and  the  intestinal 
(Lieberkiihn's)  glands. 

The  duodenal  glands  are  situated  beneath  the  mucous  membrane  and  open  by  a 
short  wide  duct  on  its  free  surface.  They  are  racemose  glands  lined  by  nucleated 
epithelium.  The  secretion  of  these  glands  is  clear,  slightly  viscid,  and  alkaline. 
Its  chemic  composition  and  functions  are  unknown. 

The  intestinal  glands  or  follicles  are  distributed  throughout  the  entire  mucous 
membrane  in  enormous  numbers.  They  are  formed  mainly  by  an  inversion  of  the 
mucous  membrane  and  hence  open  on  its  free  surface.  Each  tubule  consists  of  a 
thin  basement  membrane  lined  by  a  layer  of  spheric  epithelial  cells,  some  of  which 
undergo  distention  by  mucin  and  become  converted  into  mucous  or  goblet  cells. 
The  epithelial  secreting  cells  consist  of  granular  protoplasm  containing  a  well- 
defined  nucleus.  The  intestinal  follicles  constitute  the  apparatus  which  secretes 
the  chief  portion  of  the  intestinal  juice. 

The  surface  of  the  mucous  membrane  presents  throughout  its  entire  extent 
fine  filiform  or  conical  processes  termed  villi.  The  structure  and  function  of  the 
villi  will  be  considered  in  connection  with  the  absorption  of  food  materials. 

The  pancreas  and  liver  are  developed,  during  embryonic  life,  from  the  walls  of 
the  intestine  and  are  anatomically  and  physiologically  associated  with  it. 

PANCREAS 

The  Pancreas. — This  gland  is  long,  narrow  and  flattened  and  is  situated 
deep  in  the  abdominal  cavity,  lying  just  behind  the  stomach.     It  measures 


DIGESTION 


187 


from  fifteen  to  twenty  centimeters  in  length,  six  in  breadth,  and  two  and  a 
half  in  thickness.  It  is  usually  divided  into  a  head,  body,  and  tail.  The 
head  is  directed  to  the  right  side  and  is  embraced  by  the  curved  portion  of  the 
duodenum;  the  tail  is  directed  to  the  left  side  and  extends  as  far  as  the  spleen 


Pancreatic  ducts 


Common  bile-duct- 


Tail. 


Fig.  79. — Pancreas  and  Duodenum  Removed  from  the  Body  and  Seen  from 
Behind.     The  Gland  is  Cut  to  Show  the  Ducts. — (Landois  and  Stirling.) 

(Fig.  79).  The  pancreas  communicates  with  the  intestine  by  means  of  a 
duct.  This  duct  commences  at  the  tail  and  runs  transversely  through  the 
body  of  the  gland.     As  it  approaches  the  head  of  the  gland  it  gradually  in- 

crea  ses  in  size  until  it  measures 
about  two  or  three  millimeters 
in  diameter.  It  then  curves 
downward  and  forward  and 
opens  into  the  duodenum.  In 
its  course  through  the  gland  it 
receives  branches  which  enter 
it  nearly  at  right  angles.  The 
pancreas  is  richly  supplied  with 
blood-vessels  and  nerves,  the 
latter  coming  from  the  solar 
plexus. 

Histologic  Structure. — In 
its  structure  the  pancreas  re- 
sembles the  salivary  glands.  It 
consists  of  a  connective-tissue 
framework  which  divides  the 
gland  tissue  into  lobules.  Each 
lobule  is  composed  of  a  number 
of  acini  or  alveoli,  more  or  less 
elongated  or  tubular  in  shape.  Each  acinus  gives  origin  to  a  small  duct 
which,  uniting  with  adjoining  ducts,  forms  the  lobular  duct,  which  becomes 
tributary  to  the  main  duct.  The  acinus  is  lined  by  a  layer  of  cylindric 
epithelial  cells  characterized  by  a  difference  in  structure  between  their  cen- 
tral and  peripheral  ends  (Fig.  80).  The  central  end,  that  bordering  the 
lumen  of  the  acinus,  is  dark  in  appearance  and  filled  with  dark  granules, 
while  the  peripheral  end  is  clear  and  homogeneous.  The  relative  depth  of 
these  two  zones  varies  according  to  the  functional  activity  of  the  gland. 


Fig.  80.  Fig.  81. 

One  Saccule  of  the  P.ancreas  of  the  Rabbit 
IN  Different  States  of  Activity,  fig.  80. — After 
a  period  of  rest,  in  which  case  the  outh'nes  of  the  cells 
are  indistinct  and  the  inner  zone — /.  e.,  the  part  of  the 
cells  (a)  next  the  lumen  (c) — is  broad  and  filled  with 
fine  granules.  Fig.  8i . — After  the  gland  has  poured 
out  its  secretion,  when  the  cell  outlines  (d)  are  clearer, 
the  granular  zone  (a)  is  smaller,  and  the  clear  outer 
zone    is   wider. — (Kuhne  and  Lea.^ 


i88  TEXT-BOOK  OF  PHYSIOLOGY 

During  the  intervals  of  digestion  the  granular  layer  is  very  deep  and  oc- 
cupies almost  the  entire  cell;  after  active  digestion  the  granular  layer  is 
very  narrow,  while  the  clear  zone  is  largely  increased  in  depth  (Fig.  8i). 
The  blood-vessels  of  the  pancreas  are  arranged  around  the  acini  in  a 
manner  similar  to  that  observed  in  the  salivary  glands.  The  ultimate  ter- 
minations of  the  nerves  in  the  epithelium  are  probably  by  means  of  the 
usual  end-tufts. 

The  Islands  of  Langerhans. — Throughout  the  body  of  the  pancreas  and 
especially  in  the  outer  extremity  there  are  found  between  and  among  the 
acini  collections  of  globular  cells  arranged  in  the  form  of  rods  or  columns, 
separated  from  the  acini  and  from  one  another  by  layers  of  connective 
tissue  in  which  ramify  large  tortuous  capillary  blood-vessels.  These  colum- 
nar bodies,  seen  in  cross-section  in  Fig.  83,  have  been  named,  after  their 
discoverer,  the  islands  of  Langerhans. 

Embryologic  investigations  have  shown  that  these  cells  are  outgrowths 
from  the  primitive  acini,  to  which  they  remain  attached  for  some  time  by 


Fig.  82.^Section  of  Human 
Pancreas,  including  Several  Acini 
AND  Two  Ducts.  The  Cells  Pre- 
sent A  Central  Granular  and  a 
Peripheral  Clear  Zone. — {Piersol.) 


Fig.  83. — Section  of  Human  Pan- 
creas showing,  a,  a,  Island  of 
Langerhans,  and  b,  the  Usual  Acini. — 
(Piersol.) 


means  of  a  foot-stalk.  This  subsequently  becomes  constricted  by  the 
connective  tissue  and  the  cells  become  completely  detached.  The  cells 
then  assume  the  columnar  arrangement,  after  which  vascularization  takes 
place. 

From  the  fact  that  complete  extirpation  of  the  pancreas  as  well  as  its 
various  diseases  is  followed  by  serious  disturbances  of  the  carbohydrate 
metabolism  it  has  been  suggested  that  the  islands  of  Langerhans  have  a  func- 
tion separate  and  distinct  from  that  of  the  glandular  portion  of  the  pancreas; 
that  they  secrete  a  specific  material  which  partakes  of  the  nature  of  an 
internal  secretion  which  is  absorbed  by  the  blood  circulating  around  them 
and  carried  to  different  tissues.  The  effect  on  the  metabolism  of  the  body 
which  follows  extirpation  of  the  pancreas  will  be  referred  to  in  a  subsequent 
chapter. 

Pancreatic  Juice. — The  pancreatic  juice  may  be  obtained  by  intro- 
ducing a  silver  cannula,  through  an  opening  in  the  abdominal  wall,  into  the 
duct,  and  securing  it  by  a  ligature.     In  a  short  time  the  juice  flows  from 


DIGESTION  189 

the  distal  end  of  the  cannula,  when  it  can  be  collected.  According  to 
Bernard,  normal  juice  can  be  obtained  only  during  the  first  twenty-four 
hours  of  the  experiment.  The  juice  obtained  from  a  temporary  fistula  is 
clear,  slightly  opalescent,  viscid,  of  a  decidedly  alkaline  reaction,  and  has  a 
specific  gravity  (in  the  dog)  of  1.040.  When  cooled  too°C.,  it  assumes  a 
gelatinous  consistence.  At  ioo°C.  it  completely  coagulates.  When  obtained 
from  a  permanent  fistula,  the  juice  is  watery  and  the  solid  constituents  are 
very  much  diminished  in  amount. 

The  chemic  composition  of  the  pancreatic  juice  of  the  dog  as  determined 
by  Schmidt  is  as  follows:  water,  900.76;  organic  matter,  90.44;  inorganic 
salts,  8.80.  Of  the  inorganic  salts,  sodium  carbonate  is  probably  the  most 
essential,  as  it  is  this  salt  which  gives  to  the  juice  its  alkaline  reaction. 

Human  pancreatic  juice  obtained  from  a  fistula  of  the  duct  was  found  to 
be  clear  and  limpid,  resembling  water,  alkaline  in  reaction  and  with  a  sp.  gr. 
of  1.007.  "^^^  total  solids  of  two  specimens  amounted  to  about  1.270  and 
1.244,  grams  in  100  grams  of  the  juice.  The  amount  of  juice  collected 
varied  from  420  c.c.  to  884  c.c.  daily. 

Mode  of  Secretion. — The  secretion  of  the  juice  is,  in  the  rabbit  and 
dog  at  least,  almost  continuous  during  a  period  of  twenty-four  hours  after  a 
single  average  meal,  though  the  rate  of  flow  varies  considerably  during  this 
period.  Shortly  after  the  food  enters  the  stomach  the  flow  of  the  pancreatic 
juice  begins,  and  steadily  increases  in  amount  until  about  the  third  hour, 
when  it  reaches  its  maximum;  after  this  period  the  flow  diminishes  until  the 
sixth  hour,  when  it  again  increases  for  about  an  hour.  It  then  gradually 
diminishes  until  it  ceases  entirely.  During  the  period  of  secretory  activity 
the  blood-supply  is  very  much  increased,  from  a  dilatation  of  the  blood- 
vessels. 

The  secretion  and  discharge  of  the  pancreatic  juice  is  associated  with 
the  introduction  of  food  into  the  mouth  and  stomach  and  its  early  passage  into 
the  duodenum  and  is  brought  about  by  the  action  of  a  primary  and  a  second- 
ary stimulus. 

The  primary  stimulus  is  a  psychic  state  according  to  Pavlov  induced  by 
the  sight,  odor  and  taste  of  food  and  which  leads  to  the  discharge  of  nerve 
impulses  from  nerve-cells  in  the  medulla  oblongata  and  their  transmission  by 
efferent  nerves  in  the  trunk  of  the  vagus  nerve,  to  the  cells  of  the  acini.  It  is 
probable  that  the  impressions  made  by  the  food  on  the  terminal  filaments  of 
the  afl'erent  fibers  in  the  vagus  nerve  develop  nerve  impulses  which,  when 
transmitted  to  the  medulla,  occasion  the  discharge  of  nerve  impulses  that  not 
only  excite  the  secretion  but  increase  the  blood-supply  as  well.  The  vaso- 
motor nerve  impulses  reach  the  blood-vessel  supplied  to  the  gland,  by 
way  of  the  great  splanchnic  nerve  and  the  post-ganglionic  fibers  from  the 
semilunar  ganglion.  That  the  vagus  nerve  contains  secretor  fibers  for  the 
pancreas  has  been  established  by  Pavlov.  This  investigator  states  indeed 
that  the  vagus  nerve  contains  two  classes  of  fibers  for  the  pancreas,  secreto- 
motor  and  secreto-inhibitor,  as  well  as  vaso-dilatator  fibers  for  the  blood- 
vessels, and  therefore  the  effects  of  stimulation  are  often  contradictory  and 
confused,  but  if  the  nerve  be  di^•ided  and  time  given  for  the  degeneration  of 
the  secreto-inhibitor  and  vaso-dilatator  nerves,  usually  a  period  of  four  or 
five  days,  then  stimulation  of  the  peripheral  end  of  the  nerve  with  induced 
electric  currents  is  followed  after  a  latent  period  of  two  to  three  minutes  by 


iQo  TEXT-BOOK  OF  PHYSIOLOGY 

a  discharge  of  the  juice.     Stimulation  of  the  splanchnic  nerve  under- similar 
conditions  also  gives  rise  to  a  secretion. 

Inasmuch  as  various  agents,  such  as  mineral  and  organic  acids,  placed 
on  the  duodenal  mucous  membrane  excite  the  flow,  it  is  quite  possible  that 
the  passage  of  the  acid  contents  of  the  stomach  through  the  duodenum 
acts  as  a  powerful  stimulus  to  this  nerve  mechanism.  But  as  the  secretion 
and  discharge  of  the  juice  is  excited  by  the  same  conditions  after  the  division 
of  all  related  nerves,  other  explanations  have  been  sought  for  and  found 
in  a  secondary  stimulus  discovered  by  Bayliss  and  Starling. 

The  secondary  stimulus  is  chemic  in  character  and  developed  in  the 
glands  of  the  mucous  membrane  of  the  duodenum  by  the  action  of  the  acids 
of  the  chyme,  that  is,  of  the  digested  foods,  coming  through  the  pylorus. 

These  investigators  made  the  discovery  that  if  an  extract  of  the  gland 
portion  of  the  duodenal  mucous  membrane,  made  with  hydrochloric  acid  0.4 
per  cent,  is  injected  into  the  blood  it  evokes  a  profuse  discharge  of  pancreatic 
juice.  As  hydrochloric  acid  alone  will  not  produce  this  effect  they  assumed 
that  the  extract  contained  an  agent  that  excited  or  aroused  the  pancreas  to 
secretor  activity  and  to  which  therefore  they  gave  the  name  secretin.  This 
agent  resists  the  temperature  that  usually  destroys  enzymes  and  therefore 
is  not  regarded  as  a  member  of  this  class  of  agents.  Since  hydrochloric  acid 
appears  to  be  necessary  to  the  development  of  secretin,  the  further  assumption 
has  been  made  that  it  is  a  derivative  of  a  preexisting  compound  to  which  the 
name  prosecretin  is  given.  The  secretin  thus  developed  is  absorbed  into  the 
blood  and  carried  eventually  to  the  pancreas  and  brought  into  relation  with 
the  cells  on  which  it  exerts  its  stimulating  action.  To  an  agent  of  this  class 
Starling  has  given  the  name  hormone. 

Histologic  Changes  in  the  Cells  during  Secretor  Activity. — 
Reference  has  already  been  made  to  the  fact  that  the  cells  Hning  the  acini 
consist  of  two  zones:  an  outer  one,  clear  and  homogeneous;  and  an  inner  one, 
dark  and  granular.  The  position  of  the  nucleus  of  the  cell  varies,  being  at 
one  time  in  the  outer,  at  another  time  in  the  inner,  zone.  If  the  pancreas  be 
examined  microscopically  during  the  intervals  of  digestion,  it  will  be  observed 
that  the  inner  zone  is  broad,  highly  granular,  occupying  nearly  the  entire  cell, 
while  the  outer  zone  is  narrow  and  clear.  If,  however,  the  gland  be  examined 
shortly  after  a  period  of  active  secretion,  the  reverse  conditions  will  be 
observed;  that  is,  the  inner  zone  will  be  narrow,  containing  relatively  few 
granules,  while  the  outer  zone  will  be  clear  and  wide.  This  change  in  the 
cell  has  been  witnessed  in  the  pancreas  of  the  living  animal — rabbit — by 
Kiihne  and  Lea.  They  observed  that  as  soon  as  digestion  set  in,  the  granules 
of  the  broad  inner  zone  began  to  pass  toward  the  lumen  of  the  acinus  and  to 
disappear  gradually  as  the  secretion  was  poured  out,  while  the  outer  zone 
increased  in  width  until  almost  the  entire  cell  became  clear  and  homogeneous. 
(See  Fig.  81.)  After  secretion  ceased  the  granules  again  made  their  appear- 
ance, the  result,  in  all  probability,  of  metabolic  activity. 

Physiologic  Action  of  Pancreatic  Juice. — Experimental  investi- 
gations have  demonstrated  the  fact  that  pancreatic  juice  is  the  most  complex 
in  its  physiologic  action  of  all  the  digestive  fluids.  By  virtue  of  its  contained 
enzymes,  pancreatic  juice  acts: 

I.  On  starch.  When  normal  pancreatic  juice  or  a  glycerin  extract  of 
the  gland  is  added  to  a  solution  of  hydrated  starch,  the  latter  is  speedily 


DIGESTION  191 

transformed  into  maltose,  passing  through  the  intermediate  stage  of  dextrin. 
The  process  is  in  all  respects  similar  to  that  observed  in  the  digestion  of 
starch  by  saliva.  Pancreatic  juice,  however,  is  more  energetic  in  this  respect 
than  saliva.  •  The  enzyme  which  effects  this  change  is  termed  amylopsin. 
When  the  starch  which  escapes  salivary  digestion  passes  into  the  small 
intestine  and  mingles  with  pancreatic  juice,  it  is  very  promptly  converted  into 
maltose  by  the  action  or  in  the  presence  of  this  enzyme. 

2.  On  protein.  When  protein  compounds  are  subjected  to  the  action  of 
artificial  pancreatic  juice,  they  are  transformed  into  peptones  which  do  not 
differ  in  essential  respects  from  those  formed  by  the  action  of  gastric  juice. 
The  intermediate  stages,  however,  are  believed  to  be  somewhat  different. 
The  enzyme  which  effects  this  change  is  termed  trypsin. 

When  fibrin,  for  example,  is  added  to  trypsin  in  a  solution  rendered 
alkaline  by  sodium  carbonate,  it  does  not  swell  and  become  translucent, 
as  it  does  when  treated  with  hydrochloric  acid  and  pepsin.  On  the  con- 
trary, it  becomes  corroded  on  the  surface,  fragile,  and  in  a  short  time  under- 
goes solution.  The  first  product  is  a  compound  termed  alkali-protein. 
After  solution  has  taken  place,  various  chemic  changes  are  initiated  which 
eventuate  in  the  production  of  peptone.  The  intermediate  stages  in  this  process 
have  not  been  satisfactorily  determined.  At  no  time  during  artificial 
pancreatic  digestion  is  there  any  evidence  of  the  presence  of  the  primary 
proteoses.     The  secondary  proteoses,   however,   are  usually  present. 

When  the  proteins  which  have  escaped  digestion  in  the  stomach  pass 
into  the  small  intestine  and  mingle  with  the  pancreatic  juice,  they  are 
doubtless  digested  in  the  course  of  the  intestinal  canal,  passing  through  the 
stages  just  described.  When  proteins  are  artificially  digested  for  a  long 
time  there  appears  a  number  of  compounds  such  as  leucin,  tyrosin,  aspartic 
acid,  arginin,  etc.,  which  are  representatives  of  a  group  of  simple  chemical 
substances  known  as  amino-acids.  It  would  appear  that  the  pancreatic 
juice  had  the  power  under  such  circumstances  of  reducing  the  proteoses  or 
the  peptones  to  their  ultimate  constituents.  Whether  this  is  due  to  the  action 
of  trypsin  or  to  the  action  of  another  enzyme  erepsin  is  not  very  clear. 

The  action  of  trypsin  on  proteins  in  an  alkaline  medium  may  be  illustrated 
by  the  following  scheme: 

Protein 

I 
Alkali-protein 

Proteoses  secondary 

1 
Peptone 


I     .  I  I  I    .  I     . 

Leuan  Tyrosin  Aspartic  add  Arginin        Ammonia 

The  view  that  the  final  stage  in  the  digestion  of  proteins  is  the  formation 
of  peptones,  which  in  due  time  are  absorbed  and  synthesized  into  blood 
albumin,  has  been  generally  abandoned  for  there  is  an  ever  increasing 
evidence  that  the  final  stage  is  the  formation  of  the  nitrogen-holding  com- 
pounds above  mentioned;  in  other  words,  that  the  cleavage  of  the  proteins 
is  far  more  complete  than  has  heretofore  been  assumed.  Indeed  it  is  now 
believed  that  they  are  reduced,  if  not  to  their  ultimate  constituents,  the 
amino-  and  diamino-acids,  at  least  to  one  or  more  of  the  different  polypeptid 


192  TEXT-BOOK  OF  PHYSIOLOGY 

stages.  Ever  since  the  discovery  by  Cohnheim  of  the  existence  in  the 
intestinal  juice  of  a  substance  termed  by  him  erepsin,  which  is  capable  of 
splitting  proteoses  and  peptones  into  simple  nitrogen-holding  compounds, 
there  has  been  slowly  developing  the  idea  that  normally  during  intestinal 
digestion  the  proteoses  and  peptones  are  reduced  by  this  agent  to  leucin, 
tyrosin,  histidin,  arginin,  aspartic  acid,  ammonia,  etc.,  which  in  turn  are 
absorbed  and  transported  by  the  blood  direct  to  the  tissues.  The  discovery 
by  Vernon  of  erepsin  in  pancreatic  juice  lends  further  support  to  this  view. 

3.  On  fat.  If  pancreatic  juice  be  added  to  a  perfectly  neutral  fat — 
olein,  palmitin,  or  stearin — and  kept  at  a  temperature  of  about  ioo°F. 
(38°C.),  it  will  at  the  end  of  an  hour  or  two  be  partially  decomposed  into 
glycerin  and  the  particular  fat  acid  indicated  by  the  name  of  the  fat  used 
— e.g.,  oleic,  palmitic,  stearic.  The  oil  will  then  exhibit  an  acid  reaction. 
The  reaction  is  represented  in  the  following  formula: 

C3H,(C,,H330J3     +     3H2O     =     sC.sHj.O^        -I-     CjH.COH), 
Triolein.  Water.  Oleic  Acid.  Glycerin. 

If  to  this  acidified  oil  there  be  added  an  alkali,  e.g.,  potassium  or  sodium 
carbonate,  the  latter  will  at  once  combine  with  the  fat  acid  to  form  a 
salt  known  as  a  soap.     The  reaction  is  expressed  in  the  following  equation: 

Sodium  Carbonate.  Oleic  Acid.  Sodium  Oleate.  Carbonic  Acid. 

Na2C03         -I-         CigHj.Oj       -        2NaC,8H330,  +         H^CO, 

Coincident  with  the  formation  of  the  soap,  the  remaining  portion  of  the 
neutral  oil  will  undergo  division  into  globules  of  microscopic  size,  which  are 
held  in  suspension  in  the  soap  solution,  forming  what  has  been  termed  an 
emulsion,  which  is  white  and  creamy  in  appearance.  The  cause  of  this 
minute  subdivision  of  the  fat  and  the  necessity  for  it  is  unknown.  It  may 
be  assumed  that  by  virtue  of  the  subdivision  a  greater  surface  is  exposed  to 
the  action  of  the  pancreatic  enzyme  and  the  digestion  of  the  fat  thereby 
facilitated.  The  action  of  the  pancreatic  juice  may  then  be  said  to  consist 
in  the  cleavage  of  the  neutral  fats  into  fatty  acids  and  glycerin,  after  which 
the  formation  of  the  soap  and  the  division  of  the  fat  takes  place  spontane- 
ously. The  enzyme  which  produces  the  cleavage  of  the  neutral  fats  has 
been  termed  steapsin  or  lipase.  The  extent  to  which  the  cleavage  of  the 
fat  takes  place  in  the  intestine  has  not  been  definitely  determined.  There  are 
some  who  think  the  amount  is  relatively  small,  while  others  consider  that 
it  is  large,  practically  all  of  the  fat  undergoing  this  decomposition,  with 
the  formation  of  soap  and  glycerin  prior  to  their  absorption. 

According  to  Pavlov  the  relative  amounts  of  the  pancreatic  enzymes 
produced,  are  conditioned  by  the  character  and  amounts  of  the  food  principles 
consumed.  Thus,  if  chyme  contains  an  excess  of  either  starch,  protein,  or 
fat,  there  is  a  corresponding  increase  in  the  amount  of  either  amylopsin, 
trypsin,  or  steapsin  produced.  The  pancreas  apparently  adapts  its  activities 
to  the  character  of  the  food.  Though  it  is  probable  that  each  enzyme  is  a 
derivative  of  a  special  zymogen,  it  is  positively  known  that  this  is  the  case 
only  with  trypsin.  This  enzyme  is  a  derivative  of  the  zymogen,  trypsinogen, 
the  production  of  which  is  thought  to  be  the  special  function  of  secretin. 
The  pancreatic  juice  at  the  moment  of  its  discharge  into  the  intestine  does 
not  contain  trypsin  but  trypsinogen.     The  transformation  of  the  latter  into 


DIGESTION  193 

the  former  is  accomplished,  according  to  Pavlov,  by  a  special  activating 
ferment  secreted  by  the  epithelium  of  the  small  intestine  and  termed  entero- 
kinase.^ 

The  rapidity  with  which  pancreatic  juice  in  the  presence  of  bile  and 
hydrochloric  acid  (under  conditions  such  as  are  present  in  the  duodenum) 
can  develop  sufficient  fatty  acid  to  form  an  emulsion  was  determined  by 
Rachford  to  be  two  minutes.  The  activity  of  steapsin  is  thus  shown  to  be 
very  great. 

Intestinal  Juice. — This  fluid  is  a  product  of  the  activities  of  the  cells 
lining  the  follicles  or  glands  of  Lieberkuhn.  Owing  to  its  admixture  with 
other  secretions  and  to  the  profound  disturbance  of  the  digestive  function 
caused  by  the  establishment  of  intestinal  fistulae,  this  fluid  has  rarely  been 
obtained  in  a  state  of  purity  or  in  quantities  sufficient  for  accurate  analyses 
or  for  experimental  purposes.  Its  physiologic  properties  and  functions  are 
therefore  imperfectly  known.  Various  attempts  have  been  made  by  physi- 
ologists, by  the  employment  of  different  methods,  to  obtain  this  secretion. 
The  method  usually  employed  is  that  of  Thiry  and  Vella.  This  consists  in 
dividing  the  intestine  at  two  places,  about  eight  or  ten  inches  apart,  restoring 
the  continuity  of  the  intestine,  and  then  uniting  the  two  ends  of  the  resected 
portion  to  the  edges  of  two  openings  in  the  abdominal  walls.  The  resected 
portion,  being  supplied  with  blood-vessels  and  nerves,  maintains  its  nutrition 
and  secretes  a  more  or  less  normal  juice. 

When  obtained  from  a  dog  under  these  circumstances  the  intestinal 
juice  is  watery  in  consistence,  slightly  opalescent,  light  yellow  in  color, 
alkaline  in  reaction,  with  a  specific  gravity  of  i.oio.  Chemic  analysis 
reveals  the  presence  of  proteins,  mucin,  and  sodium  carbonate. 

The  intestinal  juice  obtained  by  Tubbey  and  Manning  from  a  small 
portion  of  the  human  intestine  (ileum)  was  opalescent,  occasionally  brownish 
in  color,  alkaline,  and  had  a  specific  gravity  of  1.006.  On  the  addition  of 
hydrochloric  acid,  carbonic  acid  was  given  off,  showing  the  presence  of 
carbonates.     It  contained  proteins  and  mucins. 

Physiologic  Action  of  the  Intestinal  Juice. — The  part  played 
by  the  intestinal  juice  in  the  digestive  process  is  yet  a  subject  of  discussion, 
as  the  results  obtained  by  different  observers  are  in  some  respects  con- 
tradictory, due  to  the  fact  that  animals,  as  well  as  human  beings,  have  been 
the  subjects  of  experimentation.  By  reason  of  its  contained  enzymes  the 
intestinal  juice  acts: 

I.  On  Proteoses  and  Peptones. — ^Even  though  the  proteins  have  been  reduced 
by  the  action  of  the  gastric  and  pancreatic  juices  to  the  stage  of  proteoses 
and  peptones  they  are  not  yet  in  a  condition  to  be  absorbed.  The 
further  stage  in  their  digestion  appears  to  be  their  reduction,  as  stated 
in  a  foregoing  paragraph,  to  amino-acids  or  their  immediate  anteced- 
ents. This  change  has  been  attributed  to  the  action  of  the  intestinal 
juice  or  a  contained  enzyme  to  which  the  name  erepsin  was  given  by  its 
discoverer  Cohnheim. 

^  An  activator  may  be  defined  as  an  agent,  which  secreted  by  some  one  organ,  is  capable  of 
converting  an  inactive  agent,  secreted  by  some  correlated  organ,  near  or  remote,  into  an  active 
agent  by  some  chemical  interaction.  Thus  the  antecedents  of  many  enzymes,  either  before  or 
after  their  discharge  by  gland  cells  are  inactive  and  require  to  be  modified  in  some  unknown  way 
before  they  become  functionally  active.  The  term  activator  is  usually  applied  to  inorganic 
agents,  the  term  kinase  to  organic  agents. 

13 


194  TEXT-BOOK  OF  PHYSIOLOGY 

2.  On    Compound  Sugars. — Saccharose,    maltose    and    lactose,    the    three 

compound  sugars,  are  believed  by  most  observers  to  be  not  only  non- 
absorbable, but  also  non-assimilable  and  therefore  are  required  to 
undergo  some  digestive  change  before  they  can  be  absorbed  and  assimi- 
lated. An  extract  of  the  intestinal  mucous  membrane  or  the  intestinal 
juice  of  the  dog  added  to  a  solution  of  saccharose  will  cause  it  to  com- 
bine chemically  with  water  after  which  a  cleavage  into  dextrose  and 
levulose  will  take  place,  which  together  constitute  invert  sugar.  The 
enzyme  to  which  this  action  is  attributed  has  been  termed  invertase  or 
saccharase.  Maltose  undergoes  a  similar  change.  After  its  combina- 
tion with  water  it  undergoes  a  cleavage  into  two  molecules  of  dextrose. 
Lactose  appears  to  be  unaffected  by  the  pure  juice.  As  it  is  non-assimi- 
lable it  has  been  supposed  to  undergo  conversion  into  dextrose  and 
galactose  while  passing  through  the  epithelial  cells  of  the  intestinal  mu- 
cosa. In  either  case  the  transformation  is  brought  about  by  two  fer- 
ments known  respectively  as  maltase  and  lactase. 

3.  On   Trypsinogen. — ^This  zymogen  when  first  discharged  from  the  pan- 

creatic duct  is  inactive  and  incapable  of  effecting  the  necessary  digestive 
changes  in  the  proteins.  Shortly  after  its  entrance  into  the  intestine,  it 
becomes  quite  active  and  efficient,  a  change  attributed  to  an  agent 
entero-kinase  secreted  by  the  mucosa  in  the  upper  part  of  the  intestine. 

THE  LIVER 

The  Liver. — ^This  highly  vascular  conglomerate  gland  is  situated  in  the 
right  hypochondriac  region  and  connected  with  the  intestine  by  a  duct. 

Inasmuch  as  the  liver  performs  several  functions  related  to  both  secretion 
and  excretion,  a  consideration  of  its  structure  and  its  various  functions  will 
be  deferred  to  a  subsequent  chapter.  In  this  connection  only  the  bile,  its 
physical  properties,  chemic  composition,  and  physiologic  action  in  relation 
to  the  digestive  process,  will  be  considered.  This  fluid  is  a  product  of  the 
secretor  activity  of  the  liver  cells.  As  it  is  poured  into  the  intestine  in  man 
and  most  mammals  at  a  point  corresponding  to  the  orifice  of  the  pancreatic 
duct,  and  most  abundantly  at  the  time  the  food  is  passing  through  the  duo- 
denum, it  is  usually  regarded  as  a  digestive  fluid  possessing  an  influence 
favorable  if  not  necessary  to  the  completion  of  the  general  digestive  process. 

Anatomic  Relations  of  the  Biliary  Passages. — After  its  forma- 
tion by  the  liver  cells  the  bile  is  conveyed  from  the  liver  by  the  bile  capillaries, 
which  unite  finally  to  form  the  main  hepatic  duct.  This  duct  emerges 
from  the  liver  at  the  transverse  fissure.  At  a  distance  of  about  5  centimeters 
it  is  joined  by  the  cystic  duct,  the  distal  extremity  of  which  expands  into  a 
pear-shaped  reservoir,  the  gall-bladder  in  which  the  bile  is  temporarily 
stored.  The  duct  formed  by  the  union  of  the  hepatic  and  cystic  ducts,  the 
common  bile-duct,  passes  downward  and  forward  for  a  distance  of  about 
7  centimeters,  pierces  the  walls  of  the  intestine  and  passes  obliquely  through 
its  coats  for  about  a  centimeter  and  opens  into  a  small  receptacle,  the  ampulla 
of  Vater.  The  ampulla  in  turn  opens  on  a  small  papilla  into  the  intestine. 
The  walls  of  the  biliary  passages  are  composed  of  a  mucous  membrane 
internally,  a  fibrous  and  muscular  coat  externally.  The  termination  of  the 
common  bile-duct  is  provided  with  a  distinct  band  of  circularly  disposed 


DIGESTION  195 

muscle-fibers,  which  when  in  action  completely  close  the  orifice  and  prevent 
the  discharge  of  bile.  It  may  therefore  be  regarded  as  a  true  sphincter 
muscle,  the  structure  and  function  of  w^hich  were  first  pointed  out  by  Oddi. 
Small  racemose  glands  are  embedded  in  the  mucous  membrane  of  the  main 
ducts. 

Physical  Properties. — ^The  bile  obtained  directly  from  the  liver  through 
a  cannula  inserted  into  the  hepatic  duct  is  always  thin  and  watery,  while 
that  obtained  from  the  gall-bladder  is  more  or  less  viscid  from  admixture 
with  mucin,  the  degree  of  the  viscidity  depending  on  the  length  of  time  it 
remains  in  this  reservoir.  The  specific  gravity  of  human  bile  varies  within 
normal  limits  from  i.oio  to  T.020.  The  reaction  is  invariably  alkaline  in 
the  human  subject  when  first  discharged  from  the  liver,  but  may  become 
neutral  in  the  gall-bladder.  The  alkalinity  depends  on  the  presence  of 
sodium  carbonate  and  sodium  phosphate.  When  fresh,  it  is  inodorous;  but 
it  readily  undergoes  putrefactive  changes,  and  soon  becomes  offensive.  Its 
taste  is  decidedly  bitter.  When  shaken  with  water,  it  becomes  frothy — a 
condition  which  lasts  for  some  time  and  which  is  due  to  the  presence  of  mucin. 
In  ox  bile  the  mucin  is  replaced  by  a  nucleo-protein. 

The  color  of  bile  obtained  from  the  hepatic  duct  is  variable,  usually  a 
shade  between  a  greenish-yellow  and  a  brov/nish-red.  In  different  animals 
the  color  varies.  In  the  herbivorous  animals  it  is  usually  green;  in  the  car- 
nivorous animals  it  is  orange  or  brown.  In  man  it  is  green  or  a  golden 
yellow.  The  colors  are  due  to  the  presence  of  pigments.  Microscopic 
examination  fails  to  show  the  presence  of  structural  elements. 

Chemic  Composition. — Human  bile  obtained  from  an  accidental  biliary 
fistula  was  shown  by  Jacobson  to  contain  the  following  ingredients,  viz. : 

COMPOSITION  OF  HUMAN  BILE 

Water 977  .40 

Sodium  glycocholate 9-94 

Sodium  taurocholate a  trace 

Cholesterin o .  54 

Free  fat o .  10 

Sodium  palmitate  and  stearate i  .36 

Lecithin. 0.04 

Organic  matter,  and  pigments  bilirubin  and  biliverdin 2  .26 

Sodium  chlorid 5-45 

Potassium  chlorid 0.28 

Sodium  phosphate i  .33 

Calcium  phosphate o -37 


Sodium  carbonate. 


0-93 


1000.00 


In  this  analysis  the  solid  ingredients  constitute  22.6  parts  per  1000,  of  which 
two-thirds  are  organic  and  one-third  inorganic.  The  amount  of  solid  varies 
according  to  the  animal  from  which  the  bile  is  obtained. 

Sodium  Glycocholate  and  Taurocholate. — Of  the  various  ingredients 
of  the  bile  none  are  more  important  than  these  two  salts,  usually  known  as 
the  bile  salts.  The  sodium  glycocholate  is  found  most  abundantly  in  the 
bile  of  herbivora,  the  sodium  taurocholate  in  the  bile  of  the  carnivora.  These 
salts  are  compounds  of  sodium  and  glycocholic  and  taurocholic  acids. 
When  separated  from  the  sodium,  the  acids  will  crystallize  in  the  form  of 
fine  acicular  needles.  Under  the  influence  of  hydrolizing  agents,  such  as 
dilute  acids  and  alkalies,  both  acids  will  undergo  cleavage  into  their  re- 


196  TEXT-BOOK  OF  PHYSIOLOGY 

spective  components — e.g.,  glycocoll  and  cholic  acid,  taurine  and  cholic  acid. 
GlycocoU  and  taurine  are  crystallizable  nitrogenized  compounds  known 
chemically  as  amido-acetic  and  amido-isethionic  acids  respectively. 

From  the  results  of  the  hydrolysis  of  the  bile  acids  it  may  be  inferred  that 
their  formation  in  the  liver  cell  is  the  result  of  an  opposite  process,  viz.:  a 
synthesis  of  glycocoll  and  taurin  with  cholic  acid.  Glycocoll  is  an  amino- 
acid  and  there  is  much  evidence  that  taurin  is  a  derivative  of  cystin,  another 
amino-acid,  both  found  in  the  small  intestine.  The  origin  of  cholic  acid  is, 
however,  not  so  clear.  The  absorption  of  the  foregoing  compounds  from 
the  intestine  and  their  transmission  to  the  liver  cells  by  the  portal  vein  fur- 
nishes them  with  the  necessary  materials  for  the  formation  of  the  bile  acids. 
During  the  period  the  bile  remains  in  the  biliary  passages,  the  biliary  salts 
hold  cholesterin  in  solution. 

There  is  also  good  evidence  for  the  view,  that  after  their  discharge  into 
the  intestine,  the  bile  salts  are  absorbed,  with  the  exception  of  a  portion  de- 
stroyed by  bacteria,  and  carried  by  the  portal  vein  to  the  liver  and  again 
excreted.  By  this  circulation  from  liver  to  intestine  and  from  intestine  to 
the  liver,  the  work  of  the  liver  cells  in  the  synthesis  or  secretion  of  bile  acids, 
is  supposed  to  be  reduced  to  a  minimum.  It  is  also  probable  that  a  portion 
of  the  acids  enters  the  general  circulation  and  influences  favorably  the  gen- 
eral nutrition.  It  is  stated  by  some  investigators  that  the  activities  of  the 
liver  cells  are  decidedly  increased  by  the  circulation  of  the  bile  salts  and  that 
they  are  to  be  regarded  as  the  natural  stimuli  to  the  secretion. 

The  presence  of  the  bile  salts  can  be  demon- 
strated by  the  employment  of  Pettenkofer's  test 
or  reaction.  It  was  shown  by  this  investigator 
that  if  to  a  solution  of  bile  salts  a  small  quantity 
of  a  10  per  cent,  solution  of  cane-sugar  be  added 
and  subsequently  a  small  quantity  of  strong  sul- 
phuric acid,  a  brilliant  red  color  appears  which 
soon  passes  into  a  rich  purple.  To  secure  the 
Fig.  84.— Cholesterin  ^gg^  results  in  the  performance  of  this  test  care 

Crystals.  —  (Landots    and      ,        ,  i  ,  •       1         1  ^  ^     ^ 

Stirling.)  should  be  exercised  to  keep  the  temperature  below 

7o°C.;  the  characteristic  colors  appear  to  be  due 
to  the  action  of  the  sulphuric  acid  on  the  cane-sugar  by  which  a  sub- 
stance, furfurol,  is  produced,  which  in  turn  reacts  with  the  cholalic  acid. 
This  test  can  be  applied  to  bile  directly;  thus  if  to  bile  in  a  test-tube  cane- 
sugar  be  added  and  the  mixture  thoroughly  shaken,  a  portion  of  the  bile 
becomes  quite  frothy.  If  now  sulphuric  acid  be  carefully  added,  the  red 
and  purple  colors  present  themselves  at  once  in  the  white  froth — an  indica- 
tion that  the  bile  salts  are  distributed  through  it. 

Cholesterin. — Cholesterin  can  be  obtained  directly  from  bile  or  better 
from  white  gall-stones  by  the  employment  of  chemic  methods  described  in 
works  on  chemistry.  When  obtained  in  the  pure  state  it  undergoes  crystal- 
lization when  it  presents  itself  in  the  form  of  fiat  rectangular  crystals  which 
are  soluble  in  ether  and  hot  alcohol.  In  the  bile  it  is  held  in  solution  by 
the  bile  salts.  Chemic  analysis  shows  that  it  is  a  monatomic  unsaturated 
alcohol  with  the  formula  C27H46OH.  Cholesterin,  though  a  constant  con- 
stituent of  bile,  is  not  confined  to  this  fluid  as  it  has  been  shown  to  be  a 
normal  constituent  of  all  animal  and  vegetable  cells,  though  it  is  particularly 


DIGESTION  197 

abundant  in  the  myelin  of  nerve-fibers.  Though  cholesterin  has  for  a  long 
time  been  regarded  merely  as  one  of  the  products  of  the  katabolism  of  living 
material,  it  has  come  to  be  believed  that  it  is  necessary  to  the  vitality  of  tissue 
cells  and  especially  to  the  blood  cells.  Entering  into  the  composition  of 
the  surface  layer  of  cells,  it  prevents  the  entrance  of  certain  toxins  which 
would  have  a  destructive  influence  on  their  structure  or  composition.  In 
the  metabolism  of  cells  it  is  set  free  after  which  it  passes  into  the  blood  to  be 
secreted  by  the  liver.  In  the  bile  it  frequently  undergoes  crystallization  and 
forms  one  of  the  forms  of  gall-stones.  In  the  intestine  it  is  converted  into 
stercorin  and  discharged  in  the  feces. 

Bilirubin,  Biliverdin. — These  two  pigments  impart  to  the  bile  its 
red  and  green  colors  respectively.  Bilirubin  is  present  in  the  bile  of  human 
beings  and  the  carnivora,  biliverdin  in  the  bile  of  the  herbivora.  As  the 
former  pigment  readily  undergoes  oxidation  in  the  gall-bladder,  giving  rise 
to  the  latter  pigment,  almost  any  specimen  of  bile  may  present  any  shade 
of  color  between  red  and  green.  Bilirubin  is  regarded  as  a  derivative  of 
hematin,  one  of  the  cleavage  products  of  hemoglobin,  the  coloring-matter 
of  the  blood.  In  the  liver  the  hematin  combines  with  water,  loses  its  iron, 
and  is  changed  to  bilirubin.  By  continuous  oxidation  there  are  formed 
biliverdin,  bilicyanin,  and  choletelin.  After  their  discharge  into  the  intes- 
tine the  bile  pigments  are  finally  reduced  to  hydrobilirubin  or  an  allied  sub- 
stance, stercobilin,  which  becomes  one  of  the  constituents  of  the  feces.  A 
portion  of  the  latter  is  absorbed  into  the  blood  and  ultimately  discharged 
into  the  urine  where  it  is  known  as  urobilin.  The  two  substances  are  re- 
garded as  identical.  An  oxidation  of  the  bilirubin  can  be  produced  by 
nitroso-nitric  acid.  If  this  agent  is  added  to  a  thin  layer  of  bile  on  a  porce- 
lain surface,  a  series  of  colors  will  rapidly  succeed  one  another,  commencing 
with  green  and  passing  to  blue,  orange,  purple,  and  yellow.  This  is  the 
basis  of  the  well-known  test  for  bile  pigments  suggested  by  Gmelin. 

Lecithin. — Lecithin  is  regarded,  because  of  its  physical  properties  and 
chemic  composition,  as  a  complex  fat.  When  pure  it  presents  itself  gener- 
ally as  a  white  crystalline  powder,  though  very  frequently  as  a  white  waxy 
mass  which  is  soluble  in  ether  and  alcohol.  Its  chemic  formula  is  CiiH.^^- 
NPO9.  Lecithin  is  widely  distributed  throughout  the  body,  being  found  in 
blood,  lymph,  red  and  white  corpuscles,  nerve-tissue,  yolk  of  egg,  semen, 
milk,  and  bile.  Lecithin  has  been  regarded  as  one  of  the  decomposition 
products  of  nerve-tissue,  removed  from  the  blood  by  the  liver  and  thus  be- 
coming one  of  the  constituents  of  the  bile,  in  which  it  is  held  in  solution  by 
the  bile  salts.  Lecithin  can  be  readily  decomposed  by  various  agents  yield- 
ing glycophosphoric  acid,  a  fat  acid  and  cholin. 

The  Mode  of  Secretion  and  Discharge  of  Bile. — The  manner  in  which 
the  bile  flows  from  the  liver  into  the  main  hepatic  ducts,  the  variations  in  the 
rate  of  its  discharge  into  the  intestine,  as  well  as  the  total  quantity  secreted 
daily,  have  been  approximately  determined  by  fistulous  openings  either 
in  the  hepatic  ducts  or  in  the  gall-bladder.  Although  the  liver  presents 
some  physiologic  peculiarities,  there  is  no  reason  to  believe  that  the  condi- 
tions of  secretion  therein  are  different  from  those  in  any  other  secretor 
organ,  or  that  any  other  structure  than  the  cell  is  engaged  in  this  process. 
As  shown  by  chemic  analysis,  the  bile  consists  of  compounds,  some  of  which, 
like  the  bile  salts,  are  formed  in  the  liver  cells,  out  of  material  furnished  by 


iqs  text-book  of  physiology 

the  blood  by  a  true  act  of  secretion,  while  others,  such  as  cholesterin  and 
lecithin,  principles  of  waste,  are  merely  excreted  from  the  blood  to  be  finally 
eliminated  from  the  body.  The  bile  is  thus  a  compound  of  both  secretory 
and  excretory  principles. 

The  flow  of  bile  from  the  liver  is  continuous  but  subject  to  considerable 
variation  during  the  twenty-four  hours.  The  introduction  of  food  into  the 
stomach  at  once  causes  a  slight  increase  in  the  flow,  but  it  is  not  until 
about  two  hours  later  that  the  amount  discharged  reaches  its  maximum; 
after  this  period  it  gradually  decreases  up  to  the  eighth  hour,  but  never 
entirely  ceases.  During  the  intervals  of  digestion  though  a  small  quantity 
passes  into  the  intestine,  the  main  portion  is  diverted  into  the  gall-bladder, 
because  of  the  closure  of  the  common  bile-duct  by  the  sphincter  muscle  near 
its  termination,  where  it  is  retained  until  required  for  digestive  purposes. 
When  acidulated  food  passes  over  the  surface  of  the  duodenum,  there  is  an 
increase  in  the  secretion  or  at  least  the  discharge  of  bile,  and  as  this  takes 
place  after  the  nerves  distributed  to  the  liver  are  divided,  the  assumption  is 
that  an  agent,  possibly  secretin,  is  developed  in  the  duodenal  mucous  mem- 
brane, which,  absorbed  into  the  blood,  is  ultimately  distributed  to  the  liver 
cells  and  by  which  they  are  excited  to  activity.  At  the  same  time  there  is 
excited,  through  reflex  action,  a  contraction  of  the  muscle  walls  of  the  gall- 
bladder and  ducts,  a  relaxation  of  the  sphincter,  and  a  gush  of  bile  into  the 
intestine,  the  discharge  continuing  intermittently  until  digestion  ceases  and 
the  intestine  is  emptied  of  its  contents. 

The  Influence  of  the  Nerve  System. — ^The  storage  and  the  discharge 
of  bile,  brought  about  by  the  alternate  contraction  and  relaxation  of  the 
muscle  walls  of  the  gall-bladder  and  of  the  sphincter  are  regulated  by  the 
nerve  system.  As  the  result  of  his  experiments  Doyon  concludes,  that  dur- 
ing the  intervals  of  intestinal  digestion  the  vagus  nerve  is  carrying  nerve 
impulses  which  on  the  one  hand  augment  the  contraction  of  the  sphincter 
and  inhibit  the  contraction  of  the  walls  of  the  gall-badder,  thus  establishing 
the  conditions  for  the  storage  of  bfle;  but  when  intestinal  digestion  is  in- 
augurated the  splanchnic  nerve  carries  nerve  impulses  which  inhibit  the 
sphincter  and  augment  the  contraction  of  the  walls  of  the  gall-bladder,  thus 
establishing  the  condition  for  the  discharge  of  the  bile. 

The  total  quantity  of  bile  secreted  daily  has  been  estimated  to  be  from 
500  to  800  grams. 

Physiologic  Action  of  Bile. — Notwithstanding  our  knowledge  of  the 
complex  composition  of  bile,  the  quantity  discharged  daily,  and  the  time 
and  place  of  its  discharge,  its  exact  relation  to  the  digestive  process  has  not 
been  fully  determined.  No  specific  action  can  be  attributed  to  it.  It  has 
but  a  slight,  if  any,  diastatic  action  on  starch.  It  is  without  influence  on 
proteins  or  on  fats  directly.  But  indirectly  and  by  virtue  of  the  bile  salts  it 
contains,  it  plays  an  important  part  in  increasing  the  action  of  the  pancreatic 
enzymes.  Thus  the  amylolytic  or  amyloclastic  power  of  the  pancreatic 
juice  is  almost  doubled  and  the  same  is  true  for  its  proteoclastic  power, 
while  its  lipoclastic  or  fat-splitting  power  is  tripled. 

The  bile  salts  also  dissolve  insoluble  soaps  which  may  be  formed  during 
digestion  and  thus  favors  the  digestion  of  fat.  If  it  be  excluded  from  the 
intestine  there  is  found  in  the  feces  from  22  to  58  per  cent,  of  the  ingested 
fats.     At  the  same  time  the  chyle,  instead  of  presenting  the  usual  white 


DIGESTION  199 

creamy  appearance,  is  thin  and  slightly  yellow.  The  manner  in  which  the 
bile  promotes  fat  digestion  is  yet  a  subject  of  investigation.  If  all  the  fat 
is  converted  into  fat  acids  and  glycerin,  with  the  formation  of  soaps,  as 
seems  probable,  the  action  of  the  bile  becomes  more  apparent  from  the  fact, 
already  stated,  that  it  dissolves  and  holds  in  solution  the  soaps  so  formed 
which  would  be  necessary  to  their  absorption  by  the  epithelial  cells.  This 
action  has  been  attributed  to  the  presence  of  the  bile  salts.  As  an  aid  to 
digestion  the  bile  has  been  regarded  as  important,  for  the  reason  that  its 
entrance  into  the  intestine  is  attended  by  a  neutralization  and  precipitation 
of  the  proteins  which  have  not  been  fully  digested  and  are  yet  in  the  stage 
of  acid-albumin.  In  this  way  gastric  digestion  is  arrested  and  the  foods  are 
prepared  for  intestinal  digestion. 

Though  bile  possesses  no  antiseptic  properties  outside  the  body,  itself 
undergoing  putrefactive  changes  very  rapidly,  it  has  been  believed  that  in 
the  intestine  it  in  some  way  prevents  or  retards  putrefactive  changes  in  the 
food.  There  can  be  no  doubt  that  if  the  bile  is  prevented  from  entering 
the  intestine  there  is  an  increase  in  the  formation  of  gases  and  other  products 
which  impart  to  the  feces  certain  characteristics  which  are  indicative  of 
putrefaction.  As  to  the  manner  in  which  bile  retards  this  process  nothing 
definite  can  be  stated.  It  has  been  supposed  to  be  a  stimulant  to  the 
peristaltic  movements  of  the  intestine,  inasmuch  as  these  movements 
diminish  when  bile  is  diverted  from  the  intestine. 

Though  no  definite  nor  specific  action  on  any  of  the  different  classes  of 
food  principles  can  be  attributed  to  the  bile,  there  is  abundant  evidence  to 
show  that  its  presence  in  the  alimentary  canal  during  digestion  is  essential 
to  the  maintenance  of  the  nutrition  of  the  body.  That  the  bile  as  a  whole, 
or  at  least  part  of  its  constituents,  favorably  influences  digestion  and  general 
nutrition  is  evident  from  the  phenomena  which  follow  its  total  exclusion 
from  the  intestine,  as  when  the  common  bile-duct  is  ligated  and  a  fistula  of 
the  gall-bladder  is  established.  The  following  phenomena  were  observed 
in  a  young  dog  so  prepared  by  Professor  Flint.  During  the  first  five  days 
succeeding  the  operation  the  abdomen  was  tumid  and  there  was  some  rum- 
bling in  the  bowels.  Though  the  animal  ate  every  day,  the  discharge  of 
fecal  matter  became  infrequent,  the  matter  passed  being  grayish  in  color  and 
highly  offensive.  After  two  weeks  the  alvine  discharges  took  place  three 
and  four  times  daily.  For  four  days  the  weight  remained  normal;  afterward 
it  began  to  diminish,  and  from  this  time  the  animal  continued  to  lose  strength 
and  weight  until  its  death,  thirty-eight  days  after  the  operation.  Ten  days 
after  the  operation  the  appetite,  which  had  been  very  good,  increased,  but 
did  not  become  ravenous  until  a  few  days  before  death.  The  animal  usually 
ate  about  a  pound  and  a  half  of  beef-heart  daily,  but  always  refused  fat. 
There  was  an  absence  at  all  times  of  jaundice,  fetor  of  the  breath,  and  fall- 
ing of  the  hair.  Post-mortem  examination  showed  that  the  bile-duct  was 
obliterated,  and  there  was  no  evidence  that  any  bile  could  have  passed  into 
the  intestine.  The  results  of  this  and  similar  cases  go  to  show  that  that  por- 
tion of  the  bile  which  is  secretory  in  character  is  essential  to  digestion  and 
the  nutrition  of  the  body — that,  though  large  quantities  of  food  are  con- 
sumed, progressive  diminution  of  weight  takes  place  until  nearly  40  per 
cent,  of  the  body  is  consumed.     In  some  instances  the  breath  becomes 


200  TEXT-BOOK  OF  PHYSIOLOGY 

fetid  and  there  is  a  falling  of  the  hair,  showing  some  profound  disturbance 
of  the  general  nutritive  process. 

The  Movements  of  the  Small  Intestine. — During  the  period  of  intes- 
tinal digestion,  the  walls  of  the  intestine  exhibit  a  series  of  movements  which 
triturate  the  food,  mix  it  with  the  intestinal  secretions,  gradually  transfer  it 
from  the  upper  to  the  lower  portions  and  promote  the  absorption  of  the  pre- 
pared food  materials.  The  movements  of  the  small  intestine  have  been 
studied  by  means  of  the  Rontgen  rays  by  Cannon.  The  method  adopted 
was  to  mix  with  the  food  subnitrate  of  bismuth,  which  being  opaque  rendered 
the  movements  of  the  intestinal  contents  and  thereby  the  movements  of  the 
intestinal  walls  visible  on  the  fluorescent  screen.  There  investigations  re- 
vealed the  presence  of  two  forms  of  activity,  one  of  which  is  more  or  less 
stationary  and  due  to  rhythmic  contraction  of  circular  muscle-fibers,  the  other 
progressive,  passing  from  above  downward  and  due  to  the  contraction  of 
circular  and  longitudinal  muscle-fibers.  The  former  activity,  which  is  by 
far  the  more  common,  results  in  a  division  of  the  intestinal  contents  into 
small  segments  and  for  this  reason  was  termed  by  Cannon  rhythmic  seg- 
mentation; the  latter  activity  is  the  well-known  peristaltic  wave. 

Rhythmic  Segmentation. — When  the  abdominal  cavity  is  investigated  by 
the  method  above  mentioned,  it  is  observed  that  after  the  food  has  passed 
into  the  intestine  and  formed  a  more  or  less  consistent  mass  of  variable 
length,  bands  of  circular  muscle-fibers,  situated  at  regular  distances  one 
from  another,  begin  to  contract  and  divide  a  mass  of  food  into  segments, 
after  which  they  at  once  relax  to  be  followed  by  contraction  of  other  bands  in 
the  segments  of  the  intestines  overlying  the  segments  of  food.  The  result  is 
again  a  division  of  the  food  into  two  new  segments  (Fig.  85).  The  lower 
half  of  each  segment  then  unites  with  the  upper  half  of  the  segment  of  food 
below  to  commingle  with  it  and  expose  new  surfaces  of  the  food  mass  to 
contact  with  the  actively  absorbing  mucosa.  The  continual  repetition  of 
this  process  results  in  a  thorough  mixing  of  the  food  with  the  digestive  juices. 
From  the  manner  in  which  these  contractions  make  their  appearance  it  would 
seem  that  the  mere  presence  of  a  segment  of  food  in  the  lumen  of  the  intestine 
is  sufficient  to  excite  the  overlying  fibers  to  activity. 

In  certain  regions  of  the  intestine  rhythmic  segmentation  may  continue 
for  half  to  three-quarters  of  an  hour  without  moving  the  food  forward  to  any 
marked  extent.  In  the  cat  the  segmentation  may  proceed  at  the  rate  of 
thirty  divisions  a  minute. 

Peristalsis. — After  the  food  has  been  prepared  by  the  process  described 
in  the  foregoing  paragraph,  it  is  then  slowly  carried  downward  by  what  is 
known  as  the  vermicular  or  peristaltic  wave.  This  wave  is  characterized  by 
a  contraction  of  the  circular  fibers  behind  the  mass  of  food  and  a  relaxation 
of  the  fibers  in  advance  of  it.  The  result  is  a  movement  forward  of  the  food, 
and  as  it  moves  it  is  followed  by  a  ring  of  constriction  and  preceded  by  a 
ring  of  relaxation  or  inhibition.  The  rate  of  movement  of  the  peristaltic  wave 
is  usually  extremely  slow  except  in  the  duodenal  region  where  it  is  quite  rapid. 

The  first  of  these  two  movements  to  make  its  appearance  is  the  peristaltic 
which  arises  at  the  upper  portion  of  the  duodenum  from  which  it  sweeps 
downward  with  considerable  rapidity,  carrying  with  it  the  food  which  has 
been  discharged  from  the  stomach  and  which  has  accumulated  in  this  region. 
From  the  appearance,  as  shown  by  Rontgen  ray  examination,  which  the 


DIGESTION 


20I 


extreme  upper  portion  of  the  duodenum  presents,  due  to  its  distension  by 
food,  it  has  been  designated,  the  cap  or  pilleus  ventriculi. 

After  the  peristaltic  wave  has  advanced  the  food  a  variable  distance,  it 
disappears  and  the  food  comes  to  rest.  By  this  procedure  the  incoming  food 
from  the  stomach  is  readily  accommodated  in  the  duodenal  portion  of  the 
intestine.  With  the  disappearance  of  the  peristaltic  wave,  rhythmic  seg- 
mentation again  arises  in  the  portion  of  the  intestine  corresponding  to  the 
new  situation  of  the  segment  of  food.  This  in  turn  is  succeeded  by  another 
peristaltic  wave  which  advances  the  food  to  a  more  distant  region  of  the 
intestine.  This  continues  until  at  the  end  of  gastric  digestion  a  more  or  less 
continuous  column  of  food  occupies  the  lumen  of  the  small  intestine  from  the 
stomach  to  the  ileo-cecal  valve. 

In  addition  to  this  characteristic  physiologic  movement  it  has  also  been 
observed  by  different  experimenters  that  the  intestine  manifests  under  special 
circumstances  two  other  forms  of  moving  waves,  waves  moving  downward 
as  well  as  upward  from  their  point  of 
origin,  but  without  being  preceded  by 
an  inhibition  or  relaxation.  These  waves 
are  therefore  not  regarded  as  true  peris- 
taltic waves.  To  avoid  confusion,  the 
term  diastalsis  has  recently  been  em- 
ployed (Cannon)  to  designate  the  true 
peristaltic  movement,  viz.:  progressive 
contraction  preceded  by  inhibition,  and 
the  terms  kaiastalsis  and  anastalsis  to 
designate  the  descending  and  ascending 
contractions  respectively,  that  occur 
without  a  forerunning  inhibition. 

Rush  Peristalsis. — Under  conditions 
that  are  perhaps  not  strictly  physiologic, 
a  rapid  and  far-reaching  peristalsis  is 
developed  which  may  pass  over  the  in- 
testine, from  the  duodenum  to  the  cecum 
without  stopping  in  the  course  of  15 
seconds,  in  the  rabbit,  and  which  has 
been  designated  rush  peristalsis.  It  is 
characterized  by  a  wave  of  constriction  preceded  by  a  completely  inhibited 
long  section  of  intestine.  The  contents  of  the  intestine  are  carried  along 
with  extreme  rapidity  and  vigor.  The  contractions  may  be  increased  by 
purgative  salts,  ergot,  barium  chlorid,  etc.,  the  inhibition  may  be  increased 
by  calcium  chlorid,  magnesium  chlorid,  etc.  A  combination  of  ergot  and 
calcium  chlorid  develops  in  the  rabbit  a  pronounced  rush  peristalsis  (Meltzer). 
This  movement  appears  to  be  under  the  control  of  the  central  nerve  system 
as  it  fails  to  develop  after  division  of  the  vagus  nerves. 

Bayliss  and  Starling  state,  from  observations  made  on  the  exposed 
intestine  of  a  dog,  that  in  addition  to  the  usual  peristaltic  movement  the 
intestinal  coils  exhibit  a  swaying  or  pendulum  movement  accompanied  by 
slight  waves  of  contraction  which  may  arise  apparently  at  any  point  and 
pass  down  the  intestine.  These  contractions  may  occur  from  ten  to  twelve 
times  a  minute  and  travel  at  a  rate  varying  from  two  to  five  centimeters  a 


Fig.  85. — The  Divisive  or  Segment- 
ing Movements  OF  THE  Small  Intestine. 
A,  surface  of  a  portion  of  the  intestine, 
showing  six  constrictions  which  divide  the 
contents  into  five  segments,  as  showrn  in 
B:  as  these  constrictions  pass  away  new 
ones  come  in  between  them  and  divide 
each  segment  of  the  contents  into  two,  the 
adjoining  halves  of  neighboring  segments 
fusing  to  make  the  new  segments  shown  in 
C.  Repetition  of  this  process  results  in  the 
condition  shown  in  D. — {Modified,  after 
Hough  and  S edgewic k  ,  "  The  Human 
Mechanism.") 


202  TEXT-BOOK  OF  PHYSIOLOGY 

second.  In  how  far  this  movement  represents  the  normal  movement  as  it 
takes  place  under  physiologic  conditions  and  as  observed  by  Cannon,  remains 
for  further  investigations  to  decide. 

The  Nerve  Mechanism  of  the  Small  Intestine. — The  causes  of  these  two 
forms  of  intestinal  activity,  rhythmic  segmentation  and  peristalsis,  have  been 
the  subject  of  much  investigation.  Because  of  the  presence  of  a  network  or 
plexus  (Auerbach's  and  Meissner's)  of  nerve-cells  and  nerve-fibers  in  the 
walls  of  the  intestines  and  in  close  relation  to  the  muscle  cells,  the  so-called 
myenteric  plexus  and  because  of  the  fact  that  the  intestines  will  contract 
for  some  time  after  removal  from  the  body  of  the  animal,  it  has  been  difficult 
to  decide  whether  the  contractions  are  myogenic  or  neurogenic. 

As  the  rhythmic  contractions  continue  though  the  peristaltic  are  abol- 
ished by  the  introduction  of  nicotin  into  the  blood,  an  agent  which  tempo- 
rarily paralyses  peripheral  nerve-cells,  it  was  concluded  by  Bayliss  and 
Starling  that  the  rhythmic  contractions  are  myogenic  and  that  the  peristaltic 
contractions  are  reflex  in. character,  the  coordination  being  carried  out  by 
the  local  nerve  mechanisms  and  initiated  by  stimulation  of  the  intestine. 
This  observation  has  been  corroborated  by  Cannon  who  has  shown  that  if 
the  myenteric  plexus  is  divided  by  incisions  extending  around  the  intestine, 
at  intervals  of  1.5  to  2  cm.  for  a  distance  of  45  cm.,  incisions  reaching  to  the 
submucous  coat,  the  peristaltic  movement  is  totally  abolished  though 
rhythmic  segmentation  develops  as  usual  and  continues  for  long  periods. 

As  the  segmentation  activity  continues  after  interruption  of  the  myenteric 
plexus,  the  inference  is  justifiable  that  it  is  purely  myogenic  and  is  the  re- 
sponse to  a  stimulus  within  the  intestine  such  as  distension  by  food,  when 
the  intestinal  wall  possesses  the  requisite  degree  of  tonicity. 

Though  the  orderly  and  coordinated  contractions  and  relaxations  of  the 
muscle  coat,  which  constitutes  a  peristaltic  movement,  are  mediated  by  the 
myenteric  plexus,  and  therefore  termed  a  myenteric  reflex,  nevertheless  the 
contraction  may  be  augmented  or  inhibited  by  the  central  nerve  system 
through  the  vagus  and  splanchnic  nerves  respectively. 

Stimulation  of  the  vagus  is  followed  by  an  augmentation  of  the  contrac- 
tion, though  not  infrequently  there  is  a  primary  inhibition  of  short  duration. 
Stimulation  of  the  splanchnic  is  followed  by  a  relaxation  or  inhibition  of  the 
contraction,  though  according  to  some  observers  there  is  at  times  an  opposite 
effect. 

The  extent  to  which  the  contraction  is  initiated  or  augmented  by  stimula- 
tion of  the  vagus  depends  upon  the  extent  to  which  the  contraction  at  the 
moment  is  inhibited  by  the  splanchnic.  Thus  after  the  splanchnics  are 
divided,  stimulation  of  the  vagus  causes  a  much  more  pronounced  contrac- 
tion than  would  otherwise  be  the  case;  a  fact  that  indicates  that  the 
splanchnic  nerve-center  is  in  a  state  of  tonus  or  tonic  activity  and  therefore 
exerting  a  constant  inhibitor  effect  on  the  muscle-fibers.  Stimulation  of  the 
peripheral  end  of  the  divided  splanchnic  causes  an  arrest  or  inhibition  of 
a  preexisting  contraction. 

The  nerve-centers  regulating  the  contraction  and  relaxation  of  the 
muscle  walls  of  the  intestine  are  doubtless  excited  to  activity  by  nerve  impulses 
transmitted  through  afferent  nerves,  probably  the  vagus,  from  the  mucous 
surface  of  the  small  intestine.  These  centers  are  also  influenced  by  nerve 
impulses  descending  from  the  cerebrum,  though  the  route  they  take  is  not 


DIGESTION  203 

clearly  defined.  It  is  well  known  that  mental  states  markedly  influence  the 
contraction  in  one  direction  or  another. 

It  has  also  been  experimentally  determined  that  the  introduction  of 
various  acids  and  gases  into  the  intestinal  canal  is  followed  by  an  increase  in 
the  contraction.  It  is  probable  therefore  that  the  gases,  acids,  and  perhaps 
other  compounds  as  well,  developed  by  bacterial  action  also  act  as  excitants 
to  muscle  activity. 

The  Large  Intestine. — The  large  intestine  is  that  portion  of  the  ali- 
mentary canal  situated  between  the  termination  of  the  ileum  and  the  anus. 
It  varies  in  length  from  one  and  a  quarter  to  one  and  a  half  meters,  in 
diameter  from  three  and  a  half  to  seven  centimeters.  It  is  divided  into 
the  cecum,  the  colon  (subdivided  into  an  ascending,  transverse,  and  descend- 
ing portion,  including  the  sigmoid  flexure),  and  the  rectum. 

The  cecum  is  situated  in  the  right  iliac  fossa.  It  is  that  dilated  portion 
of  the  large  intestine  below  the  orifice  of  the  small  intestine.  The  posterior 
and  inner  wall  presents  a  small  opening  which  leads  into  a  narrow  round 
process  about  ten  centimeters  in  length — the  vermiform  appendix.  The 
opening  of  the  small  intestine  into  the  cecum  is  narrow  and  elongated  and 
bordered  by  two  folds  of  mucous  membrane  strengthened  by  fibrous  and 
muscle-tissue.  These  folds  constitute  the  so-called  ileo-cecal  valve.  When 
the  cecum  is  distended  the  margins  of  these  folds  are  approximated  and 
effectually  prevent  the  return  of  material  into  the  small  intestine. 

The  closure  of  this  opening  is  now  attributed  to  the  activity  of  a  sphincter 
muscle,  the  ileo-colic,  the  contraction  of  which  is  regulated  by  the  nerve 
system,  by  way  of  the  splanchnic  nerves. 

The  colon  ascends  to  the  under  surface  of  the  liver,  where  it  bends  at  a 
right  angle,  forming  the  hepatic  flexure,  crosses  the  abdominal  cavity  to 
the  spleen,  forming  the  splenic  flexure,  descends  to  the  left  iliac  fossa,  and 
bends  again.  At  this  point  it  turns  upon  itself  to  form  the  sigmoid  flexure. 
The  rectum  is  a  dilated  pouch,  situated  within  the  true  pelvis.  It  measures 
from  15  to  18  centimeters  in  length.  Within  three  centimeters  of  its  termina- 
tion at  the  anus  it  presents  a  constriction  formed  by  a  circular  band  of  smooth 
muscle-fibers  known  as  the  internal  sphincter.  The  margin  of  the  anus  is 
also  surrounded  by  bands  of  striated  muscle-fibers  known  collectively  as  the 
external  sphincter. 

The  walls  of  the  large  intestine  consist  of  three  coats:  viz.:  serous, 
muscular,  and  mucous. 

The  serous  is  a  reflection  of  the  general  peritoneal  membrane. 

The  muscle  is  composed  of  both  longitudinal  and  circular  fibers.  The 
longitudinal  fibers  are  collected  into  three  narrow  bands  which  are  situated 
at  points  equidistant  from  one  another.  At  the  rectum  they  spread  out  so 
as  to  surround  it  completely.  As  the  longitudinal  bands  are  shorter  than 
the  intestine  itself,  its  surface  becomes  sacculated,  each  sac  being  partially 
separated  from  adjoining  sacs  by  narrow  constrictions.  The  circular  fibers 
are  arranged  in  the  form  of  a  thin  layer  over  the  entire  intestine.  Between 
the  sacculi,  however,  they  are  more  closely  arranged.  The  sacculi  have  been 
termed  haustra  from  their  supposed  function,  that  of  absorbing  or  drawing 
water  from  the  intestinal  contents  thus  imparting  to  them  a  certain  degree 
of  consistency.     In  the  rectum  the  circular  fibers  are  well  developed,  and  at 


204  TEXT-BOOK  OF  PHYSIOLOGY 

a  point  two  or  three  centimeters  above  the  anus  they  form,  as  stated  above, 
the  internal  sphincter. 

The  mucous  membrane  of  the  large  intestine  possesses  neither  villi  nor 
valvulae  conniventes.  It  contains  a  large  number  of  tubules  consisting  of  a 
basement  membrane  lined  by  columnar  epithelium.  They  resemble  the 
follicles  of  Lieberkiihn.  The  secretion  of  these  glands  is  thick  and  viscid 
and  contains  a  large  quantity  of  mucin. 

The  Movements  of  the  Large  Intestine. — As  a  result  of  the  actions  of 
saliva,  of  gastric,  intestinal,  and  pancreatic  juice,  and  of  the  bile,  the  food  is 
disintegrated  and  liquefied.  The  nutritive  principles,  protein,  starches,  sugars, 
and  fats,  undergo  chemic  changes  and  are  transformed  into  amino-acids  and 
peptids,  dextrose,  soap  and  glycerin,  fat  acids,  under  which  forms  they  are 
absorbed.  After  the  more  or  less  complete  digestion  and  absorption  of  these 
nutritive  substances  the  residue  of  the  food,  comprising  the  indigestible  and 
undigested  matter,  passes  out  of  the  small  intestine  into  the  large  intestine 
and  forms  a  portion  of  its  contents.  This  residue  consists  of  the  hard  parts 
of  the  cereals,  vegetable  seeds,  cellulose,  etc.,  the  quantity  and  variety  of 
which  depend  on  the  nature  of  the  food.  These  substances,  passing  into 
the  large  intestine  along  with  the  excrementitious  matter  of  the  bile,  become 
incorporated  with  the  mucous  secretions  and  assist  in  the  formation  of  the 
feces. 

Under  the  influence  of  a  peristaltic  movement  similar  to  that  wit- 
nessed in  the  small  intestine,  all  this  excrementitious  matter,  deprived  by 
absorption  of  the  excess  of  its  contained  water  and  nutritive  material,  is 
gradually  carried  downward  to  the  sigmoid  flexure,  where  it  accumulates 
prior  to  its  extrusion  from  the  body.  The  effects  of  the  peristaltic  waves  are 
to  some  extent  interfered  with  by  anti-peristaltic  or  anastaltic  waves  which, 
beginning  in  the  transverse  colon,  run  toward  and  to  the  cecum.  An  anti- 
peristaltic wave  occurs  in  the  cat  about  every  fifteen  minutes  and  lasts  for 
about  five  minutes.  The  intestinal  contents  are  thereby  driven  back  toward 
the  cecum.  The  effect  is  a  still  further  admixture  with  the  secretions  and 
exposure  to  the  absorbing  mucosa.  There  is  some  evidence  also  that  the 
anti-peristaltic  waves  may  force  some  of  the  liquefied  contents  through  the 
ileo-colic  opening  into  the  small  intestine  because  of  the  relaxation  of  the  ilio- 
colic  sphincter  muscle.  It  is  questionable  if  this  ascending  movement  is  a 
true  peristalsis  inasmuch  as  the  advancing  contraction  is  apparently  not  pre- 
ceded by  an  area  of  inhibition  or  relaxation.  It  resembles  rather  the  corre- 
sponding movement  manifested  by  the  small  intestine  to  which  the  term 
anastalsis  has  been  given,  and  is  propagated  along  the  muscle  coat  independ- 
ently of  the  myenteric  plexus. 

In  addition  to  this  anastaltic  wave,  haustral  contractions  have  been  ob- 
served which  resemble  the  segmentation  contractions  of  the  small  intestines 
and  which  promote,  it  is  believed,  the  absorption  or  drawing  of  water  from 
the  intestinal  contents. 

The  Function  of  the  Large  Intestine. — ^The  large  intestine,  by  reason 
of  its  anatomic  relation  to  the  small  intestine,  serves  as  a  receptacle  for  the 
temporary  storage  of  the  indigestible  residue  of  the  food  together  with  certain 
excretions  of  the  intestinal  glands  both  of  which  have  descended  from  the 
small  intestine.  Inasmuch  as  the  contents  of  the  large  intestine,  in  that 
portion  known  as  the  ascending  colon  are  quite  liquid,  while  the  contents 


DIGESTION  205 

in  the  portion  known  as  the  descending  colon  are  more  or  less  solid,  it  is 
apparent  that  an  absorption  of  liquid  must  take  place  during  the  transit 
from  the  cecum  to  the  rectum.  This  is  made  possible  by  the  retardation  of 
the  intestinal  contents  caused  by  the  antiperistaltic  or  the  anastaltic  wave, 
together  with  the  haustral  contractions.  Between  the  two  modes  of  activity 
different  portions  of  the  intestinal  contents  are  exposed  to  the  mucosa  and 
by  which  the  liquids  are  in  large  measure  absorbed.  In  the  distal  portion 
of  the  large  intestine,  the  characteristic  movement  is  peristaltic  by  which  the 
more  solid  contents  are  carried  to  the  sigmoid  flexure  where  they  accumulate' 
until  the  desire  to  evacuate  the  mass  is  experienced.  By  the  contraction  of 
the  pelvic  portion  of  the  descending  colon  the  mass  is  forced  into  the  rectum 
from  which  it  is  discharged  from  the  body.  The  function  of  the  large 
intestine  is  therefore  to  receive,  to  reduce  to  a  proper  consistency,  to  tempor- 
arily store  and  subsequently  discharge  its  contents,  consisting  of  the  indi- 
gestible residue  of  the  food,  together  with  excretions  of  intestinal  glands 
which  have  descended  from  the  smaU  intestine  and  which  constitute  in  part 
the  feces. 

The  Nerve  Mechanism  of  the  Large  Intestine. — The  nerve  mechan- 
ism of  the  large  intestine  includes  both  motor  and  inhibitor  nerves.  The 
motor  nerves  comprise  both  pre-  and  post-gangHonic  fibers;  the  former 
have  their  origin  in  the  spinal  cord,  from  which  they  emerge  in  the  third  and 
fourth  sacral  nerves  and  pass  by  way  of  the  pelvic  nerve  to  the  pelvic 
ganglia  around  the  cells  of  which  their  fibers  arborize;  the  latter  (post- 
ganglionic) fibers  emerge  from  the  cells  of  these  ganglia  and  are  distributed 
to  circular  and  longitudinal  muscle-fibers  of  the  intestinal  wall. 

The  inhibitor  fibers  also  comprise  both  pre-  and  post-ganglionic  fibers; 
the  former  have  their  origin  in  the  lumbar  region  of  the  spinal  cord,  from 
which  they  emerge  in  the  second  to  the  fifth  lumbar  nerves;  they  then  pass  into 
and  through  the  sympathetic  chain  and  the  inferior  splanchnic  nerves  to  the 
inferior  mesenteric  ganglion  around  the  cells  of  which  they  arborize;  the 
post-ganglionic  fibers  pass  directly  to  the  muscle-fibers  of  the  intestinal  wall. 
Stimulation  of  the  pelvic  nerve  with  induced  electric  currents  causes  con- 
traction of- the  muscle-fibers;  stimulation  of  the  hypogastric  nerves  causes 
an  inhibition  of  the  contraction. 

Intestinal  Fermentation.— Owing  to  the  favorable  conditions  in  both 
the  small  and  large  intestine  for  fermentative  and  putrefactive  processes — 
e.g.,  heat,  moisture,  oxygen,  and  the  presence  of  various  microorganisms — 
some  of  the  food,  when  consumed  in  excessive  quantity  or  when  acted  on  by 
defective  secretions,  undergoes  a  series  of  decomposition  changes  which  are 
attended  by  the  production  of  gases  and  various  chemic  compounds.  Dex- 
trose and  maltose  are  partially  reduced  to  lactic  acid;  this  to  butyric  acid, 
carbon  dioxid,  and  hydrogen.  Fats  are  reduced  to  glycerol  and  fat  acids,  the 
glycerol,  according  to  the  organisms  present,  yields  succinic  acid,  carbon 
dioxid,  and  hydrogen.  Some  of  the  protein  derivatives — the  amino-acids — 
may  be  reduced  to  still  simpler  compounds  under  the  action  of  bacteria. 
Lysin,  through  the  loss  of  a  molecule  of  CO2,  gives  rise  to  cadaverin;  orni- 
thin  to  putrescin;  tryptophan  yields  indol,  skatol;  tyrosin  yields  phenol  and 
probably  cresol. 

Indol. — This  compound  is  of  especial  interest  as  it  is  the  antecedent  of 
indican,  found  in  the  urine.     Though  it  is  in  part  discharged  in  the  feces,  it 


2o6  TEXT-BOOK  OF  PHYSIOLOGY 

is  in  part  absorbed  into  the  portal  blood  and  carried  direct  to  the  liver  where 
it  is  oxidized  to  indoxyl  and  combined  or  conjugated  with  potassium  sulphate 
forming  the  salt  potassium  indoxyl  sulphate  or  indican,  after  which  it  enters 
the  blood,  is  carried  to  and  eliminated  by  the  kidneys.  The  presence  of  this 
salt  in  the  urine  can  be  demonstrated  by  adding  hydrochloric  acid  with  a 
small  quantity  of  potassium  chlorate;  after  this  is  done  the  indican  combines 
with  water  and  undergoes  a  cleavage  into  indoxyl  and  potassium  sulphate; 
the  former  then  combines  with  oxygen  and  gives  rise  to  indigo  blue.  The 
extent  to  which  the  indican  is  present  is  taken  as  a  measure  of  the  extent  of 
intestinal  putrefaction. 

Skatol. — This  compound  is  also  a  derivative  of  the  protein  molecule,  the 
result  of  bacterial  decomposition.  It  passes  in  part  into  the  feces  and  gives 
to  them  the  characteristic  odor.  It  is  also  in  part  absorbed  into  the  portal 
blood  and  carried  direct  to  the  liver  where  it  is  oxidized  to  skatoxyl,  after 
which  it  combines  with  potassium  sulphate  to  form  potassium  skatoxyl  sul- 
phate after  which  it  enters  the  blood  and  is  carried  to  the  kidneys  to  be  elimi- 
nated in  the  urine. 

Phenol  and  Cresol. — ^These  compounds,  also  derivatives  of  the  protein 
molecule,  are  absorbed  into  the  portal  blood  and  undergo  a  similar  conjuga- 
tion.    They  too  are  finally  eliminated  by  the  kidneys. 

The  Feces. — The  feces  is  a  term  applied  to  the  mass  of  material  ejected 
from  the  rectum  through  the  anus.  They  are  characterized  by  consistency, 
color  and  odor.  The  origin  and  the  nature  of  this  material  have  both  a 
physiologic  and  a  clinic  interest. 

The  consistency  varies  from  day  to  day  from  liquid  to  solid,  depending 
partly  on  the  character  of  the  food,  the  rapidity  with  which  it  is  transported 
through  the  intestine  and  hence  the  extent  to  which  absorption  of  water  in 
the  large  intestine  takes  place.  On  a  meat  diet  the  consistency  is  firm;  on  a 
vegetable  diet  it  is  apt  to  be  soft.  The  amount  discharged  from  day  to  day 
on  a  mixed  diet  varies  from  120  to  170  grams  containing  from  30  to  42  grams 
of  dry  matter.  On  a  meat  diet  alone  the  quantity  diminishes;  on  a  vege- 
table diet,  especially  if  the  articles  of  food  are  rich  in  cellulose,  the  quantity 
will  increase  considerably  beyond  the  customary  amount. 

The  color  on  a  mixed  diet  varies  from  a  light  yellow  to  black.  The 
usual  brown  color  is  due  to  the  pigment  urobilin  or  stercobilin,  a  derivative 
of  the  pigments  of  the  bile.  On  a  meat  diet  the  color  deepens  until  it  becomes 
quite  black  due  to  the  presence  of  sulphid  of  iron,  the  result  of  the  union 
of  sulphuretted  hydrogen  with  the  iron  derived  from  hematin  contained  in 
the  meat.  On  a  vegetable  diet  the  color  lightens  and  may  become  slightly 
yellow.  If  the  contents  of  the  intestine  are  carried  forward  too  rapidly,  the 
time  may  be  insufficient  for  a  complete  reduction  of  the  bile  pigments,  hence 
they  appear  in  the  feces  imparting  to  them  a  green  color.  If  there  is  an 
obstruction  to  the  discharge  of  bile  into  the  intestine  the  feces  may  become 
yellow  or  clay-colored. 

The  odor  is  characteristic  and  due  to  the  presence  of  skatol  and  allied 
bodies  produced  by  the  putrefaction  of  proteins  by  bacterial  action.  Sul- 
phuretted hydrogen  also  contributes  to  the  odor. 

The  chemic  composition  of  the  feces  is  complex.  They  consist  of  water, 
mucin,  an  indigestible  residue  of  food,  decomposition  products,  excretions 
from  the  intestinal  glands,  and  inorganic  salts.     The  residue  of  the  food 


DIGESTION  207 

usually  consists  of  the  denser  portions  of  the  connective  tissue  of  meats  and 
the  cellulose  of  vegetables  and  cereals.  When  the  latter  are  eaten  in  large 
amounts  the  cellulose  residue  is  increased  and  by  its  mechanic  stimulation 
increases  the  peristalsis  and  hastens  the  transfer  of  the  feces  through  the 
intestine.  The  decomposition  products  are  derived  from  protein,  fat,  and 
carbohydrate  food  by  bacterial  action  and  include  skatol,  indol,  fat  acids, 
soaps,  xanthin,  ammonia,  sulphuretted  hydrogen,  etc.  The  excretion  from 
the  intestine  itself  contributes  a  considerable  portion  to  the  fecal  mass.  The 
inorganic  salts  include  phosphates  of  calcium  and  magnesium  together  with 
various  sodium  and  potassium  compounds. 

Defecation. — Defecation  is  the  final  act  of  the  digestive  process  and 
consists  in  the  expulsion  of  the  indigestible  residue  of  the  food  and  its  asso- 
ciated compounds  from  the  intestine.  This  act  usually  takes  place  in  the 
human  being  but  once  in  twenty-four  hours,  as  the  diet  contains  but  a 
minimum  quantity  of  indigestible  matter.  Previous  to  their  expulsion  the 
feces  which  have  accumulated  in  the  sigmoid  flexure  must  pass  downward 
into  the  rectum.  In  so  doing  they  develop  the  sensation  which  leads  to  the 
act  of  defecation.  The  descent  of  the  feces  is  accomplished  by  the  peristaltic 
contraction  of  the  intestinal  wall.  Coincident  with  the  passage  of  the  feces 
into  the  rectum  there  is  a  relaxation  of  the  sphincter  muscles  and  a  contrac- 
tion of  the  longitudinal  and  circular  muscle-fibers,  in  consequence  of  which 
the  feces  are  expelled.  These  complex  muscle  actions  are  also  aided  by  the 
voluntary  contractions  of  the  diaphragm  and  abdominal  muscles. 

Nerve  Mechanism  of  Defecation. — The  act  of  defecation  is  primarily 
reflex  though  somewhat  influenced  by  voluntary  efforts.  The  reflex  charac- 
ter of  the  act  is  especially  noticeable  in  young  children  in  whom  by  reason  of 
the  imperfect  development  of  the  brain  there  is  a  lack  of  volitional  control. 
During  the  intervals  of  defecation  the  anal  orifice  is  tightly  closed  by  the 
tonic  contraction  of  the  internal  non-striated  sphincter  and  the  external 
striated  sphincter  muscles,  thus  preventing  the  escape  of  gases  or  semi-liquid 
material.  The  tonic  contraction  of  both  muscles  is  maintained  by  the 
activity  of  nerve-centers  located  in  the  lumbar  region  of  the  spinal  cord. 
The  circular  and  longitudinal  fibers  of  the  rectum  proper  are  at  the  same 
time  in  a  relaxed  or  inhibited  condition,  the  result  of  an  inhibition,  or  a  want 
of  stimulation,  of  their  governing  nerve-center  or  centers  in  the  lumbar  region 
of  the  spinal  cord.  When  the  desire  to  evacuate  the  bowel  is  experienced, 
impressions  are  being  made  by  the  feces  on  the  afferent  nerve  endings  in  the 
mucous  membrane  of  the  sigmoid  flexure  and  of  the  rectum.  The  nerve 
impulses  thus  developed  are  transmitted  to  the  defecation  or  rectal  nerve- 
centers  in  the  spinal  cord  and  to  the  cerebrum  and  influence  in  one  direction 
or  another  their  activities. 

In  the  young  child  the  arrival  of  the  transmitted  impulses  in  the  spinal 
cord  is  immediately  follov.'ed  by  an  inhibition  of  the  sphincter  centers  and  a 
stimulation  of  the  rectal  muscle  centers,  as  a  consequence  of  which,  the 
sphincter  muscles  relax  and  the  expulsive  muscles  contract  thus  discharging 
the  feces. 

In  the  adult  if  the  act  of  defecation  is  to  be  permitted  the  same  mechan- 
ism is  brought  into  action.  In  their  expulsive  efforts,  these  latter  muscles 
are  assisted  by  the  contraction  of  the  diaphragm,  abdominal,  and  other 
muscles  in  response  to  volitional  efforts.     After  the  expulsion  of  the  feces. 


2o8  TEXT-BOOK  OF  PHYSIOLOGY 

there  is  a  return  to  the  former  condition,  namely,  a  relaxation  or  inhibition 
of  the  rectal  muscles  and  a  contraction  of  the  sphincter  muscles.  If  the  act  of 
defecation  is  to  be  suppressed,  the  controlling  influence  of  the  nerve-center 
on  the  contraction  of  the  external  sphincter  may  by  an  act  of  volition  be 
strengthened  and  the  action  of  the  reflex  mechanism  for  a  while  antagonized. 
The  nerve  mechanism  therefore  involves  both  efferent  and  afferent  nerves 
as  well  as  nerve  centers  in  the  lumbo-sacral  region  of  the  spinal  cord. 

Efferent  Nerves. — ^The  efferent  nerve-fibers  for  the  external  sphincter  mus- 
cle have  their  origin  in  the  spinal  cord  from  which  they  pass  by  way  of  the 
third  and  fourth  sacral  nerves,  the  pelvic  nerve  and  the  inferior  hemorrhoidal 
nerve  directly  to  the  muscle. 

The  efferent  nerve-fibers,  for  the  longitudinally  and  circularly  arranged 
muscle-fibers  of  the  rectum,  including  the  specialized  portion,  the  internal 
sphincter,  have  their  origin  in  nerve-cells  in  the  lumbo-sacral  region  of  the 
spinal  cord  and  pass  to  their  destination  by  two  paths.  The  fibers  in  the 
ff,rst  path  leave  the  spinal  cord  by  way  of  the  second  to  the  fifth  lumbar 
nerves,  then  pass  into  and  through  the  sympathetic  chain,  through  the  inferior 
splanchnics  to  the  inferior  mesenteric  ganglion  around  the  cells  of  which 
their  terminal  branches  arborize;  from  the  cells  of  this  ganglion  new  fibers 
emerge  which  pass  through  the  hypogastric  nerves  to  the  muscles.  The 
fibers  of  the  second  path  leave  the  spinal  cord  by  way  of  the  second  to  the 
fourth  sacral  nerves,  then  pass  into  the  pelvic  or  erigens  nerve  to  small 
ganglia  along  the  sides  of  the  rectum  around  the  cells  of  which  their 
terminal  branches  arborize;  from  the  cells  of  these  ganglia  new  nerve- 
fibers  emerge  which  pass  directly  to  the  muscles.  In  both  paths  the  nerves 
coming  from  the  cord  are  pre-ganglionic,  those  coming  from  the  ganglia, 
post-ganglionic. 

The  central  mechanism  that  excites  and  coordinates  the  activities  of  the 
rectal  muscles  is  situated  in  the  lumbo-sacral  region  of  the  spinal  cord  and 
is  designated  the  recto-anal  center. 

The  Afferent  Nerves. — The  afferent  nerve-fibers  that  excite  the  central 
mechanism  to  activity  are  contained  in  the  dorsal  roots  of  the  spinal  nerves. 
Although  the  anatomic  relations  of  the  various  nerves  composing  this  mechan- 
ism are  fairly  well  known,  their  physiologic  actions  are  not  clearly  defined. 
The  results  of  experimental  methods  of  investigation  are  neither  uniform  nor 
in  accord.  The  want  of  accord  lies  partly  in  anatomic  peculiarities  of  the 
animal  the  subject  of  the  investigation,  and  partly  perhaps  in  the  character 
of  the  stimulus  employed. 

Stimulation  of  the  pelvic  nerve  causes,  in  the  dog  at  least,  a  peristalic 
contraction  of  the  circular  fibers  of  the  rectum.  Stimulation  of  the  hypo- 
gastric nerve  causes  an  inhibition  or  relaxation  of  the  circular  fibers  of  the 
rectum  and  of  the  internal  sphincter  as  well.  Inasmuch  as  these  two  groups 
of  fibers  have  opposite  functions  it  may  be  assumed  that  the  nerve  centers 
controlling  them,  both  motor  and  inhibitor,  are  double  centers  and  that  they 
can  be  made  to  act  separately  and  alternately. 

Recalling  the  events  that  take  place,  it  may  be  assumed  that  peripheral 
stimulation  of  the  afferent  nerves  develops  nerve  impulses  which,  when 
transmitted  to  the  cord  cause  i,  a  stimulation  of  the  motor  center,  a  dis- 
charge of  nerve  impulses  through  the  pelvic  nerve  to  the  rectal  muscles 
calling  forth  a  contraction;  2,  a  stimulation  of  the  inhibitor  center,  a  dis- 


DIGESTION  209 

charge  of  nerve  impulses  through  the  hypogastric  nerve  to  the  internal 
sphincter  and  perhaps  the  external  sphincter  as  well,  calling  forth  their 
relaxation  or  inhibition.  With  the  discharge  of  the  feces  the  former  condi- 
tion is  re-established.  A  stimulus,  the  nature  of  which  is  not  fully  known, 
causes  a  stimulation  of  the  inhibitor  center  for  the  rectal  muscles  and  a 
stimulation  of  the  motor  center  for  the  sphincters,  the  nerve  impulses  reaching 
the  muscles  through  the  hypogastric  and  pelvic  nerv^es  respectively. 


14 


CHAPTER  XI 
ABSORPTION 

Absorption  is  a  process  by  which  nutritive  material  from  the  tissue 
spaces,  from  the  serous  cavities,  from  the  interior  of  the  lungs  and  from  the 
raucous  surfaces  of  the  body,  and  waste  materials  from  the  tissues  are  trans- 
ferred into  the  blood. 

The  absorption  of  nutritive  materials  from  the  tissue  spaces  and  from  the 
serous  cavities  may  be  regarded  as  an  act  of  resorption  or  a  return  to  the 
blood  of  nutritive  material  which  has  passed  across  the  walls  of  the  capil- 
lary blood-vessels  in  excess  of  that  needed  for  purposes  of  nutrition,  and 
which  if  not  returned  would  lead  to  an  accumulation  and  the  development  of 
edematous  conditions;  the  absorption  of  oxygen  from  the  lungs  is  essential 
to  the  maintenance  of  nutritive  activity,  to  the  oxidation  of  foods  and  the 
liberation  of  their  energy;  the  absorption  of  new  nutritive  materials  from  the 
mucous  surfaces  of  the  entire  alimentary  canal,  but  more  especially  from 
that  of  the  small  intestine,  materials  that  have  been  produced  out  of  the 
foods  by  the  digestive  process,  is  essential  to  the  maintenance  of  the 
quantity  and  quality  of  the  blood. 

The  absorption  of  the  products  of  metabolism,  of  carbon  dioxid,  urea  and 
other  nitrogen-holding  compounds  from  the  tissues  into  the  blood  is  essential 
to  the  continuance  of  their  activities  as  well  as  a  necessary  preliminary  to 
their  elimination  from  the  body. 

The  anatomic  mechanisms  involved  in  the  absorptive  process  are,  pri- 
marily, the  tissue  or  lymph-spaces,  the  blood-  and  lymph-capillaries ;  second- 
arily, the  blood-vessels  (veins,)  and  the  lymph-vessels. 

Tissue  or  Lymph-spaces. — Everywhere  throughout  the  body,  in  the 
connective-tissue  system  and  in  the  interstices  of  the  several  structures  of 
which  an  organ  is  composed,  are  found  spaces  or  clefts  of  irregular  shape 
and  size,  determined  largely  by  the  structure  of  the  organ  in  which  they 
are  found,  which  have  been  termed  tissue  or  lymph-spaces,  from  the  fact 
that  they  contain  a  clear  fluid,  the  lymph.  These  spaces  are  devoid  for 
the  most  part  of  any  endothelial  lining,  but  as  they  communicate  more  or 
less  freely  one  with  another,  there  is  a  circulation  of  lymph  through  them 
and  around  the  islets  of  tissue  (Fig.  86).  In  addition  to  the  connective- 
tissue  lymph-spaces,  different  observers  have  described  special  spaces  or 
clefts  in  organs  such  as  the  kidney,  liver,  spleen,  testicle,  and  in  all  secreting 
glands  between  their  basement  membrane  and  the  surrounding  blood- 
vessels, all  of  which  contain  a  greater  or  less  quantity  of  lymph.  Within  the 
brain,  spinal  cord,  bone,  and  other  tissues  it  has  been  shown  that  the  smallest 
blood-vessels  and  capillaries  are  bounded  and  limited  by  a  cylindrical  sheath 
containing  lymph,  which  is  known  as  a  perivascular  lymph-space.  A 
similar  sheath  surrounds  the  smallest  nerve-bundles  and  fibers,  enclosing  a 
perineural  lymph-space.  The  large  serous  cavities  of  the  body,  pleural, 
peritoneal,  pericardial,  etc.,  are  also  to  be  regarded  as  lymph-spaces.     The 

2IO 


ABSORPTION 


surfaces  of  these  cavities,  however,  are  covered  with  a  layer  of  endothelial 
cells  with  sinuous  margins.  At  intervals  between  these  cells  are  to  be  found 
small  free  openings  which  have  received  the  name  of  stomata. 

The  Blood-capillaries. — ^The  blood-capillaries  not  only  permit  of  a 
transudation  of  the  liquid  nutritive  material  from  the  blood  across  their 
delicate  walls,  but  are  also  engaged,  if  not  in  the  resorption  of  a  portion  of 
this  transudate,  at  least  in  the  absorption  of  waste  products  resulting  from 
tissue  metabolism. 

The  Blood-vessels. — ^The  blood-vessels — the  venules — that  arise  from 
the  capillary-vessels  gradually  converge  and  unite  to  form  veins  which  by 
their  union  ever  increase  in  size  until  they  terminate  in  the  inferior  and 
superior  vena  cava,  both  of  which  at  their  terminations  communicate  with 
the  right  side  of  the  heart.  The  blood  containing  the  waste  products  ab- 
sorbed from  the  tissues,  is  con- 
ducted by  these  veins  to  the 
right  side  of  the  heart,  thence 
by  the  pulmonic  artery  into  the 
lungs,  where  the  carbon  dioxid 
is  eliminated;  the  blood  carrying 
the  remainder  of  the  waste  prod- 
ucts is  then  conducted  by  the 
pulmonic  veins  to  the  left  side 
of  the  heart  by  which  it  is  dis- 
charged into  the  arteries  and  dis- 
tributed in  )^art  to  the  kidneys, 
liver,  skin  and  intestine;  by  these 
organs  the  remaining  waste  prod- 
ucts are  excreted  from  the 
blood. 

The  L5anph-capillaries. — 
The  lymph-capillaries  in  which 
the  lymph-vessels  proper  take 
their  origin  are  arranged  in  the 
form  of  plexuses  of  quite  ir- 
regular shape.  In  most  situa- 
tions they  are  intimately  inter- 
woven with  the  blood-vessels, 
from  which  they  can  be  readily 

distinguished  by  their  larger  caliber  and  irregular  expansions.  The  wall  of 
the  lymph-capillary  is  formed  by  a  single  layer  of  endothelial  cells  with 
characteristic  sinuous  outlines.  These  capillaries  anastomose  very  freely 
one  with  another  and  communicate,  on  the  one  hand,  with  the  lymph-spaces 
and  on  the  other  with  the  lymph-vessels  proper.  It  was  formerly  believed 
that  the  communication  of  the  lymph-capillary  with  the  tissue  space  was 
a  direct  one,  the  lymph  flowing  from  the  latter  into  the  former  through  an 
open  passage-way.  Recent  investigation  would  indicate  that  this  histologic 
arrangement  does  not  exist  but  that  on  the  contrary  the  lymph-capillaries 
are  closed  vessels  and  that  the  tissue  space  and  the  interior  of  the  lymph- 
capillary  are  separated  one  from  the  other  by  a  thin  partition  of  endothelial 
cells.     As  the  shape,  size,  etc.,  of  both  lymph-spaces  and  capillaries  are 


Fig.  86.— Origin  of  Lymph-vessels  from  the 
Central  Tendon  of  the  Diaphragm  Stained 
WITH  Nitrate  of  Silver,  s.  The  lymph-spaces 
and  lymph-canals,  communicating  at  x  with  the 
lymphatics,  a.  Origin  of  the  lymphatics  by  the 
confluence  of  several  juice  canals.  B.  Capillary 
blood-vessels. —  {Landois  and  Stirling.) 


212  TEXT-BOOK  OF  PHYSIOLOGY 

determined  largely  by  the  nature  of  the  tissue  in  which  they  are  found,  it 
is  not  always  possible  to  separate  one  from  the  other.  Their  function,  how- 
ever, may  be  regarded  as  similar:  viz.:  the  reception  and  collection  of  the 
excess  of  lymph  which  has  transuded  through  the  walls  of  the  blood-vessels 
and  its  transmission  onward  into  the  regular  lymph-vessels. 

Lymph- vessels. — The  lymph- vessels  constitute  a  system  of  minute, 
delicate,  transparent  vessels  found  in  nearly  all  the  organs  and  tissues  of  the 
body,  and  take  their  origin  from  the  lymph-capillaries  and  spaces  above 
described.  From  their  origin  they  gradually  converge  toward  the  trunk 
of  the  body,  and  finally  empty  into  the  thoracic  duct.  In  their  course  they 
anastomose  very  freely  with  adjoining  vessels.  The  diameter  of  a  lymph- 
vessel  varies  from  i  to  2  mm.  After  the  lymph-vessels  have  emerged  from 
the  lymph-capillaries  they  acquire  three  distinct  coats,  each  of  which  pos- 
sesses definite  histologic  features. 

The  internal  coat  is  composed  of  a  delicate  lamina  of  longitudinally  dis- 
posed elastic  fibers  covered  with  a  layer  of  flattened  nucleated  endothelial 
cells  with  wavy  outlines. 

The  middle  coat  consists  of  white  fibrous  tissue  arranged  longitudinally 
and  of  non-striated  muscle  and  elastic  fibers  arranged  transversely. 

The  external  coat  consists  of  practically  the  same  structures,  though  the 
muscle-fibers  are  longitudinally  disposed. 

The  lymph- vessels  are  provided  with  valves  which  are  so  numerous  and 
located  at  such  short  intervals  as  to  give  the  vessels  a  beaded  appearance. 
These  valves  are  arranged  in  pairs  and  consist  of  two  semi-lunar  folds  with 
their  concavities  directed  toward  the  larger  vessels.  They  are  formed  by  a 
reduplication  of  the  lining  membrane,  which  is  strengthened  by  fibrous  tissue 
derived  from  the  middle  coat. 

L)rmph-nodes5  or  glands. — In  their  course  toward  the  thoracic  duct  the 
lymph- vessels  pass  through  a  number  of  small  pisiform  bodies  termed  lymph- 
nodes  or  glands.  These  are  exceedingly  abundant  in  some  situations,  as  the 
cervical,  axillary,  and  inguinal  regions,  and  the  abdominal  cavity.  As  the 
lymph- vessels  approach  a  gland  they  divide  into  a  number  of  branches  before 
entering  it,  known  as  the  afferent  vessels.  From  the  opposite  side  of  the 
gland  the  lymphatics  again  emerge  as  efferent  vessels  to  unite  to  form  larger 
trunks.  A  section  of  a  gland  shows  that  it  consists  of  an  outer  dense  cortical 
and  an  inner  soft  pulpy  medullary  portion.  Each  gland  is  covered  exter- 
nally by  a  dense  membrane  of  fibrous  tissue  containing  in  its  meshes  non- 
striated  muscle-fibers.  From  the  inner  surface  of  this  membrane  there  pass 
inward  septa  of  connective  tissue  which,  as  they  converge  toward  the  center 
of  the  gland,  divide  its  outer  zone  into  small  conical  compartments  or 
alveoli.  When  the  septa  reach  the  medullary  portion,  they  subdivide  and 
form  bands  or  cords  which  interlace  in  every  direction  and  constitute  a  loose 
meshwork  the  spaces  of  which  communicate  with  one  another  and  with  the 
alveoli.  Within  the  meshes  of  this  framework  the  proper  gland  substance 
is  contained.  In  the  cortical  compartments  it  is  moulded  into  pear-shaped 
masses;  in  the  medullary  meshwork  it  assumes  the  form  of  rounded  cords 
which  are  connected  with  one  another.  In  both  regions,  however,  it  is 
separated  from  the  septa  by  a  space  termed  a  lymph  sinus,  through  which 
the  lymph  flows  as  it  passes  through  the  gland.  The  lymph  sinus  is  crossed 
by  a  network  of  retiform  connective  tissue  which  offers  considerable  re- 


ABSORPTION  213 

sistance  to  the  passage  of  the  lymph.  The  gland  substance  consists  also  of 
a  framework  of  retiform  connective  tissue  in  the  meshes  of  which  large 
numbers  of  lymph-corpuscles  are  contained.  The  gland  substance  is  sepa- 
rated from  the  lymph  sinus  by  a  dense  layer  of  a  reticulum,  which,  how- 
ever, does  not  prevent  lymph  and  even  corpuscles  from  passing  through  it 
into  the  lymph  sinus. 

The  lymph-glands  are  abundantly  supplied  with  blood-vessels.  The 
arteries  enter  the  gland  at  the  hilum,  penetrate  into  the  medullary  substance, 
and  terminate  in  a  fine  capillary  plexus  which  is  supported  by  the  connective 
tissue.     The  veins  arising  from  this  plexus  leave  the  gland  also  at  the  hilum. 

The  lymph-vessels  which  enter  a  gland  first  ramify  in  the  investing 
membrane  and  then  open  directly  into  the  lymph  sinus.  The  vessels  which 
leave  the  gland  are  also  in  communication  with  the  sinus.  After  the  lym- 
phatics enter  the  gland  they  lose  their  external  and  middle  coats,  retaining 
only  the  internal  or  endothelial  coat,  which  lines  the  inner  surface  of  the 
lymph  sinus.  The  current  of  lymph,  therefore,  is  from  the  afferent  vessels 
through  the  lymph  sinus  into  the  efferent  vessels.  In  addition  to  this  pri- 
mary current,  there  is  a  secondary  current  flowing  from  the  capillary  blood- 
vessels outward  and  into  the  sinus.  The  lymph  flowing  through  the  sinus 
carries  with  it  large  numbers  of  lymph-corpuscles.  It  is  quite  probable  that 
the  movement  of  the  lymph  through  this  complicated  system  of  passages  is 
aided  by  the  contraction  of  the  muscle-fibers  in  the  capsule  of  the  gland. 

The  lymph-corpuscles  or  lymphocytes  originate  for  the  most  part  in  the 
gland  substance  of  the  cortical  alveoli.  In  this  situation  there  are  groups  of 
cells,  so-called  germ  centers,  which  divide  very  rapidly  by  mitosis  and  give 
rise  constantly  to  groups  of  young  cells  which  soon  find  their  way  into  the 
lymph  stream. 

The  Thoracic  Duct. — The  thoracic  duct  is  the  general  trunk  of  the 
lymph  system,  into  which  the  vessels  of  the  lower  extremities,  of  the  abdom- 
inal organs,  of  the  trunk,  of  the  left  arm,  and  of  the  left  side  of  the  head 
empty  their  contents.  It  is  about  fifty  centimeters  in  length  and  four  milli- 
meters in  diameter.  It  extends  upward  from  the  third  lumbar  vertebra 
along  the  vertebral  column  to  the  seventh  cervical  vertebra,  where  it  empties 
into  the  venous  system  at  the  junction  of  the  internal  jugular  and  subclavian 
veins  on  the  left  side.  The  thoracic  duct  wall  has  the  same  general  layers 
as  the  wall  of  the  lymph-vessel:  viz.,  an  internal  or  endotheHal;  a  middle 
elastic  and  muscular;  an  external  or  fibrous.  It  is  also  provided  with 
numerous  valves. 

The  lymph-vessels  of  the  right  side  of  the  head,  of  the  right  arm,  and 
a  portion  of  the  right  side  of  the  trunk  terminate  in  the  right  thoracic  duct, 
which  is  about  25  to  30  mm.  in  length  and  which  empties  into  the  venous 
system  at  the  junction  of  the  internal  jugular  and  subclavian  veins  on  the 
right  side.  The  general  arrangement  of  the  lymphatic  system  is  diagram- 
matically  shown  in  Fig.  87. 

LYMPH 

Lymph  is  the  clear  fluid  found  within  the  tissue  spaces  and  within  the 
lymph-vessels.  Inasmuch  as  there  are  reasons  for  the  view  that  lymph 
varies  in  composition,  as  well  as  in  function,  in  these  different  regions  it 
will  be  found  conducive  to  clearness  to  designate  the  lymph  found  in  the 


214 


TEXT-BOOK  OF  PHYSIOLOGY 


tissue  spaces  as  intercelkdar  lymph,  and  that  found  in  the  lymph-vessels  as 
intravascular  lymph. 

The  Physical  Properties  of  Lymph. — Whether  obtained  from  tissue 
spaces  or  from  lymph-vessels,  the  lymph  presents  practically  the  same  physical 
properties.  The  lymph  obtained  from  the  thoracic  duct  during  the  intervals 
of  digestion  or  from  one  of  the  large  trunks  of  the  leg  is  a  clear,  colorless  or 
slightly  opalescent  fluid  having  an  alkaline  reaction  and  a  specific  gravity 
of  1. 020  to  1.040.     Examined  microscopically  it  is  seen  to  hold  in  suspen- 


FiG.  87. — Diagram  Showing  the  Course  of  the  Main  Trunks  of  the  Absorbent  System. 
The  lymph-vessel  of  lower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC)  at  the  recep- 
taculum  chyli  (RC),  where  the  thoracic  duct  begins.  The  superficial  vessels  are  shown  in  the 
diagram  on  the  right  arm  and  leg  (S),  and  the  deeper  ones  on  the  arm  to  the  left  (D).  The 
glands  are  here  and  there  shown  in  groups.  The  small  right  duct  opens  into  the  veins  on  the 
right  side.  The  thoracic  duct  opens  into  the  union  of  the  great  veins  of  the  left  side  of  the  neck 
(T).— (Feo'5  "  Text-hook  of  Physiology.") 

sion  a  large  number  of  corpuscles  similar  to  those  seen  m  the  lymph-glands 
and  to  the  white  corpuscles  of  the  blood.  Their  number  has  been  estimated 
at  about  8000  per  cubic  millimeter,  though  this  count  will  vary  within  wide 
limits  according  as  the  lymph  examined  has  passed  through  a  larger  or 
smaller  number  of  glands.  The  lymph-corpuscle  consists  of  a  small  quan- 
tity of  protoplasm  in  which  is  embedded  a  distinct  nucleus.  Some  of 
these  lymphocytes  contain  distinct  granules,  more  or  less  refractive,  which 
impart     to    the    corpuscle    a    granular    appearance.     When    withdrawn 


ABSORPTION  215 

from  the  vessels  lymph  undergoes  a  spontaneous  coagulation,  though 
the  coagulum  is  never  as  firm  as  that  observed  in  the  coagulation  of  the 
blood.  The  cause  of  the  coagulation  is  the  appearance  of  fibrin.  After 
a  variable  length  of  time  the  coagulum  separates  into  a  liquid  and  a  solid 
portion,  the  serum  and  the  clot. 

The  Chemic  Composition  of  Lymph. — Although  the  lymph  obtained 
from  the  tissue  spaces,  from  the  lymph-vessels,  as  well  as  from  the  so-called 
serous  cavities  has  the  same  general  chemic  composition,  there  is  reason 
for  the  view  that  it  varies  in  its  ultimate  composition  according  as  it  is 
derived  from  one  region  of  the  body  or  from  another.  The  needs  of  any 
individual  tissue  as  well  as  the  character  of  its  metabolic  products  will 
in  all  probability  change  not  only  its  normal  composition,  but  also  the 
relative  amounts  of  its  normal  constituents. 

Chemic  analysis  has  shown  that  the  lymph  from  the  thoracic  duct  con- 
tains from  3.4  to  4.1  per  cent,  of  proteins  (serum-albumin,  fibrinogen), 
0.046  to  0.13  per  cent,  of  substances  soluble  in  ether  (probably  fat),  o.i 
per  cent,  of  sugar,  and  from  0.8  to  0.9  per  cent,  of  inorganic  salts,  of  which 
sodium  chlorid  (0.55  per  cent.)  and  sodium  carbonate  (0.24  per  cent.)  are 
the  most  abundant  (Munk).  There  are  usually  in  most  specimens  small 
quantities  of  potassium,  calcium,  and  magnesium  salts.  Fibrinogen  is 
seldom  present  beyond  o.i  per  cent.,  which  will  account  for  the  feeble  and 
slow  coagulation.  Lymph  contains  both  free  oxygen  and  carbon  dioxid. 
Of  the  former,  however,  there  is  but  a  small  percentage;  of  the  latter,  about 
45  vols,  per  cent.,  partially  in  the  free  state  and  partially  combined  with 
sodium.  Urea  is  also  present  in  very  small  amounts.  This  analysis  indi- 
cates that  lymph  resembles  blood-plasma  in  the  character  of  its  constituents, 
though  their  relative  quantities  vary  considerably.  With  the  exception 
that  it  contains  no  red  corpuscles,  lymph  may  be  regarded  as  a  diluted 
blood. 

The  Production  of  Lymph. — Though  blood  is  the  common  reservoir  of 
nutritive  matenal,  the  latter  is  not  available  for  nutritive  purposes  as  long 
as  it  is  confined  within  the  blood-vessels.  The  capillary  wall,  thin  as  it  is, 
and  composed  of  but  a  single  layer  of  endothelial  cells,  would  be  sufficient 
to  prevent  its  utilization  by  the  tissues,  if  it  were  not  permeable  to  the  liquid 
portion  of  the  blood.  As  this  is  the  case,  however,  it  is  found  that  as  the 
blood  flows  through  the  capillary  vessels  a  portion  of  the  blood-plasma 
passes  across  the  capillary  wall  and  is  received  into  the  tissue-spaces, 
where  it  comes  into  intimate  contact  with  the  tissue-cells. 

The  forces  concerned  in  the  passage  of  the  constituents  of  the  blood- 
plasma  across  the  capillary  wall  have  been  the  subject  of  much  investigation. 
According  to  some  investigators,  diffusion,  osmosis,  and  filtration  are  suffi- 
cient to  account  for  all  the  phenomena.  For  a  consideration  of  the  phenom- 
ena of  diffusion,  osmosis,  and  filtration  the  reader  is  referred  to  paragraphs 
at  the  end  of  this  chapter.  It  is  assumed  that  the  capillary  wall,  being  an 
animal  membrane,  is  freely  permeable  to  water  and  crystalloid  bodies  gener- 
ally; less  so,  however,  to  colloid  bodies,  such  as  the  proteins  of  the  blood- 
plasma;  moreover,  it  is  further  assumed  that  the  physiologic  conditions  of 
the  capillary  walls  are  such  as  not  only  to  permit  of  the  passage  of  the  con- 
stituents of  the  blood  into  the  tissue  spaces,  but  also  the  passage  of  the  con- 
stituents of  the  intercellular  lymph  into  the  blood,  according  to  laws  similar 


2i6  TEXT-BOOK  OF  PHYSIOLOGY 

at  least  to  those  determining  the  passage  of  substances  through  animal 
membranes  as  determined  experimentally.  The  force  giving  rise  to  filtra- 
tion is  the  difference  of  pressure  between  that  exerted  by  the  blood  within 
the  capillary  vessels  and  that  exerted  by  the  fluid  in  the  tissue  spaces;  hence 
any  increase  or  decrease  of  this  difference  of  pressure  is  attended  by  an 
increase  or  decrease  in  the  production  of  lymph.  Thus  compression  of  the 
veins  of  a  part  which  interferes  with  the  outflow  of  blood  from  the  capillaries, 
or  a  dilatation  of  the  arterioles  which  increases  the  inflow  of  blood  to  them 
will  increase  the  capillary  pressure  and  therefore  the  production  of  lymph. 
The  reverse  conditions  will,  of  course,  diminish  the  intracapillary  pressure 
and  lymph  production.  Hemorrhages  which  lower  the  general  blood-pres- 
sure may  so  lower  the  capillary  pressure  as  not  only  to  stop  the  flow  of 
lymph  to  the  tissues,  but  may  give  rise  to  a  filtration  current  from  the  tissues 
into  the  blood. 

The  quantitative  composition  of  the  lymph  compared  with  that  of  the 
blood  indicates  that  it  is  produced  by  diffusion,  osmosis,  and  filtration.  In 
the  lymph  the  concentration  of  the  inorganic  salts  is  practically  the  same  as 
in  the  blood;  the  concentration  of  the  proteins,  however,  is  somewhat  less. 
These  facts  are  in  accordance  with  what  is  known  regarding  the  diffusibility 
of  both  crystalloids  and  colloids  through  animal  membranes. 

According  to  Heidenhain,  the  production  of  lymph  is  not  so  much 
due  to  intracapillary  pressure  as  it  is  to  the  specialized  activities  of  the 
endothelial  cells,  activities  which  indicate  that  lymph  is  a  secretion  the 
composition  of  which  varies  in  different  situations  by  virtue  of  a  difference 
in  the  molecular  structure  of  the  endothelial  cells.  As  is  the  case  with 
many  of  the  secreting  cells  of  the  body,  the  injection  of  various  substances 
into  the  blood  apparently  increases  the  activity  of  the  endothelial  cells,  as 
shown  by  an  increased  lymph  production  without  any  appreciable  increase 
of  intracapillary  pressure.  Thus  it  has  been  shown  that  the  injection  of 
peptones,  albumin,  the  extract  of  the  muscles  of  the  leech,  crab,  mussel, 
etc.,  is  followed  by  an  increase  in  the  amount  of  lymph  discharged  from 
the  thoracic  duct;  the  lymph  also  possesses  a  high  degree  of  concentration, 
being  rich  not  only  in  inorganic  but  also  in  organic  constituents.  The 
cause  of  this  increase  in  both  the  quantity  and  quality  of  the  lymph  is 
believed  to  be  an  increased  activity  in  the  secreting  power  of  the  endothelial 
cells.     These  substances  Heidenhain  regarded  as  true  lymphagogues. 

It  has  also  been  shown  that  after  the  injection  into  the  blood  of  sugar, 
sodium  chlorid,  sodium  sulphate,  urea,  etc.,  there  is  an  increase  in  the 
flow  of  lymph  from  the  thoracic  duct.  The  lymph,  however,  under  these 
circumstances  is  richer  in  water  than  is  normally  the  case.  As  the  blood 
at  the  same  time  increases  its  percentage  of  water,  it  is  assumed  that  the 
water  is  extracted  from  the  tissues,  by  reason  of  the  increased  percentage 
of  salts  and  a  higher  osmotic  pressure.  The  increased  volume  of  blood 
raises  the  intra-capillary  blood  pressure  which  in  turn  gives  rise  to  a  greater 
flow  of  lymph. 

The  more  recent  experiments  of  Starling  indicate  that  in  addition  to  the 
difference  of  pressure  between  the  blood  in  the  capillaries  and  the  lymph  in 
the  tissue  spaces,  a  new  factor  must  be  considered  and  that  is,  the  permea- 
bility of  the  capillary  wall.  This  he  finds  to  vary  considerably  in  different 
parts  of  the  vascular  apparatus,  being  greatest  in  the  capillaries  of  the 


ABSORPTION  217 

liver,  less  in  the  capillaries  of  the  intestines  and  least  in  the  capillaries  of 
the  extremities.  It  also  varies  doubtless  in  all  other  situations.  The 
increase  in  the  production  of  lymph  by  the  injection  of  peptones,  extract  of 
muscles  of  the  leech,  the  crab,  etc..  Starling  explains  by  the  assumption  that 
these  substances  alter  the  properties  of  the  capillary  wall  and  thus  increase 
its  permeability.  The  difference  of  pressure,  therefore,  between  blood  and 
lymph  taken  in  connection  with  the  degree  of  permeability  of  the  capillary 
wall  will  account  for  the  production  of  lymph  in  all  regions  of  the  body. 

Another  factor  that  has  been  invoked  to  account  for  the  passage  of  the 
constituents  of  lymph  across  the  capillary  wall,  is  an  increased  concentra- 
tion of  the  intercellular  lymph,  the  result  of  an  accumulation  of  metabolic 
products,  and  hence  an  increase  in  the  osmotic  pressure,  which  would  lead 
to  an  increase  in  the  passage  of  the  constituents  of  the  blood  into  the  lymph. 
The  activity  of  a  tissue  would  thus  indirectly  lead  to  the  formation  of 
lymph. 

The  Functions  of  Intercellular  Lymph. — The  origin  and  composition 
of  lymph,  its  situation  and  relation  to  the  tissue  cells  indicate  that  its  func- 
tion is  to  provide  the  tissue  cells  with  those  nutritive  materials  necessary  to 
their  growth,  repair,  and  functional  activities,  and  to  receive  from  the  tissue 
cells  the  waste  products  of  their  metabolism  prior  to  their  removal  by  the 
blood-  and  lymph-vessels. 

The  necessity  for  the  production  of  lymph  becomes  apparent  when  the 
chemic  changes  which  the  tissues  undergo  at  all  times  are  considered.  Thus 
whether  in  a  state  of  relative  rest  or  in  a  state  of  activity,  disintegrative 
changes  are  constantly  taking  place  and  always  in  direct  proportion  to  the 
degree  and  continuance  of  the  activity.  If  the  tissues  are  to  continue  in  the 
performance  of  their  customary  activities,  it  is  essential  that  repair  and 
restoration  be  at  once  established.  This  is  made  possible  by  the  presence 
of  lymph,  and  by  the  power  which  living  material  possesses  of  absorbing 
from  the  lymph  the  necessary  nutritive  materials,  of  assimilating  them  and 
transforming  them  into  material  like  unto  itself  and  endowing  them  with 
its  own  physiologic  properties. 

Coincidently  with  the  loss  of  nutritive  material,  the  lymph  receives  the 
products  of  the  metabolism  of  the  tissues  and  hence  changes  in  composition. 
Should  this  change  in  composition  continue  for  any  length  of  time,  the 
lymph  would  lose  its  restorative  character  and  become  destructive  to  tissue 
vitality.  Therefore  it  is  essential  that  the  nutritive  material  be  renewed  as 
rapidly  as  consumed  and  the  waste  products  be  carried  away  as  rapidly  as 
produced.  Both  these  conditions  are  fulfilled  by  the  blood-  and  lymph- 
vessels. 

The  Absorption  of  Intercellular  Lymph. — From  the  fact  that  lymph 
is  being  discharged  from  the  thoracic  duct  into  the  blood,  more  or  less  con- 
tinually, it  is  evident  that  lymph  is  being  absorbed  from  the  intercellular 
spaces;  from  which  fact  it  may  be  inferred  that  the  production  of  lymph  is  a 
continuous  process  and  that  it  is  passing  through  the  capillaries  in  amounts 
greater  than  is  necessary  for  the  immediate  needs  of  the  tissues.  Should 
this  excess  accumulate  there  would  soon  arise  the  condition  of  edema  and 
an  interference  with  the  functional  activities  of  the  tissues.  Therefore 
so  soon  as  the  accumulation  attains  a  certain  volume  it  is  absorbed  in  large 
measure  by  the  lymph-capillaries  and  transmitted  to  the  lymph-vessels  and 


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TEXT-BOOK  OF  PHYSIOLOGY 


thoracic  duct.  Because  of  the  general  belief  that  the  lymph-capillaries 
were  in  open  communication  with  the  tissue  spaces  it  was  assumed  that 
the  absorption  of  lymph  and  its  flow  through  the  lymph-vessels  was  the 
result  of  a  difference  of  pressure  between  the  lymph  in  the  tissue  spaces  and 
the  blood  in  the  innominate  veins.  But  if  the  lymph-capillaries  are  closed 
vessels,  as  recent  investigations  indicate,  then  additional  factors,  in  explana- 
tion of  lymph  absorption,  must  be  sought  for. 

It  is  quite  possible  under  even  normal  conditions  of  pressure  in  the  tissue 
spaces   that   some   of   the  more  diffusible  constituents  of  the  lymph  are 

absorbed  by  the  capillary  blood-vessels.  As 
to  whether  the  relatively  feebly  diffusible 
colloids  are  so  resorbed  is  as  yet  a  matter  of 
investigation. 

ABSORPTION  OF  FOODS 

The  most  important  of  the  absorbing  sur- 
faces, especially  in  its  relation  to  the  absorp- 
tion of  new  material,  is  the  mucous  mem- 
brane of  the  alimentary  canal,  and  more  par- 
ticularly that  portion  lining  the  small  intestine, 
provided  as  it  is  with  specialized  absorbing 
structures — the  villi.  Though  certain  sub- 
stances can  be  absorbed  from  the  mouth,  it 
is  not  probable  that  any  food  is  so  absorbed. 
From  the  changes  which  the  food  principles 
undergo  in  the  stomach  it  might  naturally  be 
inferred  that  their  absorption  would  promptly 
follow.  Experimental  researches  have  demon- 
strated, however,  that  this  takes  place,  if  at 
all,  but  to  a  slight  extent.  If,  however,  solu- 
FiG.  88.— Longitudinal  Sec-  tions  of  inorganic  salts,  sugars,  and  peptones 
tStinT  oF^THE'DoG.''mGHfY  Posscsslng  a  concentration  of  at  least  5  per 
Magnified,  a.  Columnar  epi-  Cent. — a  degree  of  Concentration  seldom  re- 
and'T  ^''^^^"j^s  g°'^^^*-'^*'^'s  (^)  alized  under  normal  conditions— are  intro- 
BlsemeS''*°membra°n?''^J.^  Kate-  ^^^^ed  into  the  stomach,  their  absorption  will 
like  connective-tissue  elements  of  be  effected,  the  rate  of  absorption  following  in 
core,    e,  e.  Blood-vessels.    /.  Ab-       general  way  the  increase,  within  limits,  in 

sorbcnt     radical     or     lacteal. —  ^  .-',,.  .        '         .      ,,  , 

iPiersoi.)  Concentration.     Water  is  practically  not  ab- 

sorbed from  the  stomach.  The  absorption 
of  the  products  of  digestion — i.e.,  dextrose,  levulose,  peptones,  amino-acids, 
soaps,  glycerin,  fat  acids,  salts,  along  with  water,  in  which  for  the  most 
part  they  are  held  in  solution — is  therefore  limited  very  largely  to  the 
small  intestine,  and  is  accomplished  by  the  villous  processes  projecting 
from  the  surface  of  the  mucous  membrane. 

Structure  of  the  Villi. — The  villi  are  small  filiform  or  conical  processes, 
from  0.5  to  I  mm.  in  length,  and  from  0.2  to  0.5  mm.  in  breadth,  covering 
the  surface  of  the  mucous  membrane  from  the  pyloric  orifice  to  the  upper 
surface  of  the  ileo-cecal  valve.  Each  villus  consists  of  a  basement  mem- 
brane (see  Fig.  88)  supporting  tall  columnar  epithelial  cells.     Each  cell  is 


ABSORPTION 


219 


^-fifflUlHv 


composed  of  granular  bioplasm  containing  a  distinct  nucleus.  At  its  free 
extremity  a  narrow  border  of  the  cell  presents  a  striated  appearance,  as  if  it 
were  composed  of  small  rods  embedded  in  some  cement  substance.  Goblet 
or  mucin-holding  cells  are  also  to  be  found  among  the  columnar  cells.  The 
body  of  the  villus,  that  portion  within  the  basement  membrane,  consists  of  a 
reticulated  connective  tissue  supporting  arteries,  capillaries,  veins,  and 
lymphoid  corpuscles.  In  the  center  of  the  villus  there  is  usually  a  single, 
though  at  times  a  double,  club-shaped  lymph-capillary,  the  walls  of  which 
are  composed  of  endothelial  cells  with  sinuous  margins.  This  capillary 
probably  begins  by  a  blind  extremity  and  opens  at  the  base  of  the  villus  into 
the  subjacent  lymph-vessels.  The 
communicating  orifice  is  guarded  by 
a  valve.  It  is  also  surrounded  by  a 
layer  of  non-striated  muscle-fibers, 
arranged  longitudinally,  derived  from 
the  muscularis  mucosae  and  attached 
to  the  apex  of  the  body  of  the  villus. 

The  arteries  which  penetrate  the 
villi  are  derived  from  those  of  the  sub- 
mucous coat  of  the  intestine,  which  are 
the  ultimate  branches  of  the  intestinal 
artery,  and  serve  the  purpose  of  de- 
livering nutritive  material  to  the  capil- 
lary plexus.  While  passing  through 
the  latter  a  portion  of  the  blood- 
plasma  transudes  through  the  capil- 
lary walls  into  the  spaces  of  the  retic- 
ulated tissue,  constituting  lymph. 
At  the  same  time  products  of  tissue 
metabolism  pass  through  the  capillary 
walls  into  the  blood.  The  blood  then 
passes  into  the  venules,  which,  leav- 
ing the  villus  at  its  base,  unite  with 
the  veins  of  the  submucous  coat  to 
form  the  intestinal  veins.  These  fi- 
nally unite  with  the  gastric  and  splenic 
veins  to  form  the  portal  vein,  which  enters  the  liver  at  the  transverse  fissure 
(Fig.  89).  The  excess  of  lymph  within  the  villus  passes  into  the  club- 
shaped  lymph-capillary,  to  be  finally  carried  by  the  lymph-vessels  of  the 
mesentery  into  the  thoracic  duct.  During  the  intervals  of  digestion  and  in 
the  absence  of  food  from  the  intestine  there  is,  of  course,  no  absorption  of 
food  nor  the  removal  from  the  villus  of  anything  but  the  excess  of  lymph 
and  metabolic  products. 

Function  of  the  Villi. — The  villi,  and  especially  the  epithelial  cells 
covering  them,  are  the  essential  agents  in  the  absorption  of  the  products 
of  digestion.  It  is  by  the  activity  of  these  cells  that  the  new  materials  are 
taken  out  of  the  alimentary  canal  and  transferred  into  the  lymph-spaces  in 
the  interior  of  the  villi,  from  which  they  are  subsequently  removed  by  the 
blood-vessels  and  lymph-vessels.  As  to  the  mechanism  by  which  the  epi- 
thelial cells  accomplish  this  result,  nothing  definite  can  be  asserted.     In- 


FiG.  89.  — Diagram  of  the  Portal 
Vein  {pv)  arising  in  the  Alimentary 
Tract  and  Spleen  (5),  and  Carrying  the 
Blood  from  These  Organs  to  the  Liver. 
— {Yeo's  "Text-hook   0}   Physiology.'') 


220  TEXT-BOOK  OF  PHYSIOLOGY 

asmuch  as  the  absorption  of  food  does  not  take  place  in  accordance  with 
the  laws  of  osmosis  as  at  present  understood,  it  has  been  suggested  that 
the  cells  possess  a  "selective  action"  dependent  on  their  organization  and 
physiologic  activity,  an  activity  which  is  to  a  great  extent  conditioned  and 
limited  by  the  degree  of  diffusibility  of  the  substances  to  be  absorbed. 

Absorption  of  Water  and  Inorganic  Salts. — Water  and  inorganic  salts 
after  their  absorption  from  the  intestine  and  transference  into  the  lymph- 
spaces  of  the  villi  pass  through  the  walls  of  the  capillary  blood-vessels  and 
are  carried,  by  the  blood  of  the  portal  vein,  into  and  through  the  liver  into 
the  blood  of  the  general  circulation.  Unless  water  be  present  in  excessive 
amounts,  there  is  no  appreciable  absorption  of  water  by  the  lymph-vessels. 

Absorption  of  Sugar. — As  previously  stated,  all  the  carbohydrates,  with 
the  exception  possibly  of  lactose,  are  transformed  by  the  digestive  fluids  into 
either  dextrose  or  levulose,  under  which  forms  they  are  absorbed  by  the 
epithelial  cells.  It  is  possible,  however  that  soluble  dextrin  may  also  be 
absorbed.  Whatever  the  form  under  which  the  carbohydrates  are  absorbed, 
they  never  leave  the  epithelial  cells  except  as  dextrose  and  levulose.  Direct 
experimentation  has  shown  that  the  sugars  are  taken  up  by  the  capillary 
blood-vessels  and  carried  direct  to  the  liver.  Analysis  of  the  blood  of  the 
portal  vein  after  the  ingestion  of  large  quantities  of  sugar  may  reveal  an 
increase  to  0.25  per  cent.;  while  after  the  injection  of  sugar  into  the  intestine 
the  percentage  may  rise  as  high  as  0.4  per  cent.  As  chemic  analysis  of 
lymph  obtained  from  the  thoracic  duct  shows  no  increase  in  the  percentage 
of  sugar  beyond  that  normally  present  (o.i  per  cent.),  it  is  assumed  that 
sugar  is  not  removed  from  the  villi  by  the  lymph-vessels. 

On  reaching  the  liver  a  portion  of  the  sugar  12  to  20  per  cent  passes  from 
the  blood  stream  across  the  walls  of  the  capillaries  into  surrounding  lymph 
spaces  and  comes  into  direct  relation  with  the  liver  cells.  Then  through  the 
agency  of  an  enzyme,  the  sugar  is  dehydrated,  converted  into  starch  and 
stored  for  a  variable  length  of  time  in  the  liver  cells  in  the  form  of  hyaline 
masses  which  can  be  readily  seen  with  the  aid  of  the  microscope.  Under 
this  form  the  carbohydrate  material  is  retained  until  the  necessity  arises 
for  its  return  to  the  blood,  and  this  happens,  when  the  percentage  of  sugar  in 
the  blood  falls  below  the  normal,  viz.:  o.io  to  0.15  per  cent.  Under  such 
circumstances  the  necessary  amount  of  the  liver  starch  is  hydrated,  con- 
verted into  sugar,  and  passed  into  the  blood  in  quantities  sufficient  to  restore 
the  normal  percentage.  The  apparent  necessity  for  this  temporary  storage 
of  sugar  in  the  liver  is  to  prevent  its  too  rapid  entrance  into  the  arterial 
blood  and  hence  a  rise  in  the  percentage  far  beyond  that  which  is  normal. 
Should  this  occur  a  condition  known  as  hyperglycemia  would  result  and 
as  a  consequence  an  elimination  of  the  excess  by  the  kidneys  giving  rise  to 
the  condition  known  as  glycosuria. 

Absorption  of  the  Products  0}  Protein  Digestion. — For  the  reason 
that  the  proteins  are  for  the  most  part  transformed  through  hydration  and 
cleavage  by  the  action  of  the  gastric  and  pancreatic  enzymes  into  peptones 
and  for  the  further  reason  that  the  peptones  are  diffusible  bodies,  it  was 
formerly  believed  that  they  represented  the  final  stages  in  the  digestion  of 
the  proteins,  and  as  such  were  absorbed  out  of  the  intestinal  contents  by 
the  action  of  the  epithelium  covering  the  villi.  Inasmuch  as  chemic  analysis 
failed  to  detect  more  than  a  trace  of  either  peptone  or  native  protein  in  the 


ABSORPTION  221 

portal  blood  or  in  the  lymph  of  the  thoracic  duct,  it  was  assumed  that  the 
epithelium  after  absorbing,  also  synthesized  them  into  some  form  of  coagu- 
lable  protein  (plasma-albumin)  which  is  readily  assimilable  by  the  blood. 
The  plasma-albumin  thus  supposed  to  be  formed  by  the  epithelial  cells  was 
believed  to  pass  into  the  interior  of  the  villus,  thence  into  the  blood  by 
which  it  was  carried  to  the  liver.  After  passing  through  this  organ  it 
entered  the  blood  of  the  general  circulation  and  became  a  part  of  the  com- 
mon store  of  protein,  out  of  which  each  tissue  cell  constructed  the  particular 
protein  characteristic  of  it.  That  such  a  conversion  of  peptones  to  plasma- 
albumin  would  be  necessary  in  case  of  their  absorption  would  appear 
from  the  fact  that  the  introduction  of  peptones  even  in  small  amounts  into 
the  blood  is  followed  by  their  elimination  unchanged  in  the  urine.  When 
injected  into  the  blood  in  large  amounts,  they  act  as  toxic  agents,  giving 
rise  to  a  fall  of  blood-pressure,  a  diminished  coagulability  of  the  blood, 
coma,  and  death. 

This  general  view  as  to  the  absorption  of  peptones  is  no  longer  enter- 
tained. There  are  reasons,  however,  for  believing  that  the  change  which 
proteins  undergo  in  digestion,  is  more  far-reaching  and  complete,  and  that 
the  peptones  in  turn  are  disintegrated  and  reduced  to  even  simpler  bodies 
such  as  polypeptids,  peptids  and  amino-acids  (see  page  191).  The  extent 
to  which  this  disintegration  proceeds  will  doubtless  depend,  to  some  extent, 
on  the  quantity  and  variety  of  proteins  consumed. 

Since  recent  investigations  make  it  highly  probable  that  the  final  stage 
in  the  digestion  of  protein  is  the  formation  of  amino-acids  it  has  come  to 
be  believed  that  the  problem  of  absorption  is  to  be  transferred  to  these 
fragmentary  bodies  rather  than  to  the  peptones.  Assuming  this  to  be  true, 
the  question  at  once  arises  as  to  what  change,  if  any,  they  undergo  during 
their  passage  through  the  intestinal  epithelium.  The  difficulty  of  detect- 
ing the  presence  of  amino-acids  in  the  blood  led  to  the  assumption  that  after 
their  absorption  they  were  synthesized,  just  as  the  peptone  molecules  were 
supposed  to  be,  and  a  protein  molecule  constructed  similar  to,  if  not  identical, 
with  plasma-albumin. 

The  recent  investigation  of  Abel,  carried  out  with  a  new  form  of 
diffusion  apparatus,  renders  it  highly  probable  that  amino-acids  are 
present  in  the  circulating  blood  in  readily  determinable  quantities.  This 
has  led  to  the  belief  that  the  amino-acids,  in  part,  at  least,  are  absorbed  as 
such,  pass  through  the  intestinal  epithelium,  the  portal  blood  and  the  liver 
unchanged,  and  enter  the  blood  of  the  general  circulation  to  be  carried 
by  it  directly  to  the  tissues  where  they  are  at  once  utilized  by  the  tissue 
cells  or  stored  for  future  use.  This  view  renders  it  much  easier  to  under- 
stand, how  out  of  the  different  proteins  of  the  foods,  varying  widely  in  their 
composition,  the  specific  proteins  of  the  tissues  are  constructed.  It  is  only 
necessary  to  assume  that  the  tissue  cells  select  from  the  variety  of  amino- 
acids  presented  to  them,  only  those  which  are  necessary  to  the  formation 
of  the  protein  by  which  they  are  characterized.  The  plasma-albumin, 
whatever  its  origin,  might  then  be  regarded  as  a  protein  surplus  to  be 
called  on  if  the  protein  ingested  should  be  insufficient.  It  may  be,  however, 
that  future  investigations  will  ascribe  other  functions  to  it. 

Under  whatever  form  the  protein  material  is  absorbed  there  is  every  reason 
to  believe  that  it  is  carried  by  the  portal  circulation  to  the  liver,  through 


222  TEXT-BOOK  OF  PHYSIOLOGY 

which  it  passes  to  enter  the  blood  of  the  general  circulation.  Ligation  of 
the  thoracic  duct  does  not  interfere  to  any  appreciable  extent  with  protein 
absorption  nor  with  the  normal  production  and  elimination  of  urea,  nor  with 
the  weight  of  the  animal. 

Many  facts  in  the  physiologic  chemistry  of  the  body  raise  the  question 
as  to  what  percentage  of  the  amino-acids  produced  in  the  intestine  daily  is 
utilized  for  tissue  repair  and  growth.  If  the  protein  requirements  of  Chit- 
tenden, viz.:  58  to  60  grams  only,  are  necessary  for  repair  and  growth, 
then  api)roximately  one-half  the  amino-acids  produced  from  the  protein 
usually  consumed  must  be  disposed  of  in  some  other  manner.  The  manner 
of  disposal  of  these  unused  (that  is,  unused  for  tissue  repair  and  growth) 
fragments  of  protein  disintegration  is  doubtless  varied;  some  of  the  amino- 
acids,  after  absorption  by  the  epithelial  cells  of  the  villi  and  mucosa,  are 
deprived  of  NH2  (the  amino-acid  nitrogen)  or  deaminized;  the  NH2  is  then 
converted  into  ammonia,  combined  with  carbon  dioxid  to  form  ammonium 
carbonate,  carried  to  the  liver,  and  changed  into  urea.  That  this  is  very 
probably  the  case  is  rendered  likely  from  the  presence  of  a  large  quantity  of 
ammonia  in  the  mucous  membrane  of  the  intestine  and  in  the  blood  of  the 
portal  vein,  in  which  after  a  meal  rich  in  protein  it  may  be  four  times  as 
great  as  in  the  arterial  blood.  The  remainder  of  the  amino-acid  molecule  is 
changed  into  some  carbonaceous  radical  and  finally  into  sugar  or  fat  and 
subsequently  utilized  by  the  organism  for  heat  production.  The  dynamic 
portion  of  the  amino-acid  is  this  deaminized  remainder.  Other  of  the  surplus 
amino-acids  are  acted  on  by  intestinal  bacteria,  and  converted  into 
simpler  compounds,  after  which  they  are  eliminated  in  the  feces  or  absorbed 
and  carried  to  the  liver  where  they  undergo  other  changes  and  eventually 
appear  in  the  urine. 

The  ammonia  set  free  during  digestion  is  absorbed,  carried  to  the  liver 
and  transformed  to  urea. 

Absorption  of  Fat. — As  previously  stated,  there  are  two  views  as  to  the 
changes  which  fats  undergo  during  digestion.  According  as  the  one  or  the 
other  is  accepted  will  depend  the  view  as  to  the  nature  of  the  absorptive 
process.  If  it  be  assumed  that  the  final  stage  in  the  digestion  of  fat  is  a 
purely  physical  one,  the  production  of  an  emulsion  in  which  the  fats  present 
themselves  as  fine  granules,  it  is  difficult  to  give  any  satisfactory  explanation 
of  the  mechanism  by  which  the  epithelial  cells  take  them  up.  Various 
theories  have  been  advanced  to  explain  the  process,  but  none  are  free  from 
serious  objections.  This  view  of  fat  absorption  has  largely  been  based  on 
the  observation  that  during  digestion  fatty  granules  can  be  seen  in  all  por- 
tions of  the  cell  apparently  passing  toward  the  interior  of  the  villus. 

If,  on  the  contrary,  it  be  admitted  that  the  final  stage  in  the  digestion  of 
fats  is  the  formation  of  soaps  and  glycerin,  both  of  which  are  soluble,  their 
absorption  can  more  readily  be  accounted  for.  According  to  this  view, 
the  soaps  and  glycerin  are  again  synthesized  by  a  process  the  reverse  of  that 
which  is  brought  about  by  the  pancreatic  enzyme,  with  the  appear- 
ance of  minute  granules  of  fat.  That  this  is  the  more  probable  view  as  to 
the  mechanism  of  fat  absorption  is  evident  from  the  fact  that  when  animals 
are  fed  with  alkaline  soaps  and  glycerin,  or  with  fatty  acids  alone,  globules 
of  fat  are  found  in  the  epithelial  cells  and  in  the  interior  of  the  villus. 

With  the  passage  of  the  fat-granules  into  the  interior  of  the  villus  they 


ABSORPTION  223 

at  once  enter  the  lymph-radicle  and  become  constituents  of  the  lymph- 
stream,  to  which  they  impart  a  white,  milky  appearance.  If  the  abdomen 
of  an  animal  in  full  digestion  be  opened,  the  lymph-vessels  of  the  mesentery 
present  themselves  as  distinct  white  threads.  An  examination  of  the  fluid 
they  contain,  known  as  chyle,  shows  the  presence  of  fat-granules  of  micro- 
scopic size.  With  the  passage  of  the  chyle  into  the  thoracic  duct  it  also 
assumes  the  same  milky  appearance.  For  this  reason  the  lymphatics  of 
the  mesentery  were  erroneously  termed  lacteals.  The  chyle  as  obtained 
from  these  lymph-vessels  possesses  the  same  qualitative  though  not  quanti- 
tative composition  as  lymph,  the  difference  being  mainly  in  the  large  excess 
of 'fat  in  the  former.  Indeed,  chyle  may  be  regarded  as  lymph  with  the 
addition  of  fat. 

With  the  absorption  of  the  last  of  the  fat-granules  the  contents  of  the 
lymph-vessels  of  the  mesentery — the  lacteals — begin  to  lose  their  white 
appearance  near  the  intestine  and  as  the  fat-granules  pass  toward  the 
thoracic  duct,  the  lymph-vessels  gradually  become  transparent  and  disappear 
from  view.  A  similar  change  in  the  appearance  of  the  thoracic  duct  ensues, 
as  the  last  portions  of  the  fat  ascend  the  duct  to  be  discharged  into  the  blood- 
stream. 

Routes  for  the  Absorbed  Food.- — ^Physiologic  experiments  have  dem- 
onstrated that  the  agents  concerned  in  the  removal  of  the  products  of 
digestion  after  their  absorption  from  the  interior  of  the  villus  are: 

1.  The  veins  of  the  gastro-intestinal  tract,  which  converge  to  form  the 

portal  vein. 

2.  The  lymph-vessels  of  the  small  intestine,  which  converge  to  empty  into 

the  thoracic  duct. 

The  products  of  digestion  find  their  way  into  the  general  circulation  by 
these  two  routes,  as  follows:     (See  Fig.  90.) 

The  water,  inorganic  salts,  proteins  or  amino-acids,  and  sugar  after 
entering  the  blood-vessels  of  the  villus  are  carried  by  the  blood  of  the  in- 
testinal veins  directly  into  the  liver  by  the  portal  vein;  after  circulating 
through  the  capillaries  of  the  liver  and  being  influenced  by  the  liver  cells, 
they  are  discharged  by  the  hepatic  veins  into  the  inferior  or  ascending  vena 
cava. 

The  fat-granules,  synthesized  in  the  epithelial  cells,  after  entering  the 
lymph-radicle  of  the  villus  are  carried  by  the  lymph-stream  of  the  intestinal 
lymph-vessels  and  emptied  into  the  receptaculum  chyli  from  which  they 
ascend  into  the  thoracic  duct,  by  which  they  are  discharged  into  the 
blood  at  the  junction  of  the  left  subclavian  and  internal  jugular  veins. 

Forces  Aiding  the  Movement  of  Lymph  and  Chyle. — The  force 
which  primarily  determines  the  movement  of  the  lymph  has  its  origin  in  the 
beginnings  of  the  lymph- vessels,  the  lymph-spaces,  and  depends  on  a  dif- 
ference in  pressure  here  and  at  the  termination  of  the  thoracic  duct.  The 
rise  of  pressure  in  the  lymph-spaces  is  due  to  the  continual  production  of 
lymph,  either  by  filtration  or  secretor  activity  of  the  capillary  walls.  As 
soon  as  the  pressure  rises  above  that  in  the  thoracic  duct  a  forward  move- 
ment of  lymph  takes  place.  Other  things  being  equal,  the  rate  of  move- 
ment will  be  proportional  to  the  difference  of  pressure.  The  first  movement 
of  the  chyle,  its  passage  from  the  lymph-capillary  in  the  villus  into  the  sub- 
jacent lymph- vessel,  has  been  attributed  to  a  shortening  of  the  villus  and  a 


224 


TEXT-BOOK  OF  PHYSIOLOGY 


Fig.  90. — Diagram  Showing  the  Routes  by  which  the  Absorbed  Foods  Reach 
THE  Blood  of  the  General  Circulation  (G.  Bachman).  I.  i.,  Loop  of  small  intestine; 
int.,  v.,  intestinal  veins  converging  to  form  in  part,  p.  u.,  the  portal  vein,  which  enters  the 
liver  and  by  repeated  branchings  assists  in  the  formation  of  the  hepatic  capillary  plexus; 
h.  u.  the  hepatic  veins  carrying  blood  from  the  liver  and  discharging  it  into,  «//.  v.  c,  the 
inferior  vena  cava;  ■int.  I.  v.,  the  intestinal  lymph  vessels  converging  to  discharge  their 
contents,  chyle,  into  rec.  c.  the  receptaculum  chyli,  the  lower  expanded  part  of  the  thoracic 
duct;  th.  d.,  the  thoracic  duct  discharging  lymph  and  chyle  into  the  blood  at  the  junction 
of  the  internal  jugular  and  subclavian  veins;  sup.  v.  c,  the  superior  vena  cava. 


ABSORPTION  225 

compression  of  the  capillary  by  the  contraction  of  the  non-striated  muscle- 
fibers  by  which  it  is  surrounded.  With  the  entrance  of  the  chyle  into  the 
subjacent  lymph-vessel  there  is  a  distention  of  the  vessel  and  a  rise  in  pres- 
sure. When  the  muscle-fibers  relax,  regurgitation  is  prevented  by  the 
closure  of  the  valves  at  the  base  of  the  villus.  The  elastic  tissue  of  the 
lymph-vessel  now  recoils  and  forces  the  chyle  toward  the  thoracic  duct. 
After  the  emptying  of  the  lymph-capillary  the  conditions  as  far  as  pressure 
is  concerned  are  favorable  for  the  absorption  of  new  material.  The  rhythmic 
contractions  of  the  intestinal  wall  undoubtedly  aid  in  the  movement  of  lymph 
and  chyle.  It  is  quite  possible  that  the  walls  of  the  general  lymphatic  system 
aid  the  forward  movement  of  lymph  by  more  or  less  rhythmic  contractions 
of  their  contained  muscle-fibers. 

Inasmuch  as  the  lymph-vessels  lie  in  situations  in  which  they  are  sub- 
ject to  compression  by  muscles  during  contraction,  it  is  probable  that  the 
fluid  in  the  vessels  will  be  forced  onward  toward  the  thoracic  duct  at  each 
compression,  a  backward  movement  being  prevented  by  the  closure  of  the 
valves  which  are  everywhere  present  in  the  vessels.  Experimental  observa- 
tions have  demonstrated  the  truth  of  this  supposition.  Alternate  contraction 
and  relaxation  of  the  muscles  of  the  leg  will.  In  an  animal  at  least,  increase 
considerably  the  flow  as  well  as  the  production  of  lymph  from  the  thoracic 
duct.     Massage  has  a  similar  influence. 

The  respiratory  movements  also  aid  the  flow  of  both  lymph  and  chyle 
from  the  thoracic  duct  and  larger  lymph-vessels  into  the  venous  blood. 
During  inspiration  the  intrathoracic  pressure  (that  is,  the  positive  pressure 
exerted  by  the  air  in  the  lungs  on  the  intrathoracic  viscera,  e.g.,  heart,  veins, 
thoracic  duct,  etc.,  which  is  less  by  about  6  millimeters  of  mercury  than 
the  pressure  in  the  lungs)  decreases.  The  decrease  is  proportional  to  the 
extent  of  the  inspiration.  With  this  decrease  of  pressure,  the  thoracic  duct 
expands  and  its  internal  pressure  falls.  As  the  intra-abdominal  portion 
of  the  thoracic  duct  and  its  tributaries  are  subjected  to  a  higher  pressure, 
practically  that  of  the  atmosphere  the  lymph  in  these  vessels  is  forced,  by 
reason  of  the  difference  in  pressure  between  these  two  regions,  into  the 
intrathoracic  portion  of  the  duct.  During  expiration,  the  rise  of  the  in- 
trathoracic pressure  to  its  former  value  leads  to  a  compression  of  the  thoracic 
duct  and  causes  the  lymph  to  be  discharged  rapidly  into  the  blood-stream. 
A  regurgitation  of  the  lymph  is  prevented  by  the  closure  of  the  numerous 
valves  throughout  the  course  of  the  duct. 

DIFFUSION.     OSMOSIS.     FILTRATION 

As  these  three  factors  are  believed  to  play  an  important  part  in  many  physiologic 
processes,  it  is  essential  to  a  better  understanding  of  these  processes,  that  certain 
elementary  facts  relating  to  these  three  factors  be  known. 

Diffusion. — By  diffusion  is  meant  the  gradual  and  spontaneous  mixture  of 
the  molecules  of  two  or  more  liquids,  or  of  two  or  more  gases,  when  brought 
into  contact  with  each  other,  without  the  application  of  an  external  force.  The 
reason  for  both  processes  lies  in  the  fact  that  the  molecules  of  a  liquid  and  of  a 
gas  are  in  constant  motion,  in  consequence  of  which  a  mutual  interpenetration  of 
the  molecules  takes  place,  which  continues  until  a  condition  of  homogeneity  is 
established. 

Again,  when  a  soluble  substance,  inorganic  or  organic,  is  placed  in  water, 
IS 


226  TEXT-BOOK  OF  PHYSIOLOGY 

the  molecules  of  the  substance  will  at  once  begin  to  separate  themselves  and 
to  diffuse  throughout  the  water  until  the  solution  becomes  homogeneous,  and 
notwithstanding  the  fact  that  the  dissolved  substance  possesses  weight,  the 
solution  remains  homogeneous.  The  force  of  gravity  is  overcome  by  the  force 
of  diffusion. 

The  velocity  with  which  the  molecules  of  a  substance  will  diffuse  through 
a  solvent  like  water,  varies  considerably.  The  experiments  of  Graham  show 
that  if  the  molecules  of  a  given  weight  of  hydrochloric  acid  diffuse  completely 
in  a  unit  of  time,  the  molecules  in  the  same  weight  of  sodium  chlorid,  cane-sugar, 
albumin  and  caramel,  will  require  for  their  diffusion  2.33,  7,  48,  and  98  units  of 
time  respectively. 

Osmosis. — Osmosis  may  be  defined  as  the  passage  of  the  molecules  of  water 
through  an  intervening  membrane.  If  the  water  on  one  side  of  the  membrane, 
parchment  for  example,  contains  in  solution  substances  such  inorganic  salts, 
their  molecules  will  also  pass  through  the  membrane  though  the  time  required 
for  this  to  take  place  may  be  much  longer  than  in  the  case  of  the  water  molecules. 
The  passage  of  the  dissolved  substance  through  the  membrane  though  usually 
included  under  the  term  osmosis  is  more  properly  termed  dialysis. 

If  the  two  volumes  of  water  on  opposite  sides  of  the  membrane  are  the  same 
in  amount,  and  if  the  one  volume  contains  a  salt  in  solution,  the  salt  molecules 
will  continue  to  pass  through  the  membrane  until  the  water  on  both  sides  contains 
the  same  number  of  molecules,  or,  in  other  words,  until  it  is  homogeneous  in  com- 
position. The  time  required  for  their  passage  being  longer  than  the  time  required 
for  the  passage  of  the  water  molecules,  there  will  be  (owing  to  factors  which  will 
be  explained  later),  a  temporary  increase  in  the  volume  of  the  water  originally  con- 
taining the  salt,  but  in  time  the  two  volumes  will  again  become  equal.  Certain 
other  substances  which  may  be  in  solution,  such  as  albumin,  starch,  etc.,  will  not 
pass  across  a  membrane,  because  of  the  large  size  of  their  molecules.  Graham 
termed  all  those  substances  which  by  virtue  of  the  small  size  of  their  molecules 
pass  through  membranes,  crystalloids,  and  all  those  which  by  virtue  of  the  large  size 
of  their  molecules  do  not  pass  through  membranes  or  to  a  very  slight  extent,  colloids. 

It  was  stated  in  the  foregoing  paragraph  that  if  two  equal  volumes  of  water 
are  separated  by  a  parchment  septum,  one  of  which  contains  in  solution  an 
inorganic  salt,  the  molecules  of  the  salt-free  water  will  osmose  through  the  septum 
into  salt- containing  water,  more  rapidly  than  they  will  in  the  opposite  direction, 
and  as  a  result,  there  will  be  a  temporary  increase  in  the  volume  of  the  water 
containing  the  salt.  If  the  membrane  were  impermeable  to  the  salt  molecules, 
the  difference  in  the  two  volumes  of  the  water  would  be  far  more  permanent  and 
striking.  The  reason  assigned  for  this  is  that  the  molecules  of  the  salt  exert  a 
pressure  against  the  outer  layer  of  the  water  molecules  and  these  in  turn  against 
the  membrane,  in  consequence  of  which  there  is  a  more  rapid  osmosis  of  the 
water  molecules  towards  the  salt  than  in  the  reverse  direction.  To  this  pressure 
is  applied  the  term 

Osmotic  Pressure. — Osmotic  pressure  may  be  defined  as  the  pressure  exerted 
by  the  molecules  of  the  substance  in  solution  against  the  outer  layer  of  the  mole- 
cules of  the  solvent.  If  the  solvent  is  enclosed  by  an  elastic  membrane  it  is 
expanded  and  in  consequence  there  is  an  osmosis  of  a  surrounding  solvent  towards 
and  through  it.  The  reason  for  this  pressure  lies  in  the  fact  that,  when  the  mole- 
cules of  a  substance  are  separated  a  certain  distance,  as  they  are  when  in  solution, 
they  repel  one  another  as  do  the  molecules  of  a  gas  and  in  their  flight  strike 
against  the  outer  layer  of  the  solvent.  The  pressure  of  the  molecules  of  a  substance 
in  solution  is  therefore  comparable  to  the  pressure  of  the  molecules  of  a  gas. 

Three  methods  may  be  employed  for  measuring  the  force  of  the  osmotic 
pressure  of  different  substances,  viz.:  i.  Physical.  2.  The  determination  of  the 
freezing  point.     3.  By  calculation. 


ABSORPTION 


227 


I.  Physical  Method. — ^For  the  purpose  of  measuring  osmotic  pressure  by 
physical  methods,  it  is  customary  to  make  use  of  an  apparatus  similar  to  that 
represented  in  Fig.  91,  which  consists  of  an  earthenware  vessel  (a),  into  the 
upper  open  end  of  which  a  tall  vertical  glass  tube  has  been  hermetically  sealed. 
The  pores  of  the  earthenware  vessel  have  been  filled  by  a  membrane  made  by 
precipitating  ferrocyanid  of  copper  within  them.  This  membrane  is  freely 
permeable  to  water,  but  impermeable  to  certain  substances  in  solution,  e.g., 
cane-sugar.  Such  a  membrane,  which  permits  the  passage  of  the  moleculesj;_of 
the  solvent  but  not  the  molecules  of  the  dissolved  substance,  is  termed  a  semi- 
permeable membrane,  and -its  use  is  absolutely  necessi- 
tated when  it  is  desired  to  obtain  the  actual  pressure 
exerted  by  any  given  substance  in  solution.  An  ap- 
paratus of  this  character  is  termed  an  osmometer. 

When,  therefore,  the  osmometer  containing  a  solution 
of  cane-sugar  is  placed  in  the  vessel  (b)  containing  water, 
the  following  phenomena  occur,  viz.:  an  ascent  of  the 
cane-sugar  solution  in  the  vertical  glass  tube,  and  a  de- 
scent of  the  level  of  the  water  in  the  vessel  b.  These 
phenomena  continue  until  the  level  of  the  fluid  in  the 
glass  tube  reaches  a  certain  height,  when  it  becomes  sta- 
tionary, and  no  further  e£fect  takes  place. 

In  explanation  of  the  foregoing  phenomena  it  may  be 
said  that  the  molecules  of  the  sugar  strike  or  press  against 
the  outer  layer  of  the  molecules  of  the  solvent,  which  at 
all  points  are  in  contact  with  the  rigid  walls  of  the  earthen- 
ware vessel,  except  at  the  open  extremity  of  the  vertical 
glass  tube.  Inasmuch  as  the  rigid  walls  of  the  osmometer 
prevent  any  outward  displacement  of  the  molecules  of  the 
water,  the  force  of  the  impact  of  the  sugar  molecule  is 
directed  against  the  molecules  at  the  extremity  of  the 
vertical  tube  which  are  in  consequence  pressed  or 
pushed  upward  a  certain  distance.  Because  of  the  loss 
of  energy  due  to  the  impact,  the  sugar  molecule  does  not 
rebound  with  the  same  velocity,  and  hence  time  is  per- 
mitted for  the  molecules  of  the  water  to  pass  into  the 
sugar  solution,  to  occupy  the  space,  and  thus  maintain 
the  level  of  the  fluid  in  the  vertical  tube.  (For  the  reason 
that  the  osmometer  is  permeable  to  water,  the  molecules 
will  pass  outward  as  well  as  inward  though  more  will 
pass  in  a  unit  of  time  in  the  latter,  than  in  the  former 
direction,  until  equilibrium  is  established.)  The  pressure 
of  the  sugar  molecules  continuing,  the  level  of  the  fluid  in 
the  glass  tube  continues  to  rise  and  the  level  of  the  fluid  in 
the  vessel,  b,  continues  to  fall  until  the  force  of  gravity 
prevents  any  further  upward  movement  of  the  molecules  of  sugar  against  the 
outer  film  of  the  molecules  of  the  water.  The  difference  in  the  level  of  the  two 
fluids  expressed  in  millimeters  of  mercury  is  taken  as  a  measure  of,  and  equal 
to,  the  pressure  of  the  sugar  in  solution.  A  i  per  cent,  solution  of  cane-sugar 
at  a  temperature  of  from  i3°C.  to  i6°C.,  as  determined  by  this  method,  exerts 
an  osmotic  pressure  of  about  535  mm.  Hg.;  a  2  per  cent,  solution  exerts  an 
osmotic  pressure  approximately  twice  this  amount. 

Experiments  made  with  this  and  similar  osmometers  show — 

1.  That  the  osmotic  pressure  of  any  substance  in  solution  is  proportional  to  the 

concentration,  providing  the  temperature  is  constant. 

2.  That  when  the  concentration  is  constant  the  osmotic  pressure  rises  with,  and 

is  proportional  to,  the  temperature. 


Sotution 
Csne  Sugar 


-a 


b- 


h^^ter 


Fig.  91. — An  Osmometer. 


228  TEXT-BOOK  OF  PHYSIOLOGY 

3.  That  when  different  substances  are  present  in  the  same  solvent  the  osmotic 

pressure  is  equal  to  the  sum  of  the  individual  or  partial  pressures. 

4.  That  whatever  the  nature  of  the  substance  in  solution  it  will  exert  the  same 

osmotic    pressure,    providing    always   the   same  number  of  molecules  are 
present;   hence  the  molecular  weights  in  grams  per  liter  of  different  sub- 
stances exert  the  same  osmotic  pressure  at  the  same  temperature. 
Because  of  the  fact  that  when  certain  substances,  e.g.,  many  inorganic  salts, 
many  acids  and  bases,  are  dissolved,  some  of  their  molecules  undergo  ionization, 
i.e.,  separation  into  parts  which  are  charged  with  electricity,  and  hence  the  two 
together,  molecules  and  ions,  exert  a  greater  osmotic  pressure  than  would  other- 
wise be  the  case;  and  because  of  the  further  fact,  that  it  is  extremely  difficult  to 
obtain  absolutely  semipermeable  membranes,  uniform  results  are  not  obtained  by 
the  employment  of  the  three  methods;  therefore,  the  osmometric  methods  as  well 
as   the   calculation   or  arithmetic  method  have  been  largely  discarded  and  the 
method  based  on  the  determination  of  the  freezing  point  has  been  adopted. 

2  The  Determination  of  the  Freezing  Point. — Because  of  the  diflEculty  in  obtain- 
ing the  exact  osmotic  pressure  by  means  of  the  osmometer  as  stated  above,  reliance 
is  now  placed  on  the  mathematic  relation  known  to  exist  between  osmotic  pressure 
and  the  freezing  point.  Thus  the  freezing  point  of  water  holding  any  substance 
in  solution  is  lower  than  water  itself  and  is  indeed  proportional  to  the  number 
of  molecules  dissolved.  As  a  standard  of  comparison  it  is  customary  to  employ 
a  gram-molecule  of  a  substance  dissolved  in  one  liter  of  water.  (A  gram-molecule 
is  the  quantity  of  a  substance  expressed  in  grams  equal  to  its  molecular  weight.) 
The  lowering  of  the  freezing  point  of  a  gram-molecule  solution  below  that  of  water 
is  constant,  viz.,  i.Sy^C.  The  osmotic  pressure  therefore  of  such  a  solution,  as 
determined  by  calculation  (see  below),  is  equal  to  22.38  atmospheres,  or  17,008 
mm.  of  Hg. 

Therefore  it  is  only  necessary  to  determine  by  means  of  a  differential  thermome- 
ter the  lowering  of  the  freezing  point  in  degrees  centigrade,  which  is  usually  expres- 
sed by  the  symbol  A.  Then  the  osmotic  pressure  is  equal  to  A  divided  by  1.87° 
C,  and  multiplied  by  22.38  atmospheres,  or  17,008  mm.  of  Hg,  Thus  if  the 
freezing  point  of  any  solution  was  found  to  be  o.83°C.  lower  than  water,  its 
osmotic  pressure  would  be  0.83-^1.87X22.38  atmospheres  or  9.847  atmospheres 
=  7,483  mm.  Hg.  If  any  two  solutions  have  the  same  freezing  point  they  contain 
the  same  number  of  molecules  and  hence  have  the  same  osmotic  pressure.  Blood 
plasma  has  a  freezing  point  of  o.56°C.  Experimentally  it  has  been  determined 
that  the  freezing  point  of  water  is  lowered  to  the  same  level,  when  it  contains  so- 
dium chlorid  to  the  extent  of  0.95  per  cent.  Hence  these  two  fluids  have  the  same 
osmotic  pressure  and  are  isotonic;  each  exerts  a  pressure  of  6.696  atmospheres. 

For  this  reason  the  sodium  chlorid  solution  can  be  employed  for  preserving, 
for  a  time  at  least,  the  form  of  blood  corpuscles  or  other  living  mammalian  cells, 
from  which  it  may  be  inferred,  that  the  contents  of  the  cells  have  an  osmotic  pres- 
sure approximately  equal  to  that  of  the  blood  plasma  or  the  salt  solution.  If  the 
salt  solution  has  a  lower  concentration  and  hence  a  lower  osmotic  pressure,  water 
will  osmose  into  the  corpuscle  and  cause  a  discharge  of  its  hemoglobin  content. 
Such  a  fluid  is  said  to  be  hypo-isotonic.  If,  on  the  contrary,  the  salt  solution  has  a 
higher  concentration  and  hence  a  higher  osmotic  pressure,  water  will  osmose  from 
the  corpuscle  causing  a  shrinkage  and  crenation  of  the  corpuscle.  Such  a  fluid 
is  said  to  be  hyperisotonic. 

3.  By  Calculation. — The  osmotic  pressure  may  also  be  obtained  by  calculation 
based  on  the  known  pressure  exerted  by  a  gram-molecule  of  hydrogen — 2  grams — 
when  compressed  to  a  volume  of  one  liter.  It  is  well  known  that  i  gram  of  hydro- 
gen at  o°C.  and  at  an  atmospheric  pressure  of  760  mm.  Hg.  occupies  a  volume  of 
11.19  liters,  and  that  2  grams  under  the  same  conditions  will  occupy  a  volume  of 
22.38  liters,  and  that  when  the  two  grams,  that  is,  one  gram-molecule  is  com- 


ABSORPTION  229 

pressed  to  a  volume  of  i  liter  the  molecules  will  exert  a  pressure  equal  to  that  of 
22.38  liters  or  22.38  atmospheres  or  17,008  mm.  of  Hg.  Since  a  gram-molecule  of 
any  substance  dissolved  in  i  liter  of  water  contains  the  same  number  of  molecules 
as  a  gram-molecule  of  hydrogen  compressed  to  one  liter,  they  have  the  same 
osmotic  pressure. 

From  this  it  is  possible  to  calculate  the  osmotic  pressure  of  an  electrolyte,  if 
the  percentage  composition  of  the  substance  in  solution  be  known.  Let  it  be 
supposed,  for  example,  that  it  is  desirable  to  know  the  osmotic  pressure  of  a  i  per 
cent,  solution  of  cane-sugar.  The  procedure  is  as  follows:  A  gram-molecule  of 
cane-sugar  (CijHjjOii)  contains  342  grams;  a  i  per  cent,  solution  contains  10 
grams  to  the  liter;  hence  its  osmotic  pressure  is  104-342X22.38  atmospheres  or 
0.65  atmosphere  which  is  equal  to  494  mm.  of  Hg. 

Filtration. — Filtration  may  be  defined  as  the  passage  of  water,  and  of  all  sub- 
stances dissolved  in  it,  through  a  membrane  as  a  result  of  a  difference  of  hydro- 
static pressure  on  the  two  sides.  The  difference  between  the  two  pressures  con- 
stitutes the  force  of  filtration,  and  hence  the  greater  the  difference,  the  greater  will 
be  the  amount  of  fluid  filtered. 

With  any  given  artificially  prepared  animal  membrane  the  quantities  of  water 
and  crystalloids  in  general  which  pass  through  the  membrane  are  proportional 
to  the  filtration  force,  and  hence  the  filtrate  will  have  a  concentration  similar  to, 
if  not  identical  with,  that  of  the  original  solution.  The  passage  of  colloids  in  solu- 
tion will  be  proportional  to  the  permeability  of  the  membrane  and  an  increase  in 
the  filtration  force.  The  filtrate,  however,  will  have  a  lower  degree  of  concentra- 
tion than  the  original  solution  for  the  reason  that  as  the  pressure  rises  the  quantity 
of  water  filtered  increases  in  a  greater  ratio  than  the  quantity  of  colloid  filtered. 

Physiologic  Applications. — In  the  animal  body  the  fluids  are  separated  by 
delicate  membranes  through  which  the  constituents  of  the  fluids,  inorganic  and 
organic,  are  continually  passing.  Thus  prepared  foods  in  the  intestine  pass  through 
the  intestinal  wall  into  blood-  and  lymph-vessels;  the  constituents  of  the  blood 
pass  through  the  wall  of  the  capillary  vessel  into  the  tissue  spaces  from  which  they 
pass  (a)  through  the  walls  of  various  glands  to  take  part  in  the  formation  of  their 
secretion;  (b)  through  the  sarcolemma  into  the  interior  of  the  muscle  fiber;  (c)  through 
the  limiting  surface  of,  and  into  the  interior  of  all  other  tissue  cells.  The  waste 
products,  the  result  of  tissue  and  food  metabohsm,  pass  from  the  interior  of  cells 
through  their  limiting  membranes  or  surfaces  into  the  tissue  spaces;  thence  through 
the  wall  of  the  capillary  vessel  into  the  blood  and  finally  through  the  wall  of  the 
capillary  vessel  and  the  epithelium  of  the  lung,  the  kidney,  the  liver,  etc.,  to  take 
part  in  the  formation  of  the  excretions.  These  and  other  processes  are  believed 
to  be  accomplished  by  the  factors,  diffusion,  osmosis,  and  filtration. 

The  statements  that  have  been  made  in  foregoing  paragraphs  in  reference  to 
diffusion,  osmosis,  and  filtration  have  been  based  on  the  results  of  experiments 
which  have  been  made  with  non-living  membranes,  and  under  conditions  purely 
physical;  and  though  it  is  quite  true  that  in  the  animal  body  the  fluids  are  separated 
by  membranes  more  or  less  permeable  to  all  their  constituents,  and  that  all  pass 
through  these  membranes,  it  is  possible  that  the  facts  which  have  been  obtained 
experimentally  are  not  strictly  paralleled  in  the  living  body,  and  hence  not  strictly 
applicable  to  the  elucidation  of  physiologic  processes.  Nevertheless  there  are 
reasons  for  thinking  that  a  thorough  understanding  of  these  factors  will  eventually 
throw  much  light  on  the  intimate  nature  of  the  process  by  which  organic  as  well  as 
inorganic  substances  in  solution  pass  through  animal  membranes  in  the  living 
condition. 


CHAPTER  XII 
THE  BLOOD 

The  blood  may  be  defined  as  the  nutritive  fluid  of  the  body  since  it  con- 
tains all  those  materials  that  are  necessary  to  the  maintenance  of  the  nutri- 
tion. The  presence  and  proper  circulation  of  the  blood  in  the  living  or- 
ganism are  essential  for  the  maintenance  of  tissue  irritability  and  for  the 
manifestation  of  the  activities  of  all  physiologic  mechanisms.  The  escape 
of  the  blood  from  the  vessels,  especially  in  the  higher  animals,  is  followed  by 
cessation  of  the  physiologic  activities  of  all  the  tissues  within  a  short  period. 
The  irritability,  however,  persists  for  a  variable  length  of  time  though  it  too 
gradually  declines  and  finally  disappears.  The  immediate  dependence  of 
the  functional  activities  of  the  tissues  and  organs  on  the  presence  of  the 
blood  can  be  demonstrated  by  the  following  experiment:  If  the  nozzle  of 
a  syringe,  adapted  to  the  size  of  the  animal,  be  introduced  through  the 
jugular  vein  into  the  right  side  of  the  heart  and  the  blood  be  suddenly  with- 
drawn, there  is  an  immediate  cessation  in  the  activity  of  all  the  organs;  the 
return  of  the  blood  to  the  vessels  within  a  limited  period  of  time  is  promptly 
followed  by  a  renewal  of  their  activity. 

Though  contained  within  a  practically  closed  system  of  vessels,  the  blood 
is  brought  into  intimate  relation  with  all  the  tissue  elements  through  the 
intermediation  of  the  capillaries.  As  the  blood  flows  through  these  delicate 
vessels,  portions  of  its  soluble  nutritive  constituents,  including  oxygen,  are 
given  up  to  the  tissues,  by  which  they  are  utilized  for  growth,  repair,  and 
the  liberation  of  heat.  At  the  same  time  the  tissues  yield  up  to  the  blood 
a  series  of  decomposition  or  katabolic  products,  resulting  from  their  activity, 
which  vary  in  quantity  and  quality  according  as  the  blood  traverses  the 
muscles,  nerv^es,  glands,  or  other  tissues. 

The  blood  may  be  regarded,  therefore,  as  a  reservoir  of  nutritive  materials 
prepared  by  the  digestive  apparatus  and  absorbed  from  the  intestinal  canal; 
of  oxygen,  absorbed  from  the  respiratory  surface  of  the  lungs;  of  katabolic 
products,  produced  by  and  absorbed  from  the  tissues.  Though  the  blood 
varies  in  composition  in  different  parts  of  the  body  in  consequence  of  the 
introduction  of  both  nutritive  material  and  katabolic  products,  it  neverthe- 
less presents  certain  average  physical,  morphologic,  and  chemic  properties 
which  distinguish  it  as  an  individual  tissue. 

The  Physical  Constitution  of  Blood. — ^A  microscopic  examination  of 
the  blood  as  it  flows  through  the  capillary  vessels  of  the  web  of  the  frog  or 
the  mesentery  of  the  rabbit  shows  that  it  is  not  a  homogeneous  fluid,  but 
that  it  consists  of  two  distinct  portions,  viz.:  (i)  a  clear,  transparent,  slightly 
yellow  fluid,  the  plasma  or  liquor  sanguinis:  (2)  small  particles  termed 
corpuscles  floating  in  it,  of  which  there  are  two  varieties,  the  red  or  the 
erythrocytes  and  the  white  or  the  leukocytes.  By  appropriate  methods  it 
can  be  shown  that  a  third  corpuscle,  colorless  in  appearance  and  smaller 
in  size  than  the  ordinary  white  corpuscle,  is  present  in  the  blood-stream 

230 


THE  BLOOD  231 

and  known  as  the  blood-platelet  or  plaque.  The  different  constituents  can 
be  roughly  separated  by  appropriate  means  when  the  blood  is  withdrawn 
from  the  body.  If  the  blood  of  the  horse  is  allowed  to  flow  directly  into  a 
tall  cylindric  glass  vessel,  surrounded  by  ice,  it  separates  in  the  course  of  a 
few  hours  into  three  distinct  layers  in  accordance  with  their  specific  gravities. 
The  lower  layer  is  dark  red  and  consists  of  the  red  corpuscles;  the  middle 
layer  is  grayish  in  color  and  consists  of  the  white  corpuscles;  the  upper 
layer  is  clear  and  transparent  and  consists  of  the  plasma.  The  red  cor- 
puscles occupy  almost  one-half,  the  white  one-fortieth,  the  plasma  a  trifle 
more  than  one-half  of  the  height  of  the  entire  blood-column,  which  indicates 
approximately  the  different  volumes  of  each.  The  same  result  can  be 
obtained  with  human  blood  by  the  use  of  the  centrifuge  or  hematocrit. 

PHYSICAL  PROPERTIES  OF  BLOOD 

1.  Color. — Within  the  blood-vessels  two  kinds  of  blood  are  distinguished 
■ — the  arterial,  the  color  of  which  is  a  bright  scarlet-red,  and  the  venous, 
the  color  of  which  is  a  dark  bluish-red  or  purple.  The  cause  of  the  color 
as  well  as  the  difference  in  color  is  the  presence  in  the  red  corpuscles  of 
coloring-matter,  hemoglobin,  in  different  degrees  of  combination  with 
oxygen.  The  intensity  of  the  color  in  either  kind  of  blood  is  dependent 
on  the  thickness  of  the  blood-stream,  for  in  the  finest  capillaries,  as  seen 
under  the  microscope,  there  is  an  almost  total  absence  of  color.  As  the 
venous  blood  passes  into  and  through  the  pulmonic  capillaries  the  hemo- 
globin absorbs  a  certain  volume  of  oxygen  after  which  it  changes  in  color 
and  on  emerging  from  the  lungs  imparts  to  the  blood  its  characteristic 
scarlet-red  color.  By  reason  of  the  union  of  the  hemoglobin  with  the 
oxygen  it  is  generally  termed  while  in  the  arteries,  oxyhemoglobin.  As 
the  arterial  blood  passes  into  and  through  the  systemic  capillaries,  the  oxy- 
hemoglobin yields  up  a  portion  of  its  oxygen  to  the  tissues  after  which  it 
again  changes  in  color  and  on  emerging  from  the  tissues  imparts  to  the 
blood  its  characteristic  bluish-red  color.  By  reason  of  the  loss  of  a  portion 
of  its  oxygen,  the  hemoglobin  is  generally  termed  while  in  the  veins,  deoxy- 
or  reduced  hemoglobin. 

2.  Opacity. — Owing  to  the  fact  that  the  corpuscles  have  a  refracting 
power  different  from  the  plasma,  the  blood,  even  in  thin  layers,  is  opaque. 
The  repeated  refractions  and  reflections  which  light  undergoes  in  passing 
through  plasma  and  corpuscles  is  attended  by  such  a  dissipation  that  it  is 
impossible  to  see  printed  matter  through  it.  That  the  opacity  is  due  to  the 
shape  of  the  corpuscles  rather  than  to  their  contained  coloring-matter  is 
evident  from  the  fact  that  when  the  hemoglobin  is  caused  to  separate  from 
the  corpuscles  by  the  addition  of  chemic  reagents,  the  blood,  though  it 
deepens  in  color,  at  once  becomes  transparent. 

3.  Odor. — When  freshly  drawn  the  blood  possesses  a  peculiar  charac- 
teristic odor  which  has  been  attributed  to  the  presence  of  a  volatile  fat  acid 
in  combination  with  an  alkaline  base.  The  intensity  of  the  odor  may  be 
increased  by  the  addition  of  concentrated  sulphuric  acid,  by  means  of  which 
the  volatile  acid  is  set  free. 

4.  Specific  Gravity. — The  specific  gravity  of  blood  lies  within  the  limits 
of  1.051  and  1.059,  averaging  in  man  1.056  and  in  woman  1.053.     Normally, 


232  TEXT-BOOK  OF  PHYSIOLOGY 

variations  from  these  values  are  only  temporary  and  are  connected  with 
variations  in  physiologic  processes.  The  specific  gravity  is  diminished  by 
the  ingestion  of  liquids  and  abstinence  from  solid  food.  It  is  increased  by 
abstinence  from  liquids,  by  the  ingestion  of  dry  food,  and  by  the  elimination 
of  large  quantities  of  water  by  the  lungs,  skin,  and  kidneys. 

Inasmuch  as  the  specific  gravity  of  the  blood  varies  from  the  normal  in 
one  direction  or  the  other  in  certain  pathologic  states,  it  is  deemed  desirable 
for  clinical  purposes  to  determine  the  extent  of  this  variation.  Among  the 
methods  suggested  for  this  purpose  that  of  Hammerschlag  is  the  one  most 
generally  resorted  to.  It  is  based  on  the  principle,  that  a  fluid  in  which  a 
drop  of  blood  neither  rises  nor  falls  must  have  the  same  specific  gravity  as 
the  blood  itself.  As  the  specific  gravity  of  the  blood  varies  in  different 
pathologic  states  it  is  essential  that  the  fluid  employed  is  of  such  a  character 
that  its  specific  gravity  can  be  quickly  varied  in  one  direction  or  the  other. 
To  meet  this  indication  a  fluid,  a  mixture  of  chloroform  (specific  gravity 
1.526)  and  benzol  (specific  gravity  0.889)  is  first  prepared  in  such  propor- 
tions that  the  mixture  has  a  specific  gravity  of  about  1.040,  With  a  pipette 
a  drop  of  blood  is  then  placed  in  the  mixture.  If  the  drop  rises  the  specific 
gravity  of  the  mixture  is  greater  than  that  of  the  blood.  Benzol  is  then 
gradually  added  until  the  drop  remains  stationary.  At  this  moment  the 
specific  gravity  of  the  mixture  is  the  same  as  that  of  the  blood.  If  the 
drop  falls  the  specific  gravity  of  the  mixture  is  less  than  that  of  the  blood. 
Chloroform  is  then  gradually  added  until  the  drop  remains  stationary. 
At  this  moment  the  specific  gravity  of  the  mixture  is  the  same  as  that  of 
the  blood.  In  either  case  the  specific  gravity  of  the  mixture  is  determined 
with  a  suitable  hydrometer  and  the  figure  observed  attributed  to  the  blood. 

5.  Reaction. — ^The  reaction  of  the  blood  has  usually  been  stated  as 
alkaline  from  its  effect  on  litmus  paper.  Thus,  if  blood  is  permitted  to 
remain  for  a  few  seconds  on  slightly  reddened  glazed  litmus  paper  and  then 
washed  off,  a  distinct  blue  color  presents  itself  against  a  red  or  violet  back- 
ground. The  alkalinity  thus  indicated  has  been  attributed  to  disodium 
phosphate,  NajHPO^,  and  sodium  carbonate,  NajCOg.  The  degree  of  the 
alkalinity  is  measured  by  the  amount  of  a  standard  acid  necessary  to  be 
added  before  the  indicator  used  shows  an  acid  reaction.  According  to 
V.  Jaksch  the  alkalinity  corresponds  to  from  260  to  300  milligrams  of  sodium 
hydrate,  NaOH,  for  every  100  c.c.  of  blood,  according  to  Lowy  from  300  to 
325  milligrams.  The  hitherto  unavoidable  error  in  these  estimates  is  about 
30  milligrams.  The  alkalinity  from  this  point  of  view  varies  but  within 
narrow  limits  in  physiologic  conditions.  It  is  increased  in  the  early  stages 
of  digestion  and  decreased  in  the  later  stages  as  well  as  after  prolonged 
exercise. 

In  accordance  with  the  ideas  of  physical  chemistry  the  acidity  of  a  fluid 
is  dependent  on  the  presence  of  hydrogen  ions,  H+,  and  the  alkalinity  on  the 
presence  of  hydroxyl  ions,  (OH).  The  reaction  of  any  fluid,  containing  a 
number  of  chemic  compounds  in  solution,  will  be  dependent  therefore  on  the 
relative  proportions  of  hydrogen  ions  and  hydroxyl  ions  that  make  their 
appearance. 

If  the  hydrogen  ions  are  in  excess  the  fluid  is  acid,  if  the  hydroxyl  ions 
are  in  excess  the  fluid  is  alkaline  in  reaction.  Tested  by  the  methods  of 
physical  chemistry  blood  and  lymph  are  found  to  possess  these  opposite 


THE  BLOOD  233 

ions  in  equal  degree  and  therefore  are  neither  acid  nor  alkaline  but  neutral 
in  reaction, 

6.  Temperature. — The  temperature  varies  in  degree  in  different  parts 
of  the  body.  If  the  temperature  variations  correspond  with  the  variations 
in  some  other  mammals,  the  highest  temperature  is  in  the  hepatic  veins 
where,  as  in  the  dog,  it  registers  about  4o.6°C.,  and  the  lowest  temperature 
is  near  the  surface  as  in  the  mouth  and  axilla,  where  it  is  approximately 

7.  Viscosity. — ^Viscosity  may  be  defined  as  the  resistance  to  the  move- 
ment of  the  molecules  of  a  fluid  homogeneous  body  among  themselves.  In 
accordance  with  the  degree  of  this  resistance,  which  may  also  be  spoken  of 
as  internal  friction,  will  the  fluid  at  a  given  temperature  be  mobile  or  viscous. 
Viscosity  varies  partly  with  the  nature  of  the  fluid  and  partly  on  its  tempera- 
ture. Thus  at  the  same  temperature  water,  syrup,  and  pitch  possess  different 
degrees  of  viscosity.  A  rise  in  temperature  of  i°C.  diminishes  the  viscosity 
about  2  per  cent.  In  all  discussions  relating  to  the  viscosity  of  fluids,  that 
of  distilled  water  is  taken  as  a  standard  and  regarded  as  unity. 

Blood  as  a  fluid  is  regarded  by  physiologists  as  possessing  \'iscosity, 
though  the  definition  in  the  foregoing  paragraph  is  not  strictly  a.pplicable, 
as  it  is  not  a  homogeneous  but  a  heterogeneous  fluid  consisting  of  plasma 
the  molecules  of  which  show  an  inner  friction  and  of  corpuscles  which  also 
show  friction.  Blood  having  a  complex  composition  as  compared  with 
water  has  naturally  a  greater  degree  of  viscosity  or  internal  friction.  Experi- 
mental investigations  render  it  certain  that  the  observed  viscosity  is  depend- 
ent on  the  corpuscular  elements  to  a  greater  extent  than  on  the  composition 
of  the  plasma.     About  two-thirds  of  the  viscosity  is  due  to  the  corpuscles. 

The  viscosity  of  blood  as  compared  with  water  may  be  determined  by 
permitting  the  two  fluids  to  flow  through  capillary  tubes  of  corresponding 
caliber  under  a  steadily  acting  pressure  and  then  determining  the  volume 
that  flows  through  each  in  a  given  time,  or  by  determining  the  distance  to 
which  each  fluid  flows  in  a  unit  of  time  in  these  capillary  tubes  and  then 
in  each  instance,  comparing  the  results  one  with  the  other.  Normal  human 
blood  is  thus  found  to  possess  a  viscosity  4.5  times  that  of  distilled  water 
at  body  temperature.  Dog's  blood  has  a  viscosity  6  times  that  of  water. 
If  the  temperature  of  blood  is  raised  the  viscosity  diminishes.  Recalling 
the  statement  that  the  viscosity  is  closely  connected  with  the  presence  of 
red  corpuscles  it  would  be  expected  that  either  an  increase  or  decrease 
in  their  number  would  change  the  viscosity  in  one  direction  or  another. 
In  a  case  of  polycythemia  in  which  the  red  corpuscle  count  was  11,000,000 
per  cubic  mfllimeter  the  viscosity  was  between  3  and  4  times  the  normal. 
In  certain  other  pathologic  states  of  the  blood  characterized  by  a  diminu- 
tion in  the  number  of  red  corpuscles  the  viscosity  diminished  one-half 
or  more.  The  ingestion  of  meat  raises  the  viscosity,  while  the  ingestion  of 
fats  and  carbohydrates  diminishes  it. 

The  determination  of  the  viscosity  for  clinical  purposes  is  accomplished 
by  the  use  of  special  forms  of  apparatus  termed  viscosimeters.  These  for 
the  most  part  consist  of  capillary  tubes  through  which  distilled  water  and 
blood  are  caused  to  flow  under  the  influence  of  a  constant  positive  or  nega- 
tive pressure.  The  distance  the  water  flows  in  a  unit  of  time,  compared 
with  that  of  blood,  represents  the  degree  of  viscosity.     Among  these  ap- 


234 


TEXT-BOOK  OF  PHYSIOLOGY 


paratus  those  of  Hess  and  Determann  are  generally  employed,  descriptions 
of  which  will  be  found  in  works  on  diagnosis. 

8.  Coagulability. — Within  a  few  minutes  after  the  blood  is  withdrawn 
from  the  vessels  of  a  living  animal  it  begins  to  lose  its  fluidity,  becomes  some- 
what viscid,  and  if  left  undisturbed  passes  rapidly  into  a  semisolid  or  jelly- 
like state.  To  this  change  in  the  physical  condition  of  the  blood  the  term 
coagulation  has  been  applied.  The  blood,  during  the  progress  of  coagula- 
tion, not  only  assumes  the  shape  of  the  vessel  in  which  it  is  contained,  but 
becomes  so  firmly  adherent  to  its  walls  that  it  may  be  inverted  without  the 
coagulum  becoming  dislodged.  If  a  portion  of  such  a  jelly-like  mass  be 
examined  microscopically,  it  will  be  found  to  be  penetrated  in  all  directions 
by  a  felt-work  of  extremely  fine  delicate  fibrils,  which,  having  made  their 
appearance  before  the  corpuscles  have  had  time  to  settle  to  the  bottom  of 
the  fluid,  have  entangled  them  in  the  meshes  so  that  the  entire  mass 
retains  its  characteristic  color.  These  fibrils  are  collectively  known  as  fibrin 
(Fig.  92). 

If  the  coagulated  blood  be  allowed  to  remain  undisturbed,  a  clear, 
transparent,  slightly  yellowish  fluid  makes  its  appearance  on  the  surface  of 
the  mass,  which  as  it  accumulates  forms  a  layer  of  varying  degrees  of  thick- 


N 


Fig.  92. — Diagram  to  Illustrate  the  Process  of  Coagulation,  i.  Fresh  blood,  plasma 
and  corpuscles.  2.  Coagulating  blood  (birth  of  fibrin).  3.  Coagulated  blood  (clot  and  serum). — 
{Waller.) 

ness.  Within  a  few  hours  the  blood-mass  detaches  itself  from  the  sides  of 
the  vessel  in  consequence  of  the  retraction  of  the  fibrils,  while  at  the  same 
time  the  clear  fluid  increases  in  amount  and  accumulates  along  the  sides 
and  bottom  of  the  vessel.  The  shrinkage  in  the  volume  of  the  red  coagulum 
and  the  increase  of  the  volume  of  the  clear  fluid  which  is  expressed  from  its 
meshes  continue  for  a  period  varying  from  ten  to  fifteen  hours,  according 
to  certain  external  conditions.  The  blood  has  now  become  separated  into 
two  distinct  portions:  viz.:  a  solid  contracted  red  mass,  termed  clot,  and  a 
clear  fluid,  termed  serum.  The  clot  consists  of  the  fibrin  containing  in  its 
meshes  the  red  and  white  corpuscles;  the  serum  consists  of  all  the  constitu- 
ents of  the  plasma  except  the  antecedents  of  the  fibrin.  The  stages  of  coagu- 
lation are  shown  in  Fig.  92. 

If  the  blood  coagulates  slowly  the  red  corpuscles,  owing  to  their  greater 
specific  gravity,  subside  to  the  bottom  of  the  blood  mass,  giving  to  it  a  deeper 
color;  the  white  corpuscles,  owing  to  their  lesser  specific  gravity,  remain  near 
the  surface  of  the  clot  and  give  to  it  a  more  or  less  whitish  appearance,  pro- 
ducing the  so-called  buffy  coat.  In  certain  inflammatory  conditions  the 
coagulating  power  of  the  blood  is  much  diminished,  and  the  corpuscles, 
having  time  to  subside,  a  well-developed  buffy  coat  is  formed  which  at  one 
time  had  much  interest  for  pathologists.     As  the  contraction  of  the  fibrin 


THE  BLOOD  235 

takes  place  more  actively  in  the  center,  there  being  here  less  resistance  than 
at  the  sides  of  the  coagulum,  the  upper  surface  usually  becomes  depressed 
or  cupped. 

Coagulation  of  Plasma. — Clear  plasma  may  be  obtained  by  means  of 
the  centrifuge  from  blood  to  which  sufficient  magnesium  sulphate  has  been 
added  to  prevent  coagulation,  or  from  horse's  blood  which  has  been  allowed 
to  flow  into  a  tall  vessel  surrounded  by  a  cooling  mixture  so  as  to  prevent 
coagulation  and  thus  permit  the  red  corpuscles  to  subside.  If  such  plasma 
is  subjected  to  room-temperature,  it  very  shortly  undergoes  coagulation, 
exhibiting  practically  the  same  phenomena  as  blood  itself.  After  a  variable 
length  of  time  it  also  separates  into  a  soft,  colorless  coagulum  or  clot  consisting 
of  fibrin,  and  a  clear  fluid,  the  serum.  The  presence  of  the  red  corpuscles 
is  therefore  not  essential  to  the  process  of  coagulation. 

Rapidity  of  Coagulation. — The  rapidity  with  which  the  blood  coagu- 
lates varies  in  different  classes  of  animals  under  the  same  conditions:  e.g., 
the  blood  of  the  pigeon  coagulates  immediately;  that  of  the  dog,  in  from  one 
to  three  minutes;  that  of  the  horse,  in  from  five  to  thirteen  minutes;  that  of 
man,  in  from  four  to  seven  minutes.  The  time,  however,  can  be  lengthened 
or  shortened  by  either  changing  the  external  conditions  or  by  altering 
temporarily  the  normal  composition  of  the  blood. 

Coagulation  is  retarded  or  prevented  by  the  following  agents,  viz.:  (i) 
A  low  temperature,  especially  that  of  melting  ice.  (2)  The  addition  of 
magnesium  sulphate  (i  volume  of  a  25  per  cent,  solution  to  3  volumes  of 
blood) ;  of  sodium  sulphate  (i  volume  of  a  saturated  solution  to  7  volumes  of 
blood),  (3)  The  addition  of  potassium  oxalate  (i  volume  of  a  i  per  cent, 
solution  to  3  volumes  of  blood).  (4)  The  injection  into  the  circulating 
blood  of  commercial  peptone.     (5)  The  mouth  secretion  of  the  leech. 

Coagulation  is  hastened  by  the  following  agents,  viz.:  (i)  a  temperature 
gradually  increasing  from  38°C.  to  5o°C.  (2)  The  addition  of  water  in 
not  too  large  amounts.  (3)  The  presence  of  foreign  bodies.  (4)  Agitation 
of  the  blood — e.g.,  stirring. 

Fibrin  and  Defibrinated  Blood. — If  freshly  drawn  blood  is  stirred  with 
a  bundle  of  twigs  or  glass  rods  for  a  few  minutes,  the  fibrin  collects  on  them 
in  the  form  of  thick  bundles  or  strands;  on  washing  it  with  water  the 
entangled  corpuscles  are  removed,  when  the  fibrin  assumes  its  natural  white 
appearance.  The  strands  can  be  resolved  into  a  large  number  of  delicate 
fibers  which  possess  extensibility  and  retractility,  and  are  therefore  elastic. 
The  chemic  features  of  fibrin  have  already  been  considered  (see  page  19). 
The  remaining  fluid,  similar  in  its  physical  appearance  to  the  blood,  is 
termed  defibrinated  blood.  When  such  blood  is  allowed  to  remain  at  rest  for 
a  few  days,  the  remaining  red  corpuscles  gradually  sink  to  the  bottom  of  the 
fluid,  above  which  will  be  found  the  clear  serum. 

CHEMIC  COMPOSITION  OF  PLASMA  AND  SERUM 

Plasma.— The  plasma  obtained  by  any  of  the  methods  previously 
described  is  a  clear,  colorless,  transparent,  slightly  viscid  fluid,  with  a  specific 
gravity  of  1.026  to  1.029.  It  is  composed  largely  of  water  holding  in  solution 
proteins,  sugar,  fat,  inorganic  salts,  urea,  cholesterin,  lecithin,  etc.  In 
composition  it  is  quite  complex,  containing  as  it  does  not  only  the  nutritive 


236  TEXT-BOOK  OF  PHYSIOLOGY 

materials  derived  from  the  digestion  of  the  food,  but  also  the  substances 
resulting  from  the  disintegration  of  the  tissues  consequent  on  their  functional 
activity. 

Serum. — The  serum  is  the  clear,  transparent,  slightly  yellow  fluid  ex- 
pressed from  the  coagulated  blood  during  the  contraction  of  the  fibrin.  It 
consists  practically  of  the  ingredients  of  the  plasma,  with  the  exception  of 
those  substances  which  entered  into  the  formation  of  fibrin. 

The  average  composition  of  plasma  is  shown  in  the  following  table* 

CHEMIC  COMPOSITION  OF  PLASMA 

Water 90 .00 

!  Plasma-albumin 4-5° 

Paraglobulin 3 .40 

Fibrinogen 0.30 

Fatty  matters 0.25 

Sugar o .  10 

Extractives 0.60 

Inorganic  salts o  .85 

100.00 

Plasma-albumin. — Of  the  protein  constituents  of  the  blood,  plasma-  or 
serum-albumin  is  the  most  abundant,  existing  to  the  extent  of  from  4  to  5 
per  cent.  It  is  readily  obtained  from  plasma  or  serum  by  saturating  either 
of  these  fluids  with  magnesium  sulphate,  when  all  the  proteins  except 
serum-albumin  are  precipitated.  After  their  removal  the  remaining  fluid 
is  subjected  to  a  temperature  of  from  70°  to  75°C.,  when  the  serum-albumin 
is  precipitated  in  a  coagulable  form,  after  which  it  can  be  removed  and  its 
chemic  features  determined.  Recent  investigations  indicate  that  plasma- 
albumin  consists  of,  at  least,  more  than  one  substance,  since  it  can  be  separated 
into  a  crystallizable  and  non-crystallizable  or  an  amorphous  portion  both 
of  which  however  respond  to  the  usual  tests  for  albumin. 

The  origin  and  function  of  plasma-albumin  are  both  obscure.  Until 
quite  recently  it  has  been  regarded,  owing  to  its  similarity  to  egg-albumin, 
and  to  its  presence  in  the  blood,  as  holding  an  important  position  as  a 
nutritive  agent.  It  was  assumed  that  it  originated  in  a  synthesis  of  the 
products  of  protein  digestion — peptones  and  amino-acids — in  the  epithelial 
cells  covering  the  villi  and  by  them  discharged  into  the  blood  of  the  portal, 
and  finally  into  the  blood  of  the  general  circulation.  On  reaching  the  capil- 
laries it  was  supposed  to  cross  the  endothelial  wall,  to  become  a  constituent 
of  the  lymph  and  then  to  be  utilized  by  the  tissue  cells  for  growth  and  repair. 
As  was  stated  in  the  chapter  on  absorption,  this  view  has  been  made  untenable 
by  reason  of  the  apparent  fact  that  the  products  of  protein  digestion— the 
amino-acids — are  absorbed  and  enter  the  blood  as  such  and  carried  direct 
to  the  tissue  cells  by  which  they  are  directly  synthesized  to  the  protein 
by  which  they  are  characterized.  Plasma-albumin  must  therefore  have, 
if  this  view  prevails,  some  other  origin  and  function.  Future  experimenta- 
tion may  disclose  both. 

Paraglobulin. — This  protein,  though  present  in  plasma,  is  best  obtained 
from  serum  when  this  fluid  is  saturated  with  magnesium  sulphate.  As  the 
line  of  saturation  is  approached  the  fluid  becomes  turbid,  and  after  a  few 
minutes  a  fine  white  precipitate  occurs.  It  can  then  be  collected  on  a  filter, 
dried,  and  its  chemic  properties  determined.  In  its  reactions  it  resembles 
the  various  members  of  the  globulin  class.     The  amount  varies  from  2  to  4 


THE  BLOOD  237 

per  cent,  in  the  blood  of  man.  As  to  the  physiologic  importance  or  ante- 
cedents of  paraglobulin  nothing  is  definitely  known.  Its  constant  presence 
in  the  blood  would  indicate  that  it  plays  an  equally  important,  though  per- 
haps different,  part  with  serum-albumin  in  the  nutrition  of  the  body. 

Fibrinogen. — This  protein  can  be  obtained  from  plasma,  lymph, 
pericardial,  and  peritoneal  fluids,  as  well  as  from  hydrocele  fluid.  It  is, 
however,  not  to  be  obtained  from  serum,  as  it  is  removed  from  the  blood 
during  the  formation  of  solid  fibrin.  It  is  normally  present  in  the  blood  in 
very  small  quantity,  amounting  to  not  more  than  0.22  to  0.33  part  per 
hundred.  Fibrinogen  may  be  obtained  from  plasma  which  has  been  pre- 
vented from  coagulating,  by  the  addition  of  magnesium  sulphate  in  certain 
quantities  or  by  the  addition  of  a  saturated  solution  of  sodium  chlorid.  In 
a  few  minutes  a  flaky  precipitate  occurs.  By  repeated  washing  and  pre- 
cipitation with  sodium-chlorid  solutions  of  varying  strength,  the  fibrinogen 
may  be  obtained  in  a  pure  state.  The  history  of  fibrinogen  is  unknown, 
though  there  is  some  experimental  evidence  for  the  belief  that  it  is  produced 
in  the  liver  though  out  of  what  has  not  been  determined.  Beyond  the  fact 
that  it  contributes  to  the  occasional  formation  of  fibrin  there  is  no  positive 
knowledge  either  as  to  its  origin,  its  nutritive  value,  or  its  final  disposition  in 
the  blood  under  normal  conditions. 

Fat. — ^The  plasma,  and  the  serum  as  well,  contains  a  very  small  quantity 
of  fat  in  the  form  of  microscopic  globules.  Though  the  percentage  is  nor- 
mally not  above  0.25,  yet  just  after  a  meal  rich  in  fat  the  amount  may  be  so 
great  as  to  give  to  the  blood  a  milky  or  opalescent  appearance.  Within  a 
few  hours,  however,  this  excess  of  fat  disappears  from  the  blood.  As 
oxidation  of  fat  in  the  blood  has  not  been  established  the  probabilities 
are  that  the  microscopic  granules  pass  across  the  walls  of  the  capillaries 
and  become  constituents  of  the  lymph.  Soaps  or  alkaline  salts  of  the 
fat  acids,  though  formed  during  the  digestion  of  fats,  are  not  present  in 
the  blood.     Lecithin  and  cholesterin  are  present  in  very  small  quantities. 

Sugar. — ^The  plasma  also  contains  a  small  quantity  of  sugar  in  the  form 
of  dextrose  and  is  to  be  regarded  as  a  normal  constituent.  The  percentage 
of  the  sugar  varies  from  o.io  to  0.20  per  cent.  To  this  condition  of  the  blood 
in  which  the  sugar  is  present  in  but  normal  amounts  the  term  glycemia  has 
been  given.  If  the  percentage  falls  below  the  normal  amount,  a  condition 
of  hypoglycemia  is  established;  if  on  the  contrary  the  percentage  is  increased 
beyond  the  normal  a  condition  of  hyperglycemia  is  established,  whereupon 
the  excess  will  be  for  the  most  part  eliminated  by  the  kidneys  giving  rise 
to  a  condition  known  as  glycosuria. 

If  the  statement  be  accepted  that  the  amount  of  blood  in  a  body  weigh- 
ing 70  kilos  is  3864  grams  (i/ipth  of  the  body  weight)  the  amount  of  sugar 
in  the  entire  volume  of  blood  is  at  most  about  7.72  grams,  an  amount  which 
does  not  materially  change  under  physiologic  conditions. 

Extractives. — The  blood  contains  a  series  of  nitrogenized  bodies,  such 
as  urea,  uric  acid,  creatin,  creatinin,  xanthin,  etc.,  which  result  from  the 
katabolic  changes  in  nerve-  and  muscle-tissues  as  well  as  from  subsequent 
chemic  combinations  and  decompositions.  Though  constantly  absorbed 
from  the  tissues,  they  seldom  accumulate  beyond  a  small  amount,  since 
they  are  constantly  being  eliminated  from  the  blood  by  the  various  ex- 
cretory organs. 


238 


TEXT-BOOK  OF  PHYSIOLOGY 


Inorganic  Salts. — The  inorganic  salts  of  the  plasma  are  chiefly  sodium 
and  potassium  chlorids,  sulphates,  and  phosphates,  together  with  calcium 
and  magnesium  phosphates.  Of  the  salts,  sodium  chlorid  is  the  most 
abundant,  amounting  to  0.56  part  per  hundred.  Calcium  phosphate  is 
present  in  small  quantity,  0.2  part  per  100.  This  salt  is  not  present  to 
the  same  extent  in  serum  for  the  reason  that  it  became  a  constituent  of 
fibrin  at  the  time  of  coagulation.  In  other  respects  serum  differs  but 
slightly  from  plasma  in  the  proportions  of  its  saline  constituents. 


HISTOLOGY  OF  THE  RED  CORPUSCLES  OR  ERYTHROCYTES 

The  histologic  features  of  the  red  corpuscles  are  readily  observed  in  a 
drop  of  freshly  drawn  blood  when  examined  microscopically.  The  field  of 
the  microscope  will  be  seen  to  be  crowded  with  red  corpuscles  floating  in  a 
clear  transparent  fluid — the  plasma.  Here  and  there  will  also  be  seen  a  white 
corpuscle,  round  or  irregular  in  shape,  and  granular  in  appearance.  Within 
a  short  time  a  characteristic  phenomenon  takes  place:  viz.,  the  arranging  of 
the  corpuscles  in  the  form  of  columns  of  varying  lengths,  resembling  rolls  of 

coins.  These  rolls  interlace  with 
each  other  at  all  angles  and  form  a 
network  in  the  meshes  of  which  lie 
individual  red  and  white  corpuscles. 
(See  Fig.  93.)  The  cause  of  this 
tendency  of  the  corpuscles  to  adhere 
to  one  another  is  not  definitely 
known.  Since  it  does  not  take  place 
in  circulating  blood,  and  since  it  is  to 
a  great  extent  prevented  by  defibrin- 
ating  the  blood,  it  has  been  supposed 
to  be  dependent  on  the  formation  of 
some  adhesive  substance  connected 
with  the  formation  of  fibrin. 

Color. — When  viewed  by  trans- 
mitted light,  a  single  corpuscle  is 
slightly  yellow  or  greenish  in  color; 
but  when  a  number  are  grouped  to- 
gether, the  color  deepens  and  the  cor- 
puscles appear  red.  In  either  case 
the  color  is  due  to  the  presence  in  the  corpuscle  of  a  specific  coloring-matter, 
hemoglobin. 

Shape. — The  red  corpuscle  is  a  circular,  flattened  disc  with  rounded 
edges.  Each  surface  is  perfectly  smooth  and  presents  a  shallow  depression 
in  its  center,  so  that  it  is  also  biconcave.  A  longitudinal  section  of  a  cor- 
puscle would  present,  when  viewed  edgewise,  an  outline  similar  to  that  of 
Fig.  94.  This  difference  in  the  thickness  of  the  peripheral  and  central 
portions  of  the  corpuscle  gives 'jise  to  differences  in  optical  appearances 
when  examined  microscopically.  At  a  certain  distance  of  the  object-glass 
the  corpuscle  presents  in  its  peripheral  portion  a  bright  rim,  and  in  its  cen- 
tral portion  a  dark  spot  If  the  objective  be  brought  nearer  and  the  center 
accurately  focused,  the  reverse  appearance  obtains;  the  central  portion 


Fig.  93. — Corpuscles  from  Human 
Subject.  A  few  colorless  corpuscles  are 
seen  among  the  colored  discs,  many  of 
which  are  arranged  in  rouleaux. — {Funke.) 


THE  BLOOD  239 

becomes  bright  and  the  peripheral  portion  dark.     The  cause  of  this  differ- 
ence in  optical  appearance  is  the  unequal  distribution  of  the  transmitted 
light  in  consequence  of  the  shape  of  the 
corpuscle.  ry — -.^^—^-^-..^^^^^ — ,_^^, 

Size. — The  diameter  of  a  typical  well-  aif \ Jb 

developed  red  corpuscle  under  normal  con-  k       ^ -^^^^    j[ 

ditions  is  0.0075   ni^i-j   its  greatest  thick-  d 

ness  is  0.0019  ^^-     Though  this  may  be  Fig.  94.— Ideal   Tr.^sverse 

assumed   as  the  average  diameter,   there  section  of  a  Human  Red  CoRprs- 

,,  r    T     •        1  11  CLE.     (Magnified   qooo  times.)     a,  o. 

is   a   small   percentage  of  dlStmctly  smaller      Diameter,     c,  d.  Thickness. 

and  a  small  percentage  of  distinctly  larger 

corpuscles.     The  following  table  shows  the  results  of  measurements  made 

by  different  observers: 

Normal  Limits.  Average  Diameter. 

Welcker diameter  0.0045-0.0095  mm 0.0070  mm. 

Hayem diameter  o  .0060-0 .0088  mm o  .0075  mm. 

Gram diameter  0.0067-0.0093  mm 0.0078  mm. 

Melassez o  .0076  mm. 

0,00747  (33V0  inch) 

Structure. — The  red  corpuscle  of  man  as  well  as  of  all  other  mammals 
possesses  neither  a  nucleus  nor  a  limiting  membrane,  but  appears  to  consist 
of  a  homogeneous  substance  more  or  less  semisolid  in  consistence.  Under 
the  influence  of  certain  reagents  the  corpuscle  separates  into  two  distinct 
portions:  viz.,  a  colorless  protoplasmic  stroma  and  a  coloring-matter  which 
diffuses  into  the  surrounding  liquid.  The  presence  of  the  former  can  be 
demonstrated  by  the  addition  of  iodin,  which  imparts  to  it  a  faint  yellow  color. 
The  stroma  is  elastic,  and  determines  not  only  the  shape  of  the  corpuscle  but 
gives  to  it  the  properties  of  extensibility  and  retractility. 

The  foregoing  is  the  classic  and  generally  accepted  view  as  to  the  shape, 
size,  and  structure  of  the  red  corpuscle.  Nevertheless  recent  investigations 
render  it  probable  that  the  statements  were  based  on  observations  of  the 
corpuscles  under  artificial  rather  than  natural  conditions,  and  therefore  not 
strictly  true.  For  many  years  histologists  from  time  to  time  have  stated 
that  the  red  corpuscle  is  not  circular  and  biconcave  in  shape,  in  the  cir- 
culating blood,  but  bell-shaped,  similar  to  that  shown  in  Fig.  95.  It  was 
not  until  1902,  after  the  publication  of  Weidenreich's  investigations,  that 
this  view]  began  to  receive  more  attention  than  had  hitherto  been  accorded 
it.  Weidenreich  preserved  in  a  moist  chamber  a  hanging  drop  of  human 
blood,  and  on  examination  found  that  the  red  corpuscles  were  bell-shaped 
though  the  depth  of  the  bell  cavity  varied  considerably.  An  examination 
of  the  capillary  circulation  in  the  omentum  of  the  rabbit  revealed  the  fact 
that  the  corpuscles  in  their  natural  medium  were  also  bell-shaped.  The 
circular  biconcave  shape  they  ordinarily  present  when  a  drop  of  blood  is 
examined  microscopically  he  attributes  to  cooling,  evaporation  and  con- 
centration of  the  drawn  blood.  Experimentally  it  was  shown  that  when 
blood  was  added  to  0.6  to  0.65  per  cent,  solution  of  sodium  chlorid  all  the 
corpuscles  were  bell-shaped,  but  if  the  solution  was  increased  or  decreased 
in  strength,  this  form  was  at  once  changed. 

The  dimensions  of  the  bell-shaped  cell  according  to  Weidenreich  are  as 
follows: — • 


240  .  TEXT-BOOK  OF  PHYSIOLOGY 

Greatest  diameter 7      microns  o  .007    mm. 

Diameter  of  cavity 3      microns  0.003    ^"i. 

Height  of  bell 4      microns  o .  004    mm. 

Height  of  cavity 2.5  microns  o .  002  5  mm. 

Thickness  of  wall  at  apex 2      microns  o  .002    mm. 

Thickness  of  wall  at  base 1.5  microns  0.0015  mm. 

The  foregoing  observations  have  been  confirmed  by  many  subsequent 
investigators.  Thus  Lewis  states  that  if  a  drop  of  blood  is  placed  immediately 
on  a  warm  slide  and  examined,  the  corpuscles  exhibit  the  bell  shape,  but  as 
the  slide  cools  they  gradually  become  biconcave  discs  of  the  conventional 
form.  He  also  observed  that  the  corpuscles  in  the  capillary  blood-vessels 
of  the  omentum  of  the  guinea-pig  were  bell-shaped  and  presenting  an 
appearance  similar  to  that  shown  in  Fig.  95.  Radasch  found  on  examina- 
tion of  fetal  tissues  such  as  the  spleen,  kidney,  liver,  placenta,  etc.,  that  the 
great  majority  of  the  corpuscles  in  all  situations  presented  the  bell  shape 
rather  than  the  circular  biconcave  shape.  This  observer  is  of  the  opinion 
that  the  bell  shape  can  not  be  due  to  the  action  of  the  fixatives  employed  in 
the  preparation  of  the  tissues. 

The  structure  of  the  corpuscle,  according  to  Weidenreich,  differs  also 
from  that  usually  stated.  He  asserts  that  the  corpuscle  is  surrounded  by  a 
structureless,  colorless  membrane  enclosing  a  colored  but  not  nucleated 
semi-fluid  mass,   which  consists  chemically  of  protein  material,  lecithin, 

cholesterin,  inorganic  salts  and  hemoglobin. 

There  is  no  evidence  of  the  existence  of  a 
z-)'^  stroma  in  the  adult  state. 

>.Cyfy^^,  -.A..>.^-^-.^....yv^  Number  of  Red  Corpuscles. — In  any 

\~^/^^  (^^^^^^C\  R\r\      given  specimen  of  blood  the  corpuscles  are 

so  numerous  and  the  spaces  between  them 
so  small  that  it  seems  almost  impossible  to 
determine  their  number.     This,  however, 

has  been  accomplished  for  a  cubic  milli- 
FiG.     05. —  Red  Corpuscles  ^  -,,      jJf  .  , 

Sketched  WHILE  Circulating   in    meter  of  blood  by  various  observers   em- 
THE  Vessels  of  the  Omentum  of  a    ploying  different  methods   with  compara- 

HiZlo'lT    ^^^'  ^'  ^'^''  ""  ^^"^'''    *^^^^>'  uniform  results.     The  average  normal 

number  of  corpuscles  in  one  cubic  milli- 
meter of  blood  is,  for  men,  5,000,000;  and  for  women,  4,500,000.  This  value, 
however,  will  vary  within  slight  limits,  with  variations  in  the  activity  of  physio- 
logic processes  and  to  a  large  extent  at  times  in  pathologic  states  of  the  blood 
or  body.  The  number  is  increased  in  the  cutaneous  veins  by  all  influences 
which  cause  a  diminution  in  the  quantity  of  water  in  the  blood — e.g.,  co- 
pious sweating,  acute  watery  diarrhea,  fasting,  abstinence  from  liquids;  the 
number  is  diminished  by  influences  which  dilute  the  blood — e.g.,  the  ingestion 
of  liquids,  the  absorption  of  fluids  from  the  tissue  spaces,  etc.  But  it  is  well 
to  remember  that  these  influences  which  produce  changes  in  the  number  of 
corpuscles  per  cubic  millimeter  do  not  necessarily  produce  corresponding 
changes  in  the  total  number  of  red  corpuscles  in  the  body.  In  women 
lactation,  menstruation,  and  the  act  of  parturition  diminish  the  number. 
High  altitudes  apparently  increase  the  number  of  corpuscles,  as  shown  by 
their  increase  in  the  blood  of  the  peripheral  vessels.  Whether  this  is  an 
indication  that  there  is  a  corresponding  increase  of  the  total  number  in  the 
general  volume  of  the  blood  is  uncertain.  The  following  table  will  show  the 
increase  in  the  count  per  cubic  millimeter  at  different  altitudes : 


THE  BLOOD 


241 


Place 


Height  Above  Sea-level 


Red  Cells 


Author 


Christiania o  meter 

Gottingen 148  meters 

Tubingen 314  meters 

Zurich 414  meters 

Auerbach 425  meters 

Reibaldsgriin 700  meters 

Arosa 1800  meters 

The  Cordilleras 4392  meters 


4,974,000 
5,225,000 
5,322,000 
5,752,000 
5,748,000 
5,900,000 
7,000,000 
8,000,000 


Laache, 

Schaper. 

Reinert. 

Steirlin. 

Koppe. 

Koppe. 

Egger. 

Viault. 

(Koppe.) 


This  increase  in  the  number  of  corpuscles  takes  place,  according  to 
Viault's  observations,  within  two  or  three  weeks,  and  is  apparently  not  con- 
nected with  either  diet  or  mode  of  life,  but  rather  with  diminished  atmos- 
pheric, if  not  oxygen,  pressure.  On  returning  to  sea-level  there  is  a  gradual 
reduction,  without  any  apparent  destruction  of  the  corpuscles,  to  their  normal 
number.     The  reason  for  these  variations  is  not  clear. 

The  method  of  counting  corpuscles  introduced  by  Vierordt  and  Welcker 
has  been  modified  by  different  observers,  and  especially  by  Thoma.     On 


S|K 



i-:;i 

it 

mJ 

tip" 

Fig.  96. — Hemocytometer.     a.  Surface;  b,  section  view;  c,  squares  on  the  surface  of  B  magnified. 
M,  G,  S,  mouth  piece,  rubber  tube  and  pipette. 

account  of  the  great  number  of  corpuscles  in  i  cubic  millimeter  of  blood,  it 
becomes  necessary  for  purposes  of  enumeration  that  the  blood  be  diluted  a 
delinite  number  of  times  and  that  the  diluted  mixture  be  placed  in  a  counting 
chamber  possessing  a  definite  capacity.  By  means  of  the  pipette  or  melang- 
eur  of  Potain  and  the  counting  chamber  of  Thoma  both  these  objects  are 
attained. 

The  pipette  consists  of  a  capillary  tube  (Fig.  96)  provided  with  an  enlarge- 
ment containing  a  freely  movable  small  glass  ball,  E.  One  end  of  the  tube,  S, 
is  pointed,  while  to  the  other  end  is  attached  a  rubber  tube,  G,  for  the  purpose 
of  facilitating  the  introduction  of  the  blood  and  the  diluting  fluid.  The  capillary 
tube,  which  is  accurately  calibrated,  carries  marks,  0.5,  i,  loi,  which  signify  that 
if  the  tube  be  filled  with  blood  up  to  the  mark  i  and  the  diluting  fluid  be 
sucked  into  the  tube  up  to  the  mark  loi,  the  blood  will  be  diluted  100  times. 
If  the  blood  be  sucked  up  to  the  mark  0.5  and  the  diluting  fluid  to  loi,  then  the 
blood  will  be  diluted  200  tiines.  In  using  the  pipette  the  point  is  introduced 
into  a  drop  of  blood  derived  from  a  small  wound  in  the  skin  of  the  lobe  of  the 
16 


242 


TEXT-BOOK  OF  PHYSI  OLOGY 


ear  or  finger  and  sucked  into  the  tube  by  introducing  the  end,  M,  of  the  rubber 
tube  into  the  mouth.  The  tube  is  then  quickly  inserted  into  a  solution  which 
will  preserve  the  shape  and  size  of  the  corpuscles,  such  as  Gowers's  sodium  sulphate 
solution,  sp.  gr.  1.025,  or  a  3  per  cent,  sodium  chlorid  solution,^  and  the  fluid 
sucked  into  the  tube  up  to  the  mark  loi.  On  shaking  the  pipette  for  a  few  minutes, 
the  admixture  will  take  place,  aided  by  the  movements  of  the  glass  ball. 

Fig.  96  shows  both  a  surface  view,  a,  and  a  section  view,  b,  of  the  counting 
chamber.  This  consists  of  an  oblong  glass  plate,  o,  on  which  are  cemented  two 
small  pieces  of  glass,  one  of  which,  WD,  has  in  the  center  a  circular  opening 
in  which  is  placed  the  other,  B,  a  circular  disc  or  stage.  Their  relation  is  such 
that  a  narrow  groove  or  moat  separates  the  one  from  the  other,  the  floor  of  which 
is  formed  by  the  glass  plate.  The  surface  of  the  circular  stage  is  exactly  o.i  mm. 
lower  than  that  of  the  cover-glass,  r.  On  the  surface  of  the  glass  stage  a  series 
of  small  squares  is  engraved,  C,  each  one  of  which  has  a  side  length  of  yV  ^^^ 
and  an  area  of  4^7^  square  mm.  To  facilitate  counting,  a  group  of  16  such 
squares  is  surrounded  by  a  thick  line.     Fig.  97.     This  group  is  separated  from 

adjoining  groups,  also  enclosed  by  thick  lines,  by 
an  intermediate  fine  line,  which  serves  as  a  guide 
in  passing  from  one  group  to  another.  When  a 
cover-glass  is  accurately  applied  to  the  glass,  b, 
each  one  of  the  small  squares  will  have  a  cubic 
capacity  of  4^^X0.1,  or  y^Vir  cubic  millimeter. 

Before  placing  the  diluted  blood  on  the  count- 
ing stage,  the  fluid  in  the  tube  of  the  pipette 
should  be  blown  out  and  discarded,  as  it  contains 
no  portion  of  the  blood.  A  small  drop  is  then 
placed  on  the  glass  stage  and  covered  with  the  cover- 
glass.  After  a  few  minutes  the  corpuscles  settle 
upon  the  ruled  spaces  and  are  ready  for  counting. 
The  number  of  corpuscles  in  at  least  five  series 
of  sixteen  small  squares  is  then  counted;  this 
number  is  then  multiplied  by  the  degree  of  dilu- 
tion (100  or  200  as  the  case  may  be)  and  this  divided  by  the  cubic  contents 
of  each  small  square  (4Vo~o) ;  the  product  is  then  divided  by  the  number  of  squares 
counted  (80  in  the  instance  given  above):  e.g.,  five  series  of  sixteen  small  squares 
contain  500  corpuscles 


"rr7  "f-T  ^^ TT^  „ ,.  ' ],   ^  7--J  —f--- 

--2 —  — -1  i "    °  r  ° °   "    —JL  sZs — . 


Fig.  97. — Microscopic  Ap- 
pearance OF  THE  Small 
Squares  and  the  Distribu- 
tion OF  THE  Corpuscles. 


SOO  X200X4000_ 

80"  " 


•  5,000,000  erythrocytes  per  c.mm.. 


The  accuracy  of  the  counting  is  proportional  to  the  number  of  squares  counted. 
If  200  squares  are  counted  with  each  of  two  different  drops,  and  the  average  taken 
the  probable  limit  of  error  will  be  less  than  2  per  cent. 

The  Preservation  of  the  Red  Corpuscles  in  the  Plasma. — Within  the 
blood-vessels  the  physical  conditions  and  chemic  composition  of  the  plasma 
are  such  that  both  the  form,  and  the  composition  of  the  corpuscle  or  the  rela- 
tion of  the  hemoglobin  to  the  stroma,  are  maintained  in  the  normal  or  physio- 

1  Various  solutions  have  been  devised  for  diluting  blood,  any  one  of  which  may  be  employed,  e.g.: 
Hayem's  Fluid:  Toisson's  Fluid: 

Hydrarg. bichlor 0.5  gm.  Aquae  destillat 160.00  parts. 


Sodii  sulphat 5 .0  gm. 

Sodii  chlorid 2  .0  gm. 

Aquae  destillat 200 .0  gm. 


Glycerinae 30 .00  parts 

Sodii  sulphat 8 .00  parts. 

Sodii  chlorid i  .00  part. 

Methyl- violet 0.025  part. 


Gower's  Fluid: 

Sodii  sulphat gr.  104 

Add.  acetic 3  j 

Aquae  dest q.  s.  ad  5iv. 


THE  BLOOD  243 

logic  condition.  The  plasma  is  preservative  of  the  structure  and  function 
of  the  corpuscle.  The  reason  assigned  for  this  is  that  the  osmotic  pressure 
of  the  sahs  in  the  plasma  and  of  the  salts  in  the  corpuscle  exactly  balance 
one  another  so  that  there  is  neither  an  absorption  of  water  from,  nor  a  yield- 
ing of  water  to  the  plasma,  on  the  part  of  the  corpuscle.  The  plasma  having 
an  osmotic  pressure  equal  to  that  within  the  corpuscle  is  said  to  be  isotonic 
with  it. 

When  blood  is  to  be  prepared  for  microscopic  examination  with  a  view  of 
determining  the  histologic  features  of  the  corpuscles  or  for  purposes  of 
enumeration,  it  must  be  diluted,  and  unless  special  precautions  are 
observed  the  condition  of  equal  osmotic  pressure  will  be  disturbed  by  the 
diluting  agent  and  the  corpuscles  will  lose  their  characteristic  form  and 
structure  from  either  an  absorption  or  loss  of  water. 

If  distilled  water  is  employed  for  this  purpose,  the  osmotic  pressure  of  the 
plasma  is  of  course  diminished,  and  in  consequence  the  osmotic  pressure  of 
the  inorganic  constituents  of  the  corpuscles  (particularly  potassium  phos- 
phate) causes  an  inflow  of  water.  The  corpuscle  therefore  swells  and 
assumes  a  more  or  less  spheric  form;  the  hemoglobin  is  dissociated  and 
discharged  into  the  surrounding  fluid  throughout  which  it  diffuses.  Such 
an  environment  having  an  osmotic  pressure  less  than  that  of  the  corpuscle  is 
said  to  be  hypotonic,  hypisotonic,  or  hypo-isotonic  to  it. 

If  on  the  contrary,  water  containing  inorganic  salts  (particularly  sodium 
chlorid)  is  added  in  amounts  which  impart  to  the  plasma  an  osmotic  pressure 
greater  than  that  within  the  corpuscle,  there  will  be  an  outflow  of  water 
from  the  corpuscle,  a  shrinkage  of  the  volume  and  a  crenation  of  its  surface. 
Such  an  environment  having  an  osmotic  pressure  greater  than  that  of  the 
corpuscle  is  said  to  be  hypertonic,  or  hyperisoionic  to  it.  It  is  essential 
therefore  in  diluting  the  plasma  with  water,  that  the  latter  contains  inorganic 
salts  in  such  amounts  that  the  resulting  mixture  (plasma  and  water)  possesses 
an  osmotic  pressure  equal  to  that  of  the  original  plasma  or  to  that  of  the  cor- 
puscle. A  diluting  agent  well  adapted  for  this  purpose  is  the  well-known 
Ringer's  mixture.  Other  solutions  which  preserve  the  form  of  the  corpuscles 
during  the  time  required  for  their  enumeration  are  the  solutions  devised  by 
Hayem,  Toisson,  and  G  owers  alluded  to  on  a  preceding  page.  Because  of  the 
fact  that  sodium  chlorid  is  the  chief  inorganic  constituent  of  the  plasma  it  is 
common  in  laboratory  work  to  dilute  the  plasma  of  mammalian  blood  and 
of  frog's  blood  with  solutions  of  sodium  chlorid  of  0.9  per  cent,  and  0.6  per 
cent,  respectively,  which  though  not  absolutely  are  sufficiently  isotonic  for 
the  purpose  desired. 

The  Effects  of  Reagents. — Many  other  saline  solutions  with  an  osmotic 
pressure  greater  or  less  than  normal  plasma,  dilute  solutions  of  acids  and 
alkalies,  bile  salts,  chloroform,  ether,  ammonium  sulphocyanid,  electricity, 
etc.,  also  destroy  the  physical  and  chemic  integrity  of  the  corpuscle  and  cause 
the  hemoglobin  to  separate  from  the  stroma  and  diffuse  into  the  plasma 
without  itself  undergoing  any  appreciable  change  in  composition.  With 
the  escape  and  diffusion  of  the  hemoglobin  the  blood  becomes  transparent 
and  changes  to  a  dark  red  color  to  which  the  term  "laky"  has  been  given. 
The  mechanism  by  which  the  hemoglobin  becomes  dissociated  and  dis- 
charged from  the  corpuscle  by  these  agents  is  unknown.  The  disinteg- 
ration of  the  corpuscle  and  the  diffusion  of  the  hemoglobin  into,  and  its 


244 


TEXT-BOOK  OF  PHYSIOLOGY 


solution  by  the  surrounding  medium,  is  termed  hemolysis  and  the  agents 
by  which  it  is  produced  are  termed  hemolytic  agents. 


CHEMIC  COMPOSITION  OF  RED  CORPUSCLES 

When  analyzed  chemically  the  red  corpuscles  are  found  to  consist  of 
water  65  per  cent,  and  solid  matter  35  per  cent.     The  solids,  moreover,  have 

been  found  to  consist  of  a  pigment  hemo- 
globin T,T),  protein  0.9,  cholesterin  and 
lecithin  0.46,  and  inorganic  salts  (chiefly 
potassium  phosphate  and  chlorid  and 
sodium  chlorid)  1.4  per  cent,  respectively. 
Of  the  total  solids  the  hemoglobin  con- 
stitutes about  94  per  cent. 

Hemoglobin. — In  the  normal  condi- 
tion of  the  corpuscle  the  hemoglobin  is  in 
an  amorphous  condition  and  is  com- 
bined in  some  unknown  way  with  the 
stroma. 

When  hemoglobin  is  decomposed  in 
the  absence  of  oxygen  it  undergoes  a 
cleavage  into  a  protein,  globin,  and  an 
iron-holding  pigment,  hemochromogen, 
which  constitutes  about  4  per  cent,  of 
the  molecule.  If  a  solution  of  hemo- 
chromogen  be  exposed  to  air  it  absorbs 
oxygen  and  is  converted  into  hematin. 
This  latter  compound  can  also  be  de- 
rived directly  from  hemoglobin  by  the 
action  of  acids  and  alkalies.  It  is  to  the 
presence  of  hemochromogen  in  combina- 
tion with  the  protein  globin  that  the 
hemoglobin  is  indebted  for  its  power  of 
absorbing  and  carrying  oxygen. 
If  blood  which  has  been  rendered  laky,  by  water  or  any  other  of  the 
known  agencies,  be  allowed  to  evaporate  slowly,  the  dissolved  hemoglobin 
undergoes  crystallization.  The  rapidity  with  which  the  crystals  form  varies 
in  the  blood  of  different  animals  under  similar  conditions.  According  to  the 
ease  with  which  crystallization  takes  place,  Preyer  has  classified  various 
animals  as  follows:  (i)  Very  difficult — calf,  pigeon,  pig,  frog;  (2)  difficult — 
man,  monkey,  rabbit,  sheep;  (3)  easy — cat,  dog,  mouse,  horse;  (4)  very  easy 
— guinea-pig,  rat. 

The  hemoglobin  cr}'Stals  vary  in  shape  according  to  the  blood  from 
which  they  are  obtained  (Fig.  98).  Those  obtained  from  the  guinea-pig 
are  tetrahedral;  those  from  man  and  most  mammals  are  prismatic  rhombs; 
those  from  the  squirrel  are  in  the  form  of  hexagonal  plates.  Notwith- 
standing these  shght  differences,  all  forms  belong  to  the  same  crystal  system, 
with  the  exception  of  those  from  the  squirrel. 

A  simple  but  very  effective  method  of  obtaining  blood-crystals  suggested 
by  Reichert  is  to  lake  defibrinated  blood,  especially  that  of  the  dog,  rat, 


Fig.  98.  —  Crystallized  H  e  m  o- 
GLOBIN.  a,  b.  Crystals  from  venous 
blood  of  man.  c.  From  blood  of  cat. 
d.  Of  guinea-pig.  e.  Of  marmot.  /. 
Of  squirrel. — {Gautier.) 


THE  BLOOD 


245 


guinea-pig,  and  horse,  with  acetic  or  ethylic  ether  and  then  add  a  solution, 
I  to  5  per  cent.,  of  ammonium  oxalate.  A  drop  of  this  mixture  placed  under 
the  microscope  will  show  crystal  formation  in  a  very  few  minutes. 

Chemic  Composition  of  Hemoglobin. — By  appropriate  methods 
hemoglobin  can  be  obtained  in  a  practically  pure  form,  and  when  subjected 
to  a  temperature  of  ioo°C.  its  water  of  crystallization  is  driven  off,  after 
which  it  can  be  analyzed.  In  the  subjoined  table  the  results  of  several 
analyses  are  given  for  100  parts  of  hemoglobin. 


Dog 

Horse 

Dog 

Guinea-pi 

53-91 

51-15 

53-85 

54.12 

22.62 

23-43 

21.84 

20.68 

6.62 

6.76 

7-32 

7-36 

15.98 

17.94 

16.17 

16.78 

0.54 

0-39 

0-39 

0.58 

0-33 

0-33 
ZinoflFsky. 

0.43 

0.48 

aquet. 

Hoppe-Seyler. 

c. 
o. 

H. 

N. 
S.. 
Fe 


The  percentage  composition  of  hemoglobin  is  thus  seen  to  vary  slightly 
in  different  animals,  suggesting  that  there  may  be  different  kinds  of  hemo- 
globin. The  molecular  composition  is  not  known.  On  the  assumption 
that  each  molecule  contains  one  atom  of  iron,  Preyer  suggested  the  following 
empirical  formula:  C6ooH98oNi540i79S3Fe,  with  a  molecular  weight  of 
I3>332;  Jaquet  has  suggested  a  different  formula :  C^sgH.jogNjgsOjigSgFe, 
with  a  molecular  weight  of  16,669.  ^^  is  very  evident  from  this  that  the 
molecule  is  of  enormous  size  and  exceedingly  complex. 

Quantity  of  Hemoglobin. — The  quantity  of  hemoglobin  in  blood  as 
determined  by  chemic,  chromometric,  and  spectro-photometric  methods 
amounts  to  about  14  per  cent,  in  man  and  13  per  cent,  in  woman.  Of  the 
chemic  methods,  that  based  on  the  amount  of  iron  is  the  one  generally 
employed.  Chemic  analysis  has  shown  that  hemoglobin  contains  0.33  per 
cent,  and  blood  0.046  per  cent,  of  iron;  with  these  two  factors  the  quantity 
of  hemoglobin  can  be  determined  by  the  following  formula:  x=^°°^°'°*  = 
14  per  cent.  The  total  quantity  of  hemoglobin  in  the  blood,  assuming 
the  latter  to  be  about  3684  grams  (one-nineteenth  of  the  body-weight,  70 
kilos)  will  therefore  amount  to  515  grams;  e.g.,x=  ^^^^j^^'*  =5iS-  The 
total  amount  of  iron  in  the  blood  is  obtained  by  the  following  formula : 
viz. ,  X  -  36A4  X 0.04 6  _  I . yo  grams . 

'  lOO  '         o 

Clinic  Methods  for  the  Determination  of  the  Percentage  of  Hemo- 
globin.— Under  normal  physiologic  conditions  the  percentage  of  hemo- 
globin undergoes  but  slight  variation.  In  pathologic  states  there  is  fre- 
quently a  great  diminution  in  the  amount,  especially  in  chlorosis,  splenic 
leukemia,  and  pernicious  anemia,  diseases  in  which  it  diminishes  to  a  con- 
siderable per  cent,  in  many  instances.  For  clinic  purposes  it  becomes  a 
matter  of  importance  to  have  some  method  by  which  the  diminution  of 
hemoglobin  can  be  determined.  In  the  various  methods  employed  the 
normal  amount  of  hemoglobin  is  considered  as  100  per  cent,  and  the  normal 


246  TEXT-BOOK  OF  PHYSIOLOGY 

number  of  red  corpuscles,  5,000,000  per  cubic  millimeter,  is  also  considered 
as  100  per  cent.  Under  such  conditions  the  corpuscles  have  a  normal  color 
known  as  the  color  index.  This  is  expressed  by  a  fraction  of  which  the 
percentage  of  hemoglobin  is  the  numerator  and  the  percentage  of  corpuscles 
the  denominator.  The  normal  color  index  is  therefore  i  or  unity.  In 
some  pathologic  states  the  hemoglobin  alone  diminishes,  the  number  of  the 
corpuscles  remaining  the  same;  in  this  instance  the  color  index  is  less  than 
unity,  e.g.,  if  the  hemoglobin  be  reduced  to  80  per  cent.,  as  determined  by  the 
method  to  be  described,  then  the  color  index  will  be  ^^^^^  =  0.8  which  indicates 
that  each  corpuscle  retains  but  eight-tenths  of  the  normal  amount  of  hemo- 
globin, or  stated  in  the  reverse  way,  each  corpuscle  has  lost  two-tenths  of  the 
normal  amount  of  its  hemoglobin.  In  other  pathologic  states  there  is  both 
a  diminution  in  the  percentage  of  the  hemoglobin  and  in  the  percentage  of 
the  corpuscles  and  the  diminution  may  be  equal  or  unequal  in  degree.  If 
the  diminution  be  equal  the  color  index  is  unity;  if  it  be  unequal  the  color 
index  is  less  or  greater  than  unity;  e.g.,  if  the  percentage  of  hemoglobin  be 
but  60  and  the  percentage  of  red  corpuscles,  as  determined  by  the  method 
of  counting  be  but  80  (4,000,000  per  cubic  millimeter)  then  the  color  index 
is  f|^  =  o.75  which  indicates  that  each  corpuscle  retains  but  three-fourths 
of  the  normal  amount  of  hemoglobin;  if  on  the  contrary  the  percentage  of 
hemoglobin  be  but  60,  and  the  percentage  of  red  corpuscles  be  but  50  then 
the  color  index  is  1.2  which  indicates  that  each  corpuscle  contains  a  larger 
percentage  of  hemoglobin  than  normally.  This  condition  is  sometimes 
observed  in  pernicious  anemia. 

For  the  determination  of  these  variations  in  the  hemoglobin  for  clinical 
purposes  two  chromometric  methods  are  at  present  largely  employed,  that 
of  Gowers  and  v.  Fleischl.  All  chromometric  methods  are  based  on  the 
principle  that  if  two  equally  thick  and  equally  well-illuminated  solutions 
present  the  same  intensity  of  color,  their  richness  in  coloring-matter  is  the 
same.  There  are  two  methods  by  which  this  can  be  done:  (i)  By  diluting 
the  blood  to  be  examined  with  water  until  the  shade  of  color  corresponds 
to  that  of  a  solution  of  hemoglobin  of  known  strength  (Gowers).  (2) 
Diluting  a  given  quantity  of  blood  with  a  given  quantity  of  water  and  then 
finding  an  identical  color  which  represents  a  previously  determined  quantity 
of  hemoglobin  (v.  Fleischl). 

Gowers'  hemoglobinometer  consists  of  two  glass  tubes  of  exactly  the 
same  size  and  similar  to  those  shown  in  Fig.  99.  One,  A,  contains  glycerin 
jelly  colored  with  picro-carmine  the  shade  of  which  corresponds  to  that  of 
normal  blood  diluted  100  times  (20  c.mm.  in  2000  c.mm.  of  water),  repre- 
senting a  I  per  cent,  solution.  The  other  tube,  B,  is  ascendingly  graduated 
with  120  divisions,  each  one  of  which  corresponds  to  20  c.mm.  With  a 
graduated  pipette,  D,  20  cubic  millimeters  of  blood  are  accurately  measured 
and  dropped  into  the  bottom  of  the  tube  B,  in  which  a  few  drops  of  distilled 
water  have  been  placed  so  as  to  prevent  coagulation.  Water  is  then  added 
drop  by  drop  until  the  color  of  the  diluted  blood  is  exactly  that  of  the  stand- 
ard. The  division  of  the  scale  reached  by  the  dilution  will  represent  the 
relative  percentage  of  hemoglobin.  If  this  tint  is  not  obtained  until  the 
dilution  reaches  100  divisions,  the  quantity  of  hemoglobin  is  normal.  If 
more  water  must  be  added,  it  is  in  excess;  if  less,  it  is  diminished.  If,  for 
example,  the  20  cubic  millimeters  of  blood  from  an  anemic  patient  gave  the 


THE  BLOOD 


247 


standard  tint  at  60  divisions,  the  blood  contained  but  60  per  cent,  of  the 
normal  amount  of  hemoglobin. 


Fig.  99. — Haldane's  Modification  of  Gowers'  Apparatus. 

Haldane's  Modification  of  Gowers*  Method.— Haldane's  hemoglobin- 
ometer,  Fig.  99,  is  a  modification  of  that  of  Gowers.     The  tube  A  contains 


Fig.  100. — ^VoN  Fleischl's  IIemometer.  K.  Red  colored  wedge  of  glass  moved  by  R. 
G.  Mixing  vessel  with  two  compartments,  a  and  a'.  M.  Table  with  hole  to  read  off  the  percentage 
of  hemoglobin  on  the  scale  P.     T.  Pinion  to  move  K.     S.  Mirror  of  plaster-of-Paris. 

also  a  I  per  cent,  solution  of  blood  having  the  normal  percentage  of  hemo- 
globin saturated  with  carbon  monoxid.  With  the  graduated  capillary 
pipette,  D,  20  cubic  millimeters  of  blood  are  then  obtained  from  a  shght 


248  TEXT-BOOK  OF  PHYSIOLOGY 

wound  in  the  finger  or  elsewhere  and  then  dropped  into  the  tube  B,  in  which 
a  small  quantity  of  distilled  water  from  E  was  previously  placed  to  prevent 
coagulation.  The  cap  of  G  is  then  attached  to  a  gas  burner,  through  which 
flows  either  pure  CO  or  a  gas  containing  CO  and  the  rubber  tube  inserted 
into  D  to  the  level  of  the  water.  After  the  gas  has  been  flowing  for  a  few 
seconds  the  rubber  tube  is  withdrawn,  and  while  the  glass  tube  is  yet  full  of 
the  gas,  it  is  closed  with  the  thumb  and  gently  shaken  so  as  to  convert  all  the 
hemoglobin  into  carbon-monoxid  hemoglobin.  This  is  then  diluted  very 
gradually  as  in  the  employment  of  the  Gowers  apparatus  until  the  tint  of 
the  solution  in  B  corresponds  to  that  in  A.  The  level  at  which  this  is 
observed  indicates  the  percentage  of  hemoglobin  in  the  blood  used.  The 
error  in  this  method  scarcely  exceeds  i  per  cent. 

Von  Fleischl's  hemometer  consists  of  a  metallic  cell  divided  into  two 
compartments,  a  and  a',  by  a  vertical  partition  (Fig.  loo).  In  the  former 
a  definite  quantity  of  blood  is  placed  and  diluted  with  a  known  quantity  of 
water.  Beneath  the  compartment  a'  is  placed  a  glass  wedge  stained  with 
the  golden  purple  of  Cassius  or  similar  pigment,  the  color  of  which  passes 
from  a  deep  red  at  one  end  to  clear  glass  at  the  other  (Fig.  loi).     To  the 

side  of  this  wedge  is  placed  a  scale 
ranging  from  o  to  120.  By  means  of 
the  screw,  R  T,  the  glass  wedge  is 
moved  until  the  color  of  the  glass  and 
diluted  blood  is  identical.  The  illum- 
ination of  the  blood  and  glass  wedge  is 
Fig.  ioi.-Tinted  Glass  Wedge  of  the  accomplished  by  lamp-light  reflected 
VON  Fleischl  Hemometer.  from     the    white    reflecting    surface 

beneath.  The  depth  of  color  of  the 
glass  opposite  100  on  the  scale  corresponds  to  that  of  normal  blood.  If, 
therefore,  the  colors  are  identical  at  75  divisions,  the  blood  contains  but  75 
per  cent,  of  the  normal  amount  of  hemoglobin. 

Absorption  Spectra. — Both  oxyhemoglobin  and  reduced  hemoglobin, 
like  other  soluble  pigments,  have  an  absorbing  influence  on  certain  waves 
of  light,  and  hence  give  rise  to  absorption  bands  which  can  be  studied 
with  the  spectroscope,  and  which  are  so  characteristic  as  to  serve  for  their 
identification. 

In  principle  a  spectroscope  consists  of  a  prism  which  decomposes  the 
light  from  a  narrow  slit  into  a  band  of  all  the  spectral  colors.  A  form  of 
spectroscope  in  common  use  is  that  shown  in  Fig.  102.  It  consists  of  a 
tube,  B,  which  has  at  one  end  a  sHt  that  can  be  narrowed  or  widened  by 
means  of  a  screw.  The  light,  having  passed  through  it,  falls  on  an  achro- 
matic convex  lens  (called  the  collimator)  at  the  opposite  end  of  the  tube 
which  renders  the  divergent  rays  of  light  parallel.  These  parallel  rays  sub- 
sequently fall  on  the  prism,  by  which  they  are  dispersed  and  directed  into  the 
tube,  A,  which  is  nothing  more  than  a  small  telescope.  On  looking  into  it 
at  the  ocular  end  the  spectral  colors  are  seen.  If  the  light  has  been  derived 
from  the  sun,  the  spectrum  will  present  vertical  dark  lines,  the  so-called 
Fraunhofer's  lines.  They  are  given  from  A  to  F  in  Fig.  103.  If  a  colored 
medium  be  held  in  front  of  the  slit  so  that  the  light  has  to  pass  through  it  first, 
certain  dark  bands  will  appear  in  the  spectrum,  owing  to  the  absorption  of 
certain  rays. 


THE  BLOOD 


249 


Dilute  solutions  of  arterial  blood  show  absorption  bands  between  the 
Fraunhofer  lines,  D  and  E,  in  the  green  and  yellow  portion  of  the  spectrum. 


Fig.  102. — The  Spectroscope.  A.  Telescope.  B.  Tube  for  the  admission^  of  light  and 
carrying  the  collimator.  C.  Tube  containing  a  scale,  the  image  of  which  when  illuminated  is 
reflected  above  the  spectrum.     D.  The  fluid  examined. — {Landois  and  Stirling.) 

(See  Fig.  103.)  The  band  nearest  D,  frequently  designated  as  alpha,  is  dark 
in  the  center  and  sharply  defined.  The  band  which  lies  toward  E,  desig- 
nated as  beta,  is  broader  and  less  sharply  defined. 


Red.     Orange. 


Yellow. 


Green. 


Cyan  Blue. 


Oxy 

hemoglobin 

0.8  %. 


Fig.  103.— Spectra  of  Hemoglobin  and  Some  of  its  Compounds. 
— {Landois  and  Stirling.) 

As  the  amount  of  light  absorbed  varies  with  the  concentration  of  the 
solution  as  well  as  its  thickness,  and  gives  rise  to  absorption  bands  of  different 


250 


TEXT-BOOK  OF  PHYSIOLOGY 


widths  and  intensities,  it  becomes  necessary,  in  order  to  obtain  the  character- 
istic bands,  to  employ  only  dilute  solutions. 

The  absorption  spectra,  as  seen  with  different  strengths  of  solution  one 
centimeter  thick,  are  shown  graphically  in  Fig.  104.  It  will  be  observed 
that  solutions  varying  in  strength  from  o.i  per  cent,  to  0.6  per  cent,  give  rise 
to  the  two  characteristic  bands,  but  with  gradually  increasing  breadths. 
With  a  percentage  greater  than  0.65  per  cent,  the  light  between  D  and  E, 
the  yellow-green,  becomes  extinguished  and  the  two  bands  fuse  together, 
forming  a  single  band  overlapping  slightly  the  lines  D  and  E.  At  the  same 
time  there  is  a  progressive  darkening  of  the  violet  end  of  the  spectrum.  At 
0.85  per  cent.,  all  the  light  is  absorbed  with  the  exception  of  a  small  amount 
of  the  red.  Solutions  less  than  o.oi  per  cent,  to  0.003  P^^  cent,  show  but  a 
single  absorption  band — that  nearest  D. 

A  solution  of  venous  blood  or  of  reduced  hemoglobin  shows  but  a  single 
absorption  band  (see  Fig.  103),  frequently  designated  as  gamma,  broader 


oBC    D       Eb    F  G     h 

Fig.  104. — Graphic  Representa- 
tion OF  THE  Absorption  of  Light  in 
A  Spectrum  by  Solutions  of  Oxy- 
hemoglobin OF  Different  Strengths. 
The  shading  indicates  the  amount  of 
absorption  of  the  spectrum,  and  the 
numbers  at  the  side  the  strength  of  the 
solution.  — (Rollet.) 


aBC    D       Eb    F  G     h 

Fig.  105. — Graphic  Representation 
OF  THE  Absorption  of  Light  in  a  Spec- 
trum BY  Solutions  of  Reduced  Hemo- 
globin of  Different  Strengths.  The 
shading  indicates  the  amount  of  absorp- 
tion of  the  spectrum,  and  the  numbers  at 
the  side  the  strength  of  the  solution. — 
(Rollet.) 

and  less  marked  between  the  lines  D  and  E,  but  extending  slightly  beyond  D. 
Fig.  105  shows  in  the  same  graphic  manner  the  increasing  breadth  of  the 
absorption  band  with  increasing  strengths  of  solution,  as  well  as  the  simul- 
taneous absorption  of  light  at  both  the  red  and  violet  ends  of  the  spectrum. 

Oxyhemoglobin;  Reduced  Hemoglobin.— The  coloring-matter  of 
the  blood  is  characterized  by  the  property  of  combining  with  and  of  again 
yielding  up  oxygen.  The  union  is  a  chemic  one,  taking  place  under  certain 
pressure  conditions.  It  therefore  may  exist  in  two  states  of  oxidation, 
distinguished  by  a  difference  in  color  and  their  absorption  spectra.  If 
hemoglobin  either  in  blood  or  in  solution  be  shaken  with  air,  it  at  once 
combines  with  oxygen  and  is  converted  into  oxyhemoglobin,  which  imparts 
to  the  blood  or  solution  a  bright  red  or  scarlet  color.  If  the  blood  or  solu- 
tion be  now  deprived  of  oxygen,  the  oxyhemoglobin  is  converted  into  reduced 
hemoglobin,  which  imparts  to  the  blood  or  solution  a  dark  bluish-red  or  pur- 
ple color. 

The  quantity  of  oxygen  absorbed  by  i  gram  of  hemoglobin  is  estimated 


THE  BLOOD  251 

at  1.56  c.c.  measured  at  o°C.  and  760  mm.  of  mercury.  The  compound 
formed  by  the  union  of  oxygen  and  hemoglobin  is  a  very  feeble  one;  for 
when  the  pressure  is  lowered  the  union  becomes  less  stable,  and  as  the  zero 
point  is  approached,  as  in  the  Torricellian  vacuum,  a  rapid  dissociation  of 
the  oxygen  takes  place.  This,  however,  is  not  due  entirely  to  a  fall  of 
pressure  but  partly  to  the  dissociation  force  of  heat,  which  increases  in  power 
as  the  pressure  falls.  The  same  dissociation  of  oxygen  can  be  brought  about 
by  passing  through  blood  indifferent  gases,  such  as  hydrogen,  nitrogen, 
carbon  dioxid,  which  lower  oxygen  pressure,  or  by  the  addition  of  reducing 
agents,  such  as  ammonium  sulphid  or  Stokes'  fluid. 

These  experimental  determinations  of  the  relation  of  oxygen  to  hemo- 
globin partly  explain  the  oxidation  and  deoxidation  of  the  hemoglobin  in  the 
lungs  and  tissues.  As  the  blood  passes  through  the  lungs  and  is  subjected 
to  the  oxygen  pressure  there,  the  hemoglobin  combines  with  a  definite 
quantity  of  oxygen,  and  on  emerging  from  the  lungs  exhibits  a  bright  red  or 
scarlet  color;  as  the  blood  passes  through  the  systemic  capillaries  where  the 
oxygen  pressure  in  the  surrounding  tissues  is  low,  the  oxyhemoglobin  yields 
up  a  portion  of  its  oxygen,  becoming  deoxidized  or  reduced,  and  the  blood 
on  emerging  from  the  tissues  exhibits  a  dark  bluish  color.  The  portion  of 
oxygen  given  up  to  the  tissues  is  termed  respiratory  oxygen.  In  100  parts  of 
arterial  blood  the  coloring-matter  presents  itself  almost  exclusively  in  the 
form  of  oxyhemoglobin.  In  passing  through  the  capillaries  about  5  per  cent, 
only  gives  up  its  oxygen  and  becomes  reduced,  so  that  both  kinds  are  present 
in  venous  blood.  In  asphyxiated  blood  only  reduced  hemoglobin  is  present. 
It  is  this  capability  of  combining  with  and  of  again  yielding  up  oxygen,  that 
enables  hemoglobin  to  become  the  carrier  of  oxygen  from  the  lungs  to  the 
tissues. 

Carbon  Monoxid  Hemoglobin. — Carbon  monoxid  is  a  constituent  of 
coal-gas  and  more  largely  of  water-gas.  From  either  source  it  is  likely  to 
accumulate  in  the  air,  and  if  inspired  for  any  length  of  time  produces  a  series 
of  eflFects  which  may  eventuate  in  death.  If  blood  be  brought  into  contact 
with  this  gas,  it  assumes  a  bright  cherry-red  color,  which  is  quite  persistent 
and  due  to  the  displacement  of  the  loosely  combined  oxygen  and  the  union 
of  the  carbon  monoxid  with  the  hemoglobin.  The  compound  thus  formed 
is  very  stable  and  resists  the  action  of  various  reducing  agents.  Tiie  passage 
of  air  or  of  some  neutral  gas  through  the  blood  for  a  long  period  of  time  will 
gradually  displace  the  carbon  monoxid  and  enable  the  hemoglobin  again  to 
absorb  oxygen.  It  is  for  this  reason  that  partial  poisoning  with  the  gas  is 
not  fatal.  It  is  to  its  power  of  displacing  oxygen  and  forming  a  stable  com- 
pound with  hemoglobin  and  thus  interfering  with  its  respiratory  function 
that  carbon  monoxid  owes  its  poisonous  properties.  Examined  spectro- 
scopically,  solutions  of  carbon  monoxid  hemoglobin  exhibit  two  absorption 
bands  closely  resembling  in  position  and  extent  those  of  oxyhemoglobin;  but 
careful  examination  shows  that  they  are  slightly  nearer  the  violet  end  of  the 
spectrum  and  closer  together.  (See  Fig.  103.)  A  useful  test  for  CO  blood 
is  the  addition  of  caustic  soda,  which  produces  a  cinnabar  red  precipitate. 

Carbo-hemoglobin. — ^When  hemoglobin  is  exposed  to  an  atmosphere 
of  oxygen  and  carbon  dioxid,  it  will  absorb  the  latter  as  well  as  the  former, 
though  the  union  of  the  oxygen  under  such  circumstances  is  not  as  strong 
as  when  oxygen  alone  is  present.     To  the  compound  formed  by  the  union  of 


252  TEXT-BOOK  OF  PHYSIOLOGY 

the  carbon  dioxid  and  the  hemoglobin  the  name  carbo-hemoglobin  has  been 
given.  The  union,  however,  is  not  very  stable  as  the  carbon  dioxid  can  be 
readily  dissociated.  As  the  carbon  dioxid  absorption  will  take  place, 
even  though  the  hemoglobin  is  practically  saturated  with  oxygen,  the  as- 
sumption has  been  made  that  the  oxygen  combines  with  hemochromogen, 
and  the  carbon  dioxid  with  the  protein  constituent — globin.  If  this  be  the 
case  the  hemoglobin  and  hence  the  red  corpuscle,  can  be  regarded  as  con- 
cerned in  the  absorption  and  transmission  of  carbon  dioxid  from  the  tissue 
to  the  lungs. 

Methemoglobin. — This  is  a  pigment,  closely  related  to  oxyhemoglobin, 
found  in  the  blood  after  the  administration  of  various  drugs,  in  cysts  and  in 
the  urine  in  hematuria  and  hemoglobinuria.  It  is  also  produced  when  a 
solution  of  hemoglobin  is  exposed  to  the  air  and  becomes  acid  in  reaction 
and  brown  in  color.  The  spectrum  shows  two  absorption  bands  similar  to 
oxyhemoglobin,  but  in  addition  a  new  and  quite  distinct  band  near  the  line 
C,  in  the  red.  If  the  acid  solution  be  rendered  alkaline  by  the  addition  of 
ammonia,  this  band  disappears  and  another  makes  its  appearance  near  the 
line  D.  The  addition  of  ammonium  sulphid  develops  reduced  hemoglobin, 
which,  on  the  absorption  of  oxygen,  produces  again  oxyhemoglobin,  as 
shown  by  the  spectroscope. 

Hematin. — Boiling  hemoglobin  or  adding  to  it  acids  or  alkalies  decom- 
poses it  and  develops  one  or  more  protein  bodies  to  which  the  general  term 
globulin  has  been  given,  and  an  iron-holding  pigment  termed  hematin. 
This  is  regarded  as  an  oxidation  product  of  hemoglobin  and  constitutes 
about  4  per  cent,  of  its  composition.  When  obtained  in  a  pure  state,  it  is  a 
non-crystallizable  blue-black  powder  with  a  metallic  luster.  According  as 
it  is  treated  with  acids  or  alkalies,  two  combinations  of  hematin  can  be 
obtained  (acid  and  alkaline),  each  of  which  has  special  properties,  giving 
rise  to  different  absorption  bands. 

Hemin. — This  pigment  is  a  derivative  of  hematin,  presenting  itself  in 
the  form  of  microscopic  rhombic  plates  or  rods  (Teichmann's  crystals), 
which  are  so  characteristic  as  to  serve  as  tests  for  blood-stains  in  medicolegal 
inquiries.  These  crystals  are  readily  obtained  by  adding  to  a  small  quantity 
of  dried  blood  on  a  glass  slide  a  few  drops  of  glacial  acetic  acid  and  a  crystal 
of  sodium  chlorid;  after  heating  gently  for  a  few  minutes  over  a  spirit  lamp 
and  then  allowing  the  mixture  to  cool,  crystallization  of  the  hemin  soon  takes 
place. 

Hematoidin. — This  term  has  been  applied  to  a  pigment  which  occurs 
in  the  form  of  yellow  crystals  in  old  blood-clots  or  in  blood  which  has  been 
extra vasated  into  the  tissues.  In  its  chemic  composition  and  in  its  reactions 
it  closely  resembles  bilirubin,  the  pigment  of  the  bile,  exhibiting  the  same 
characteristic  play  of  colors  on  the  addition  of  nitric  acid. 

The  Stroma. — The  stroma  of  the  red  corpuscles  obtained  by  the  methods 
which  dissolve  out  the  hemoglobin  has  been  shown  by  analysis  to  consist  of 
from  60  to  70  per  cent,  of  water  and  40  to  30  per  cent,  of  solid  material, 
containing  a  proteid  resembling  cell-globulin,  lecithin,  cholesterin,  and 
inorganic  salts,  among  which  potassium  phosphate  is  especially  abundant. 

Life-history  of  Red  Corpuscles. — In  the  performance  of  their  functions 
the  red  corpuscles  undergo  more  or  less  disintegration  and  finally  destruction; 
but  as  the  average  number  is  maintained  under  normal  physiologic  condi- 


THE  BLOOD  253 

tions,  there  must  be  a  constant  renewal  of  corpuscles  from  day  to  day.  The 
evidence  of  destruction  of  red  corpuscles  is  furnished  by  the  presence  in  the 
blood,  in  various  situations  of  the  body,  of  a  pigment  containing  iron  and  the 
presence  of  pigments  in  the  bile  and  urine,  all  of  which  are  believed  to  be 
derivatives  of  effete  hemoglobin.  The  blood-pigment  (hematin),  which 
contains  the  iron  of  the  hemoglobin,  is  found  in  the  capillaries  of  the  liver, 
in  the  cells  of  the  splenic  pulp,  and  in  the  marrow  of  the  bones.  Whether 
the  presence  of  the  pigment  in  these  organs  is  a  proof  that  the  corpuscles  are 
destroyed  here,  or  whether  they  are  to  be  regarded  merely  as  agents  con- 
cerned in  the  further  reduction  and  elimination  of  the  hematin,  is  uncertain. 
The  genetic  relationship  between  bile-pigment  and  hemoglobin  is  shown  by 
the  fact  that  any  artificial  destruction  of  hemoglobin  or  its  injection  into  the 
blood  is  attended  by  an  increase  in  the  quantity  of  bile-pigment  eliminated. 
It  appears  also  from  chemic  considerations  that  the  hemoglobin  will  undergo 
cleavage  into  a  globulin  body  and  hematin,  which  by  the  loss  of  its  iron  is 
readily  converted  into  the  bile-pigment,  bilirubin.  The  amount  of  this 
latter  pigment  may  therefore  be  taken  as  an  index  of  the  extent  of  corpuscular 
destruction. 

There  is  some  evidence  for  the  view  that  the  iron  set  free  by  the  reduc- 
tion of  the  hematin,  eventually  reaches  the  blood-making  organs  and  is 
utilized  by  the  developing  corpuscles  in  the  formation  of  the  necessary 
hemoglobin. 

This  gradual  decay  of  corpuscles  as  well  as  the  losses  occasioned  by 
hemorrhages  necessitate  a  continuous  formation  of  new  corpuscles,  so  that 
the  normal  number  may  be"  maintained.  The  rapidity  with  which  corpuscles 
may  be  renewed,  in  the  woman  at  least,  is  shown  by  a  computation  of  Mr. 
Charles  L.  Mix.  A  woman  loses  during  a  menstrual  period  150  c.c.  of 
blood.  At  the  end  of  twenty-eight  or  thirty  days  this  volume  is  restored,  so 
that  in  one  day  5  c.c,  or  5000  c.mm.,  of  blood  must  be  formed,  or  208 
c.mm.  per  hour  and  3|-  c.mm.  per  minute.  That  is,  during  a  certain  num- 
ber of  years  15,750,000  corpuscles  must  be  formed  every  minute,  and  this 
independent  of  the  daily  loss  due  to  functional  activity. 

At  the  present  time  there  is  a  general  agreement  among  histologists  that 
in  adult  life  the  red  corpuscles  are  derived  from  embryonic  forms,  the  so- 
called  erythroblasts,  cells  of  a  large  size  with  distinctly  reticulated  nuclei, 
which  are  found  chiefly  in  the  red  marrow  of  the  long  bones,  ^  In  this 
situation  both  arterial  and  venous  capillaries  are  relatively  large  and  the 
blood  is  separated  from  the  surrounding  marrow  by  extremely  thin  walls. 
In  the  passages  of  this  capillary  network  the  erythroblasts  make  their  appear- 
ance most  probably  by  a  transformation  of  pre-existing  marrow  cells  which 
cross  the  capillary  wall  from  without.  At  first  they  are  large,  homogeneous, 
colorless,  perhaps  slightly  tinged  with  hemoglobin  and  distinctly  nucleated. 
They  increase  in  number  by  karyokinesis  and  at  the  same  time  increase  in 
their  hemoglobin  content.  In  the  course  of  their  development  the  nucleus 
becomes  smaller  and  denser,  when  the  cells  are  known  as  normoblasts. 
Subsequently  the  nucleus  is  extruded,  carrying  with  it  a  portion  of  the  peri- 
nuclear cytoplasm,  after  which  the  remainder  of  the  corpuscle  assumes 

*  For  an  admirable  resume  of  the  various  views  regarding  the  origin  and  formation  of  red 
corpuscles  see  the  paper  of  Mr.  Charles  L.  Mix,  Boston  Med.  and  Surg.  Journal,  1892,  Nos. 
II  and  12;  also  paper  by  Prof.  W.  H.  Howell,  Journal  of  Morphology,  vol.  iv,  1892. 


254 


TEXT-BOOK  OF  PHYSIOLOGY 


the  shape  and  size  of  the  adult  corpuscle  and  is  carried  out  into  the  general 
circulation.  After  severe  hemorrhage  the  formative  processes  in  the  marrow 
may  become  so  active  that  erythroblasts  and  normoblasts  make  their  appear- 
ance in  the  blood-stream  before  the  extrusion  of  the  nucleus  has  taken 
place. 

The  Corpuscles  of  Other  Vertebrated  Animals. — In  all  mammals, 
with  the  exception  of  the  camel,  llama,  and  dromedary,  the  red  corpuscles 
present  the  same  shape  and  structure  as  the  corpuscles  of  man,  and  may  be 
described  as  circular,  flattened,  biconcave  discs.  In  the  animals  excepted 
the  corpuscles  are  oval.  The  size,  however,  varies  in  different  animals  from 
0.0092  mm.  (s^yVs"  irich)  in  the  elephant  to  0.0023  ^^^-  (t 2"i"2"5- ^^^h)  in  the 
musk-deer,  while  in  most  animals  the  average  lies  between  0.0084  mm.  and 
0.0050  mm.  Inasmuch  as  the  question  may  arise  as  to  whether  the  corpus- 
cles of  any  given  specimen  of  blood  are  those  of  a  human  being  or  of  some 
other  mammal,  a  knowledge  of  the  size  of  the 
corpuscles  is  a  matter  of  medicolegal  as  well  as  of 
physiologic  interest.  Though  the  difl'erences  in  size 
are  slight,  yet  it  is  possible  for  skilled  microscopists, 
when  examining  fresh  blood,  to  make  a  diagnosis 
between  the  corpuscles  of  man  and  those  of  the 
domesticated  animals,  with  the  exception,  perhaps, 
of  the  guinea-pig.  The  diagnosis  of  the  corpuscles 
of  dried  blood  which  have  been  altered  by  the  ac- 
tion of  various  external  agents,  even  though  capa- 
ble of  a  certain  degree  of  restoration,  is  most  dif- 
ficult, and  should  not  be  attempted  in  criminal 
cases  without  large  experience  in  microscopy,  in 
measurements  and  methods  of  preparation  of  all 
kinds  of  blood-corpuscles,  and  a  proper  conception  of  corpuscular  forms 
and  sizes.  In  the  following  table  the  average  results  of  the  measurements 
of  the  corpuscles  in  different  classes  of  animals  are  given  (abstracted  from 
Formad's  compilation) : 


Fig.  106.  Fig.  107. 
Amphibian  Coloef.d 
Blood-corpuscles.  Fig. 
106,  on  the  flat;  Fig.  107, 
on  edge.  —  {Landois  and 
Stirling.) 


Gulliver 


Inch 


Mm. 


Wormley 


C.  Schmidt 
Mallinin 


French  Medi- 
colegal Soc. 
Welcker 


Formad 


Inch 


Mm. 


Inch 


Mm. 


Inch       Mm.      Inch  !   Mm. 


Man  ...._. 1.32000.00791.32500.0078  t.3300    0.007711.3257  !o.oo78!i.320o  0.0079 

Guinea-pig 1.35380. 00  71  1.3223  0.0079  i  .3300*  o.oo77i.32i3t  0.0079  i.  3400  0.0075 

Dog.. I1.3S320.0071  1. 3561  0.0071  1. 3  63  6    0.0070  1. 3485  0.0073  1. 35  80  0.0071 

Rabbit Ii. 3  60  7  0.0070  i. 3653  0.0070  i. 3968    0.0064  i. 3653  0.0069  i  .3662  0.0069 

Ox..  ..• 1. 4267  0.0060  1. 4219  0.0060  1. 4354     0.0058  1.4545  0.0056  1. 4200  0.0060 

Pig I1.42300.00601.42680.00591.4098    0.0062  1. 4098  0.0062  1. 4250  0.0060 

Horse 1 1.4600  0.0057  1.4243  0.0059  1.4464    0.0057  1.4545  0.0056  1.4310  0.0059 

Cat ' 1 .4400  0.0058  1.4372  0.0058  1. 4545    0.0056  1.3922    0.0065 

Sheep Ii  .5300  0.0048  1 .4912  0.0031  1 .5649    0.0045  1. 5076  0.0059  1.5000  0.0051 

Goat 11.6366  0.0040  1.6189,0.0041,1.6369  jo.0040  1 .5525  10.00461 1 .610010.004a 


*  Masson. 


t  Woodward. 


In  birds,  reptiles,  and  amphibians  the  corpuscles  are  larger  than  in 
mammals,  are  oval  in  shape,  and  nucleated.  (See  Figs.  io6  and  107.) 
As  the  scale  of  animal  life  is  descended  the  corpuscles  increase  in  size,  until 
in  Proteus  and  Amphiuma  the  long  diameter  attains  an  average  length  of 
0.058  mm.  and  0.077  mm.  respectively.     In  fish  the  corpuscles  are  smaller, 


THE  BLOOD  255 

oval,  and  nucleated,  with  the  exception  of  the  lamprey  eels,  in  which  they 
are  circular,  biconcave,  and  nucleated,  though  the  nucleus  is  generally  con- 
cealed in  the  peripheral  portion  of  the  corpuscle.  As  in  these  animals  the 
corpuscles  are  almost  twice  the  size  of  the  human  red  corpuscles,  they  can, 
notwithstanding  the  similarity  of  shape,  be  readily  distinguished  from  them. 

The  Function  of  the  Red  Corpuscles. — The  red  corpuscles,  by  virtue 
of  the  capacity  of  their  contained  hemoglobin  for  oxygen  absorption,  may  be 
reg^ded  as  carriers  of  oxygen  from  the  lungs  to  the  tissues,  and  therefore 
important  factors  in  the  general  respiratory  process.  The  size  as  well  as  the 
number  of  the  corpuscles  in  different  classes  of  animals  appears  to  be  directly 
related  to  the  activity  of  the  respiratory  process.  In  those  animals  in  which 
the  corpuscles  are  small  and  numerous  and  the  total  superficial  area  large, 
respiration  is  active,  the  quantity  of  oxygen  absorbed  is  large,  and  the 
energy  liberated  through  oxidation  is  correspondingly  large.  In  those 
animals,  on  the  contrary,  in  which  the  corpuscles  are  large  and  relatively 
few  in  number,  the  reverse  conditions  obtain.  This  is  in  accordance  with 
the  fact  that  the  superficial  area  of  any  given  volume  of  substance  is  increased 
in  proportion  to  the  extent  to  which  it  is  subdivided. 

The  superficial  area  of  a  single  human  red  corpuscle  has  been  estimated 
at  0.000128  sq.  mm.  If  the  number  of  corpuscles  in  i  cubic  millimeter  of 
blood  averages  5,000,000,  the  superficial  area  would  amount  to  640  square 
millimeters;  and  if  the  amount  of  blood  in  the  body  of  a  man  weighing  70 
kilos  is  taken  as  one-nineteenth  of  this  weight — that  is,  3864  grams  (3659  c.c.) 
— the  total  area  of  the  corpuscular  surface  will  amount  to  2341  square  meters. 

HISTOLOGY  OF  THE  WHITE  CORPUSCLES  OR  LEUKOCYTES 

The  presence  of  white  corpuscles  in  the  blood  can  be  readily  observed 
under  the  same  conditions  as  the  red  corpuscles  are  observed.  Thus  when 
the  mesentery  of  the  frog  or  the  guinea-pig  is  examined  with  the  microscope 
the  white  corpuscles  are  seen  adhering  to  the  walls  of  the  blood-vessels;  in 
a  drop  of  freshly  drawn  blood  they  are  found  in  the  spaces  between  red 
corpuscles  (Fig.  93).  A  careful  examination  of  the  blood  by  the  employ- 
ment of  appropriate  methods  has  revealed  the  presence  of  several  varieties  of 
white  corpuscles,  to  which  reference  will  be  made  in  a  subsequent  paragraph: 

Shape  and  Size. — In  the  resting  condition,  whether  seen  in  the  vessel 
or  on  the  stage  of  the  microscope,  the  white  corpuscle,  as  its  name  implies,  is 
grayish  in  color,  round  or  globular  in  form,  though  often  presenting  a  more 
or  less  irregular  surface.  Its  diameter  varies  from  0.004  to  0.013  mm., 
though  the  average  is  about  o.oii  mm.  or  about  w^wo  inch. 

Structure. — A  typical  white  corpuscle  consists  of  a  ground-substance 
uniformly  transparent  and  apparently  homogeneous,  in  which  are  embedded 
a  number  of  granules  of  varying  size,  some  of  which  are  very  fine,  while 
others  are  large.  By  various  reagents  it  has  been  demonstrated  that  the 
granules  are  fatty,  protein,  and  carbohydrate  (glycogen)  in  character.  In 
the  fresh  cells  the  existence  of  a  nucleus  is  difficult  of  detection,  though  its 
presence  can  be  demonstrated  by  the  addition  of  acetic  acid,  which  renders 
the  perinuclear  cytoplasm  more  transparent  and  makes  the  nucleus  con- 
spicuous and  sharply  defined.  From  its  structure  it  is  apparent  that  the 
white  corpuscle  belongs  to  the  group  of  undifferentiated  tissues  and  resembles 


256  TEXT-BOOK  OF  PHYSIOLOGY 

the  cells  of  the  embryo  in  its  earliest  stages  as  well  as  the  unicellular  organism, 
the  amoeba. 

Chemic  Composition. — The  chemic  composition  of  the  white  corpuscles 
has  been  inferred  from  an  analysis  of  pus-corpuscles,  with  which  they  are 
practically  identical,  and  of  lymph-corpuscles  from  the  lymph-glands.  Of 
the  corpuscle  about  90  per  cent,  is  water  and  the  remainder  solid  matter 
consisting  mainly  of  proteins,  of  which  nuclein,  nucleo-albumin,  and  cell 
globulin  are  the  most  abundant.  The  two  former  are  characterized  b/  the 
presence  of  a  considerable  quantity  of  phosphorus,  amounting  to  as  much  as 
10  per  cent.  Lecithin,  fat,  glycogen,  and  earthy  and  alkaline  phosphates 
are  also  present. 

Number  of  White  Corpuscles. — The  number  of  white  corpuscles  per 
cubic  millimeter  of  blood  is  much  less  than  the  number  of  red  corpuscles,  the 
ratio  being  in  the  neighborhood  of  i  white  to  700  red.  This  ratio,  however, 
varies  within  wide  limits  in  different  portions  of  the  body  and  under  normal 


-5,. 


a^Sr 


Fig.  108. — Amceboid  Movements  of  a  White  Corpuscle  from  the  Frog.  The  form 
changes  occurred  within  ten  minutes.  The  black  particles  are  Chinese  ink  which  had  been 
injected  twenty-four  hours  before  into  the  dorsal  lymph  sac. — (Rauber-Kopsch.) 

variations  in  physiologic  conditions.  In  the  blood  of  the  splenic  artery  there 
is  but  I  white  to  2260  red,  while  in  the  splenic  vein  there  is  i  white  to  every 
60  red;  or  about  thirty-eight  times  as  many  as  in  the  artery.  In  the  portal 
vein  there  is  i  white  to  740  red,  while  in  the  hepatic  vein  there  is  i  white  to 
170  red. 

The  total  number  of  white  corpuscles  per  cubic  millimeter  has  been 
estimated  at  from  5000  to  10,000,  though  the  average  is  about  7500.  The 
number,  however,  is  influenced  by  a  variety  of  physiologic  conditions.  The 
ingestion  of  food  rich  in  protein  material  raises  the  count  from  30  to  40  per 
cent.,  as  compared  with  the  count  before  the  meal.  In  the  new-born  the 
number  is  greater  than  in  adults — ^17,000  to  20,000  per  cubic  millimeter. 
Cabot  states  that  30,000  is  never  a  high  count  after  a  meal  in  infants  under 
two  years.  In  the  later  months  of  pregnancy,  especially  in  primiparae,  the 
number  increases  to  16,000  to  18,000.  Many  pathologic  conditions  of  the 
body  also  influence  the  count  very  considerably. 

Fasting  for  a  few  days  always  lowers  the  count,  and  in  a  case  of  total 
abstinence  of  food  for  a  week,  reported  by  Luciani,  the  count  fell  to  861  per 


THE  BLOOD 


257 


cubic  millimeter,  after  which  it  rose  to  1530,  where  it  practically  remained 
for  the  succeeding  three  weeks  of  the  fasting  period. 

When  the  number  of  leukocytes  present  in  the  peripheral  blood  exceeds 
the  normal,  i.e.,  10,000  per  cubic  millimeter  the  condition  is  termed  leuko- 
cytosis; when  the  number  falls  below  the  normal  the  condition  is  termed 
leukopenia.  Both  conditions,  however,  may  be  only  temporary  and  therefore 
physiologic,  or  they  may  be  permanent,  associated  with  certain  diseased  states 
of  the  body  and  therefore  pathologic.  It  is  therefore  permissible  to  speak 
of  a  physiologic  and  a  pathologic  leukocytosis  and 
leukopenia. 

The  method  for  counting  the  white  corpuscles 
is  similar  to  that  used  in  counting  the  red  corpus- 
cles. The  given  volume  of  blood  should,  however, 
be  diluted  with  10  or  20  volumes  of  a  one  per  cent, 
solution  of  acetic  acid,  which  disintegrates  the  red 
corpuscles  and  thus  facilitates  the  counting  of  the 
white.  The  pipette  should  have  a  larger  bore  than 
that  used  for  the  red,  and  a  much  greater  number 
of  squares  in  the  counting  chamber  should  be 
counted,  so  as  to  diminish  the  percentage  of  error. 

Physiologic  Properties. — The  white  corpuscles 
and  especially  the  leukocytes  possess  the  character- 
istic property  of  exhibiting  movements  similar  to 
those  exhibited  by  the  amoeba,  and  are  therefore 
termed  amoeboid.  These  movements  consist  in  al- 
ternate protrusions  and  retractions  of  portions  of  the 
cell  body,  as  a  result  of  which  they  present  from  mo- 
ment to  moment,  a  great  variety  of  forms.  (See 
Fig.  108.)  The  protruded  process,  the  pseudopod, 
can  also  attach  itself  to  some  point  of  the  surface 
on  which  it  rests,  and  then  draw  the  body  of  the 
corpuscle  after  it.  By  a  repetition  of  this  process 
the  corpuscle  can  slowly  creep  about  and  change 
its  position  in  reference  to  its  environment.  By 
virtue  of  these  amoeboid  movements  the  corpuscle 
can  appropriate  small  particles  of  pigment,  such 
as  indigo  or  carmine,  and  after  a  short  time  elimi- 
nate them  from  various  parts  of  the  surface.  It 
is  also  capable  of  thrusting  a  process  into  and  through  the  wall  of  the 
capillary  vessel,  after  which  the  remainder  of  the  corpuscle  follows  (Fig. 
109).  This  continues  until  the  corpuscle  is  outside  the  vessel  and  in  the 
lymph-space,  where  it  resumes  its  original  shape  and  movement.  This 
process  is  best  observed  in  inflammatory  conditions,  when  the  blood  has 
come  to  rest  and  the  vessels  are  occluded  with  both  red  and  white  corpuscles. 
To  this  passage  of  the  white  blood-corpuscles  through  the  capillary  wall 
the  term  diapedesis  is  given.  The  movements  of  the  white  corpuscles  are 
increased  by  a  rise  in  temperature  up  to  40°C.,  beyond  which  they  cease, 
owing  to  the  coagulation  of  the  cell-substance.  A  low  temperature  also 
arrests  the  movements.  Induced  electric  currents  also  cause  contraction 
and  death  of  the  cell.  Moisture  and  oxygen  are  necessary  to  their  activity. 
17 


Fig.  109. — Small  Ves- 
sel SHOWING  Various 
Stages  in  the  Diapedesis 
OF  Leukocytes.— (G.  Bach- 
vian.) 


258  TEXT-BOOK  OF  PHYSIOLOGY 

From  their  similarity  to  lower  organisms  the  white  corpuscles  may  be  re- 
garded as  independent  organisms  living  in  the  animal  iluids,  just  as  the 
amoeba  lives  in  its  natural  liquid  medium. 

Varieties  of  Leukocytes. — A  detailed  study  of  the  blood  with  the  aid 
of  the  triacid  staining  fluid  of  Ehrlich  or  any  of  the  various  eosin  and  methy- 
lene-blue  stains,  reveals  the  presence  of  five  distinct  varieties  of  leukocytes 
and  transitional  forms  which  may  be  classified  as  follows: 

1.  SmaU  lymphocytes,  so  called  from  their  resemblance  to  the  corpuscles 

of  the  lymph-glands,  consisting  of  a  deeply  staining  and  relatively 
large  round  nucleus,  encircled  by  a  narrow  rim  of  cytoplasm.  Found 
in  from  20  to  25  per  cent,  of  all  leukocytes.  They  vary  in  size  from 
0.004  to  0.007  ni^- 

2.  Large  lymphocytes  or  hyaline  cells,  which  are  believed  by  some  to  represent 

the  preceding  type  at  a  later  stage  of  development,  by  others  to  have  an 
independent  origin,  are  distinguished  by  a  round  or  ovoid  nucleus 
staining  faintly  and  surrounded  by  a  relatively  larger  layer  of  cytoplasm 
than  is  seen  in  the  small  lymphocyte.  The  large  lymphocyte  is  present 
to  the  extent  of  from  4  to  8  per  cent.  Transitional  forms,  usually  pre- 
sent from  I  to  2  per  cent,  are  very  much  like  the  large  lymphocyte  in 
appearance  and  size,  with  the  exception,  however,  that  they  possess  a 
cresentic  or  indented  nucleus  and  have  a  somewhat  greater  affinity  for 
basic  dyes.  They  are  usually  counted  with  the  large  lymphocytes. 
Both  varieties  of  lymphocytes  are  characterized  by  a  cytoplasm 
which  is  devoid  of  granules.  Rarely,  basophilic  granules  may  be 
present. 

3.  Polymorphonuclear  neutr ophites.     The  nucleus  of  this  cell  is  irregular 

and  assumes  a  great  variety  of  shapes  in  different  cells,  a  feature  which 
has  suggested  the  name  given  to  the  cell.  The  perinuclear  cystoplasm 
contains  a  large  number  of  fine  granules  which  are  neutrophilic  or 
faintly  acidophihc  in  their  staining  reaction.  They  make  up  about  60 
to  70  per  cent,  of  the  whole  numbeT  of  the  white  blood-cells.  They 
vary  in  size  from  0.007  to  o.oio  of  a  mm. 

4.  Eoslnophile  cells.     The  nucleus  resembles  in  many  respects  that  of  the 

preceding  variety;  it  is,  however,  less  apt  to  stain  so  deeply.  It  is  also 
very  irregular  in  shape  and  many  cells  possess  several  apparently  dis- 
tinct nuclei.  The  cytoplasm  is  ill-defined  but  its  presence  is  easily 
revealed  through  the  large,  intensely  acidophilic  granules  which  it 
possesses. 

It  is  present  to  the  extent  of  0.5  to  2  per  cent. 

5.  Basophile  cells,  the  nucleus  of  which  is  round  or  slightly  irregular.     The 

granules,  which  may  be  large  or  small,  are  basophilic  and  stain  more 
deeply  than  the  nucleus,  though  they  have  the  same  color.  It  is  rare 
for  this  cell  to  be  present  above  0.5  per  cent,  of  all  leukocytes. 

In  abnormal  states  of  the  blood  other  forms  of  leukocytes  are  fre- 
quently present,  e.g.,  myelocytes,  leukoblasts,  myeloplaxes,  etc.,  the 
significance  of  which  is  not  always  apparent. 
Origin  of  the  White  Corpuscles. — Of  the  various  theories  advanced  to 
explain  the  origin  of  leukocytes,  that  formulated  by  Ehrlich  has  found  the 
most  credence.     According  to  this  theory  the  leukocytes  may  genetically  be 
classed  into  two  groups.     In  the  first  group  are  the  large  and  small  lympho- 


(Triacid  Stain.) 

I,  2,  3,  4.  Small  Lymphocytes. 

Contrast  the  faintly  colored  protoplasm  of  these  cells  in  the  triple  stained  si)ccinien,  with 
their  intensely  basic  protoplasm  in  the  film  stained  with  eosin  and  methylene-blue,  17  and  18. 
The  cell  body  of  i  is  invisible.     Note  the  kidney-shajjcd  nucleus  in  4. 

5,  6.  Large  Lymphocytes. 

With  this  stain  the  nucleus  reacts  more  strongly  than  the  protoplasm;  with  eosin  and  meth- 
ylene-blue (19,  20),  on  the  contrary,  the  protoplasm  is  so  deeply  stained  that  the  nucleus 
appears  pale  lay  contrast.  This  peculiarity  is  also  observed  in  the  smaller  forms  of 
lymphocytes. 

7,  8    Transitional  Forms. 

Note  the  moderately  basic  and  indented  nucleus,  and  the  almost  hyaline  non-granular 
protoplasm.  Compare  8  with  the  myelocyte,  7,  Plate  I,  these  cells  differing  chiefly  in  that 
the  myelocyte  contains  neutrophile  granules. 

9,  10,  II.  Polynuclear  Neutrophils. 

These  cells  are  characterized  by  a  polymorphous  or  polynuclear  nucleus,  surrounded  by 
a  cell-body  tilled  with  fme  neutrophile  granules.  In  11  the  nuclear  structure  is  obviously 
separated  into  four  parts;  in  9  it  is  moderately,  and  in  10  markedly,  polymorphous. 

12,  13.  Eosinophiles. 

The  nuclei  are  not  unlike  those  of  the  polynuclear  neutrophile,  except  that  they  are  some- 
what less  convoluted,  and  poorer  in  chromatin,  staining  less  intensely.  The  protoplasm  is 
filled  with  coarse  eosinophile  granules,  the  characteristics  of  which  are  clearly  illustrated 
by  13,  a  "fractured"  eosinophile. 

14.  Eosinophile  Myelocyte. 
Compare  with  15. 

15,  16.  Myelocytes.     {Neutrophilic) 

These  cells  are  morphologically  similar  to  14,  except  that  they  contain  neutrophile  instead 
of  eosinophile  granules.  Note  that  the  granules  of  the  myelocyte  are  identical  with  those 
of  the  polynuclear  neutrophile.     A  dwarf  form  of  myelocyte  is  represented  by  16. 

{Eosin  and  Methylene-blue.) 

17,  18.  Small  Lymphocytes. 

Note  the  narrow  rim  of  pseudo-granular  basic  protoplasm  surrounding  the  nucleus,  and 

the  pale  appearance  of  the  latter. 
19,  20.  Large  Lymphocytes, 

Budding  of  the  basic  zone  of  protoplasm  is  represented  by  20.     Both  of  these  cells  belong 

to  the  same  type  as  5  and  6. 
21,  22.  Large  Mononuclear  Leukocytes. 

Compared  with  19  and  20,  these  cells  have  a  decidedly  less  basic  protoplasm,  but  a  somewhat 

more  basic  nucleus.     In  the  triple  stained  film  these  differences  cannot  be  detected,  so 

that  they  must  be  classed  as  large  lymphocytes. 

23.  Transitional  Form. 

The  distinction  between  this  cell  and  24  is  not  marked;  the  nucleus  of  the  latter  simply 
being  somewhat  more  basic  and  convoluted. 

24,  25,  26,  27.  Polynuclear  Neutrophiles. 

With  this  stain  these  cells  show  a  feebly  acid  protoplasm,  and  lack  granules.     Note  that 
the  more  twisted  the  nucleus  the  deeper  it  is  stained.     Compare  with  9,  10  and  11. 
28,  29.  Eosinophiles. 

Compare  with  12  and  13. 

30.  Eosinophilic  Myelocyte. 

Compare  with  14. 

31.  Basophile.     {Finely  granular.) 

This  cell  is  characterized  by  the  presence  of  exceedingly  fine  ^-granules,  staining  the  pure 
color  of  the  basic  dye.  The  nucleus  is  markedly  convoluted  and  deficient  in  chromatin. 
The  cell  here  shown  was  found  in  normal  blood. 

32.  33,  34,  35,  36.  Mast  Cells. 

The  granules  take  a  modified  basic  color,  as  shown  by  their  royal-purple  tint  in  this  illus- 
tration. Note  their  unusually  large  size  aind  ovoid  shape  in  35,  their  pecuUar  distribution 
in  35  and  36,  and  their  irregularity  in  size  in  32  and  36.  With  the  triacid  mixture  these 
granules,  as  well  as  those  of  the  finely  granular  basophile,  31,  remain  unstained,  showing 
as  dull-white  stippled  areas  in  the  cell-body.  The  nuclear  chromatin  of  the  mast  cell  is  so  deli- 
cate and  so  feebly  stained  that  it  is  barely  visible.  These  cells  were  found  in  the  blood  of  a 
case  of  splenomedullary  leukemia. 


PLATE  I. 


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The  Leukocytes. 

(2-16,  Triacid  Stain;  17-36,  Eosin  and  M ethylene -bliM.) 

(E.  F.  Faber,  jec.) 

{From  DaCosia's  "  Clintcal  Hematology.") 


THE  BLOOD  259 

cytes  which  take  their  origin  entirely  from  the  lymph-adenoid  tissues  of  the 
body,  e.g.,  the  lymph-glands,  solitar}'  and  agminated  follicles  of  the  intes- 
tines, etc.  As  the  lymph  flows  through  these  structures  the  lymph-corpus- 
cles, as  the  future  lymphocytes  of  the  blood  are  called  in  these  situations,  are 
washed  oul;  and  carried  by  way  of  the  lymph-stream  into  the  general 
circulation. 

In  the  second  group  are  the  transitional  forms,  the  polymorphonuclear, 
eosinophile  and  basophile  leukocytes  which  originate  from  the  bone-marrow 
only.  The  immediate  ancestors  of  these  cells  are  known  as  myelocytes  and 
are  normally  found  in  the  red  bone-marrow.  These  cells,  through  transi- 
tional stages,  assume  the  characteristics  of  the  leukocytes  just  mentioned  and 
pass  directly  into  the  capillaries  of  the  marrow  whence  they  are  distributed 
throughout  the  body. 

Several  attempts  have  been  made  by  different  investigators  to  trace  all 
varieties  of  leukocytes  to  a  common  mother  cell.  While  this  is  believed  to 
take  place  during  embr}-onal  life,  the  proofs  of  such  an  origin  of  leukocytes  in 
the  normal  adult  are  insufficient  and  unconvincing. 

After  an  unknown  period  of  life  the  leukocytes  undergo  dissolution  and 
disappear. 

Functions. — The  functions  of  the  white  corpuscles  are  but  imperfectly 
known,  and  at  present  no  positive  statements  can  be  made.  It  has  been 
suggested  that  wherever  found  in  the  body,  whether  in  blood  or  tissues,  they 
are  engaged  in  the  removal  of  more  or  less  insoluble  particles  of  disintegrated 
tissues,  in  attacking  and  destroying  more  or  less  effectively  various  forms  of 
invading  bacteria  and  thus  protecting  the  body  against  their  deleterious 
activity.  This  they  do  by  surrounding,  enveloping,  and  incorporating  either 
the  tissue  particle  or  bacterium  and  digesting  it.  On  account  of  this  swallow- 
ing action  these  cells  were  termed  by  Metchnikoff  phagocytes  and  the  process 
phagocytosis.  In  their  capacity  as  phagocytes  it  has  been  assumed  that 
they  are  made  more  or  less  efficient  by  the  presence  in  the  plasma  of  an 
agent  or  agents  which  in  some  unknown  manner  render  the  'bacteria  less 
resisting  and  thus  make  them  more  susceptible  to  the  attacks  of  the  leuko- 
cytes. These  agents  which  are  supposed  to  be  secreted  by  the  tissue  cells 
are  termed  opsonins,  from  their  supposed  function,  that  of  preparing  the 
bacteria  for  leukocytic  digestion.  The  cells  engaged  in  this  process  are  the 
polymorphonuclear  leukocytes  and  the  large  and  the  small  lymphocytes. 
He  regards  them  as  the  general  scavengers  of  the  body.  It  has  been  sug- 
gested that  they  are  also  engaged  in  the  absorption  of  fat  from  the  lym- 
phoid tissue  of  the  intestine.  In  their  dissolution  they  contribute  to  the 
blood-plasma  certain  protein  materials  which  assist  under  favorable 
circumstances  in  the  coagulation  of  the  blood. 

HISTOLOGY  OF  THE  BLOOD-PLATELETS 

The  blood-platelets  or  plaques  are  small  histologic  elements  circulating 
in  the  blood-plasma.  They  were  discovered  and  described  in  1845  by  Arnold. 
Hayem,  later  applied  to  them  the  term  hematoblasts,  on  the  supposition 
that  they  were  the  early  stages  in  the  development  of  the  red  corpuscles. 
This  is  now  known  to  be  erroneous.  On  account  of  their  specific,  distinct 
characters,  and  their  constant  presence  in  the  blood  of  living  animals  (guinea- 


26o  TEXT-BOOK  OF  PHYSIOLOGY 

pig  and  bat),  they  arc  now  regarded  as  normal  constituents  of  the  blood  and 
designated  sometimes  as  the  third  corpuscle.  When  blood  is  freshly  drawn 
from  the  body,  the  plaques  rapidly  undergo  disintegration  and  disappear; 
but  by  treating  the  blood  with  osmic  acid,  the  form  and  structure  of  the 
plaque  may  be  retained.  They  may  also  be  preserved  by  preparing  and 
staining  the  tissues  with  Wright's  blood  stain. 

The  blood-platelet  may  be  defined  as  a  colorless,  grayish-white,  homo- 
geneous or  finely  granular  protoplasmic  disc,  varying  in  diameter  from  1.5 
to  3.5  micro-millimeters.  The  edges  are  rounded  and  well  defined,  but  it 
is  not  certain  whether  they  are  only  flattened  or  are  slightly  biconcave. 
There  is,  however,  no  nucleus,  though  the  central  portion  is  granular  and 
the  peripheral  portion  clear.  The  ratio  of  the  plaques  to  the  red  corpuscles 
is  I  to  18  or  20,  and  the  total  number  per  cubic  millimeter  has  been  estimated 
to  be  250,000  to  300,000  or  more. 

When  blood  is  shed  they  tend  to  adhere  to  each  other  and  form  irregular 
masses  known  as  Schultze's  granular  masses.  If  threads  are  suspended  in 
blood,  the  plaques  accumulate  in  enormous  numbers  upon  them  and  appear 
to  form  a  center  from  which  fibrin  filaments  radiate  as  coagulation  proceeds. 
The  white  thrombi  which  form  in  blood-vessels  in  consequence  of  diseased 
states — e.g.,  endocarditis,  atheromatous  ulceration,  etc. — are  composed  very 
largely  of  blood-plaques  and  fibrin  threads. 

The  blood-plaques  can  be  seen  with  high  powers  of  the  microscope  in  the 
blood-vessels  of  the  omentum  of  the  guinea-pig  and  rat,  especially  when  the 
blood-stream  begins  to  slow.  They  are  also  readily  seen  in  the  blood-vessels 
of  subcutaneous  connective  tissue  of  various  animals,  and  especially  in  that 
of  the  new-born  rat.  A  small  quantity  of  this  tissue  moistened  with  normal 
saline  and  examined  microscopically  with  suitable  powers  will  show  large 
numbers  of  plaques  within  the  blood-vessels. 

As  to  the  origin  of  the  blood-platelets  there  has  been  much  difference  of 
opinion.  Many  theories  have  been  proposed,  none  of  which  have  been 
accepted.  As  a  result  of  long-continued  observations  Wright  has  recently 
published  results  which  make  it  probable  that  they  are  fragments  or  detached 
portions  of  the  cytoplasm  of  giant  cells,  megakaryocytes,  found  in  the 
marrow  of  the  bones.  The  cytoplasm  is  prolonged  into  pseudopod-like 
processes  which  become  detached,  and  as  they  are  in  close  relation  to 
the  blood  channels  they  are  soon  taken  up  and  carried  into  the  blood 
of  the  general  circulation  when  they  are  known  as  blood-platelets  or 
plaques. 

The  function  of  the  blood-plaques  is  unknown,  but  it  has  been  surmised 
that  in  some  way  they  are,  like  the  leukocytes,  concerned  in  the  coagula- 
tion of  the  blood.  Whenever  they  are  diminished  in  number,  as  in  purpura 
and  hemophilia,  coagulation  takes  place  very  slowly. 

THE  TOTAL  QUANTITY  OF  THE  BLOOD;  ITS  GENERAL 
COMPOSITION 

The  determination  of  the  total  quantity  of  the  blood  in  an  animal  is  best 
made  by  the  chromometric  method,  somewhat  modified  at  present,  of 
Welcker.  This  consists,  first,  in  bleeding  an  animal,  collecting  all  the  blood 
it  yields,  and  weighing  it;  second,  in  washing  out  the  vessels  with  a  normal 


THE  BLOOD  261 

saline  solution  until  the  fluid  comes  from  the  veins  clear  and  free  from  blood; 
third,  in  mincing  the  tissues  of  the  body,  after  removal  of  the  contents  of  the 
alimentary  canal,  soaking  them  in  water  for  twenty-four  hours,  and  then 
expressing  them.  All  the  v/ashings  are  collected  and  weighed.  A  given 
volume  of  the  normal  defibrinated  blood,  treated  with  carbon  monoxid  so  as 
to  give  it  uniform  color,  is  then  diluted  with  water  until  its  tint  is  identical 
with  that  of  the  washings  similarly  treated  with  carbon  monoxid.  From  the 
quantity  of  water  necessary  to  dilute  the  blood  the  quantity  of  blood  in  the 
washings  is  readily  determined.  The  animal  having  been  previously 
weighed  and  the  weight  of  the  contents  of  the  alimentary  canal  deducted,  the 
ratio  of  the  total  weight  of  the  blood  to  the  weight  of  the  body  at  once  be- 
comes apparent.  By  this  method  it  has  been  shown  that  the  ratio  of  blood 
to  body-weight  in  a  human  adult  is  i  :  13;  in  an  infant,  i  :  19;  in  a  dog,  i  :  13; 
in  a  cat,  1:21. 

The  more  recent  investigations  of  Haldane  and  Smith  and  of  Plesch 
with  the  employment  of  a  different  method  make  it  probable  that  the  ratio 
is  approximately  i  iiq.  Thus  a  man  weighing  70  kilos  would  have  3684 
grams  of  blood. 

The  amount  of  blood  in  the  different  organs  has  been  determined  by 
ligating  the  blood-vessels  in  the  living  animal,  removing  the  organ,  and  after 
allowing  the  blood  to  escape  subjecting  the  tissues  to  the  chromometric 
methods  described  above.  According  to  Ranke,  the  volume  of  the  blood  is 
distributed  as  follows:  Heart,  lungs, arteries,  and  veins,  ^;  liver,  \;  muscles,  \; 
other  organs,  \. 

General  Composition. — The  results  of  the  analyses  of  the  blood  will 
vary  with  the  animal  and  the  methods  employed.  The  following  table, 
taken  from  Gad,  shows  the  average  composition,  expressed  in  whole  numbers, 
of  horse's  blood.  In  essential  respects  the  ratio  of  the  constituents  in  human 
blood  would  not  be  materially  different. 

One  thousand  parts  of  blood  contain: 

r  Water 200 200 

Cells 328  ]                                       r  Hemoglobin 116 

[  Solids 128  ]  Other  organic  matter 10 

[  Salts 2 

Water 604 604 

Fibrin 7 

Albumin 52 

Fat 1 

Other  organic  matter 3 

Potassium  and  sodium  salts 4 

Calcium  and  magnesium  salts i 


Plasma 672 

•■Solids 68^ 


CHEMISTRY  OF  COAGULATION 

The  changes  which  eventuate  in  the  formation  of  fibrin,  and  hence  all 
the  subsequent  phenomena  of  coagulation,  are  chemic  in  character;  but  as 
these  changes  take  place  in  organic  compounds  the  composition  of  which  is 
but  imperfectly  known,  the  intimate  nature  of  the  process  is  quite  obscure. 
All  the  theories  which  have  been  advanced  in  explanation,  though  approxi- 
mating the  truth,  are  more  or  less  incomplete  and  in  some  respects  contra- 
dictory. Since  the  coagulation  is  coincident  with  the  appearance  of  the 
fibrin,  the  antecedents  of  this  substance,  the  physical  and  chemic  conditions 


262  TEXT-BOOK  OF  PHYSIOLOGY 

which  condition  its  development,  and  the  succession  of  chemic  changes 
involved  must  be  determined,  before  any  consistent  theory  can  be  established. 

Extra-vascular  Coagulation. — At  present  it  is  generally  believed 
that  the  immediate  factors  concerned  in  extra-vascular  coagulation  are 
fibrinogen,  a  calcium  salt,  and  an  agent  thrombin.  As  to  the  manner  in 
which  these  three  bodies  react  one  with  another  there  is  a  diversity  of  opinion. 

As  an  outcome  of  a  long  series  of  experiments  that  have  been  performed 
to  determine  the  nature  and  the  succession  of  the  chemic  phenomena 
underlying  the  coagulation  of  the  blood,  the  following  facts  seem  to  be  well 
established,  viz:  the  immediate  cause  of  the  coagulation  is  the  appearance 
of  fibrin,  a  derivative  of  an  antecedent  substance  always  present  in  the  blood 
termed  fibrinogen;  the  cause  of  the  conversion  of  the  soluble  fibrinogen  into 
the  insoluble  fibrin  is  the  presence  and  activity,  under  the  circumstances,  of 
an  agent  termed  thrombin,  the  chemic  nature  of  which  is  a  subject  of  discus- 
sion. By  some  chemists  it  is  regarded  as  a  ferment  which  causes  a  mo- 
lecular rearrangement  of  the  fibrinogen;  by  others  it  is  regarded  as  a  definite 
organic  colloidal  body  which  unites  in  some  physico-chemic  manner  with 
the  fibrinogen  to  form  fibrin. 

The  crux  of  the  problem  is  the  source  and  the  conditions  necessary  for 
the  production  of  the  thrombin.  It  is  generally  conceded  that  thrombin  is  a 
derivative  of  an  antecedent  substance  prothrombin  or  thrombogen,  a  substance 
always  present  in  the  blood  plasma,  a  product  of  the  decomposition  of  blood- 
platelets  and  leukocytes.  With  prothrombin  there  is  physiologically  associ- 
ated a  calcium  salt,  the  presence  of  which  is  absolutely  essential  for  coagula- 
tion or  the  conversion  of  prothrombin  into  thrombin  as  was  conclusively 
shown  by  Arthus  and  Pages:  For  if  it  is  precipitated  by  the  addition  of 
oxalate  of  potassium,  coagulation  will  not  take  place.  At  all  times  then, 
there  are  present  in  the  blood,  prothrombin,  a  calcium  salt  and  fibrinogen. 
Given  the  two  former  factors,  the  question  arises,  why  do  they  not  react  to 
form  thrombin  in  the  circulating  blood,  and  why  do  thev  so  react  in  shed 
blood  ? 

The  answer  of  Morawitz  is,  that  prothrombin  requires  an  activating 
agent,  a  kinase  which  is  wanting  in  circulating  blood  but  is  present  in  shed 
blood.  It  is  supposed  to  develop  in  the  disintegration  of  the  cell  elements  of 
the  blood,  leukocytes  and  blood-platelets,  and  perhaps  from  the  cell  elements 
of  the  injured  tissues  as  the  blood  flows  over  them.  Shortly  after  its  appear- 
ance the  kinase,  with  the  aid  of  the  calcium  salt  converts  the  prothrombin  into 
thrombin,  after  which  it  unites  with  fibrinogen  to  form  fibrin.  For  this 
reason  the  kinase  has  been  termed  thrombo-kinase. 

The  answer  of  Howel!  to  the  foregoing  question  is  somewhat  diflferent  and 
based  on  a  long  series  of  experiments  recently  published.  From  the  results 
of  these  experiments  the  answer  given  is  that  prothrombin  is  prevented  from 
reacting  with  the  calcium  salt  to  form  thrombin  in  the  circulating  blood,  by 
reason  of  the  presence  and  possible  union  with  prothrombin  of  an  agent 
termed  anti-thrombin.  So  long  as  this  relation  is  not  disturbed  the  blood 
remains  fluid.  When  blood  is  shed  there  is  supposed  to  develop  from  the  cell 
elements  of  the  blood,  the  leukocytes  and  blood-platelets,  and  perhaps  from 
the  cell  elements  of  the  injured  tissues  as  well,  a  plastin,  the  specific  action  of 
which  is  to  combine  with  the  anti-thrombin  and  thus  set  free  the  prothrombin. 
This  having  been  accomplished  the  calcium  salt  activates  the  prothrombin, 


THE  BLOOD  263 

and  converts  it  into  thrombin,  after  which  it  combines  with  the  fibrinogen. 
For  this  reason  the  plastic  agent  has  been  termed  thromho-plastin.  Experi- 
ments indicate  that  this  agent  belongs  to  the  group  of  bodies  known  as 
phosphatids  and  similar  in  its  properties  to  one  member  of  this  group,  viz., 
kephalin. 

Intra-vascular  Coagulation. — So  long  as  the  relations  of  the  blood 
and  the  vascular  apparatus  remain  physiologic,  no  coagulation  occurs  in  the 
vessels.  The  reasons  assigned  for  this  are:  (i)  the  absence  of  thrombo- 
kinase  in  sufiicient  amounts;  or  (2)  the  presence  of  anti-thrombin.  On  either 
assumption  the  reaction  between  prothrombin  and  calcium  with  the  forma- 
tion of  thrombin  does  not  take  place.  If  the  vessels  are  injured  as  they  are 
when  ligated  or  torn  or  in  any  way  impaired,  coagulation  promptly  takes 
place  with  the  subsequent  occlusion  of  the  vessel.  As  to  whether  the  injured 
tissues  or  the  blood-cells  now  generate  an  agent,  thrombo-kinase,  which 
activates  the  prothrombin  and  calcium,  or  whether  they  generate  an 
agent  thrombo-plastin,  which  neutralizes  an  anti-thrombin,  is  a  subject  of 
discussion. 

Under  pathologic  conditions  of  the  circulatory  apparatus,  especially  of 
the  internal  lining,  intra-vascular  coagulation  frequently  arises,  though  the 
process  cannot  be  considered  as  identical  with  extra-vascular  coagulation. 
Many  pathologists  assert  that  in  its  origin,  mode  of  formation,  and  structure 
the  intra-vascular  coagulum  or  thrombus  is  not  a  true  coagulum  as  ordinarily 
understood,  but  rather  a  conglutination  of  blood-plaques  and  leukocytes. 
Whenever  the  integrity  of  the  internal  wall  of  the  vessel  is  impaired  by 
disease  or  by  the  introduction  of  foreign  bodies,  there  is  primarily  a  deposition 
and  accumulation  of  blood-plaques  at  the  injured  area  or  on  the  foreign  body 
which  constitutes  to  a  large  extent  the  mass  of  the  thrombus  which  at  once 
forms.  The  thrombi  which  form  on  the  surface  of  atheromatous  ulcers,  on 
the  valves  of  the  heart,  and  in  the  veins  in  consequence  of  diseased  states,  on 
threads  or  needles  passed  through  the  vessels,  at  the  orifices  of  torn  blood- 
vessels, consist  largely  of  blood-plaques.  A  thrombus  so  formed  may  con- 
tain a  number  of  delicate  fibrin  threads,  which,  however,  present  a  different 
appearance  from  the  fibrin  of  the  extra-vascular  clot.  In  the  thrombi  which 
form  around  foreign  bodies  there  is  a  larger  quantity  of  fibrin  than  in  those 
originating  from  causes  wholly  within  the  vessel. 


CHAPTER  XIII 
THE  CIRCULATION  OF  THE  BLOOD 

Each  organ  and  tissue  of  the  body  is  the  seat  of  a  more  or  less  active 
metabolism,  the  maintenance  of  which  is  essential  to  its  physiologic  activity. 
This  metabolism  is  characterized  by  the  assimilation  of  food  materials  and 
the  production  of  waste  products;  that  it  may  be  maintained  it  is  imperative 
that  there  shall  be  a  continuous  supply  of  the  former  and  a  continuous 
removal  of  the  latter.  Both  conditions  are  subserved  by  the  blood.  In 
order,  however,  that  this  fluid  may  fulfil  these  functions  it  must  be  kept  in 
continuous  movement,  must  flow  into  and  out  of  the  tissues  in  volumes  vary- 
ing with  their  activity,  with  a  certain  velocity  and  under  a  given  pressure. 

The  apparatus  by  which  these  results  are  attained  is  termed  the  circula- 
tory apparatus.  This  consists  of  a  central  organ,  the  heart;  a  series  of 
branching  diverging  tubes,  the  arteries;  a  network  of  minute  passageways 
with  extremely  delicate  walls,  the  capillaries;  a  series  of  converging  tubes, 
the  veins.  These  structures  are  so  arranged  as  to  form  a  closed  system  of 
vessels  within  which  the  blood  is  kept  in  continuous  movement  mainly  by  the 
pressure  produced  by  the  pumping  action  of  the  heart,  though  aided  by  other 
forces.     (See  Fig.  no.) 

In  this  system  a  particle  of  blood  which  passes  any  given  point  will 
eventually  return  to  the  same  point,  no  matter  how  intricate  or  tortuous  the 
route  may  be  through  which  it  in  the  meanwhile  travels;  for  this  reason 
the  blood  is  said  to  move  in  a  circle,  and  the  movement  itself  is  termed 
the  circulation. 

In  order  to  understand  the  reasons  for  the  movement  of  the  blood  in  one 
direction  only,  as  well  as  for  many  other  phenomena  connected  with  the 
circulation,  a  knowledge  of  the  structure  of  the  heart  and  its  internal  mechan- 
ism is  of  primary  importance. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  HEART 

The  heart  is  a  conic  or  pyramid-shaped  hollow  muscle  situated 
in  the  thorax  just  behind  the  sternum.  The  base  is  directed  upward  and  to 
the  right  side;  the  apex  downward  and  to  the  left  side,  extending  as  far  as  the 
space  between  the  cartilages  of  the  fifth  and  sixth  ribs.  In  this  situation 
the  heart  is  enclosed  and  suspended  in  a  fibro-serous  sac,  the  pericardium, 
attached  to  the  great  vessels  at  its  base. 

The  heart  is  a  hollow,  double  muscle  organ,  consisting  of  a  right  and  a 
left  half,  separated  by  a  musculo-membranous  septum.  The  general 
cavity  of  each  side  is  subdivided  by  an  incomplete  transverse  fibrous  septum 
into  two  smaller  cavities,  an  upper  and  a  lower,  known  respectively  as  the 
auricle  and  the  ventricle.  The  heart  may  therefore  be  said  to  consist  of  four 
cavities,  the  walls  of  which  are  composed  of  muscle-tissue.     Of  these  four 

264 


THE  CIRCULATION  OF  THE  BLOOD 


265 


cavities,  the  right  auricle  and  the  right  ventricle  constitute  venous  the  heart; 
the  left  auricle  and  the  left  ventricle,  the  arterial  heart. 

The  right  auricle  is  quadrangular  in  shape  and  presents  on  its  posterior 
aspect  two  large  openings,  the  terminations 
of  the  two  final  trunks  of  the  venous  system, 
the  superior  and  inferior  vena  cava  (Fig,  1 1 1 ) . 
Below  the  auricle  communicates  with  the 
ventricle  by  a  large  opening  which,  from  its 
position,  is  termed  the  auriculo -ventricular 
opening.  The  walls  of  the  auricle  are  ex- 
tremely thin,  not  measuring  more  than  two 
millimeters  in  thickness. 

The  right  ventricle,  as  shown  on  cross- 
section,  is  crescentic  in  shape  owing  to  the 
projection  of  the  ventricular  septum.  It  pre- 
sents at  its  upper  left  angle  a  cone-shaped 
prolongation,  the  cmius  arteriosus.  From  this 
prolongation,  and  continuous  with  it,  arises 
the  pulmonic  artery.  The  wall  of  the  ven- 
tricle measures  in  the  middle  about  four  milli- 
meters in  thickness.  The  inner  surfaces  of 
the  ventricle  show^:  (i)  a  complicated  system 
of  muscle  ridges  and  bands,  the  columncB  cor- 
nea (fleshy  columns),  and  (2)  a  set  of  muscle 
projections,  the  musculi  papillares  (papillary 
muscles),  which  arise  by  a  broad  base  from 
the  walls  of  the  ventricle  and  project  upward 
toward  the  auriculo-ventricular  opening. 
From  the  apex  of  each  papillary  muscle  there 
are  given  off  fine  tendinous  cords,  the  chordce 
iendinecB,  which  become  attached  above  to  the 
under  surface  of  the  auriculo-ventricular  valve. 

The  left  auricle,  similar  in  general  shape  to 
the  right,  presents  posteriorly  four  openings, 
the  terminations  of  the  four  final  trunks  of  the 
venous  system  of  the  lungs,  the  pulmonary 
veins.  Below  is  found  the  corresponding 
auriculo-ventricular  opening.  The  wall  of  the 
auricle  measures  about  3  mm.  in  thickness. 
The  left  ventricle  (Fig.  112)  is  conic  in  shape 
from  above  downward  and  oval  or  circular  in 
shape  on  cross-section.  At  its  upper  inner 
angle  it  presents  a  circular  orifice,  the  mar- 
gins of  which  give  attachment  to  the  walls  of  tremities.  4.  Spleen.  5.  Intestines, 
the  aorta,  the  main  arterial  trunk  of  the  sys-  ^;  S!^(i//J'SSL™''"' 
temic  circulation.     The  inner  surfaces  of  the 

ventricle  show  a  similar  though  better-developed  system  of  columnae  car- 
neas,  musculi  papillares,  chordae  tendineae,  etc.  The  wall  of  the  left  ven- 
tricle measures  about  11.5  mm.  in  thickness  in  the  middle. 


Fig.  no. — Diagram  of  the 
Circulation,  i.  Heart.  2. 
Lungs.     3.    Head    and    upper  ex- 


266 


TEXT-BOOK  OF  PHYSIOLOGY 


The  Endocardium. — The  cavities  of  both  the  right  and  left  sides  of  the 
heart  are  Hned  by  a  thin,  firm  connective-tissue  membrane,  closely  adherent 
to  the  muscle-tissue,  termed  the  endocardium.  It  contains  also  elastic  fibers 
and  smooth  muscle-fibers.  Its  entire  surface  is  covered  with  a  layer  of 
polygonal  endothelial  cells.     This  membrane  serves  partially  to  resist  undue 


Fig.  III. 


Fig.  112. 


Fig.  III. — Interior  of  Right  Auricle  and  Ventricle,  Exposed  by  the  Removal  of 
A  Part  of  Their  Walls,  i,  Superior  vena  cava;  2,  inferior  vena  cava;  2',  liepatic  veins;  3,  3',  3", 
inner  wall  of  right  auricle;  4,  4,  cavity  of  right  ventricle;  4',  papillary  muscle;  5,  5',  5",  flaps  of 
tricuspid  valve;  6,  pulmonic  artery  in  the  wall  of  which  a  window  has  been  cut;  7,  on  aorta 
near  the  ductus  arteriosus;  8,  9,  aorta  and  its  branches;  10,  11,  left  auricle  and  ventricle. — {Allen 
Thomson.) 

Fig.  112. — Left  Auricle  and  Ventricle,  Opened  and  Part  of  Their  Walls  Removed 
TO  Show  Their  Cavities,  i,  Pulmonic  vein  cut  short;  i',  cavity  of  left  auricle;  3,  3",  thick 
wall  of  left  ventricle;  4,  portion  of  the  same  with  papillary  muscle  attached;  5,  the  other  papillary 
muscles;  5',  wall  of  the  ventricle;  6,  6',  the  segments  of  the  mitral  valve;  7,  the  figure  in  aorta  is 
placed  over  the  semilunar  valves;  7',  aorta;  8,  pulmonic  artery;  10,  branches  of  aorta. — {Allen 
Thomson.) 

distention  of  the  heart  during  contraction  and  to  prevent  separation  of  the 
muscle-fibers.  The  endocardium  is  continuous  with  the  lining  membrane 
of  the  blood-vessels. 

The  inter-auricular  septum  is  quite  thin  and  composed  of  the  two  layers 
of  the  endocardium,  between  which  is  a  layer  of  muscle-fibers.  It  presents 
at  its  lower  portion  an  oval  depression,  the  fossa  ovalis. 

The  inter-ventricular  septum  is  quite  thick  and  well  developed,  and  com- 
posed of  the  two  layers  of  the  endocardium  enclosing  the  muscle-fibers.     In 


THE  CIRCULATION  OF  THE  BLOOD  267 

the  upper  and  central  portion  of  the  septum,  there  is,  however,  a  small  region 
which  is  thin  owing  to  the  absence  of  muscle-tissue  and  composed  of  endo- 
cardium only.     This  region  is  known  as  the  pars  memhranacea  septi. 

The  Cardio-pubnonic  Vessels. — Though  the  two  sides  of  the  heart 
are  separated  from  each  other  by  the  auriculo-ventricular  septum,  they  are 
anatomically  and  physiologically  connected  by  the  intermediation  of  the 
pulmonic  system  of  vessels:  viz.,  the  pulmonic  artery,  capillaries,  and 
veins  (Fig.  no). 

The  pulmonic  artery  arises  from  the  conus  arteriosus  of  the  right  ven- 
tricle. After  a  short  upward  course  it  divides  into  a  right  and  a  left  branch, 
which  enter  the  corresponding  lungs.  The  vessel  at  once  divides  and  sub- 
divides into  a  number  of  branches,  which,  after  following  the  bronchial  tubes 
to  their  termination,  give  origin  to  capillaries  that  surround  the  air-cells  of 
the  pulmonic  lobules. 

The  capillaries  in  this  situation  are  extremely  abundant  and  well  developed. 
They  lie  close  to  the  inner  surfaces  of  the  air-cells.  The  blood  is  thus 
brought  into  intimate  relationship  with  the  intra-pulmonic  air,  and  the 
exchange  of  gases — the  excretion  of  carbon  dioxid  and  the  absorption  of 
oxygen — for  which  the  cardio-pulmonic  vessels  exist,  is  readily  accom- 
plished. 

The  pulmonic  veins  which  return  the  arterialized  blood  to  the  heart  are 
formed  by  the  convergence  and  union  of  the  small  veins  which  emerge 
from  the  capillary  system.  The  final  trunks  thus  formed,  the  four  pulmonic 
veins — two  from  each  lung — enter  the  posterior  wall  of  the  left  auricle. 

The  Course  of  the  Blood  through  the  Heart. — ^There  is  thus  estab- 
lished a  pathway  between  the  venae  cavae  on  the  right  side  and  the  aorta  on 
the  left  side,  by  way  of  the  right  side  of  the  heart,  the  cardio-pulmonic 
vessels,  and  the  left  side  of  the  heart. 

The  venous  blood  flowing  toward  the  heart  is  emptied  by  the  superior 
and  inferior  vena  cava  into  the  right  auricle,  from  which  it  passes  through 
the  auriculo-ventricular  opening  into  the  right  ventricle  (Fig.  no);  thence 
into  and  through  the  pulmonic  artery  and  its  branches  to  the  pulmonic 
capillaries,  where  it  is  arterialized  by  the  exchange  of  gases — the  giving  up  of 
a  portion  of  carbon  dioxid  to  the  lungs  and  the  absorption  of  oxygen — and 
changed  in  color  from  bluish-red  to  scarlet-red.  The  arterialized  blood, 
flowing  toward  the  heart,  is  emptied  by  the  pulmonic  veins  into  the  left 
auricle,  from  which  it  passes  through  the  auriculo-ventricular  opening  into 
the  left  ventricle;  thence  into  the  aorta  and  its  branches  to  the  systemic 
capillaries,  where  it  is  de-arterialized  by  a  second  but  opposite  exchange  of 
gases — the  giving  up  of  a  portion  of  its  oxygen  to  the  tissues  and  the  absorp- 
tion of  carbon  dioxid  from  the  tissues — and  changed  in  color  from  scarlet  to 
bluish-red.  The  venous  blood  is  again  returned  by  the  systemic  veins  to  the 
vense  cavas.  Though  the  blood  is  thus  described  as  flowing  first  through  the 
right  side  and  then  through  the  left  side,  it  must  be  kept  in  mind  that  the  two 
sides  fill  synchronously;  that  while  the  blood  is  flowing  into  the  right  side 
from  the  vense  cavae,  it  is  also  flowing  into  the  left  side  from  the  pulmonic 
veins  in  equal  quantities  and  velocities. 

Though  there  is  but  one  set  of  capillaries,  as  a  rule,  between  arteries  and 
veins,  there  is  an  exception  in  the  case  of  the  arteries  and  veins  of  some  of  the 
abdominal   viscera.     Thus  the  veins  emerging  from  the  capillaries  of  the 


268 


TEXT-BOOK  OF  PHYSIOLOGY 


stomach,  intestines,  pancreas,  and  spleen,  instead  of  passing  directly  to  the 
inferior  vena  cava,  unite  to  form  a  large  vein — the  portal  vein — which  enters 
the  liver.  In  this  organ  the  portal  vein  divides  to  form  a  second  capillary 
system  which  is  in  close  relation  to  the  liver  cells  and  from  which  arise  the 
veins  which  unite  to  form  the  hepatic  veins.  These  latter  vessels  empty 
and  discharge  the  blood  into  the  inferior  vena  cava  just  below  the  diaphragm. 
From  the  foregoing  facts  physiologists  frequently  divide  the  general 
circulation  into: 
I.  The  pulmonic  circulation,  which  includes  the  course  of  the  blood  from 

the  right  side  of  the  heart  through  the  lungs  to  the  left  side  of  the 

heart. 


Fig.     113. — Right     Cavities     of     the  Fig.  114. — Right  Cavities  of  the  Heart. 

Heart. — Auricuio-ventricular    valve    open,   Auriculo-ventricular     valve      closed,    semilunar 
semilunar  valves  closed. —  (Dalton.)  valves  open. — {Dalton.) 

2.  The  systemic  circulation,  which  includes  the  course  of  the  blood  from 

the  left  side  of  the  heart  through  the  aorta  and  its  branches,  through 
the  capillaries  and  veins,  to  the  right  side  of  the  heart. 

3.  The  portal  circulation,  which  includes  the  course  of  the  blood  from  the 

capillaries  of  the  stomach,  intestines,  pancreas,  and  spleen  through 
the  portal  vein,  to  the  liver. 

Orifices  and  Valves. — The  movement  of  the  blood  along  the  path  of 
the  circle  above  outlined  is  accomplished  by  the  alternate  contraction  and 
relaxation  of  the  muscle  walls  of  the  heart.  That  the  movement  may  be  a 
progressive  one,  that  there  shall  be  no  regurgitation  during  either  the  con- 
traction or  the  relaxation,  it  is  essential  that  some  of  the  orifices  of  the 
heart  be  closed  during  each  of  these  periods.  This  is  accomplished  by  the 
heart  valves. 

The  right  auriculo-ventricular  opening  is  surrounded  and  strengthened 
by  a  ring  of  fibrous  tissue  to  which  is  attached  a  membrane  partially  sub- 
divided into  three  portions  or  cusps,  which  during  the  period  of  relaxation 
are  directed  into  the  ventricle  (Fig.  113);  during  the  period  of  contraction 
they  are  raised  and  placed  in  complete  apposition,  when  they  act  as  a  valve 


THE  CIRCULATION  OF  THE  BLOOD 


269 


preventing  a  backward  flow  into  the  auricle  (Fig.  114).  In  the  former 
position  the  valve  is  open;  in  the  latter,  shut.  For  these  reasons  this  struc- 
ture is  known  as  the  tricuspid  valve.  This  valve  is  formed  of  fibrous  tissue 
derived  from  the  fibrous  ring,  and  some  muscle-fibers,  and  covered  over  by  a 
reduplication  of  the  endocardium.  To  the  under  surface  and  to  the  edges 
of  this  valve  the  tendinous  cords  of  the  papillary  muscles  are  firmly  and 
intricately  attached.  These  cords  are  just  sufficiently  long  to  permit  closure 
of  the  valve  and  to  prevent  its  being  floated  into  the  auricle. 

The  orifice  of  the  pulmonic  artery  is  also  surrounded  by  a  ring  of 
fibrous  tissue  to  which  are  attached  three  semilunar  or  pocket-shaped  mem- 
branes, the  semilunar  valves.  Each  valve  is  formed  by  a  reduplication  of 
the  endocardium  strengthened  by  fibrous  tissue.  In  the  center  of  the  free 
edge  of  the  valve  there  is  a  small  nodule  of  fibro-cartilage  (the  corpus 
Aurantii).  The  outer  edge  of  the 
valve  is  strengthened  by  a  delicate 
fibrous  band.  A  similar  band 
strengthens  the  convex  attached 
portion  of  the  valve  just  where  it  is 
joined  to  the  fibrous  ring.  A  third 
set  of  fibers  pass  toward  the  nod- 
ule, interlacing  in  all  directions. 
Two  narrow  crescentic-shaped  areas 
(the  lunulas)  near  the  free  edge  are 
devoid  of  these  fibers.  During  the 
period  of  relaxation  of  the  heart  the 
edges  of  the  valves  are  in  close  ap- 
position and  prevent  a  return  of 
the  blood  into  the  ventricle  (Fig. 
113);  during  the  contraction  they 
are  directed  into  the  artery  (Fig. 
114).  In  the  former  position  they 
are  shut;  in  the  latter,  they  are  open- 

The  left  auriculo-ventricular  opening  is  provided  with  a  similar  though 
better  developed  fibrous  ring  and  membranous  valve.  It  is,  however, 
subdivided  into  but  two  portions  or  cusps,  and  is  therefore  termed  the 
bicuspid  valve,  or,  from  its  fancied  resemblance  to  a  bishop's  mitre,  the 
mitral  valve.  The  general  arrangement,  connections,  and  mode  of  action 
of  this  valve  are  similar  in  all  respects  to  those  of  the  tricuspid  valve.  The 
orifice  of  the  aorta  is  also  surrounded  by  a  ring  of  fibrous  tissue  to  which 
are  attached  three  semilunar  or  pocket-shaped  valves  (Fig.  112),  which  in 
their  arrangement,  connections,  and  mode  of  action  are  similar  in  all  respects 
to  those  at  the  orifice  of  the  pulmonic  artery.  The  anatomic  relations  of 
the  cardiac  orifices  one  to  the  other  and  the  appearance  presented  by  the 
valves  when  closed  are  represented  in  Fig.  115. 

The  Heart  Muscle-fibers  and  Their  Arrangement. — The  muscle- 
fibers  of  the  heart  represent  in  their  structure  a  type  between  the  ordinary 
striated  muscle  and  the  smooth  muscle.  A  longitudinal  section  of  the 
heart-muscle  shows  a  reticulated  arrangement  of  the  fibers,  the  outcome  of 
a  similar  reticulated  condition  of  the  mesodermic  material  in  which  they 
develop.     The    mesodermic    reticulum    containing    numerous    nuclei    is 


Fig.  1 1 5.— Valves  of  the  Heart,  i.  Right 
auriculo-ventricular  orifice,  closed  by  the  tri- 
cuspid valve.  2.  Fibrous  ring.  3.  Left  auric- 
ulo-ventricular orifice,  closed  by  the  mitral 
valve.  4.  Fibrous  ring.  5.  Aortic  orifice  and 
valves.  6.  Pulmonic  orifice  and  valves.  7,8,9. 
Muscular  fibers. — {Bonamy  and  Beau.) 


270 


TEXT-BOOK  OF  PHYSIOLOGY 


lateral  union 


I 


capil- 
lary 


t-i 


s 

^ 

termed  a  syncytium.  As"  the  heart  develops  the  muscle-fibers  make  their 
appearance  in  the  protoplasm  and  assume  an  arrangement  which  corre- 
sponds to  that  of  the  trabeculae  composing  the  reticulum  (Fig.  116).  In 
the  adult  heart  the  intermediary  spaces  are  reduced  to  narrow  clefts  in 
consequence  of  the  multiplication  of  the  muscle-fibers.  The  clefts  are 
occupied  with  connective  tissue,  blood-vessels,  lymphatics,  etc.  The  indi- 
vidual fiber  consists  of  alternate  dim 
and  light  bands  similar  to  the  corre- 
sponding bands  of  the  ordinary  skeletal 
muscles,  though  it  is  devoid  of  a  sar- 
colemma.  Among  the  fibers  large  oval 
nuclei  are  distributed.  At  varying  inter- 
vals the  fibers  are  interrupted  by  inter- 
calated discs.  When  the  heart  muscle 
is  treated  with  caustic  potash  the  trabec- 
ulae separate  at  the  level  of  these  discs, 
forming  what  has  hitherto  been  termed 
the  muscle  cell  or  fiber. 

The  arrangement  of  the  muscle- 
fibers  is  quite  complicated  and  in  ac- 
i  ordance  with  the  functions  of  the  in- 
dividual portions  of  the  heart.  In  the 
;iuricles  the  fibers  are  arranged  in  two 
sets:  an  outer  transverse  set,  which 
pass  from  auricle  to  auricle,  and  an 
inner  longitudinal  set,  which  pass  over 
the  auricles  and  are  attached  anteriorly 
and  posteriorly  to  the  connective  tissue 
of  the  transverse  auriculo-ventricular 
septum.  The  longitudinal  fibers  of  the 
auricles  are  practically  independent  of 
each  other.  Circularly  arranged  fibers 
are  present  near  the  terminations  of 
the  venae  cavae  and  pulmonic  veins. 
In  the  ventricles  the  muscle-fibers  are  also  arranged  in  two  sets,  a 
superficial  longitudinal  and  a  deep  transverse,  though  their  arrangement  is 
somewhat  more  complicated  than  that  observed  in  the  auricles.  In  a 
general  way  it  may  be  said  that  the  superficial  longitudinal  fibers  on  both 
the  anterior  and  posterior  surfaces  take  their  origin  in  the  connective  tissue 
of  the  auriculo-ventricular  septum.  The  superficial  fibers  on  the  anterior 
surface  of  the  heart  pass  obliquely  downward  and  forward  from  right  to  left 
toward  the  apex,  where  they  turn  backward  and  inward  in  a  vertical  manner 
after  which  they  ascend  to  terminate  in  the  wall  of  the  septum,  the  columnae 
carneae  and  musculi  papillares.  The  superficial  fibers  of  the  posterior  sur- 
face of  the  heart  pass  obliquely  downward  from  left  to  right,  wind  around 
the  apex,  turn  upward  and  end  in  the  same  structures  as  do  the  fibers  from 
the  anterior  surface.  The  fibers  from  the  base  of  the  right  ventricle  termi- 
nate in  the  structures  of  the  left  ventricle,  while  those  from  the  left  ventricle 
terminate  in  the  structures  of  the  right  ventricle.  Longitudinal  fibers  are 
also  found  on  the  inner  surface.     The  transverse  fibers  are  very  abundant 


\ 


Intercalated 
disc. 


Nucleus  of      Nucleus  of 
a  muscle      a  connective 
fiber.  tissue  cell. 

Fig.  116. — From  a  Longitudinal  Sec- 
tion OF  A  PAPILLARY  MuSCLE  OF  THE  HUMAN 

Heart.     X  360. — (Stdhr.) 


THE  CIRCULATION  OF  THE  BLOOD  271 

and  surround  each  ventricle  separately  though  they  are  continuous  with 
each  other  across  the  septum.  Between  the  superficial  longitudinal  and  deep 
transverse  fibers  there  are  several  layers  of  fibers  which  possess  varying 
degrees  of  obliquity.  The  general  arrangement  of  the  fibers  is  such  as  to 
insure  a  complete  and  simultaneous  discharge  of  blood  from  both  auricles 
as  well  as  from  both  ventricles  (Fig.  117). 

I  II 


Fig.  117. — Arr.\ngement  OF  Ventricul.'^  Muscle-fibers. -T-(4/'<er  ilfacCa//Mm.)  I  and 
II,  Superficial  fibers  of  the  left  ventricle  and  conus  arteriosus;  III,  deep  layers  of  the  left  ven- 
tricle; LAV,  mitral  orifice;  RAV,  tricuspid  orifice;  PA,  pulmonic  artery. — {From  Hirschf elder.) 

THE  MUSCLE  CONNECTION  BETWEEN  THE  AURICLES  AND 

VENTRICLES 

The  Muscle  Band  of  His,  or  the  Auriculo-ventricular  Bundle. — In 

the  mammalian  heart  there  is  no  continuity  of  the  muscle-fibers  across  the 
auriculo-ventricular  groo-ve,  uniting  auricles  and  ventricles,  such  as  exists  in 
the  frog  or  turtle  heart.  The  muscle-fibers  of  the  auricles  and  ventricles  are 
completely  separated  from  each  other  by  the  transverse  fibrous  septum  to 
which  they  are  attached.  This  fact  has  for  a  long  time  made  it  difficult  to 
understand  how  the  contraction  process  which  begins  in  the  auricles  (and  to 
which  there  will  be  occasion  to  refer  in  subsequent  paragraphs)  is  conducted 
to  the  ventricles.  The  physiologic  necessity  for  the  existence  of  a  muscle 
connection  between  the  auricles  and  ventricles  led  to  a  series  of  investigations 
which  have  resulted  in  the  discovery  of  an  elaborate  system  of  muscle-fibers 
by  which  they  are  united  both  anatomically  and  physiologically. 

In  1893  Wilhelm  His,  Jr.,  discovered  the  existence  of  a  band  or  bundle 
of  muscle-fibers  which  apparently  took  its  origin  from  the  posterior  part  of 
the  right  side  of  the  auricular  septum,  from  which  point  it  passed  forward 
just  above  the  auriculo-ventricular  septum  to  a  point  near  the  aortic  opening, 
where  it  divided  into  two  portions,  a  right  and  a  left,  of  which  the  latter 
apparently  ended  in  the  basis  of  the  aortic  leaflet  of  the  mitral  valve.  This 
bundle  has  been  termed  "the  muscle-bundle  of  His."  In  1904  Retzer  and 
Braunig,  working  independently,  corroborated  the  existence  of  this  bundle 
and  described  its  anatomic  course  more  completely.  The  investigations  of 
Braunig  led  to  the  conclusion  that  this  bundle  of  muscle-fibers  which  was 
constandy  present  in  all  animals  examined,  including  man,  began  on  the 
right  side  of  the  auricular  wall  below  the  fossa  ovalis,  from  which  point  it 
passed  forward,  and  anteriorly  penetrated  the  auriculo-ventricular  septum 
to  become  connected  with  the  musculature  of  the  ventricular  septum  just 
below  the  pars  membranacea  septi.  Though  both  these  observers  state  that 
the  bundle  divides  into  a  right  and  left  limb  as  it  enters  the  ventricular 


272  TEXT-BOOK  OF  PHYSIOLOGY 

septum,  the  ultimate  distribution  and  termination  of  these  limbs  was  not 
clearly  determined.  Retzer  estimated  that  this  bundle  was  i8  mm.  long, 
2.5  mm.  broad,  and  1.5  mm.  thick.  By  these  investigators  this  bundle  was 
termed  the  "  auriculo- ventricular  bundle." 

In  1906  Tawara  published  the  results  of  an  extended  series  of  investiga- 
tions made  on  the  embryonic  and  adult  hearts  of  many  mammals  including 
man,  which  resulted  in  a  further  increase  of  knowledge  concerning  the 
development,  anatomic  course,  and  histologic  features  of  this  bundle,  and 
established  beyond  doubt  that  it  is  the  pathway  along  which  the  contraction 
process  is  conducted  from  the  auricles  to  the  ventricles. 

A  brief  summary  of  Tawara's  account  of  this  bundle  is  as  follows:  It 
arises  near  the  opening  of  the  coronary  sinus  where  it  is  connected  with  the 
true  auricular  fibers.  From  their  origin  the  fibers  converge  to  form  a  dis- 
tinct bundle  which  then  passes  forward  on  the  right  side  of  the  auricular 
septum  between  the  lower  edge  of  the  fossa  ovalis  and  the  auriculo-ventricu- 
lar  septum;  just  above  the  insertion  of  the  median  cusp  of  the  tricuspid 
valve  the  bundle  presents  a  very  complicated  network  of  muscle-fibers  which 
has  been  designated  as  a  knot  or  the  auriculo-ventricular  node  or  the  node 
of  Tawara;  from  the  anterior  portion  of  the  node  a  bundle  of  fibers  turns 
downward  and  penetrates  the  auriculo-ventricular  septum,  beyond  which  it 
passes  below  the  pars  membranacea  septi  to  the  upper  limit  of  the  muscle 
portion  of  the  ventricular  septum.  It  then  divides  into  two  limbs  or 
branches  which  descend  on  either  side  of  the  septum  under  the  endocar- 
dium, the  right  limb  lying  somewhat  deeper  than  the  left.  Each  of  these 
limbs  is  enclosed  by  a  layer  of  connective  tissue  which  isolates  it  from  the 
musculature  of  the  ventricular  septum  as  far  as  the  lower  third  of  the  ven- 
tricular cavities.  In  this  region  they  divide  into  a  number  of  bundles,  some 
of  which  enter  the  papillary  muscles,  while  others,  forming  tendon-like 
strands,  branch  freely  beneath  the  endocardium  and  spread  in  all  direc- 
tions over  the  entire  inner  surface  of  the  ventricle  and  enter  into  histologic 
connection  with  the  true  cardiac  muscle-fibers. 

The  fibers  composing  this  system,  and  termed  by  Tawara  from  its  sup- 
posed function  the  "  conduction  system  "  are  histologically  different  from  the 
cardiac  fibers,  in  so  far  as  they  are  poorer  in  sarcoplasm  and  similar  in  their 
appearance  to  embryonic  muscle-fibers.  In  the  auricular  portion  of  the 
bundle  the  fibers  exhibit  a  more  or  less  reticular  arrangement;  in  the  ven- 
tricular portion,  the  fibers  are  more  regularly  arranged,  are  richer  in  sar- 
coplasm and  present  a  number  of  fibrillae  near  their  periphery.  In  asso- 
ciation with  the  muscle-fibers  composing  the  auriculo-ventricular  bundle 
there  is  a  special  collection  of  nerve-cells  and  nerve-fibers.  Their  function 
is  unknown. 

The  ultimate  termination  of  the  system,  beneath  the  endocardium,  con- ' 
stitutes  the  so-called  Purkinje  fiber  layer.     In  the  sheep,  calf,  and  in  other 
animals  these  fibers  are  abundant  and  readily  recognized;  though  they  are 
not  so  well  developed,  they  are  nevertheless  present  and  extensively  dis- 
tributed in  the  human  heart. 

The  Keith-Flack  Node  or  the  Sino-Auricular  Node. — This  is  a 
small  body,  discovered  by  the  investigators  whose  names  it  bears,  situated 
in  the  sulcus  terminalis  "just  below  the  fork  formed  by  the  junction  of  the 
upper  surface  of  the  auricular  appendix  with  the  superior  vena  cava."     It 


THE  CIRCULATION  OF  THE  BLOOD  273 

appears  to  be  a  remnant  of  primitive  muscle  tissue  at  what  was  formerly  the 
junction  of  the  sinus  venosus  and  the  auricle.  In  its  structure  it  resembles 
the  auriculo-ventricular  (Tawara's)  node,  in  that  it  consists  of  peculiar 
muscle-fibers,  nerve-cells,  and  nerve-fibers  enclosed  by  connective  tissue.  It 
is  also  provided  with  an  abundant  blood-supply.  In  the  human  heart,  the 
muscle-fibers  of  this  remnant  are  striated,  possess  well  marked  and  elongated 
nuclei  and  are  plexiform  in  arrangement.  From  the  node  the  muscle-fibers 
extend  downward  along  the  sulcus  terminalis  for  about  two  centimeters. 
The  thickness  of  the  bundle  is  about  two  millimeters.  Superiorly  the  node 
appears  to  be  connected  with  or  continuous  with  fibers  in  the  superior  vena 
cava;  inferiorly  it  is  connected  with  the  true  auricular  fibers.  The  dissection 
of  this  node  shows  that  the  terminal  branches  of  the  vagi  and  sympathetic 
nerves  are  in  histologic  relation  with  the  nerve-cells.  The  situation,  struc- 
ture and  relations  of  this  neuro-muscle  node  appear  to  justify  the  assump- 
tion that  it  is  directly  concerned  in  the  initiation  of  the  heart-beat. 

THE  MECHANICS  OF  THE  HEART 

Methods  of  Observation. — The  movements  of  the  heart,  as  well  as 
many  phenomena  connected  with  the  flow  of  blood  through  its  cavities,  have 
been  determined  by  observation  of,  and  experimentation  on,  the  exposed  heart 
of  a  mammal — e.g.,  dog,  cat,  rabbit — supplemented  and  corrected  by  experi- 
ments on  the  heart  in  its  normal  relations.  Valuable  information  as  to  the 
heart-beat  and  the  influences  which  modify  it  has  been  obtained  from  experi- 
ments made  on  the  isolated  heart  of  the  turtle,  frog,  and  allied  animals. 

If  the  thorax  of  a  dog,  completely  anesthetized,  is  opened  and  artificial 
respiration  established,  the  heart  will  be  observed  in  active  movement  inside 
the  pericardium.  If  this  sac  is  divided  and  turned  aside,  the  heart  will  be 
fully  exposed  to  view.  At  the  normal  rate  of  movement  of  the  heart 
characteristic  of  the  dog  it  will  be  almost  impossible  to  determine  either  the 
succession  of  events  or  their  duration.  But  by  observing  the  heart  under 
different  conditions  at  different  rates  of  movement  and  with  instrumental 
aids,  physiologists  have  succeeded  not  only  in  analyzing  the  movements,  but 
in  describing  their  sequence  and  in  estimating  their  time  duration. 

Phenomena  Observed. — From  many  observations  and  experiments  it 
has  been  determined  that  the  heart  at  each  beat  presents  two  distinct  move- 
ments which  alternate  with  each  other  in  quick  succession.  One  is  the 
movement  of  contraction,  or  the  systole,  by  which  the  blood  contained  within 
its  cavities  is  ejected  into  the  arteries — pulmonic  artery  and  aorta;  the 
other  is  the  movement  of  relaxation,  or  the  diastole,  followed  by  a  pause 
during  which  the  cavities  again  fill  up  with  blood  from  the  venae  cavae  and 
pulmonic  veins. 

The  contraction  of  any  part  of  the  heart  is  termed  the  systole;  the  relaxa- 
tion, the  diastole.  As  each  side  of  the  heart  has  two  cavities  the  walls  of 
which  contract  and  relax  in  succession,  it  is  customary  to  speak  of  an  auricu- 
lar systole  and  diastole,  and  a  ventricular  systole  and  diastole.  As  the  two 
sides  of  the  heart  are  in  the  same  anatomic  relation  to  each  other,  they 
contract  and  relax  in  the  same  periods  of  time. 

It  has  also  been  ascertained  that  the  contraction  of  the  auricles  and 
ventricles  as  well  as  their  subsequent  relaxations,  though  occurring  with 


274  TEXT-BOOK  OF  PHYSIOLOGY 

extreme  rapidity,  do  not  take  place  simultaneously  but  successively;  that 
the  contraction  process  passes  over  the  heart  in  the  form  of  a  wave;  that  it 
begins,  indeed,  at  the  terminations  of  the  great  veins,  viz.,  the  vence  cavcB,  then 
passes  to  and  over  the  auricles,  thence  to  and  over  the  ventricles  from  base 
to  apex  with  great  rapidity,  but  occupying  in  these  different  regions  unequal 
periods  of  time;  that  the  relaxation  immediately  succeeds  the  contraction, 
in  the  same  order,  and  that  at  the  close  of  the  ventricular  relaxation  there 
is  a  period  during  which  the  whole  heart  is  in  repose,  passively  filling  with 
blood. 

The  immediate  cause  of  the  movement  of  the  blood  through  the  vessels 
is  the  contraction  and  relaxation  of  the  muscle-walls  of  the  heart,  and  more 
particularly  of  the  walls  of  the  ventricles,  each  of  which  plays  alternately 
the  part  of  a  force-pump,  and  possibly  to  a  slight  extent  of  a  suction-pump. 
The  motive  power  is  furnished  by  the  heart  itself,  by  the  transformation 
of  potential  energy,  stored  up  during  the  period  of  rest,  into  kinetic 
energy — i.e.,  heat  and  mechanic  motion. 

Changes  in  Position  and  Form. — It  is  also  apparent  under  the  condition 
of  the  foregoing  observation  that  the  heart  during  each  pulsation  undergoes 
changes  of  both  position  and  form.  In  the  diastolic  condition,  during  which 
the  heart  is  in  repose,  the  apex  is  directed  obliquely  downward  and  to  the 
left;  the  body  of  the  heart  is  enlarged  and  its  walls  relaxed.  As  the 
systole  begins  and  reaches  its  maximum,  the  apex  is  tilted  upward,  the 
entire  heart  is  rotated  on  its  axis  from  left  to  right  and  forced  forward  by 
the  expansion  and  elongation  of  the  pulmonic  artery  and  aorta.  As  the 
diastole  begins  and  rapidly  passes  to  its  completion  a  reverse  series  of 
movements  is  presented,  viz. :  an  ascent  of  the  heart  due  to  the  recoil  and 
shortening  of  the  pulmonic  artery  and  aorta,  a  rotation  of  the  heart  on 
its  axis  from  right  to  left,  and  a  fall  of  the  apex.  With  the  completion  of 
this  latter  event,  the  heart  for  a  brief  period  is  in  repose. 

It  is  probable,  however,  that  these  movements  are  not  permitted  to 
the  same  extent  in  the  unopened  chest,  for  the  following  reasons:  the 
heart  is  enclosed  in  the  pericardium,  is  supported  posteriorly  by  the 
expanded  lungs,  and  both  posteriorly  and  inferiorly  by  the  diaphragm,  all 
of  which  cooperate  in  keeping  the  heart,  and  more  particularly  the  right 
ventricle,  in  close  contact  with  the  chest  wall  and  limiting  its  movements. 
By  means  of  needles  inserted  into  the  apex  of  the  heart,  through  the  chest 
walls,  it  has  been  shown  by  their  slight  movement  that  the  apex  is 
practically  a  fixed  point. 

In  the  diastolic  condition  the  shape  of  the  heart  near  the  base  is  elliptic 
on  cross-section,  the  long  diameter  extending  from  side  to  side.  In  the 
completed  systolic  condition  the  shape  of  the  same  cross-section  approxi- 
mates that  of  a  circle.  In  passing  from  the  diastolic  to  the  systolic  condition 
the  transverse  diameter  diminishes  while  the  antero-posterior  diameter 
increases,  and  the  whole  heart  becomes  somewhat  more  conic  in  shape. 
It  is  questionable  if  the  vertical  diameter  perceptibly  shortens.  During  the 
systole  the  heart  hardens,  increases  in  convexity,  and  is  more  forcibly  pressed 
against  the  chest  wall.  As  this  takes  place  suddenly,  it  gives  rise  to  a 
marked  vibration  of  the  chest  wall,  known  as — 

The  Cardiac  Impulse. — This  impulse  is  principally  observed  in  the 
space  between  the  fifth  and  sixth  ribs  about  an  inch  internal  to  a  line  drawn 


THE  CIRCULATION  OF  THE  BLOOD  275 

vertically  from  the  middle  of  the  clavicle.  The  cardiac  impulse  is  synchron- 
ous with  the  cardiac  systole. 

The  cardiac  impulse  may  be  recorded  with  an  appropriate  apparatus 
known  as  a  cardiograph;  the  record  obtained  with  it  is  known  as  a  cardiogram. 
A  cardiograph  consists  of  a  tambour  covered  with  a  thin  rubber  membrane 
provided  w^ith  a  button.  The  tambour  is  supported  by  a  metallic  frame 
which  permits  of  an  easy  and  accurate  adjustment  of  the  button  over  the 
seat  of  the  cardiac  impulse.  A  rubber  tube  connects  the  cardiographic 
tambour  with  a  second  tambour  provided  with  a  recording  lever  and  thus 
transmits  all  variations  in  the  pressure  of  the  air  in  the  former  to  the  latter. 

When  all  adjustments  are  carefully  made  a  tracing  similar  to  that  shown 
in  Fig.  118  will  be  obtained,  in  which  the  slight  elevation   a   represents  the 
contraction  of  the  auricle  which,  completing 
the  filling  of  the  ventricle,  causes  the  apex  of 
the  heart  to  press  more  vigorously  against  the 
chest  wall ;  h-c  represents  the  contraction  of 
the  ventricles,  at  which  moment  the  apex  is 
suddenly  and  forcibly  driven  against  the  chest 
wall;  c-d  represents  the  systolic  plateau,  the 
time  during  which  the  ventricle  is  discharging 
blood  into  the  aorta;  d-e  represents  the  relaxa- 
tion of  the  ventricle;  while  e^  represents  the 
time  of  the  diastole,  during  which  the  heart 
cavities  are  enlarging  with  the  incoming  of  a         Fig.  118.— a  Cardiogram. 
new  volume  of  blood  in  consequence  of  which  —{After  Pachon.) 

the  heart  is  pressing  against  the  chest  walls.  The  systolic  plateau  is  charac- 
terized by  one  or  more  elevations  and  depressions,  the  true  cause  of  which  is 
unknown. 

The  Cardiac  Cycle. — The  term  cardiac  cycle  is  employed  to  express  the 
sequence  of  events  from  the  beginning  of  one  auricular  systole  and  the 
beginning  of  the  auricular  systole  which  immediately  follows  it.  An  examina- 
tion of  the  heart  shows  that  each  pulsation  may  be  divided  into  three  phases, 
viz.: 

1.  The  auricular  systole. 

2.  The  ventricular  systole. 

3.  The  pause  or  period  of  repose  during  which  both  auricles  and  ven- 
tricles are  at  rest. 

The  Graphic  Record  of  the  Cardiac  Cycle. — For  the  purpose  of  obtaining  accurate  in- 
formation as  to  the  sequence  of  events,  their  time  relations,  as  well  as  of  the  pressure  within  the 
heart  cavities  during  each  phase  of  its  activit}',  it  is  necessary  to  obtain  graphic  records  of  the 
entire  cardiac  cycle. 

This  was  first  successfully  accompUshed  by  Chauveau  and  Marey,  by  means  of  sounds  or 
tambours  (Fig.  119)  introduced  through  the  jugular  vein  into  the  cavities  of  the  right  heart.  Each 
tambour  consists  of  a  metallic  frame  covered  by  a  thin  rubber  membrane.  By  means  of  flexible 
tubes,  a.  v.,  the  interior  of  each  tambour  can  be  placed  in  communication  with  the  interior 
of  a  second  tambour  provided  with  a  recording  lever.  Pressure  applied  to  the  cardiac  tambour 
will  be  followed  by  a  movement  of  the  enclosed  air  toward  the  recording  tambour  indicated 
by  an  outward  movement  of  its  membrane  and  a  rise  of  the  lever;  removal  of  the  pressure  will 
be  followed  by  a  movement  of  the  enclosed  air  toward  the  cardiac  tambour  indicated  by  an 
inward  movement  of  the  membrane  and  a  fall  of  the  lever. 

When  the  tambours  are  introduced  into,  and  carefully  adjusted  to  the  interior  of  the  right 
heart,  the  auricular  and  ventricular  contractions  will  exert  pressure  on  their  enclosed  tambours 
as  indicated  by  the  rise  of  the  levers  of  the  recording  tambours,  which  continues  so  long  as 
the  pressure  lasts.     With  the  relaxation  of  the  auricular  and  ventricular  walls   the  pressure   is 


-276 


TEXT-BOOK  OF  PHYSIOLOGY 


removed  and  the  levers  fall  to  their  former  position.  When 
the  levers  are  applied  to  the  surface  of  a  recording  cylinder 
a  record  of  auricular  and  ventricular  contractions  is  obtained 
such  as  that  shown  in  Fig.  120. 

A  similar  record  would  be  obtained  if  the  tambours  were 
placed  in  the  cavities  of  the  left  side  of  the  heart. 

In  the  graphic  record,  Fig.  120  obtained 
by  the  foregoing  method,  it  is  apparent  that 
during  the  period  of  repose  there  is  a  gradual 
ascent  of  the  tips  of  the  recording  levers,  the 
result  of  a  gradual  increase  of  pressure  due  to 
the  accumulation  of  blood  within  the  heart 
cavities.  When  this  reaches  a  certain  level  the 
auricular  contraction  occurs  rather  suddenly, 
followed  by  an  equally  sudden  relaxation,  after 
which  the  auricular  walls  remain  at  rest  for  a 
relatively  long  period,  though  the  pressure  with- 
in the  auricle  undergoes  variations  both  in 
the  way  of  increase  and  decrease  as  shown  by 
small  undulations  on  the  curve. 

With  the  close  of  the  auricular  systole,  the 
ventricular  systole  occurs  quickly  and  energet- 
ically and  endures  for  some  time,  after  which 
the  ventricular  walls  quickly  relax  and  remain 
at  rest  until  the  close  of  the  next  auricular  con- 
FiG.   119.— Cardiac  Sounds,  traction.     The  summit  of  the  ventricular  trac- 
V,  Tambours  to  be  inserted  into  j^g  generally  spoken  of  as  the  plateau  presents 
leryd^taoTerurict^rmlbe;  a  series  of  elevations  and  depressions  as  stated 

membrane    surrounding    metal    in  a  foregoing  paragraph. 


frame- work;  a,  v,  ends  of  tubes  in  ^  comparison  of  the  twO  traces   showS   that 

TuZyT  "''^  ^^"^b^^'^^-  between  the  close  of  the  auricular  and  the  be- 
ginning of  the  ventricular  systole  there  is  a  slight 
pause  known  as  the  inter  systolic  pause  (Chauveau).     The  tracings  also  show 
that  between  the  close  of  the  ventricular  contraction  and  the  beginning  of 


■■■■■■■■■■■■RSUHHIHI 

■ji"SS™Sa3S8™""™*"' 

HHUHUHHIHHmHI 

HHUmUHIHIiHBBBBI 

IHISISSSiHUHHIBWSlBIIUH 

niimsiimmiHHHiBi 

HHIMBBIINHIHIIUHBHI 

SBSSSBBBSiBara 

iBsassaiaBaBsa 

Fig.  120. — A  Graphic  Record  of  (i)  the  Intra-auricular  Pressure;  and  (2)  the  Intra- 
ventricular Pressure  of  the  Horse. — {Chauveau  and  Marey.) 

the  succeeding  auricular  contraction  there  is  a  period  during  which  the 
whole  heart  is  at  rest  and  during  which  the  cavities  are  filling  with  blood. 


THE  CIRCULATION  OF  THE  BLOOD 


277 


For  the  purpose  of  obtaining  the  time  of  all  these  events,  the  recording 
surface  was  divided  into  equal  spaces  by  vertically  drawn  lines.  The  rate 
of  movement  of  the  surface  was  such  that  each  division  corresponded  to 
one-tenth  of  a  second.  The  record  thus  indicates  that  the  auricular  con- 
traction lasted  approximately  0.2  second,  the  ventricular  contraction  0.4 
second,  and  the  pause  0.4  to  0.6  second. 

From  similar  experiments  made  on  other  animals,  e.g.,  the  dog,  similar 
results  have  been  obtained;  but  by  reason  of  the  employment  of  more 
sensitive  and  more  quickly  responsive  tambours,  the  curve  of  the  auricular 
contraction  exhibits  variations  not  recorded  by  the  forms  of  tambour  used 
in  earlier  experiments.  Reference  to  these  variations  will  be  alluded  to  in 
subsequent  paragraphs.  The  results  obtained  by  recent  observers  now 
generally  accepted  are  in  accord  with  the  results  obtained  by  Chauveau  and 
Marey  by  means  of  their  cardiac  tambours  as  shown  in  Fig.  120. 

The  Movement  of  the  Blood  Dur- 
ing the  Cycle. — From  the  characteris- 
tic features  of  the  foregoing  record  it  is 
apparent  that  with  the  relaxation  of  the 
auricular  walls,  blood  at  once  flows 
from  the  venae  cavae  and  the  pulmonic 
veins  into  the  auricular  cavities  and 
continues  so  to  do  throughout  the  entire 
auricular  diastole.  With  the  relaxation 
of  the  ventricular  walls,  however,  the 
blood  that  has  accumulated  in  the 
auricles  up  to  this  time,  or  its  equiva- 
lent coming  from  the  vens  cavas  and  pulmonic  veins,  now  flows  into  the 
ventricles  until  they  are  nearly  filled.  Before  they  are  filled,  however,  the 
auricular  diastole  comes  to  an  end,  the  auricular  walls  again  contract  and 
force  some  of  their  contained  blood  into  the  ventricles  and  thus  rapidly  com- 
plete the  filling.  The  ventricular  systole  immediately  follows,  during  which  the 
blood  is  driven  into  the  pulmonic  artery  and  aorta.  This  having  been  ac- 
complished, the  ventricles  relax,  and  the  blood  that  has  been  accumulating 
in  the  auricles  begins  to  flow  into  the  ventricles,  after  which  the  same  series 
of  events  follows  as  in  the  previous  cycle. 

The  Curve  of  the  Systole  and  Diastole  of  the  Heart. — In  the  study 
of  the  volume  changes  of  the  heart  by  means  of  a  specially  devised  cardi- 
ometer,  Henderson  was  enabled  to  record  the  contraction  and  relaxation 
movements  of  the  heart,  obtaining  thereby  a  curve  that  resembled  the  curve 
of  the  contraction  and  relaxation  of  a  skeletal  muscle.  If  this  curve  is  re- 
versed and  superposed  on  a  curve  of  the  intra-ventricular  pressure,  their 
relation  one  to  an  other  becomes  apparent,  Fig.  121. 

The  chief  characteristic  of  the  systolic  portion  of  the  curve,  viz.:  its 
steep  ascent,  though  less  than  that  of  the  pressure  curve,  shows  that  with 
the  opening  of  the  semilunar  valves,  b,  a  large  portion  of  the  volume  of  the 
blood  in  the  ventricles,  90  per  cent,  according  to  Henderson,  is  quickly  and 
in  a  uniform  manner  discharged,  after  which  the  outflow  slows  and  finally 
ceases  as  indicated  by  the  rounded  apex  of  the  curve.  The  chief  character- 
istic of  the  diastolic  portion  of  the  curve,  viz. :  its  steep  descent,  though  less 
than  that  of  the  pressure  curve,  shows  that  with  the  opening  of  the  auriculo- 


FiG.  121. — The  Volume  Curve  of 
THE  Heart  Contrasted  with  the  Curve 
OF  the  Intraventricular  Pressure. 


2  78  TEXT-BOOK  OF  PHYSIOLOGY 

ventricular  valves,  </,  a  large  portion  of  the  volume  of  the  blood  in  the  auricles 
fiov^^s  quickly  into  the  ventricles  during  the  early  part  of  the  diastole,  nearly, 
if  not  entirely  retuUing  them,  a  procedure  occupying  a  period  of  time  ap- 
proximately that  required  for  the  systolic  discharge.  In  some  experiments 
the  portion  of  the  curve,  recording  the  increase  of  volume  does  not  reach  the 
abscissa  but  runs  off  parallel  with  it.  In  other  experiments  it  gradually 
approximates  the  abscissa  or  shows  a  slight  fall  at  the  time  of  the  succeeding 
auricular  contraction.  According  to  Henderson  blood  has  ceased  to  flow 
from  the  auricles  into  the  ventricles  during  the  foregoing  period  and  the 
contraction  of  the  auricles  adds  but  little  to  the  volume  of  the  blood  already 
in  the  ventricles.  Other  investigators  attach  more  importance  to  this 
event. 

The  portion  of  the  volume  curve  between  the  apex  and  the  beginning 
of  the  succeeding  ventricular  systole,  embracing  the  entire  diastolic  period, 
has  been  subdivided  into  two  portions,  viz. :  (a)  from  the  apex  to  the  be- 
ginning of  the  horizontal  portion,  corresponding  to  the  period  of  relaxation 
and  refilling — the  diastole  proper — and  (b)  the  period  of  rest  or  the  diastasis. 
This  latter  period  is  of  rather  short  duration  in  the  normal  rate  of  heart 
beat.     It  lengthens  if  the  rate  decreases,  and  shortens  if  the  rate  increases. 

The  Action  of  the  Valves  During  the  Cycle. — As  previously  stated, 
the  forward  movement  of  the  blood  is  permitted  and  regurgitation  prevented 
by  the  alternate  action  of  the  semilunar  and  the  auriculo-ventricular  valves. 
As  a  point  of  departure  for  a  consideration  of  the  action  of  the  valves  and 
their  relation  to  the  systole  and  diastole  of  the  heart,  the  close  of  the  ventricu- 
lar systole  may  be  conveniently  selected. 

At  this  moment,  if  the  blood  is  not  to  be  returned  to  the  ventricles,  the 
semilunar  valves  must  be  instantly  and  completely  closed.  This  is  accom- 
plished in  the  following  manner:  During  the  outflow  of  blood  from  the 
ventricles  the  valves  are  pushed  outward  toward  the  walls  of  the  vessels, 
though  not  coming  into  contact  with  them,  for  behind  them  are  the  pouches 
of  Valsalva,  containing  blood,  continuous  with  and  under  the  same  pressure 
as  that  in  the  vessels  themselves.  With  the  cessation  of  the  outflow  and  the 
beginning  of  the  relaxation  the  pressure  of  the  blood  behind  the  valves 
suddenly  forces  them  inward  until  their  free  edges,  including  the  lunulae, 
come  into  complete  apposition.  By  this  means  the  orifices  of  the  pulmonic 
artery  and  aorta  are  securely  closed  and  a  return  flow  prevented.  Reversal 
of  the  valves  is  prevented  by  their  mode  of  attachment  to  the  fibrous  rings  of 
the  orifices. 

During  the  ventricular  systole  the  relaxed  auricles  have  been  filling  with 
blood.  With  the  ventricular  relaxation  this  volume,  or  its  equivalent,  flows 
readily  into  the  empty  and  easily  distensible  ventricles,  its  place  being  taken 
by  an  additional  volume  of  blood  flowing  from  the  venae  cavae  and  pulmonic 
veins.  Whether  the  ventricles  exert  a  suction  power  at  the  moment  of 
their  relaxation  is  an  undecided  question.  A  steady  stream  of  blood  into 
the  auricles  and  ventricles  continues  throughout  the  entire  period  of  rest  until 
both  cavities  are  filled.  The  tricuspid  and  bicuspid  valves  which  hang  down 
into  the  ventricular  cavities  are  now  floated  up  by  currents  of  blood  welling 
up  behind  them  until  they  are  nearly  closed.  The  auricles  now  contract, 
forcing  their  contained  volumes,  or  at  least  the  larger  portions  of  them,  into 
the  ventricles,  which  become  fully  distended. 


THE  CIRCULATION  OF  THE  BLOOD  279 

With  the  cessation  of  the  auricular  systole  the  ventricular  systole  begins. 
If  the  blood  is  not  to  be  returned  to  the  auricles  at  this  moment,  the  tricuspid 
and  mitral  valves  must  be  suddenly  and  accurately  closed.  This  is  readily 
accomplished  by  reason  of  the  position  of  the  valves,  which  have  been 
floated  up  and  placed  almost  in  apposition  by  the  blood  itself.  With  the 
beginning  of  the  ventricular  pressure  the  blood  is  forced  upward  against  the 
valves  until  their  free  edges  are  brought  together  and  the  orifices  closed. 
Reversal  of  these  valves  into  the  auricles  is  prevented  by  their  attachment  to 
the  chorda  tendinece,  and  the  latter  are  kept  from  moving  bodily  upward 
during  the  ventricular  contraction  by  the  compensatory  downward  pull  of 
the  papillary  muscles.  The  blood  now  confined  in  the  ventricle  between  the 
closed  auriculo-ventricular  and  semilunar  valves  is  subjected  to  pressure 
from  all  sides.  As  the  pressure  rises  proportionately  to  the  vigor  of  the  con- 
traction, there  comes  a  moment  when  the  intra-ventricular  pressure  exceeds 
the  pressure  in  the  aorta  and  in  the  pulmonic  artery.  As  soon  as  this 
occurs  the  semilunar  valves  of  both  vessels  are  thrown  open  and  the  blood 
discharged.^  Both  contraction  and  outflow  continue  until  the  ventricles 
are  practically  empty,  after  which  ventricular  relaxation  sets  in.  W^ith 
the  discharge  of  the  blood  the  pressure  in  both  the  pulmonic  artery  and  aorta 
rises,  passing  from  a  minimal  to  a  maximal  value.  Coincidently  the  pres- 
sure in  the  ventricles  rises  and  even  exceeds  that  in  the  pulmonic  artery  and 
aorta  and  so  continues  until  near  the  close  of  the  systole  when  the  two 
opposing  pressures  are  approximately  equal.  With  the  onset  of  the 
ventricular  relaxation  the  intra-ventricular  pressure  suddenly  falls,  and  so 
soon  as  it  falls  below  the  positive  pressure  of  the  blood  in  the  sinuses  of 
Valsalva  the  semilunar  valves  are  again  closed,  the  column  of  blood  is 
supported,  and  regurgitation  is  prevented.  In  the  meantime  and  while 
the  ventricles  are  contracting,  blood  is  again  flowing  into,  and  accumulat- 
ing in  the  auricles  and  thereby  distending  them  preparatory  to  the  next 
systole.  With  the  accumulation  of  blood  in  the  auricles  and  ventricles 
the  cardiac  cycle  is  completed. 

The  approximate  changes  in  the  shape  of  the  heart,  the  variations  in  the 
size  of  its  cavities  and  in  the  size  of  the  blood-vessels  arising  from  them, 
and  the  relative  position  of  the  valves  during  systole  and  diastole  are  shown 
in  Fig.  123. 

Heart-sounds. — Tv/o  sounds  accompany  each  pulsation  of  the  heart, 
both  of  which  may  be  heard  by  applying  the  ear  or  the  stethoscope  to  the 
chest  walls,  especially  over  the  region  of  the  heart.  One  of  these  sounds  is 
low  in  pitch,  dull  and  prolonged;  the  other  is  high  in  pitch,  clear  and  short. 
These  sounds  can  be  approximately  reproduced  by  pronouncing  the  syllables 
hlbb-diip,  lilbbdup.  The  long  dull  sound  occurs  with  the  systole,  the  first 
phase  of  a  new  cardiac  cycle,  and  is  therefore  termed  the  first  sound;  the 
short  clear  sound  occurs  at  the  beginning  of  the  diastole,  with  the  second 
phase  of  the  cardiac  cycle,  and  is  therefore  termed  the  second  sound.  The 
first  sound  is  the  systolic,  the  second  the  diastolic.  With  the  ear  it  can 
readily  be  determined  that  there  is  a  brief  pause  between  the  first  and  second 
sounds,  and  a  longer  pause  between  the  second  and  the  first  sounds.     The 

^  The  discharge  of  the  blood  by  the  contraction  of  the  ventricular  walls  is  probably  aided  by 
the  simultaneous  downward  displacement  of  the  more  central  portion  of  the  auriculo-ventricular 
septum,  due  to  the  contraction  of  the  papillary  muscles. 


28o  TEXT-BOOK  OF  PHYSIOLOGY 

duration  of  the  first  sound  is  almost  equal  to  the  duration  of  the  systole— viz., 
0.3  second;  the  duration  of  the  second  sound  is  not  more  than  o.i  second. 
The  systoHc  sound  is  heard  most  distinctly  over  the  body  of  the  heart;  the 
diastolic  sound  is  heard  most  distinctly  in  the  neighborhood  of  the  third  rib 
to  the  right  of  the  sternum. 

The  causes  of  the  heart-sounds  have  enlisted  the  attention  of  clinicians 
and  physiologists  for  years,  and  many  factors  have  been  assigned  for  their 
production.  At  present  it  is  generally  believed  that  the  first  sound  is  the 
product  of  at  least  two,  possibly  three,  factors:  viz.,  the  contraction  of  the 
muscle  walls  of  the  ventricles,  the  simultaneous  closure  and  subsequent 
vibration  of  the  tricuspid  and  mitral  valves,  and  the  sudden  increase  of 
pressure  of  the  apex  of  the  heart  against  the  chest  wall. 

That  the  contraction  of  the  ventricular  muscle  gives  rise  to  a  sound  is 
certain  from  the  fact  that  it  is  perceptible  in  an  excised  heart  when  the 
cavities  are  free  from  blood  and  when  the  valves  are  prevented  from  closing. 
The  explanation  of  this  sound  is  extremely  difficult,  as  the  contraction, 
though  prolonged,  is  not  of  the  nature  of  a  tetanus  and  therefore  not  charac- 
terized by  rapid  variations  of  tension.  The  apex  element  may  be  eliminated 
by  placing  the  individual  in  the  recumbent  position. 

The  second  sound  is  the  product  of  the  simultaneous  closure  and  subse- 
quent vibration  of  the  aortic  and  pulmonic  valves  which  occur  at  the 
beginning  of  the  ventricular  diastole  as  the  blood  surges  back  against  the 
closed  valves.  This  has  been  definitely  proved  by  the  fact  that  the  sound 
disappears  when  the  valves  are  destroyed  or  held  back  by  hooks  introduced 
into  the  aorta  and  pulmonary  artery.  It  is  also  possible  that  the  vibration 
of  the  column  of  blood  produces  an  additional  tone  which  adds  itself  to  that 
produced  by  the  valves. 

The  foregoing  events  are  shown  in  their  order  in  the  following  diagram. 


/"   Sound  y^  Soantf 

Fig.  122. — Diagram  Showing  the  Event  in  the  Cardiac  Cycle. 

Relative  Functions  of  Auricles  and  Ventricles. — Though  both 
auricles  and  ventricles  are  essential  to  the  continuous  movement  of  blood, 
they  possess  unequal  values  in  this  respect.  The  passage  of  the  blood 
through  the  pulmonic  and  systemic  vessels  is  accomplished  by  the  driving 
power  of  the  right  and  left  ventricles  respectively,  aided,  however,  by  minor 
extra-cardiac  forces.     They  may  be  regarded  therefore  d.s  force-pumps. 

If  the  heart  consisted  of  ventricles  only,  the  flow  of  blood  from  the  venee 
cavae  and  pulmonic  veins  would  be  temporarily  arrested  during  their  systole 
and  their  subsequent  refilling  delayed.  This  is  obviated,  however,  by  the 
addition  of  the  auricles;  for  during  the  ventricular  systole  the  blood  continues 
to  flow  into  the  auricles,  in  which  it  is  temporarily  stored  until  the  ventricular 
relaxation  sets  in.  With  this  event  the  accumulated  blood  passes  into  the 
ventricles,  which  are  thus  practically  filled  before  the  auricular  systole  occurs 
by  which  the  fiUing  is  completed.     By  this  means  there  is  no  delay  in  the 


THE  CIRCULATION  OF  THE  BLOOD 


281 


filling  of  the  ventricles,  and  hence  their  effective  working  as  force-pumps  is 
more  readily  secured.  The  auricles  may  therefore  be  regarded  as  feed- 
pumps. For  this  reason  it  is  probable,  notwithstanding  the  contraction  of 
the  circular  muscle-fibers  at  the  terminations  of  the  venous  system,  that  the 
flow  of  blood  into  the  auricles  is  never  entirely  arrested,  but  merely  retarded. 
Regurgitation  in  these  vessels  does  not  occur  for  the  reason  that  the  re- 
tardation develops  a  side  pressure  during  the  auricular  contraction^which 
is  equal  to  if  it  does  not  exceed  the  pressure  in  the  auricle. 

Synchronism  of  the  Two  Sides  of  the  Heart. — If.  the  balancejof  the 
circulation  is  to  be  maintained,  the  two  sides  of  the  heart  must  act  synchron- 
ously. That  they  do  so  can  be  shown  by  attaching  levers  to  their  walls,  and 
thus  recording  their  activities.     The  synchronism  is  so  perfect  that^until 


S.a.-D.v. 


D.a.-S.v. 


Fig.  123. — Diagrammatic  Representation  of  the  Axiricular  Systole,  S.a.,  with  the 
Ventricular  Diastole,  D.  v.,  and  of  the  Atjricular  Diastole,  D.  a.,  with  the  Ventricular 
Systole,  S.v.  C.s.  and  C.i.  Superior  and  inferior  cavse;  A.d.  (atrium  dextrum)  right  auricle; 
A.s.  (atrium  sinistrum)  left  auricle;  V.d.  (ventriculus  dexter)  right  ventricle;  V.s.  (ventriculus 
sinister)  left  ventricle;  P.  pulmonic  artery;  A.  aorta;  P.P.  papillary  muscles. — {Landois.) 


recently  it  was  generally  believed  to  be  dependent  on  nerve  connections;  but 
Porter  has  shown  that  if  the  ventricles  are  cut  away  from  the  auricles,  in 
which  the  nerve  mechanism  seems  to  lie,  the  synchronism  of  the  former  is 
not  interfered  with;  that  the  apical  halves  of  the  ventricles  will  beat  syn- 
chronously if  perfused  with  blood  through  an  artery;  that  a  very  small  bridge 
of  muscle-tissue  will  carry  the  wave  of  excitation  from  one  part  to  neighboring 
parts  of  the  ventricle.  It  is  therefore  probable  that  the  synchronism  is 
accomplished  through  muscle  connections  only.  The  left  ventricle,  in 
keeping  with  the  greater  work  it  has  to  do,  has  a  greater  development  than 
the  right,  and  therefore  contracts  more  energetically.  The  ratio  between 
the  energy  of  the  left  and  right  sides  is  approximately  3  to  i. 


282 


TEXT-BOOK  OF  PHYSIOLOGY 


max  valve 


to  manometer 


min  valve 


Intra-ventricular  Pressure. — i.  Positive  Pressure. — It  has  been  stated 
that  during  the  pause  of  the  heart  when  its  cavities  are  filling  with  blood 
the  semilunar  valves  are  kept  closed  by  the  pressure  of  the  blood  in  the 
pulmonic  artery  and  aorta,  a  pressure  due  to  the  resistance,  as  will  be  ex- 
plained later,  offered  to  the  flow  of  the  blood  mainly  by  the  smaller  arteries 
and  capillaries;  that  they  are  opened  only  when  the  pressure  of  the  blood 
within  the  ventricle  exceeds  that  in  the  arteries.  It  is  apparent  therefore 
that  there  must  be,  during  the  time  of  the  systole,  an  intraventricular  posi- 
tive pressure  sufficiently  high  to  overcome  and  even  exceed  the  pressure 
ordinarily  present  in  the  aorta  and  pulmonic  artery.  It  becomes,  there- 
fore, a  matter  of  interest  to  determine  the  extent  of  this  pressure  as  well 
as  its  variations  during  the  course  of  a  cardiac  cycle.  This  can  be  done 
by  inserting  a  long  catheter  into  either  the  right  or  left  ventricle,  through 
the  jugular  vein,  or  the  carotid  artery  respectively,  and  connecting  its  free 

extremity  with  a  mercurial  manometer. 
By  the  interposition  of  a  double  valve  such 
as  represented  in  Fig.  124,  it  becomes  possi- 
ble, according  to  the  direction  in  which  the 
blood  is  permitted  to  flow,  to  obtain  either 
the  maximal  or  the  minimal  pressure  that 
occurs  in  the  heart  during  a  series  of 
cycles.  By  the  employment  of  a  device  of 
this  character  Goltz  found  in  the  left  ven- 
tricle of  the  dog  a  maximal  pressure  of 
114  to  135  mm.  of  mercury;  in  the  right 
ventricle,  a  maximal  pressure  of  35  to  62 
mm. 

The  maximal  pressure  in  the  ventricles 
during  the  systole,  though  always  higher 
than  that  in  the  arteries,  is  neither  a  fixed 
nor  an  invariable  pressure,  as  it  rises  and 
falls  with  the  latter  from  moment  to  mo- 
ment. Within  limits  the  cardiac  power, 
and  therefore  the  intra-ventricular  pressure,  is  capable  of  considerable  in- 
crease. The  function  of  the  heart  is  to  drive  the  blood  through  the  vessels 
with  a  given  velocity.  This  is  possible  only  by  first  overcoming  the  resis- 
tance to  the  flow  offered  by  the  vessels,  as  indicated  by  the  arterial  pres- 
sure. As  this  is  a  variable  factor,  rising  and  falling  very  considerably  at 
times,  the  heart  must  meet  and  exceed  each  rise,  within  limits  if  the  circu- 
lation is  to  be  maintained.  This  it  does  by  calling  on  the  reserve  power 
with  which  it  is  endowed.  The  power  put  forth  by  the  heart  is  propor- 
tional to  the  work  it  has  to  perform.  If  the  arterial  pressure  continues 
higher  than  the  average  for  any  length  of  time,  the  heart  meets  the  condition 
by  an  hypertrophy  of  its  walls,  but  in  so  doing  it  encroaches  on  the  reserve 
power  proportionally  and  when  the  latter  has  become  exhausted  the  heart 
may,  on  some  sudden  rise  of  pressure  in  the  aorta,  be  unequal  to  the  dis- 
charge of  blood  from  its  cavities  and  hence  remain  in  a  state  of  permanent 
diastole. 

2.  Negative  Pressure. — It  has  also  been  demonstrated  by  the  employ- 
ment of  the  minimal  valve  that  there  is  brief  moment  in  the  cycle  when  the 


Fig.    124. — V. 


to  heart 
Frank's 


Valve. 


This  is  placed  in  the  course  of  the 
tube  between  heart  and  manometer, 
so  that  the  latter  may  be  used  as  a 
maximum,  minimum,  or  ordinary' 
manometer  according  to  the  tap  which 
is  left  open. — {Starling.) 


THE  CIRCULATION  OF  THE  BLOOD  283 

intra-ventricular  pressure  is  less  than  the  pressure  of  the  atmosphere,  be- 
coming indeed  negative  to  it.  This  moment  must  be  that  between  the  be- 
ginning of  the  relaxation  of  the  ventricles  and  the  opening  of  the  auriculo- 
ventricular  valves.  The  extent  of  the  negativity,  its  duration  and  frequency 
have  never  been  satisfactorily  determined.  Goltz,  however,  found  in  the 
left  ventricle  of  the  dog  a  minimal  pressure  of  —  23  to  —  50  mm.  of  mercury. 
The  cause  of  the  negative  pressure,  its  influence  on  the  opening  of  the 
auriculo-ventricular  valves,  and  on  the  entrance  of  blood  into  the  ventricles 
are  equally  unknown.  A  probable  cause  is  an  expansion  of  the  base  of 
the  ventricles  due  to  the  enlargement  of  the  aorta  and  pulmonic  artery. 
That  it  is  not  due  to  the  expansion  of  the  thorax  is  evident  from  the  fact 
that  it  occurs  when  the  thorax  is  open  and  the  heart  exposed. 

The  Intra-ventricular  Pressure  Curve  of  the  Dog. — It  was  stated  in 
a  previous  paragraph  that  the  contraction  of  the  auricles  and  ventricles  of 
animals  other  than  the  horse  have  been  graphically  recorded.  This  is 
especially  true  of  the  heart  of  the  dog.  A  graphic  record  of  the  intra- 
ventricular pressure,  its  course,  its  variations,  and  time  relations  is  necessary 
for  the  interpretation  of  the  heart  mechanisms.     With  such  a  record  may  be 


Fig.  125. — V.  CuRX'E  of  the  Pressutue  in  the  Ventricle  of  the  Dog.  A.  Curve  of 
THE  Pressure  in  the  Aorta.  The  curves  were  taken  simultaneously,  s,  Tuning-fork  vibrations 
each  corresponding  to  i/ioo  of  a  second,  a-b,  line  of  atmospheric  pressure.  The  ordinates 
0-5  correspond  in  the  two  records,  o.  Closure  of  the  auriculo-ventricular  valve;  i,  opening 
of  the  semilunar  valves;  2,  point  of  maximum  pressure;  3,  beginning  of  the  ventricular  relaxa- 
tion; 4,  closure  of  the  semilunar  valves;  5,  opening  of  the  auriclo- ventricular  valve.     {Hiirthle.) 

compared  the  records  of  the  pressures  in  the  venae  cavae  and  auricles  on  the 
one  hand,  and  in  the  aorta,  on  the  other  hand,  and  their  relations  one  to 
another  accurately  defined. 

The  intra-ventricular  pressure  has  been  obtained  by  specially  devised 
manometers  or  tonometers  or  tono graphs,  as  they  are  variously  termed,  the 
construction  of  which  is  such  as  to  enable  them  to  respond  instantly  to  the 
very  rapid  variations  of  the  pressure  which  occur  during  the  brief  cardiac 
cycle.  One  of  the  best  is  that  of  Hiirthle.  This  consists  of  a  small  metallic 
tambour  5  or  6  millimeters  in  diameter,  covered  by  a  thin  rubber  membrane. 
A  small  button  resting  on  the  membrane  plays  against  an  elastic  steel  spring, 
by  the  tension  of  which  the  pressure  of  the  blood  is  counterbalanced.  The 
movements  of  the  membrane  are  taken  up,  magnified,  and  recorded  by  a 
suitable  lever.  A  long  cannula  is  inserted  into  the  right  ventricle  through 
the  jugular  vein  or  into  the  left  ventricle  through  the  carotid  artery.     Both 


284  TEXT-BOOK  OF  PHYSIOLOGY 

cannula  and  tambour  are  filled  with  an  alkaline  solution  to  prevent  coagula- 
tion of  the  blood,  and  then  made  air-tight.  The  pressure  of  the  blood  in  the 
ventricle  is  thus  transmitted  by  a  liquid  column  to  the  tambour  and  to  its 
attached  lever.  With  such  a  manometer  a  curve  is  registered  similar  to  that 
shown  in  Fig.  125,  To  obtain  the  absolute  value  of  this  curve  in  millimeters 
of  mercury  it  is  necessary  to  graduate  the  instrument  previously.  An 
examination  of  the  curve  shows  that  previous  to  the  ventricular  contraction 
there  is  a  very  slight  rise  of  pressure  above  that  of  the  atmosphere,  repre- 
sented by  the  line  a — b.  This  may  be  due  to  the  inflow  of  blood  from  the 
auricle  during  the  diastole.  At  o  the  pressure  suddenly  rises,  passes  quickly 
to  its  maximum  value,  (2),  which  is  maintained  with  slight  variations  for 
some  time,  and  then  suddenly  (3)  begins  to  fall,  and  rapidly  reaches  the 
line  of  atmospheric  pressure,  or  even  passes  below  it,  becoming  negative  in 
fact  for  a  short  period.  The  curve  may  also  be  taken  as  a  record  of  the 
ventricular  contraction,  for  there  are  reasons  to  believe  that  the  two  closely 
coincide  throughout  their  entire  course.  A  characteristic  feature  of  this 
curve  is  the  more  or  less  horizontal  portion  comprised  between  the  points 
2  and  3,  marked  by  several  elevations  and  depressions,  which  has  been 
termed  the  systolic  plateau. 

With  other  forms  of  elastic  manometers,  especially  those  in  which  the 
transmission  of  the  intra-ventricular  pressure  is  effected  by  air  or  by  a  com- 
bination of  air  and  liquid,  this  portion  of  the  curve  is  represented  by  a  single 
peak,  which  is  taken  as  an  indication  that  the  maximal  pressure  once  reached 
is  not  maintained,  but  immediately  begins  to  fall  to  its  original  level,  not- 
withstanding the  continued  contraction  of  the  ventricle.  Those  who  adhere 
to  this  view  attribute  the  plateau  to  the  closure  of  the  orifice  of  the  catheter 
by  the  contracting  and  approximating  walls  of  the  ventricle.  There  are 
reasons  for  believing,  however,  that  the  former  curve  is  the  more  correct  repre- 
sentation of  the  course  of  the  intra-ventricular  pressure.  Bayliss  and  Star- 
ling photographed  on  a  moving  surface  the  oscillations  of  a  fluid,  a  solu- 
tion of  sodium  sulphate,  in  a  capillary  glass  tube  one  end  of  which  was 
closed,  the  other  end  placed  in  connection  with  an  intra-cardiac  catheter, 
the  oscillations  representing  the  variations  in  pressure.  The  photogram 
thus  obtained  resembles  the  curve  obtained  by  Hiirthle's  membrane  ma- 
nometer. 

The  Relation  of  the  Intra-ventricular  Pressure  Curve  to  the  Intra- 
cardiac Mechanisms. — By  itself  the  curve  of  the  intra-ventricular  pressure 
affords  no  indication  as  to  events  occurring  within  the  heart:  i.e.,  as  to  the 
times  during  the  systole,  of  the  closure  of  the  auriculo-ventricular  valves  and 
the  opening  of  the  semilunar  valves,  or  the  times  during  the  diastole,  of  the 
closure  of  the  semilunar  valves  and  the  opening  of  the  auriculo-ventricular 
valves. 

By  registering  the  curve  of  pressure  in  the  aorta  simultaneously  with  the 
pressure  in  the  left  ventricle  (Fig.  125),  and  by  comparing  these  with  the 
curve  of  the  successive  differences  of  pressure  in  these  two  cavities  as  deter- 
mined by  the  "differential  manometer,"  it  becomes  possible  to  mark  on  the 
ventricular  pressure  curve  the  points  at  which  the  foregoing  events  take 
place.  As  the  outcome  of  many  observations  and  determinations,  the 
following  statements  may  be  made :  As  a  point  of  departure  for  a  considera- 
tion of  the  relation  of  the  intra-ventricular  pressure  to  the  time  of  action 


THE  CIRCULATION  OF  THE  BLOOD  285 

of  the  valves,  the  close  of  the  ventricular  systole  may  be  conveniently 
selected. 

During  the  systolic  plateau  the  blood  is  passing  from  the  ventricle  into  the 
aorta.  Independent  of  the  slight  elevations  and  depressions  there  is  an 
absolute  fall  of  pressure  between  the  beginning  and  the  end  of  the  plateau. 
There  is  also  a  corresponding  fall  in  the  aortic  pressure,  corresponding  to 
these  two  points.  The  curve  of  the  difference  of  pressure  shows,  however, 
that  the  ventricular  pressure  is  slightly  higher  than  the  aortic.  This  fall  in 
both  ventricular  and  aortic  pressures  is  due  to  the  escape  of  blood  from  the 
arterial  into  and  through  the  capillary  system.  At  3  (see  Fig.  125),  however, 
whether  completely  emptied  or  not,  the  ventricle  suddenly  relaxes,  and  its 
pressure  soon  falls  below  that  in  the  aorta.  As  soon  as  this  takes  place 
the  semilunar  valves  must  close,  if  regurgitation  into  the  ventricular  cavity 
is  to  be  prevented.  A  comparison  of  the  aortic  pressure  curve  shows  a 
slight  notch,  the  "dicrotic  notch,"  just  preceding  a  slight  elevation,  the 
"dicrotic"  wave.  This  notch  occurs  at  the  moment  when  the  semilunar 
valves  close.  The  corresponding  point  on  the  ventricular  pressure  curve 
has  been  placed  just  where  the  ordinate  4  cuts  the  descending  portion. 
As  yet,  however,  the  pressure  is  higher  in  the  ventricle  than  in  the  auricle, 
and  continues  so  until  near  the  line  of  atmospheric  pressure.  At  this 
point  the  pressure  in  the  auricle,  due  to  the  accumulation  of  blood 
during  the  ventricular  systole,  now  forces  open  the  mitral  valve  and  the 
blood  flows  into  the  ventricle.  The  opening  of  the  mitral  valve  occurs  about 
the  point  where  the  ordinate  5  cuts  the  curve. 

The  ventricular  pressure  curve  affords  but  slight,  if  any,  indication  of  the 
auricular  systole.  It  apparently  does  not  give  rise  to  any  noticeable  increase 
m  the  ventricular  pressure.  The  slight  rise  in  the  pressure  curve,  which 
just  precedes  the  abrupt  rise  due  to  the  ventricular  systole,  may  be  taken  as 
an  indication  of  an  increasing  pressure  due  to  the  inflow  of  blood  from  the 
auricle,  as  a  result  of  the  auricular  systole.  Immediately  following  this 
event  the  ventricular  systole  begins  and  as  soon  as  the  pressure  in  the  ven- 
tricle exceeds  that  in  the  auricle  the  mitral  valve  closes.  This  is  marked 
on  the  curve  where  the  ordinate  cuts  it,  at  o.  With  the  closure  of  the  mitral 
valve  the  blood  becomes  imprisoned  within  a  closed  cavity,  closed  at  one 
orifice  by  the  mitral  valve  and  at  the  other  orifice  by  the  semilunar  valves. 
As  the  blood  is  incompressible  the  intra-ventricular  pressure  under  the  force 
of  the  ventricular  contraction  rapidly  rises  and  continues  so  to  do  until  the 
pressure  in  the  ventricle  exceeds  that  in  the  aorta,  at  which  moment  the  semi- 
lunar valves  are  suddenly  opened  and  the  blood  discharged.  A  comparison 
of  the  aortic  curve  shows  that  for  a  short  time  during  the  ventricular  systole 
the  pressure  is  falling,  but  at  one  point  the  curve  turns  at  a  sharp  angle  and 
rapidly  rises.  This  is  an  indication  that  the  semilunar  valves  are  suddenly 
thrown  open  and  the  blood  begins  to  pass  into  the  aorta.  This  event  occurs  at 
a  moment  marked  on  the  ventricular  curve  by  the  ordinate  i.  Beyond  this 
point  the  pressure  continues  to  rise,  for  the  aortic  pressure  must  not  only  be 
exceeded,  but  a  certain  velocity  must  be  imparted  to  the  blood.  Between 
the  ordinates  i  and  4,  the  semilunar  valves  remain  open  and  the  blood 
passes  into  the  aorta. 

In  accordance  with  the  foregoing :  the  ventricular  systole  may  be  sub- 
divided into  two  periods: 


286  TEXT-BOOK  OF  PHYSIOLOGY 

1.  The  period  of  rising  tension,  from  the  beginning  of  the  systole  and  the 

closure  of  the  auriculo-ventricular  valves  to  the  opening  of  the  semi- 
lunar valves,  the  pre-sphygmic  period,  occupying  from  0.02  to  0.04 
second. 

2.  The  period  of  ejection,  the  sphygmic-period,  from  the  opening  of  the 

semilunar  valves  to  the  end  of  the  systole,  occupying  about  0.2  second. 
The  ventricular  diastole  may  also  be  divided  into  two  periods: 

1.  The  period  of  falling  tension  or  relaxation,  the  post-sphygmic  period, 

from  the  end  of  the  systole  and  the  closure  of  the  semilunar  valves  to 
the  opening  of  the  auriculo-ventricular  valves,  occupying  about  0.05 
second. 

2.  The  period  of  filling,  from  the  opening  of  the  auriculo-ventricular  valves 

to  the  beginning  of  the  succeeding  auricular  systole. 
The  Time  Relations  of  the  Successive  Periods  of  the  Ventricular 
Activity  of  the  Human  Heart. — ^The  duration  of  each  of  the  periods  of 
ventricular  activity  as  stated  in  the  foregoing  paragraph  holds  true  only  for 
the  animal  the  subject  of  the  experiment.  The  time  relations  of  each  period 
vary  somewhat  with  the  animal  as  well  as  with  the  rate  of  the  heart  during 
the  time  of  the  experiment.  In  human  beings  the  same  holds  true.  As  the 
outcome  of  different  methods  of  investigation  the  average  duration  of  each 
period  has  been  approximately  estimated  as  follows: 

Period.  Rate  70.  Rate  80. 

1.  Presphygmic 0.055  1  Systole  0.051  1   Systole 

2.  Sphygmic 0.26S  f  0.323  0.25410.305 

3.  Post-sphygmic 0.050  1  Diastole  0.050  1  Diastole 

4-  Pause 0.490/0.540  0.395/0.445 

Total  Duration 0.863  0.750 

The  Intra-auricular  Pressure. — During  the  auricular  systole  the 
pressure  within  the  auricle  undergoes  variations  as  shown  by  direct  examina- 
tion by  means  of  a  cannula  inserted  into  the  auricular  cavity  and  connected 
externally  with  a  recording  tambour,  or  by  indirect  examination  by  means 
of  an  exploratory  tambour  placed  over  the  right  jugular  vein  in  close  relation 
to  the  clavicle.  The  pressure  variations  in  the  jugular  vein  which  are  thus 
recorded  by  means  of  a  tambour  provided  with  a  writing  lever  are  believed 
to  be  caused  by,  closely  follow  and  reproduce  the  pressure  variations  in  the 
auricle. 

Among  the  most  important  of  the  direct  examinations  of  the  auricular 
pressure  are  those  of  Porter,  carried  out  by  the  insertion  of  a  large  cannula  in 
the  auricular  appendix,  or  in  a  pulmonary  vein  close  to  the  auricle  and  con- 
nected by  its  free  extremity  with  a  Hiirthle  tambour.  The  curve  of  pressure 
thus  obtained,  shown  in  Fig.  126,^  is  characterized  by  three  positive  and  three 
negative  waves.  Among  the  more  important  of  the  indirect  determinations 
of  the  auricular  pressure  variations  are  those  of  Bachmann,  carried  out  with 
highly  sensitive  recording  tambours.  The  curve  of  pressure  variations  in 
the  jugular  vein  thus  obtained,  by  Bachmann,  Fig.  127,  is  placed  in  juxta- 
position for  purposes  of  comparison. 

The  first  positive  wave,  a,  is  caused  by  the  systole  of  the  auricle  and 
amounts  to  about  9  millimeters  of  mercury.  The  first  negative  wave  is  due 
to  the  relaxation  of  the  auricle. 


THE  CIRCULATION  OF  THE  BLOOD 


287 


The  second  positive  wave,^  s,  is  not  of  auricular  origin,  but  is  due  to 
the  systole  of  the  ventricle  in  its  early  stage  corresponding  to  the  period 
between  the  closure  of  the  auriculo-ventricular  valve,  and  the  opening  of 
the  semilunar  valves,  the  period  of  rising  tension,  and  amounts  to  about 
5  mm.  of  Hg.  It  is  probably  due  to  the  bulging  of  the  auriculo-ventricular 
valve  into  the  auricular  cavity,  by  the  still  higher  ventricular  pressure,  thus 
diminishing  its  size  and  raising  the  pressure. 


Fig.  126. — Curve  of  Pressure  Variations 
IX  THE  Auricles.     (Enlarged.) — {Porter.) 


Fig.  127. — Curve  of  Pressure  Variations  in 
THE  Jugular  Vein.    (Enlarged.) — {Bachmann.) 


The  second  negative  wave,  af,  begins  with  the  opening  of  the  semi- 
lunar valves,  determined  by  comparison  with  a  simultaneously  recorded 
curve  of  intra-ventricular  pressure,  and  is  due  in  part  to  the  relaxation  of 
the  auricular  walls,  but  more  especially  to  a  descent  of  the  more  central 
portions  of  the  auriculo-ventricular  septum,  into  the  ventricular  cavity, 
due  to  the  contraction  of  the  papillary  muscles  during  the  ventricular 
systole.  The  hollow  cone  thus  formed  enlarges  the  auricular  cavity, 
withdraws  some  of  its  contained  blood,  and  hence  lowers  the  pressure, 
thus  contributing  materially  to  the  filling  of  the  auricle.  This  negative 
pressure  amounts  to  about  —10  mm.  of  Hg. 

The  third  positive  wave,  v,  occurs  toward  the  end  of  the  ventricular 
systole  and  is  probably  caused  by  an  inflow  of  blood  from  the  veins  as 
well  as  by  a  return  of  the  auriculo-ventricular  septum  to  its  normal  position, 
the  result  of  a  relaxation  of  the  papillary  muscles  at  a  time  when  the  intra- 
ventricular pressure  is  still  higher  than  the  intra-auricular  pressure.  It 
amounts  to  about  5  mm.  of  Hg. 

The  third  negative  wave,  vf,  appears  very  shortly  after  the  relaxation  of 
the  ventricle  and  though  there  is  at  this  moment  a  rapid  fall  of  intra- 
ventricular pressure,  on  opening  of  the  auriculo-ventricular  valves  and  a 
descent  of  blood  into  the  ventricle,  the  fall  of  auricular  pressure  seldom 
amounts  to  more  than  0.5  mm.  of  Hg. 


Fig.  128. — Curve  of 
Pressure  Varlations  in 
THE  Auricle. — {Porter.) 


'  The  original  tracing  obtained  by  Porter  is  shown  in  the  ac- 
companying Fig.  128.  The  letters  designating  the  waves  have 
the  following  significance.  A,  systolic  rise;  AB,  first  diastolic 
fall;  BC,  first  diastolic  rise;  CD,  second  diastolic  fall;  E,  second 
diastoUc  rise;  F,  third  diastolic  fall;  G,  pause.  In  Fig.  126 
the  tracing  has  been  enlarged  and  the  waves  relettered  and  named 
in  accordance  with  the  terminology  in  vogue  in  the  literature  of 
clinical  medicine. 


*The  corresponding  wave  on  the  curve  of  the  pressure  variations  in  the  jugular  vein  is 
believed  by  Mackenzie  to  be  due  to  the  impact  of  the  expanding  carotid  artery,  and  hence  calls  it 
the  carotid,  c,  wave;  inasmuch  as  it  occurs  in  point  of  time  with  the  beginning  of  the  ventricu- 
lar systole,  it  is  also  called  the  systolic,  s,  wave. 


288  TEXT-BOOK  OF  PHYSIOLOGY 

A  Graphic  Record  of  the  Auricular  and  Ventricular  Contractions 
of  the  Human  Heart. — From  the  similarity  of  the  anatomic  arrangement 
of  the  human  heart  to  that  of  mammals  in  general  it  is  permissible  to  assume 
that  a  graphic  record  of  the  auricular  and  ventricular  contractions  of  the 

human  heart  would  resemble  in  its  general 
features  that  of  the  hearts  of  mammals  hereto- 
fore experimented  on,  and  that  the  same  series 
of  events  present  themselves  in  the  human  heart 
during  each  cycle,  though  by  reason  of  the 
difference  in  the  rate  of  the  beat,  the  duration 
of  each  event  in  the  cycle  is  somewhat  different. 
The  nearest  approach  to  obtaining  a  graphic 
record  of  the  auricular  and  ventricular  con- 
tractions of  the  human  heart  by  the  direct  ap- 
plication of  exploratory  tambours  was  made  by 
Fig.  129.— Tracings  of  the  Franfois  Franck  on  a  woman  whose  heart  was 
CoS^s  "pKOM  T  woi:"  congenitally  displaced  into  the  abdominal  cavity. 
WITH  Ectopia  of  the  Heart,  a,  An  investigation  revealed  the  fact  that  this 
Auricular;  v  ventricular.— (Fraw-  woman  had  a  large  opening  in  the  anterior 
gots-  ranc  .)  portion  of  the  diaphragm  through  which  the 

ventricle  had  passed  and  formed  a  large  protrusion  in  the  epigastric 
region.  Through  thin  and  relaxed  abdominal  walls  the  ventricular  pul- 
sations could  be  distinctly  felt  as  well  as  the  pulsation  of  what  ap- 
peared to  be  the  inferior  portion  of  the  right  auricle.  A  fibrous  ring 
around  the  edge  of  the  opening  in  the  diaphragm  supported  the  heart 
at  the  auriculo-ventricular  groove.  On  the  application  of  exploratory 
tambours  in  connection  with  recording  tambours  one  to  the  right 
ventricle,  the  other  to  the  right  auricle,  the  record  shown  in  Fig.  129  was 
obtained  of  which  the  upper  line  represents  the  contraction  of  the  auricle 
and  the  lower  line  the  contraction  of  the  ventricle.  A  comparison  of  the 
record  with  that  obtained  from  the  horse.  Fig.  120,  p.  276,  shows  that  the 
relation  of  the  auricular  to  the  ventricular  systole  is  the  same  in  the  former 
as  in  the  latter  and  that  in  their  general  features  the  two  records  correspond, 
from  which  it  may  be  inferred  that  in  the  human  heart  the  events  occurring 
during  the  cycle  are  practically  identical  with  those  occurring  in  the  hearts 
of  other  mammals.  The  small  size  of  the  auricular  curve  and  the  absence  of 
undulations  are  probably  due  to  the  fact  that  the  tambour  was  placed  on 
only  a  portion  of  the  auricle. 

A  Schematic  Representation  of  the  Events  of  a  Cardiac  Cycle  in 
Man. — From  graphic  studies  of  the  cardiac  impulse,  of  the  pressure  changes 
in  the  auricle  and  ventricle  as  indicated  by  pressure  changes  in  the  jugular 
vein  and  carotid  artery  respectively  it  has  become  possible  to  construct  a 
diagram  of  the  cardiac  cycle  of  the  human  heart,  to  designate  on  the  ventricu- 
lar curve  the  time  of  the  opening  and  closing  of  the  valves,  as  well  as  the 
time  relations  of  the  entire  series  of  events.  A  scheme  of  this  character  is 
shown  in  Fig.  130,  based  on  that  constructed  by  Fredericq. 

Though  the  numerical  values  given  for  the  duration  of  the  auricular  and 
ventricular  systole  and  diastole,  viz.:  auricular  systole  o.io  second;  auricular 
diastole  0.70  second;  ventricular  systole  0.33  second;  ventricular  diastole  and 
pause  0.47  second,  it  must  be  borne  in  mind  that  they  are  true  only  for  the 


THE  CIRCULATION  OF  THE  BLOOD 


289 


heart  beating  approximately  75  times  per  minute.  If  the  number  of  beats 
increases,  not  only  does  the  entire  cycle  diminish  in  duration,  but  its  dif- 
ferent subdivisions,  auricular  systole,  ventricular  systole,  and  diastole  also 


4.SYST<-        AURICULAR      D/ A  STOLE 


Pause  of  entire  Mean 


'Openi/iff  of 
Scmilun ai'  Dalocs. 

■  CIosuTc  of 
■iuricuJo  vtntricular 
valve 


Vosuic  ofSc?nilu,7ia 
'-•,  valves. 

T^.-Opcitiiicf   of 

hiricidoven 

valve. 


'Ope?ii?tff    of 
Semilu/iar  valves. 

Closure   of 
^uriculovcfitriadctr 
valve. 


\iMRICULAn  Sy STOLE 


VENTRICULAR  DIASTOLE 


1'-'  Soufid. 


SecoTids 


m 


I  I      Z""- Sound. 


I  I  I 


0       J       .2       .3       .i-      .S      .6       .7       .8      .1      .Z      .3       .i:      .S 

Fig.  130. — A  Schematic  Representation  of  the  Events  of  a  Cardiac  Cycle. 

diminish  in  duration,  though  in  unequal  degrees.  Thus  it  has  been  de- 
termined that  with  each  increase  of  10  beats  above  70,  the  ventricular  systole 
shortens  by  about  0.02  second  and  the  ventricular  diastole  by  about  o.io 
second.  The  opposite  holds  true  if  the  num- 
ber of  beats  decreases  below  70  per  minute. 
d  The  Relation  of  the  Cardiogram  to  the 
Events  of  the  Cardiac  Cycle. — A  compari- 
son of  a  typical  cardiogram,  such  as  is  seen 
in  Figs.  118  and  131,  with  the  curve  of  intra- 
ventricular pressure,  shows  that  they  corre- 
spond in  essential  features.  The  slight  eleva- 
tion {a)  on  the  cardiogram  represents  the  q  q- 
contraction  of  the  auricle,  which  completing       Fig.  131.- 

the   filling   of   the   ventricle  causes  it  to  press   ricularstysole;  6,  c,  «f,  ventricular  sy^- 
•  1  •■ii.-L^         iiL        tole:  d,  e,  ventncular  diastole;  C,  O, 

more  Vigorously  agamst  the  chest  wall;  b-c  dosing  and  opening  of  the  auriculo- 

representS  the  contraction   of   the  ventricles,    ventricular  valves;  O',  C,  opening  and 

at  which  moment  the  apex  is  suddenly  and  closing  of  the  semilunar  valves;  C,0' 

,        .,,       J   .  .         ^,  ,  11  T   period  of  rising  tension;  C'-L',  period  of 

torcibly   driven   against   the    chest   wall;   C-d   ventriculardischarge;CC',  timeof  the 
represents  the  SVStolic  plateau,  the  time  dur-   occurrence    of   the  first  and   second 

ing  which  the  ventricle  is  discharging  blood  sounds  respectively. 
into  the  aorta;  d-e  represents  the  relaxation  of  the  ventricle,  while  e-/ rep- 
resents the  time  of  the  diastole  during  which  the  heart  cavities  are  enlarg- 
ing with  the  incoming  of  a  new  volume  of  blood,  in  consequence  of  which 
the  heart  is  pressing  against  the  chest  walls.  The  systolic  plateau  is  charac- 
terized by  one  or  more  elevations  and  depressions,  the  true  cause  of  which 
is  unknown. 


CO 

Cardiogram,     a,  Au- 


290  TEXT-BOOK  OF  PHYSIOLOGY 

From  the  correspondence  of  the  curve  of  cardiac  pressure  against  the 
chest  wall  with  the  curve  of  intra-ventricular  pressure  it  becomes  possible 
to  indicate  with  approximate  accuracy  the  time  of  the  opening  and  closing 
of  the  auriculo-ventricular  valves  and  the  semilunar  valves  and  hence 
the  time  of  occurrence  of  the  heart  sounds  and  other  features  of  the 
cardiac  cycle.     Such  a  construction  is  shown  in  Fig.  131. 

The  Pulse  Volume. — The  pulse  volume  or  the  systolic  output  or  the 
amount  of  blood  discharged  by  the  ventricle  at  each  systole  has  long  been  a 
subject  of  investigation,  but  by  reason  of  the  inherent  dif35culties  of  the 
problem  the  results  that  have  been  obtained  have  varied  within  wide  limits, 
viz.:  from  180  c.c.  to  50  c.c.  The  methods  that  have  been  employed  for 
the  determination  of  this  volume  are  compHcated  and  need  not  be  detailed 
here.  Suffice  it  to  say  that  the  results  of  the  more  recent  experiments 
would  indicate  that  the  volume  varies  from  80  c.c.  to  100  c.c.  If  the  pulse 
volume  be  assumed  to  weigh  ico  grams  and  the  total  volume  of  blood  in  a 
man  weighing  70  kilograms  to  weigh  3864  grams  then  the  pulse  volume  will 
be  about  one-thirty-eighth  of  the  total  weight  of  blood,  or  about  0.00142  of 
the  body  weight.  In  38  heart  beats  therefore  the  entire  amount  of  blood 
will  have  passed  through  the  heart.  The  systolic  output  is  conditioned  by 
the  factors  which  increase  or  decrease  the  length  of  the  diastole  and  hence 
the  filling  of  the  ventricle. 

The  Frequency  of  the  Heart-beat. — The  frequency  of  the  heart-beat 
varies  with  a  variety  of  conditions:  e.g.,  age,  sex,  posture,  exercise,  etc. 

Age. — The  most  important  normal  condition  which  modifies  the  activity 
of  the  heart  is  age.     Thus: 

Before  birth,  the  number  of  beats  a  minute  averages 140 

During  the  first  year  it  diminishes  to 128 

During  the  third  year  it  diminishes  to 95 

From  the  eighth  to  the  fourteenth  year  it  averages 84 

In  adult  males  it  averages 72 

Sex. — The  heart-beat  is  more  rapid  in  females  than  in  males.  Thus 
while  the  average  beat  in  males  is  72,  in  females  it  is  usually  8  or  10  beats 
more. 

Posture. — Independent  of  muscle  efforts  the  rate  of  the  beat  is  influenced 
by  posture.  It  has  been  found  that  when  the  body  is  changed  from  the  lying 
to  the  sitting  and  to  the  standing  position,  the  beat  will  vary  as  follows — 
from  66  to  71  to  81  on  the  average. 

Exercise  and  digestion  also  temporarily  increase  the  number  of  beats. 

A  rise  in  blood-pressure  from  any  cause  whatever  is  usually  attended  by 
a  decrease,  while  a  fall  in  blood-pressure  is  attended  by  an  increase  in  the 
rate. 

The  Electric  Currents  of  the  Heart. — ^The  wave  of  contraction  of  the 
heart  muscle,  like  the  wave  of  contraction  of  the  skeletal  muscle  (see  page 
81)  is  accompanied  or  perhaps  preceded  by  the  development  of  electric 
currents  the  existence  of  which  can  be  demonstrated  by  connecting  the  base 
and  apex,  by  means  of  non-polarizable  electrodes,  with  a  suitable  galvan- 
ometer. When  the  base  and  apex  of  the  frog's  heart  are  thus  connected,  it 
will  be  found  that  with  each  contraction,  the  galvanometer  needle  will  deviate 
from  its  position  of  rest  and  in  a  direction  that  indicates  that  there  is  not 
only  a  change  in  their  electric  potential  but  also  that  the  base  becomes 


THE  CIRCULATION  OF  THE  BLOOD 


291 


primarily  electronegative  to  the  apex  which  remains  electropositive. 
With  the  development  of  the  negativity  a  current  passes  down  the  heart 
to  the  apex,  out  through  the  galvanometer  circuit  and  back  to  the  point 
of  origin.     This  current  has  been  termed  the  action  current. 

If  the  heart  is  connected  with  a  more  quickly  responsive  apparatus,  e.g., 
a  capillary  electrometer  (see  appendix)  it  will  be  found  that  the  mercury 
will  pulsate  twice,  but  in  opposite  directions,  with  each  contraction,  indi- 
cating that  two  action  currents  are  developed,  the  first  indicating  that  the 
base  is  primarily  negative  to  the  apex  which  is  electropositive,  and  the 
second  that  the  apex  is  secondarily  negative  to  the  base  w^hich  has  become 
again  electropositive:  in  other  words  the  changes  in  electric  potential  are 
diphasic.     With  the  capillary  electrometer  Waller  demonstrated  that  the 


Fig.  132. — The  Electronegative  and  ELECTROPOSixrvE  Areas  of  the  Body  Surface. 
A  and  B  respectively  represents  apex  and  base  of  the  ventricular  mass.  Then,  if  at  any  moment 
a  difference  of  potential  should  arise  between  A  and  B,  a  current,  ccc,  will  be  established  along 
and  around  the  axis  C  C;  the  line  O  0  will  represent  the  plane  of  zero  potential  or  equator; 
the  lines  a  a  a,  b  b  b  will  represent  equipotential  curves  around  A  and  B.  A  difference  of 
potential  between  A  and  B  will  be  manifested  if  the  two  leading-off  electrodes  are  applied  on 
opposite  sides  of  the  equator,  0  0;  no  such  difference  will  be  manifested  if  both  electrodes 
are  on  the  same  side  of  the  equator.  The  equator  0  0  will  divide  the  body  into  two  asym- 
metrical parts,  (i)  a  portion  b  b  b  including  the  head  and  right  upper  extremity,  (2)  a  portion 
aaa  including  the  three  other  extremities. — {Waller,  Hitman  Physiology.) 

human  heart  also  exhibits  similar  changes  in  its  electro-potential  with  each 
beat  giving  rise  to  electric  currents  which  find  their  way  through  the  sur- 
rounding tissues  to  the  surface  of  the  body  from  which  they  can  be  led 
off  to  the  electrometer  and  then  back  to  the  surface  of  the  body  and  finally 
to  the  base  of  the  heart.  He  was  enabled  to  determine  and  locate  the  points 
which  are  electronegative  and  electropositive.  The  results  of  his  experi- 
ments are  embodied  in  the  accompanying  diagram  Fig.  132,  which,  with 
the  legend  is  self-explanatory. 

If  any  two  points,  a  and  b,  on  opposite  sides  of  the  equator  O,  O,  are 
connected  with  the  electrometer  the  mercury  will  pulsate  twice  with  each 
heart-beat  indicating  the  presence  of  two  action  currents  which  travel  suc- 
cessively in  opposite  directions.  With  suitable  apparatus  the  oscillations, 
of  the  mercury  can  be  photographically  recorded.     By  reason  of  the  rapid. 


292 


TEXT-BOOK  OF  PHYSIOLOGY 


action  of  the  heart  and  the  inertia  of  the  mercury,  the  exact  changes,  of  the 
electric  potential  recorded  are  difficult  of  interpretation. 

By  means  of  the  highly  sensitive  and  quickly  responsive  string-galvan- 
ometer devised  by  Einthoven  (see  appendix)  not  only  can  the  variations  of  the 
electric  potential  of  the  heart  be  detected,  but  the  deviations  of  the  string 
caused  by  the  passage  of  the  most  delicate  current,  can  be  magnified  and  a 
shadow  projected  on  a  sensitive  surface  and  photographed.  The  record 
thus  obtained  is  termed. 

The  Electrocardiogram. — An  electrocardiogram  obtained  by  photo- 
graphing the  oscillations  of  the  galvanometer  string  during  the  cardiac  cycle, 
when  different  portions  of  the  body  surface,  a  and  h,  are  connected  with  the 
terminals  of  the  string,  is  shown  in  Fig.  133,  the  different  features  of  which 
are  designated  by  the  letters  P,  Q,  R,  S,  T.  This  curve,  which  represents 
the  relative  strength,  duration,  succession  and  direction  of  the  currents  during 
the  cycle,  it  is  believed  furnishes  a  correct  picture  of  the  strength,  duration, 

succession  and  direction  of  the  physio- 
logic processes — the  excitation  and  con- 
traction processes— as  they  arise  in  the 
heart  muscle  during  its  activity. 

The  interpretations  of  the  electrocar- 
diogram and  the  significance  of  its  various 
features  are  not,  in  some  respects,  in  as 
complete  accord  as  seems  desirable.  In 
general,  it  may  be  said  that  P  corres- 
ponds to  the  auricular  systole  and  that 
Q,  R,  S,  T  correspond  to  the  ventricular 
systole.  R  and  T  are  always  present  in 
physiological  conditions  at  least,  while  Q 
and  S,  may,  either  one  or  the  other  or 
both,  be  wanting. 

Einthoven  is  of  the  opinion  that  the  general  form  of  the  electrocardio- 
gram indicates  the  path  and  propagation  of  the  stimulus  or  the  excitation 
process  through  the  heart,  as  well  as  the  order  of  contraction  of  dift'erent 
portions  of  the  ventricular  walls,  both  of  which  are  in  harmony  with  the 
origin,  course  and  ultimate  distribution,  of  the  auriculo-ventricular  con- 
duction system  as  determined  by  Tawara  (see  page  272). 

The  record  presents  first  a  horizontal  portion — the  image  of  the  gal- 
vanometer string — ^which  indicates  that  the  string  is  at  rest  by  reason  of 
the  fact  that  all  portions  of  the  heart,  during  the  diastolic  pause  are  isoelectric 
or  of  equal  electropotential. 

P  is  the  result  of  the  contraction  of  the  auricle,  indicating  a  condition 
of  negativity  toward  the  ventricle  which  is  now  electropositive  in  conse- 
quence of  which,  the  electric  currents  pass  down  the  heart,  thence  to  the 
body  surface,  thence  through  the  galvanometer  circuit  and  back  to  the  heart, 
causing  the  string  as  they  pass  through  it,  to  move  outward.  With  the 
cessation  of  the  auricular  contraction  the  string  returns  to  its  position  of 
rest.  The  more  or  less  horizontal  line  between  P  and  Q  indicates  that  both 
auricles  and  ventricles  are  now  at  rest  and  in  the  same  state  of  electric 
potential.  The  passage  of  the  excitation  process  through  the  conduction 
system,  which  is  now  taking  place,  from  its  beginning  to  its  terminations, 


[-] 

-1 

^ 

nfi 

r-1 

n 

r- 

[— r 

p 

1—] 

[— ] 

: 

^ 

:  ^ 

J  ■ 

>X 

^:S^ 

-:-/ 

'> 

i 

:  S: 

■■     1 

v« 

:>: 

: 

r 

•    1" 

:   ' 

j    1 

:      : 

i 

U 

f 

i 

V 

\^ 

1 

■ 

1 

1 

i 

\ 

' 

1 

1 

1 

1 

II 

if 

*i« 

\ 

ii 

1 

1 

1 

1 

1 

(f 

.S| 

Fig.  133. — Scheme  of  the  Electro- 
CAIUJIOGJRAM. — {Einthoven.) 


THE  CIRCULATION  OF  THE  BLOOD 


293 


is,  in  and  of  itself,  too  feeble  to  cause  a  deviation  of  the  thread  under  the 
conditions  of  tension  necessary  for  the  registration  of  the  other  features  of 
the  electrocardiogram. 

The  system  of  points  or  waves,  negative  and  positive,  Q,  R,  S  in  the  first 
part  of  the  ventricular  portion  of  the  electrocardiogram  permits  of  the  infer- 
ence that  the  stimulus  arrives  and  the  contraction  arises,  at  once  or  almost 
at  once  in  different  places  of  the  ventricular  masses.  The  first  point  Q,  indi- 
cates that  the  stimulus  arrives  first  at  a  point  lying  near  to  the  apex  and 
calls  forth  a  contraction  of  moderate  degree  establishing  a  certain  degree 
of  electronegativity  toward  the  base  which  is  now  electropositive.  The 
direction  of  the  point,  below  the  horizontal  portion,  is  due  to  the  direction 
of  the  current  through  the  heart  and  galvanometer,  which  is  of  course  the 
reverse  of  the  current  that  originates  in  the  auricle  and  which  gives  rise  to  P. 
The  point  R,  which  immediately  follows,  indicates  that  the  stimulus  has 
arrived  at  the  base  of  the  ventricle  calling  forth  a  more  pronounced  contrac- 
tion and  a  greater  degree  of  electronegativity  than  that  present  at  the  apex. 
The  point  5  indicates  that  soon  there- 
after, the  stimulus  reaches  regions 
lying  nearer  to  the  apex  of  the  ven- 
tricle calling  forth  a  contraction  and 
establishing  an  electronegativity  which 
soon  gains  the  upper  hand.  The 
horizontal  portion  of  the  electrocar- 
diogram, between  the  system  of  points 
Q,  R,  S,  on  the  one  side,  and  the  point 
T,  on  the  other  side,  represents  a  con- 
traction state  in  which  the  entire 
muscle  mass  of  the  two  ventricles 
participates  in  the  same  measure  and 
hence  all  portions  are  in  a  condition 
of  equal  negativity  for  which  reason 
the  string  remains  stationary.  It  is  during  this  period  that  the  ventricles 
are  engaged  in  driving  the  blood  out  of  their  cavities  into  the  aorta  and 
pulmonic  artery. 

T,  the  last  peak  of  the  ventricular  portion  of  the  electrocardiogram,  is 
directed  upward  and  hence  reveals  the  fact  that  the  base  of  the  ventricle  is, 
at  this  moment,  in  a  condition  of  greater  electronegativity  than  are  por- 
tions of  the  heart  lying  nearer  the  apex.  The  origin  of  the  T  peak  has  been 
a  subject  of  much  discussion  but  the  investigations  of  Gotch  have  made  it 
apparent  that  it  is  due  to  the  contraction  of  muscle-fibers  which  have  not 
before  been  brought  into  action,  that  is,  fibers  near  the  root  of  the  aorta, 
in  that  portion  of  the  ventricle  which  is  the  homologue  of  the  bulbus  arteriosus 
in  the  lower  animals.  The  contraction  of  this  region  completes  the  discharge 
of  the  blood  from  the  ventricular  cavity. 

The  electrocardiogram  has  been  further  analyzed  and  elaborated  by 
Kraus  and  Nicolai.  The  results  of  their  investigations  are  embodied  in 
Fig.  134.  A  somewhat  different  terminology  for  the  different  features  of  the 
general  record  has  been  introduced.  Thus  in  this  scheme  the  letters 
A ,  Ja,  J,  Jp,  and  F,  replace  the  letters  P,  Q,  R,  S,  T,  of  the  Einthoven  scheme. 


^ 

t    ~ 

t                       ^ 

-»                             4^^  wL              7i 

""^■fc«'"       5_         aw       m           jC 

1  1  1  r      '  li  ^                J  1 1  t  h  rf  P                 ^^      ^  i  1  1 

!■               r               IP                        ^B  11 

^^^         \J                                                                 I'i'yy                                                       XT 

X-J/l        I,              -4-                       ''f 

'^'^        ^ 

Fig.  134. — Scheme  and  Interpretation  of 
THE  Electrocardiogram. — {Nicolai.) 


294  TEXT-BOOK  OF  PHYSIOLOGY 

A,  expresses  auricular  activity,  J^  and  F  express  respectively  the  initial  and 
final  activity  of  the  ventricle.  The  letters  Ap  indicate  a  negative  wave 
following  auricular  activity;  h,  represents  the  time  during  which  the  excita- 
tion process  is  passing  through  the  auriculo-ventricular  bundle  (bundle  of 
His)  from  its  origin  to  its  terminations.  J  designates  a  wave  which  initiates 
the  ventricular  contraction  and  is  the  result  of  the  contraction  of  the  system 
of  papillary  muscles.  It  is  preceded  and  followed  by  negative  waves  Ja 
and  Jp.  The  horizontal  portion  of  the  record,  /,  represents  the  contraction 
of  the  middle  layers  of  the  muscle  fibers  of  the  ventricle.  F  represents  the 
contraction  of  the  muscle  fibers  in  the  region  of  the  conus  arteriosus;  and  is 
likewise  preceded  and  followed  by  negative  waves  Fa,  and  Fp.  P  represents 
the  pause  in  the  heart  activities. 

The  activities  of  the  different  parts  of  the  heart  indicated  by  the  lettering 
may  be  summarized  as  follows:  (Ott.) 

Phase  of  Electrocardiogram.  Approximately. 

A .  Activity  <f  ^l^j;^^^^^^^^  }  presystole.  Period  of  filling  of  the  heart  (ventricle). 

/.        "  "  papillary  system — intersystole.  Period  of  distention  of  the  heart. 

t.  "         "  middle    layer    of  1 

ventricular  muscle  [  systole.        Period  of  ventricular  discharge. 
F.         "        "  conus  arteriosus     J 
P.    Rest  of  the  whole  heart — pause  of  heart.  Period  of  filling  of  the  heart. 

In  pathologic  conditions  of  the  heart  muscle  there  are  many  deviations 
from  the  normal  electrocardiogram  which  are  more  or  less  characteristic 
and  of  diagnostic  value  and  importance. 

THE  BLOOD -SUPPLY  TO  THE  HEART 

The  nutrition  of  the  heart,  its  irritability  and  contractility,  the  force 
and  frequency  of  the  beat,  are  dependent  on  and  maintained  by  the  intro- 
duction of  arterialized  blood  into  and  the  removal  of  waste  products  from 
its  tissue. 

In  frogs  and  allied  animals  the  heart  muscle  is  nourished  by  the  blood 
flowing  through  its  cavities.  During  the  diastole  the  blood,  under  the 
influence  of  the  slight  pressure  developed,  passes  from  the  interior 
of  the  heart  into  a  system  of  irregular  passage-ways  or  channels,  which 
penetrate  the  heart-wall  in  all  directions  and  thus  comes  into  direct  contact 
with  the  heart-cells.  With  the  beginning  of  the  systole  the  blood  is  forced 
out  of  these  channels  into  the  interior  of  the  ventricle,  bringing  with  it  the 
products  of  tissue  metabolism. 

In  mammals  the  entire  inner  surface  of  the  heart,  as  shown  by 
the  investigations  of  Pratt,  also  presents  a  series  of  openings,  the  foramina 
of  Thebesius,  which  lead  into  a  similar  series  of  passage-ways  penetrating 
in  various  directions  the  heart-walls,  and  there  are  reasons  for  believing 
that  the  heart  of  the  mammal  may  be  to  some  extent  nourished  in  a  manner 
similar  to  the  manner  by  which  the  frog  heart  is  nourished.  Thus,  if  a 
glass  tube  be  inserted  and  fastened  into  the  aortic  opening  of  the  excised 
heart  of  a  cat  and  the  interior  of  the  ventricle  filled  with  warm  defibrinated 
blood  of  the  same  animal,  under  a  pressure  of  about  75  mm.,  the  heart  will 
recommence  and  continue  to  beat  for  a  period  varying  from  one  to  several 
^  /  has  the  significance  here  of  /. 


THE  CIRCULATION  OF  THE  BLOOD  295 

hours,  thus  showing  that  the  mammahan  heart  may  to  some  extent  so 
receive  nutritive  material.  By  reason  of  the  fact  that  the  metabolism  of  the 
heart  of  the  mammal  is  so  much  more  active  than  that  of  the  heart  of  the 
frog,  this  method  is  far  from  being  sufficient  for  nutritive  purposes  and  hence 
a  more  perfect  and  active  blood-supply  is  necessitated  for  furnishing 
nutritive  material  and  the  removal  of  the  waste  products.  These  results  are 
accomplished  by  the  coronary  arteries,  on  the  one  hand,  and  the  coronary 
veins,  on  the  other. 

The  Coronary  Vessels. — ^The  coronary  arteries,  two  in  number,  the 
right  and  left,  arise  from  the  aorta  in  the  pouches  of  Valsalva  just  above 
the  right  and  left  semilunar  valves.  Turning  in  opposite  directions,  they 
form  a  circle  around  the  base  of  the  ventricles  and  ultimately  anastomose. 
From  both  the  right  and  left  artery  branches  are  given  off  which  run  over 
the  walls  of  both  auricles  and  ventricles,  the  most  important  of  which  in 
man  are  the  anterior  and  posterior  inter-ventricular.  These  main  vessels 
lie  in  grooves  on  the  surface  of  the  heart  beneath  the  visceral  pericardium, 
surrounded  by  connective  tissue  and  fat.  From  their  relation  to  the  outer 
surface  of  the  heart  they  may  be  designated  extra-mural  vessels.  From  these 
vessels  small  branches  are  given  off  which  penetrate  the  walls  of  the  heart, 
in  which  they  divide  into  many  branches  and  because  of  their  relation  to 
the  heart-muscle  they  may  be  designated  tntra-mural  vessels.  These  vessels 
are  spoken  of  as  terminal  arteries  in  the  sense  that  "the  resistance  in  the 
anastomosing  branches  is  greater  than  the  blood  pressure  in  the  arteries 
leading  to  those  branches"  (Pratt).  From  these  small  arteries  arise  a 
rich  network  of  capillary  blood-vessels  which  closely  invest  the  individual 
muscle  cells,  and  which  permit  here  a  rapid  and  extensive  interchange  of 
nutritive  and  waste  materials.  The  coronary  veins  arise  from  the  union 
of  the  small  veins  which  emerge  from  the  capillary  areas.  The  veins  follow 
the  course  of  the  arteries  and  finally  terminate  in  the  coronary  sinus,  located 
in  the  auriculo-ventricular  groove  on  the  posterior  surface  of  the  heart. 
This  sinus  opens  into  the  right  auricle  between  the  opening  of  the  inferior 
vena  cava  and  the  auriculo-ventricular  opening.  Its  orifice  is  guarded  by 
a  valve,  which  is  usually  single,  though  sometimes  double. 

While  by  far  the  larger  portion  of  the  blood  is  returned  by  the  coronary 
veins,  it  is  also  certain  that  some  of  it  is  returned  by  small  veins  which  open 
into  little  pits  or  depressions  on  the  inner  surface  of  the  heart-walls,  known 
as  the  foramina  Thebesii.  It  has,  however,  been  shown  by  Pratt  that  these 
foramina  are  present  not  only  in  the  auricular  walls,  as  generally  stated,  but 
in  the  walls  of  the  ventricular  cavities  as  well.  They  communicate  through 
a  capillary  plexus  with  both  arteries  and  veins,  and  by  special  large  passages 
with  the  veins  alone. 

The  Filling  of  the  Coronary  Arteries. — The  period  of  time  in  the 
cardiac  cycle  during  which  the  coronary  (the  extra-mural)  arteries  are  filled 
with  blood,  whether  during  the  systole  or  the  diastole,  has  been  a  subject  of 
much  discussion.  Thus  it  was  asserted  and  maintained  by  Briicke  that 
this  event  must  occur  during  the  diastole,  because  of  the  supposed  fact  that 
the  semilunar  valves  during  the  systole  are  so  closely  pressed  against  the 
walls  of  the  aorta  and  over  the  openings  of  the  coronary  arteries  as  to  prevent 
the  entrance  of  blood  into  them;  but  with  the  diastole  and  the  return  of  the 
valves  to  their  former  position  the  blood  under  the  recoil  of  the  walls  of  the 


296  TEXT-BOOK  OF  PHYSIOLOGY 

aorta  would  flow  freely  into  them  as  well  as  into  and  through  the  intra- 
mural arteries,  capillaries  and  veins.  According  to  Briicke  the  partial  empty- 
ing of  the  coronary  arteries  and  their  intra-mural  branches,  and  the  consequent 
fall  of  pressure  within  them  toward  the  end  of  the  diastole  facilitates  the 
systole  of  the  ventricles,  while  the  filling  of  the  vessels  and  the  consequent 
rise  of  pressure  promotes  the  diastole.  This  anatomic  mechanism  and  its 
associated  functional  activity  constituted  according  to  Briicke  an  apparatus 
by  which  the  activity  of  the  heart  could  be  self-regulated.  This  view  as  to 
the  time  of  the  filling  of  the  extra-mural  coronary  arteries  has,  however,  been 
disproved. 

At  the  present  time  it  is  generally  believed  as  the  result  of  many  forms 
of  experimentation  that  the  extra-mural  coronary  arteries  are  filled  during 
the  time  of  the  systole.  For  it  has  been  shown  that  the  semilunar  valves  do 
not  close  the  openings  of  the  coronary  arteries  by  reason  of  the  presence  of 
blood  behind  them  under  a  high  pressure;  that  a  division  of  one  of  the 
branches  of  these  arteries  is  followed  by  a  spurt  of  blood  synchronous  with 
the  systole.  Moreover,  if  a  kymographic  trace  of  the  pressure  within  the 
coronary  artery  be  compared  with  the  trace  of  the  pressure  within  the 
carotid  artery,  it  will  be  found  that  there  is  a  complete  agreement  between 
them  as  the  pressures  in  the  two  vessels  rise  and  fall  simultaneously  and 
as  a  corollary  are  filled  during  the  systole.  Because  of  the  pressure  which 
the  heart-muscle  must  exert  upon  the  smaller  arteries  and  veins  within  its 
own  substance  during  systole,  it  is  probable  that  there  is  a  temporary  retarda- 
tion of  the  flow  of  the  blood  during  the  systole  in  the  coronary  (the  extra- 
mural) vessels,  followed  by  a  return  of  the  velocity  during  the  period  of 
diastolic  repose. 

During  the  diastole  the  blood  flows  freely  from  the  extra-mural  vessels 
into  the  intra-mural  arteries  and  capillaries.  It  is  at  this  time  too  that  the 
heart-muscle  receives  from  the  capillary  blood-vessels  its  nutritive  material 
and  returns  to  the  blood  the  products  of  its  metabolism.  During  the  systole 
the  intra-mural  capillaries  and  veins  are  compressed  and  the  blood  driven 
into  the  extra-mural  veins.  The  greater  the  force  and  frequency  of  the  beat, 
the  larger  the  volume  of  blood  passing  through  the  coronary  system. 

Vaso-motor  Fibers  for  the  Coronary  Arteries. — The  presence  in  the 
vagus  and  sympathetic  nerves,  of  vaso-motor  fibers  for  the  coronary  arteries 
has  been  a  subject  of  much  investigation  and  discussion.  By  reason  of  the 
fact  that  stimulation  of  these  nerves  modifies  the  rate  and  the  force  of  the 
heart-beat,  and  these  in  turn  modify  the  flow  of  blood  through  the  vessels,  it 
is  difl&cult  to  state  whether  the  observed  effects  are  the  result  of  changes  in 
the  caliber  of  the  arteries  or  to  a  change  in  the  character  of  the  heart-beat. 
Moreover  owing  to  the  anatomic  relation  which  the  arteries  bear  to  the  heart 
muscle,  the  rapidity  of  the  flow  through  them  must  vary  with  each  contrac- 
tion and  relaxation  and  thereby  the  difficulty  of  interpretation  is  increased. 
The  results  of  direct  experimental  investigations  of  Porter,  however,  lead  to 
the  conclusion  that  the  existence  of  vaso-motor  (constrictor)  fibers  for  the 
coronary  arteries  is  highly  probable. 

These  investigations  have  been  corroborated  by  the  investigations  of 
Barbour  and  Prince  who  have  found  that  when  isolated  monkey  hearts  are 
perfused  with  autogenous  hirudin  blood  diluted  with  Locke's  solution  con- 
taining minute  doses  of  epinephrin,  the  coronary  flow  is  decreased.     From 


THE  CIRCULATION  OF  THE  BLOOD  297 

previous  experiments  on  rings  of  the  coronary  artery  of  the  human  heart 
the  conclusion  is  drawn,  that  in  man  and  the  monkey,  epinephrin  constricts 
the  coronary  arteries  even  though  it  dilates  them  in  the  dog,  cat,  rabbit, 
and  other  animals.  From  these  result  the  further  conclusion  is  drawn  that 
in  man  and  monkeys  the  coronary  arteries  are  supplied  with  vaso-constrictoi 
nerves  of  true  sympathetic  (thoracico-lumbar)  origin. 

The  Effects  of  Ligation  of  the  Coronary  Arteries. — As  stated  in  a 
foregoing  paragraph  the  nutrition  of  the  heart-muscle,  its  irritability  and 
contractility,  depend  on  the  blood-supply  derived  from  the  coronary  vessels. 
This  is  shown  by  the  effects  which  follow  its  withdrawal.  Ligation  of  both 
coronary  arteries  in  the  dog  is  followed  by  a  diminution  in  the  force  and 
frequency  of  the  heart-beat,  and  in  a  few  minutes  by  complete  cessation. 
Ligation  of  even  a  single  branch  of  a  coronary  artery  of  the  dog  heart,  pro- 
vided it  supply  a  sufficiently  large  territory — e.g.,  the  arteria  circumfiexa — 
is  sufficient  to  cause  arrest  in  at  least  80  per  cent,  of  animals  (Porter).  With 
the  ligation  of  this  vessel  there  occurs  a  gradual  diminution  in  the  force  and 
frequency  of  the  systole.  As  the  power  of  coordinate  contraction  declines 
the  heart-muscle  frequently  exhibits  a  series  of  independent  contractions  of 
individual  fibers  and  cells  known  as  fibrillary  contraction.  All  the  results 
which  follow  ligation  are  to  be  attributed  in  the  light  of  experiment  to  the 
sudden  anemia  which  is  thus  established.  The  removal  of  the  ligature  and 
the  return  of  the  blood  will  restore  the  nutrition  and  re-establish  coordinate 
contractions.  The  excised  heart  of  the  mammal  which  has  passed  into  the 
condition  of  fibrillary  contraction  may  be  again  made  to  beat  rhythmically 
and  vigorously  by  first  cooling  it  with  normal  saline,  and  then  perfusing  it 
with  warm  defibrinated  blood  through  the  coronary  vessels  under  a  suitable 
pressure.  The  same  result  can  be  brought  about  by  first  perfusing  it  with 
a  I  per  cent,  solution  of  potassium  chlorid  until  the  heart  comes  to  rest  and 
then  perfusing  it  with  Ringer's  solution. 

The  Beat  of  the  Excised  Heart. — The  beat  of  the  heart,  its  frequency 
and  regularity,  its  continuance  from  the  early  stages  of  fetal  development  till 
death,  has  long  been  an  interesting  subject  for  physiologic  investigation. 
Though  related  to  the  functional  activities  of  the  body  at  large,  the  activity 
of  the  heart  is  in  a  sense  independent  of  them,  for  it  will  continue  for  a 
variable  length  of  time  after  they  have  ceased.  The  heart  of  the  frog  or 
the  turtle  will  continue  to  beat  under  appropriate  conditions  for  hours  after 
separation  of  all  anatomic  connections  and  removal  from  the  body.  The 
heart  of  the  dog  or  cat  will,  however,  beat  but  for  a  few  minutes.  The 
human  heart  would  in  all  probability  act  in  the  same  way.  Nevertheless 
there  are  good  reasons  for  believing  that  though  the  heart  has  ceased  to 
respond  to  its  customary  stimulus,  the  irritability  yet  endures  though 
perhaps  in  lessened  degree,  by  reason  of  the  absence  of  blood,  in  the  mam- 
malian heart,  in  the  coronary  system  of  vessels.  For  if,  after  the  heart  has 
ceased  to  beat  for  some  time,  warm  defibrinated  and  oxygenated  blood 
or  Locke's  modification  of  Ringer's  solution  be  passed  through  the  coronary 
vessels  the  beat  will  reappear  and  continue  at  its  usual  rate  for  some  hours. 
(See  paragraph  relatmg  to  the  action  of  inorganic  salts  on  the  mammalian 
heart,  page  307.) 

The  reason  for  the  longer  continuance  of  the  beat  of  the  excised  heart 
of  the  cold-blooded  animal    beyond    that  of    the   warm-blooded   animal 


298  TEXT-BOOK  OF  PHYSIOLOGY 

lies  probably  in  the  difference  in  the  rate  of  their  respective  metabolisms. 
There  is  reason  to  believe  that  each  cell  of  the  heart-muscle,  in  common  with 
other  tissue  cells,  during  life  stores  up  and  holds  in  reserve  a  larger  quantity 
of  nutritive  material  than  is  necessary  for  its  immediate  needs.  When  sepa- 
rated from  the  general  blood-supply,  the  cells  begin  to  utilize  this  reserved 
material.  With  its  consumption  the  irritability  declines  and  after  a  variable 
period  of  time  the  contraction  ceases.  As  the  metabolism  is  far  more  rapid 
in  the  warm-blooded  than  in  the  cold-blooded  animal,  it  is  probable  that  the 
reserved  nutritive  material  is  utilized  more  quickly  in  the  former  than  in  the 
latter  other  conditions  being  equal.  So  long  as  it  lasts  in  either  class,  the 
irritability  and  contractility  persist. 

Whatever  the  immediate  or  exciting  cause  of  the  heart  contraction  may  be, 
the  fundamental  condition  for  its  manifestation  is  the  maintenance  of  the 
irritability.  So  long  as  this  persists  at  a  sufficiently  high  level  the  heart- 
muscle  will  contract  in  response  to  the  appropriate  stimulus. 

THE  PHYSIOLOGIC  PROPERTIES  OF  THE  HEART-MUSCLE 

The  physiologic  properties  of  the  heart-muscle  on  which  its  efficiency  as 

a    pumping   organ   depends,    viz.,   irritability,    conductivity,    rhythmicity, 

tonicity,  automaticity,  have  been  largely  determined  by  a  study  of  the  heart 

of  the  frog.    As  some  of  the  facts  to  be  stated  in  subsequent  paragraphs  have 

reference  to  this  heart,  it  will  be  found  conducive  to  clearness  if  its  anatomic 

structure  and  physiologic  action  be  understood.     For  this  reason  a  brief 

account  of  the  frog  heart  will  be  found  in  the  appendix. 

I.  Irritability. — The  heart-muscle  in  common  with  other  muscles  possesses 

irritability,  by  virtue  of  which  it  responds  by  a  change  of  form  to  the 

action  of  a   stimulus.     Whatever  the   stimulus,   here,   as  elsewhere, 

there  is  a  conversion  of  potential  into  kinetic  energy — heat,  electricity, 

and  mechanic  motion.     The  normal  physiologic  stimulus  has  not  been 

positively  determined.     In  common  with  other  forms  of  muscle-tissue, 

the  heart-muscle  may  be  made  to  contract  by  artificial  stimuli — e.g., 

mechanic,  thermic,  chemic,  and  electric. 

For  the  demonstration  of  this  fact  it  is  necessary  to  eliminate  the 
action  of  the  physiologic  stimulus  and  to  bring  the  heart  to  rest  in  the 
condition  of  diastole.  This  can  be  done  with  the  frog's  heart,  by 
ligating  the  tissues  at  the  sino-auricular  junction,  a  procedure  which 
prevents  the  passage  of  the  contraction  wave,  which  originates  in  the 
sinus,  over  the  auricles  and  ventricles  (a  fact  that  will  be  more  fully 
alluded  to  in  a  subsequent  paragraph).  With  the  heart  thus  prepared 
and  while  still  in  situ,  the  apex  may  be  connected  with  a  recording 
lever  and  its  evoked  contractions  registered  on  a  recording  surface.  In 
this  condition  it  will  respond  by  a  contraction  to  any  form  of  an  ade- 
quate stimulus,  such  as  the  induced  electric  current. 

In  its  irritability,  contractility,  and  manner  of  response  to  stimuli, 
the  heart  of  the  mammal  corresponds  in  all  essential  respects  to  the 
heart  of  the  frog  or  turtle. 

The  irritability  of  the  heart-muscle  depends  primarily  on  the  blood- 
supply  and  secondarily  on  the  maintenance  of  a  normal  temperature, 
and  so  long  as  both  conditions  are  maintained  the  muscle  will  respond 


THE  CIRCULATION  OF  THE  BLOOD  299 

by  a  contraction  to  any  adequate  stimulus,  physiologic  or  artificial. 
a.  The  Blood-supply. — The  supply  of  blood  to  the  mammalian  heart  is 
derived  from  the  coronary  arteries  which,  though  filled  during  the 
systole,  deliver  the  blood  to  the  intra-mural  arterioles  and  capillaries 
during  the  diastole.  The  facts  relating  to  the  blood-supply  have  been 
presented  fully  in  a  foregoing  paragraph  (page  294). 
h.  The  Influence  oj  Temperature. — For  the  manifestation  of  the  irrita- 
bihty  and  contractility  it  is  essential  that  the  heart-muscle  be  kept  at  a  suf- 
ficiently high  temperature  in  order  that  the  physiologic  or  a  given  arti- 
ficial stimulus  may  evoke  a  maximal  contraction.  This  is  accomplished 
by  immersing  the  suspended  heart  in  a  bath  of  Ringer's  solution  the  tem- 
perature of  which  can  be  readily  decreased  or  increased  by  appropriate 
means.  The  optimum  temperature  for  the  frog  heart  is  about  2  5°C. 
As  the  temperature  is  lowered  both  rate  and  force  decrease  until  at 
about  from  4°C.  to  o°C.  both  cease.  Beyond  35°C.  it  also  ceases  to 
contract,  because  of  a  coagulation  of  the  muscle  substance.  The 
mammalian  heart  attains  its  maximum  activity  at  a  temperature  of 
39°C.  to  4i°C.  It  ceases  to  beat  at  about  47°C.  on  the  one  hand  and 
at  about  i7°C.  on  the  other  hand. 
2.  Conductivity. — Conductivity  of  living  material  may  be  defined  as  the 
ability  to  transmit  through  itself  a  condition  of  activity  due  to  the 
action  of  a  stimulus.  In  muscle  material  the  condition  of  activity  is 
characterized  by  a  molecular  process  known  as  the  excitation  process, 
followed  almost  immediately  by  a  change  of  shape  known  as  the  con- 
traction wave. 

In  skeletal  muscle  conductivity  is  developed  to  a  high  degree.  Thus 
if  a  stimulus,  e.g.,  an  induced  electric  current,  be  sent  transversely 
through  one  end  of  a  muscle  an  excitation  process  is  developed,  followed 
by  a  contraction  wave,  both  of  which  are  conducted  through  the  muscle 
without  interruption  to  the  other  end  with  a  speed,  in  the  frog  muscle, 
of  about  10  meters  per  second.  In  the  cardiac  muscle  the  physiologic 
stimulus  acts  at  or  near  the  terminations  of  the  venae  cavae,  from  which 
point  an  excitation  process  and  a  subsequent  contraction  wave  are 
conducted  over  the  auricles,  thence  to  the  ventricles  from  base  to  apex 
with  extreme  rapidity.  It  is  evident  therefore  that  the  heart-muscle 
also  possesses  conductivity  to  a  high  degree.  It  is  now  generally  believed 
that  the  propagation  of  both  processes  is  accomplished  by  muscle-tissue 
alone,  independently  of  the  nerve  system.  The  conductivity,  however, 
is  not  equally  well  developed  in  every  part  of  the  heart. 

In  the/rog  heart  this  is  especially  true  of  the  tissue  at  both  the  sino- 
auricular  and  the  auriculo-ventricular  junctions.  At  these  points  the 
contraction  wave  is  delayed  for  an  appreciable  period  (a  condition  at- 
tributed to  the  embryonic  character  of  the  muscle-tissue),  so  that  what 
would  otherwise  be  a  single  wave  becomes  divided  into  three  smaller 
waves,  so  that  it  becomes  possible  to  observe  and  distinguish  the  con- 
traction of  the  different  chambers  of  the  heart.  In  the  frog's  heart 
the  excitation  process  and  the  contraction  wave  begin  in  the  sinus  veno- 
sus,  from  which  they  are  conducted  to  the  auricles,  thence  to  the  ven- 
tricles. The  successive  contractions  of  the  walls  of  the  subdivisions  of 
the  heart  can  be  readily  recorded  with  suitable  apparatus.     In  Fig.  135, 


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which  is  a  graphic  record  of  the  heart-beat,  the  two  elevations  of  the 
lever  on  the  up-stroke,  a  and  b,  represent  the  contraction  of  the  sinus 
and  the  auricle  respectively,  followed  by  the  vigorous  and  long  con- 
tinued contraction  of  the  ventricle,  while  the  two  depressions,  c  and  d, 
indicate  the  delay  in  the  transmission  of  the  contraction  wave  at  the 
two  junctions.  There  is  here  an  anatomic  obstacle  to  the  conduction 
of  the  contraction  wave.     The  block  between  the  sinus  and  the  auricle 

may  be  artificially  increased  to  such  an  ex- 
tent as  to  prevent  absolutely  the  passage  of 
the  contraction  wave  by  ligation  of  the  tis- 
sue between  them,  a  procedure  introduced 
by  Stannius  and  now  known  as  the  first 
Stannius  ligature.  Under  such  circum- 
stances the  auricles  and  ventricle  remain  at 
rest  while  the  sinus  continues  to  beat  at  its 
usual  rate.  The  obstacle  between  the  auri- 
cles and  ventricle  may  be  increased  by  the 
same  method  also  introduced  by  Stannius, 
and  now  known  as  the  second  Stannius 
ligature,  or  better  by  means  of  a  suitable 
and  adjustable  clamp.  By  carefully  regulating  the  pressure  of  the  clamp 
it  is  possible  to  so  block  the  wave  that  three  or  four  auricular  contrac- 
tions may  occur  before  the  excitation  process  forces  the  block  and  ex- 
cites a  ventricular  contraction.  (Fig.  136.)  If  the  block  is  complete, 
rather  than  partial,  the  ventricle  will  come  to  rest  and  so  remain. 
From  the  foregoing  facts  it  is  evident  that  the  physiologic  stimulus  ex- 
erts its  action  in  the  sinus  venosus  and  that  the  auricular  and  ven- 
tricular beats  are  in  turn  dependent  on  it. 


Fig.  135.- 
Contraction 
Heart. 


-Record    of    the 
OF    the     Frog's 


Fig.  136. — Record  of  the  Auricular  and  Ventricular  Contractions  before 

AND  after  the  CLOSURE  OF  THE  ClAMP  AT  a. 


In  the  mammalian  heart  the  seat  of  the  stimulus  and  the  point  of 
origin  of  the  excitation  process  and  the  subsequent  contraction  wave 
have  been  a  subject  of  much  investigation  and  discussion.  For  some 
time  it  has  been  believed  that  these  processes  originate  at  the  termina- 
tions of,  or  between  the  terminations  of  the  venae  cavse  in  a  region 
corresponding  to  the  sinus  venosus  in  the  frog  heart\  from  which  they 

'In  the  mammalian  heart  the  sinus  venosus  as  a  distinct  chamber  has  been  obliterated, 
but  it  is  represented  by  the  following  remnants:  (i)  The  termination  of  the  superior  vena 


THE  CIRCULATION  OF  THE  BLOOD  301 

pass  over  the  auricles,  thence  to  the  ventricles.  On  the  basis  of  this  belief 
it  has  been  assumed  that  there  is  a  specialized  area  in  which  the  stimulus 
arises  and  which  determines  the  rate  and  rhythm  of  the  entire  heart. 
At  present  it  is  believed  that  this  area  is  identical  with  the  region  occu- 
pied by  the  sino-auricular  node,  the  lower  portion  of  the  sulcus  terminalis. 
With  the  view  of  determining  the  truth  of  this  assumption  Flack  per- 
formed a  number  of  experiments  on  the  hearts  of  dogs,  cats,  and  rabbits, 
some  of  the  results  of  which,  abstracted  from  his  paper,  are  as  follows: 
The  application  of  cold  either  through  metallic  tubes  or  by  means  of  an 
ethyl  chlorid  spray,  the  remainder  of  the  heart  being  protected,  caused 
slowing  of  both  auricles  and  ventricles.  Weak  electric  stimulation 
caused  marked  inhibition  of  both  auricles  and  ventricles;  slightly 
stronger  stimulation  caused  a  mixed  effect  of  inhibition  and  acceleration, 
the  latter  usually  predominating;  still  stronger  stimulation  gave  rise  to 
marked  acceleration  of  the  whole  heart  rhythm  or  an  altered  rhythm  of 
auricles  and  ventricles.  When  electric  stimuli  were  applied  to  other 
regions  of  the  superior  vena  cava  or  sulcus  no  effects  were  noticeable. 

Mechanic  stimulation  as  pinching  the  node  with  forceps  called  forth 
similar  results.  Destruction  of  the  node,  however,  had  no  effect  on 
the  rhythm.  The  application  of  a  weak  solution  of  atropin  abolishes 
the  customary  effects  of  both  vagus  and  sympathetic  nerve  stimulation. 
From  the  foregoing  facts  it  may  be  assumed  that  the  usual  seat  of  origin 
of  the  stimulus  to  the  cardiac  contraction  is  the  sino-auricular  node,  but 
as  the  heart  continues  to  contract  after  the  node  is  destroyed,  it  is 
evident  that  some  other  portion  or  portions  of  the  auricular  wall  are 
also  capable  of  developing  under  the  circumstances  an  adequate  stimulus, 

A  further  proof  that  the  sino-auricular  node  is  the  initiator  of  the  car- 
diac contraction  is  found  in  its  change  of  electric  potential.  It  has  long 
been  established  that  when  any  portion  of  living  material  enters  into  a 
state  of  activity  it  becomes  electro-negative  to  all  other  portions  which 
are  at  the  same  instant  electro-positive.  Lewis  with  special  electrodes 
in  connection  with  a  string  galvanometer  found  in  a  series  of  determina- 
tions that  with  the  beginning  of  a  cardiac  contraction,  the  sino-auricu- 
lar node  was  the  point  of  initial  electro-negativity,  a  fact  that  is  in  accord 
with  the  general  truth  that  the  region  of  greatest  activity  exhibits  the 
greatest  degree  of  negativity.  The  sino-auricular  node  may  therefore 
be  regarded  as  the  primary  seat  of  the  stimulus  or  excitation  process 
and  the  initiator  of  the  beat. 

From  the  sino-auricular  node  the  excitation  process  is  conducted  to 
the  auricles  and  ventricles  in  quick  succession,  though  between  the  end 
of  the  auricular  contraction  and  the  beginning  of  the  ventricular  con- 
traction there  is  also  a  perceptible  interval  similar  to  that  observed  in  the 
frog  heart.  For  a  long  time  it  was  assumed  that  the  excitation  process 
and  the  contraction  wave  passed  directly  from  auricles  to  ventricles 
across  the  auriculo- ventricular  junction  as  in  the  frog  and  that  the  interval 

cava  (the  right  duct  of  Cuvier);  (2)  the  coronary  sinus  (the  left  duct  of  Cuvier) ;  (3),  a 
stratum  submerged  beneath  auricular  tissue  at  the  taenia  terminalis;  (4)  the  remnants  of 
the  venous  valves,  i.e.,  the  Thebesian  and  Eustachian  valves  (Flack).  In  addition  there 
is  a  remnant  of  primitive  tissue  at  the  sino-auricular  jimction,  that  is,  where  the  superior 
vena  cava  joins  the  tsenia  terminalis  of  the  right  auricle,  and  known  as  the  sino-auricular 
node. 


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between  the  auricular  and  ventricular  contractions  was  due  to  an  interfer- 
ence with  the  passage  of  the  contraction  wave  across  the  junction  because 
of  the  extreme  scarcity  of  the  muscle  fibers  in  this  region  or  to  their 
embryonic  character.  In  recent  years,  however,  this  view  has  been 
abandoned  because  the  real  bond  of  union  between  the  auricular  and 
ventricular  tissues,  across  which  the  excitation  process  passes,  has  been 
found,  as  stated  on  page  271,  in  the  system  of  muscle-fibers,  described 
in  part  by  His,  Retzer  and  Braunig,  and  Tawara  and  in  part  by  Keith 
and  Flack  and  known  as  the  conduction  system  of  the  heart,  page  272,  This 
system  it  is  believed  constitutes  the  anatomic  and  physiologic  path 
across  which  the  excitation  process  passes  from  auricles  to  ventricles. 
The  excitation  process  originating  in  the  sino-auricular  node  passes 
first  to  the  auricular  walls,  exciting  them  to  contraction  and  then  into 
and  through  the  auriculo-ventricular  bundle  to  the  ventricular  walls, 
exciting  them  to  contraction.     The  supposition  that  this  was  the  case 

has  been  demonstrated  by  Hering  and  others 
V  y/^^''^s:=:— ^        who  succeeded  in  dividing  the  muscle-bundle 

■^•'—  /^  _  ^)  in  the  excised  hearts  of  rabbits  and  dogs,  kept 
actively  beating  by  perfusion  with  Ringers' 
solution.  On  division  of  the  bundle  both 
auricles  and  ventricles  continued  to  beat 
though  with  different  rates  and  independently 
of  each  other.  These  and  other  experiments 
of  a  similar  character  have  demonstrated  be- 
yond question  that  the  auriculo-ventricular 
bundle  with  its  widespread  ramifications  is 
the  true  conducting  system  between  auricles 
and  ventricles.  In  this  system  the  sino-auric- 
ular node  is  regarded  as  the  primary  domi- 
nating "pace  maker"  of  the  rate  and  rhythm 
of  the  heart.  Inasmuch,  however,  as  the 
heart  will  continue  to  beat,  after  the  destruc- 
tion of  the  sino-auricular  node  it  is  evident 
that  it  is  not  the  only  region  that  can  initiate 
the  contraction.  Whether  the  contraction  under  such  circumstances 
is  due  to  an  excitation  arising  in  some  other  portion  of  the  auricular  wall 
or  in  the  subsidiary  auriculo-ventricular  node  is  a  subject  of  discussion. 
The  cause  assigned  by  Tawara,  for  the  interval  between  the  auricular 
and  ventricular  contraction  is  not  so  much  the  embryonic  character  of 
the  fibers  of  the  system,  as  it  is  the  length  of  the  system  as  a  whole, 
which  he  estimates  at  from  4  to  6  centimeters.  This  time,  estimated 
from  the  beginning  of  the  auricular  systole  to  the  beginning  of  the 
ventricular  systole  amounts  to  from  o.i  to  0.2  second.  The  interval 
between  these  two  events,  determined  from  the  time  between  the  oc- 
currence of  the  a  and  the  c  or  5  waves  on  the  jugular  pulse  tracing  is 
known  as  the  a-c  interval,  or  the  As-Vs  interval. 

With  the  mammalian  heart  as  with  the  frog  heart  it  is  possible  to 
increase  the  length  of  the  interval  between  the  auricular  and  the  ven- 
tricular contraction,  the  inter-systolic  period,  by  compression  of  a  por- 
tion of  the  tissues  between  auricles  and  ventricles  including  presum- 


FiG.  137. — The  Erlanger 
heart-block  clamp  compress- 
ing the  auriculo-ventricular 
bundle  (AVE ) .  SM,  Septum 
membranaceum;  MV,  mitral 
valve. — {Hirschf elder.) 


THE  CIRCULATION  OF  THE  BLOOD  303 

ably  the  central  part  of  the  conducting  system,  the  muscle  bundle  of 
His.  This  has  been  accomplished  in  the  dog  by  Erlanger  by  means 
of  a  specially  devised  hook  clamp  (Fig.  137),  which  consists  of  an  L- 
shaped  hook  of  steel  wire  the  arm  of  which  can  be  made  to  approach 
a  brass  block  by  means  of  a  bolt  and  screw.  The  L-shaped  hook  is 
inserted  into  the  right  wall  of  the  aorta,  then  passed  downward  and 
backward  into  the  left  ventricle,  then  pushed  through  the  ventricular 
septum  into  the  right  ventricle.  In  this  position  it  lies  under  the 
auriculo-ventricular  bundle.  Compression  is  now  brought  about  by 
approximating  the  hook  to  the  brass  block  by  means  of  the  nut. 
When  the  compression  is  brought  about  suddenly  and  completely  the 
ventricles  at  once  cease  beating,  though  the  auricles  continue  to  beat 
with  their  customary  rate  and  regularity.  After  a  variable  period  of 
time,  varying  from  a  few  seconds  to  70  seconds,  during  which  the 
ventricles  are  relaxed  and  gradually  filling  with  blood  from  the  auri- 
cles, the  ventricular  beat  returns,  at  first  slowly  but  with  a  gradually 
increasing  frequency  until  a  definite  but  a  comparatively  slow  rate  is 
attained.  The  rhythm  thus  developed  is  termed  the  ideo-ventricular 
rhythm. 

In  experiments  on  the  dog  heart  performed  by  Erlanger  the  following 
results  were  obtained  when  the  auriculo-ventricular  bundle  was  com- 
pletely crushed. 

Aur.  rate  per  minute.  Ven.  rate  per  minute.         Ratio  of  Aur.  to  Ven. 

Max.  216  -  Max.  6g.8  3.09 

Min.  117. 8  Min.   34.8  3.38 

Ave.   166.9  Ave.    52.3  3.19 

The  reason  assigned  for  the  cessation  of  the  ventricular  contraction 
is  the  non-arrival  of  the  excitation  process  at  the  ventricular  end  of  the 
conducting  system,  because  of  the  blocking  or  compression.  Under 
physiologic  conditions  the  ventricular  beat  is  directly  dependent  on  the 
arrival  of  the  excitation  process  from  the  auricles  and  if  it  fails  to  arrive 
the  ventricle  does  not  contract  for  some  seconds.  The  return  of  the 
beat  during  complete  blocking  is  attributed  to  the  development  of  a 
hitherto  dormant  inherent  rhythmicity.  When  this  is  established  both 
auricles  and  ventricles  continue  to  beat  though  with  totally  different 
rhythms. 

The  effects  which  follow  gradual  compression  of  the  muscle-bundle 
are  somewhat  different  from  those  which  follow  sudden  compression. 
If  the  clamp  is  accurately  adjusted  and  the  compression  gradually 
applied,  the  first  perceptible  effect  is  a  lengthening  of  the  normal  pause, 
the  inter-systolic,  between  the  auricular  and  the  ventricular  contraction. 
With  an  increase  in  the  compression  there  will  come  a  moment  when 
one  of  the  auricular  contraction  waves  fails  to  reach  the  ventricle,  or  if  it 
does,  it  is  so  enfeebled  that  it  is  incapable  of  exciting  the  ventricle, 
which  in  consequence  fails  to  contract.  This  dropping  out  of  a  ven- 
tricular contraction  may  occur  once  in  every  10,  9,  8,  7,  6,  etc.,  auricular 
beats,  in  accordance  with  the  degree  of  compression.  With  a  further 
tightening  of  the  clamp,  the  blocking  of  the  excitation  process  may  be 
still  further  increased  so  that  only  every  second,  third,  or  fourth  auricular 
beat  is  capable  of  developing  a  ventricular  beat,  establishing  what  has 


304  TEXT-BOOK  OF  PHYSIOLOGY 

been  termed  the  2  :i,  3  :i,  4:1,  rhythms  respectively;  and  finally 
when  the  blocking  is  complete  no  excitation  process  can  reach  the 
ventricle. 

Owing  to  the  capability  of  the  mammalian  ventricle  to  develop  an 
independent  rhythm  when  not  stimulated  by  the  auricles  for  a  few 
seconds  or  more,  it  is  not  always  possible  to  state  at  what  particular 
moment  in  the  successive  stages  of  compression  the  independent  ventricu- 
lar rhythm  becomes  manifest.  Usually  when  the  rhythm  is  of  the  3  :  i 
type,  i.e.,  when  the  third  auricular  contraction  fails  to  reach  the  ventricle, 
it  will  begin  to  beat  of  itself.  Under  such  circumstances  the  auricles 
and  ventricles  become  dissociated  even  though  the  block  is  not  quite 
complete. 

These  experimental  facts  have  afforded  an  explanation  of  the  altered 
rhythm  between  auricles  and  ventricles  often  found  in  that  pathologic 
condition  known  as  Adams-Stokes  disease.  In  this  disease  the  rhythm 
may  be  any  one  of  the  rhythms  stated  in  the  foregoing  paragraph.  In 
two  instances  the  following  ratio  of  the  ventricle  to  the  auricle  was 
observed  by  Erlanger. 

Aur.  rate  per  minute.  Ven.  rate  per  minute.         Ratio  of  Aur.  to  Ven. 

79-6  22.4  3.5s 

84.6  31.0  2.73 

In  a  few  cases  of  death  from  this  disease  a  post-mortem  examination 
showed  a  lesion  of  the  auriculo-ventricular  bundle. 
3.  Rhythmicity. — Rhythmicity  may  be  defined  as  the  ability  to  act  in 
regularly  recurring  cycles  or  the  property  of  anything  so  acting.  As 
the  heart-beat  recurs  in  regular  cycles  or  at  regular  intervals,  it  may 
therefore  be  said  that  the  heart-muscle  is  characterized  by  rhythmicity. 
The  beat  of  the  heart  as  well  as  each  phase  of  the  beat  occupies  a 
regular  measure  of  time  and  is  therefore  rhythmic  in  character.  Experi- 
mental procedures,  however,  show  that  the  rhythmic  power  or  at  least 
the  frequency  of  the  rhythm  varies  in  each  of  its  subdivisions  when  they 
are  separated  one  from  the  other.  Thus  if  the  tissue  between  the  sinus 
and  auricle  in  the  frog  or  turtle  heart  be  divided,  the  auriculo-ventricular 
portion  at  once  ceases  to  beat,  while  the  sinus  continues  to  beat  as  usual. 
In  a  short  time,  however,  the  auricles  and  ventricles  begin  again  to  beat, 
but  with  a  slower  rhythm.  Division  of  the  tissue  between  auricles  and 
ventricles  is  again  followed  by  rest.  In  a  short  time  the  auricles  begin 
to  beat,  while  the  ventricle  remains  quiescent.  If  the  ventricle  now  be 
stimulated  in  a  rhythmic  manner  it  may  resume  rhythmic  activity. 
These  facts  are  taken  as  an  indication  that  the  rhythmic  power  is 
greatest  in  the  sinus,  less  in  the  auricles,  and  least  in  the  ventricles. 

In  the  warm-blooded  animal,  e.g.,  dog,  cat,  rabbit,  there  is  also  a 
difference  in  the  rhythmicity  of  the  auricles  and  ventricles.  This  is 
shown  by  the  effects  which  follow  division  of  the  auriculo-ventricular 
bundle,  or  sudden  and  complete  compression  of  that  portion  of  the 
auriculo-ventricular  tissue  containing  it.  In  either  case  the  ventricle 
for  a  short  time  remains  at  rest,  though  the  auricles  continue  to  beat  at 
their  usual  rate.     After  a  variable  number  of  seconds  the  ventricle 


THE  CIRCULATION  OF  THE  BLOOD  305 

develops  a  rhythm  of  its  own,  though  it  never  attains  that  of  the  auricle. 
From  these  facts  it  is  probable  that  in  each  division  of  the  heart  a  stimu- 
lus similar  to  that  acting  in  the  sinus  is  developed  when  the  heart 
chambers  are  separated  one  from  the  other. 

4.  Tonicity. — Tonicity  may  be  defined  as  a  condition  of  muscle  material 

characterized  by  a  slight  degree  of  contraction  which  varies  in  extent, 
however,  from  time  to  time  under  physiologic  conditions.  Whatever 
the  cause  of  the  tonicity  may  be  in  any  given  form  of  muscle,  the 
slight  degree  of  contraction  which  characterizes  it  not  only  resists  undue 
extension  but  permits  of  a  quicker  response  to  the  action  of  a  stimulus 
and  a  more  effective  performance  of  work.  The  heart-muscle,  like 
the  skeletal  muscle,  maintains  continuously  a  certain  degree  of  contrac- 
tion, which  not  only  prevents  undue  expansion  of  the  heart  during  the 
period  of  diastole,  but  increases  its  efficiency  as  a  pumping  organ  at  the 
beginning  and  during  the  systole.  The  results  obtained  from  subjecting 
the  heart  muscle  to  the  action  of  a  calcium  salt  renders  it  probable  that  the 
tonicity  is  in  large  measure  the  result  of  or  is  associated  with  the  action  of 
such  a  salt  (see  page  306).  The  tonicity  may,  however,  be  increased 
or  decreased  by  the  action  of  various  external  agents.  Thus  the  passage 
of  dilute  solutions  of  various  drugs — e.g.,  alkalies,  digitalis — through 
the  cavities  of  the  excised  heart  will  so  increase  the  tonicity,  or  the 
contractile  power,  that  complete  relaxation  is  prevented,  until  finally 
the  heart  comes  to  a  standstill  in  the  condition  of  systole.  The  passage 
of  dilute  solutions  of  lactic  acid,  muscarine,  etc.,  through  the  heart  will, 
on  the  contrary,  so  decrease  the  tonicity  or  the  contractile  power  that 
the  normal  contraction  is  not  attained.  The  relaxation  therefore 
gradually  increases  until  the  heart  finally  comes  to  a  standstill  in  the 
condition  of  extreme  diastole.  In  the  first  instance  the  tonicity  is  said 
to  be  increased;  in  the  second  instance,  decreased. 

5.  Automaticity. — Automaticity  may  be  defined  as  the  power  of  maintain- 

ing activity  by  a  self-acting  cause  or  the  power  of  acting  independent 

of  external  causes.     Inasmuch  as  the  heart  continues  to  contract  in 

a  perfectly  rhythmic  manner  after  removal  from  the  body  and  apparently 

without  the  aid  of  an  external  stimulus,  it  is  said  that  the  heart-muscle 

is  automatic  or  spontaneous  in  action.     Strictly  speaking,  however,  this 

is  not  the  case,  for  the  reason  that  all  movement,  that  of  the  heart 

included,  is  the  resultant  of  the  action  of  natural  causes  though  their 

true  nature  may  be  beyond  the  reach  of  present  methods  of  investigation. 

The  Nature  of  the  Stimulus. — As  the  heart  continues  to  beat  after 

removal  from  the  body,  it  is  evident  that  the  stimulus  does  not  originate  in 

the  central  nerve  system  but  in  the  heart  itself.     Two  views  have  been  held 

as  to  its  origin  and  nature: 

1.  That  it  originates  in  the  nerve-cells  found  in  various  parts  of  the  heart- 

muscle;  that  it  is  a  nerv'e  impulse  rhythmically  and  automatically 
discharged  by  these  cells  and  transmitted  by  their  axons  to  the  heart- 
muscle  cells. 

2.  That  it  originates  in  the  muscle-cells  themselves;  that  it  is  chemic  in 

character  and  due  to  a  reaction  between  the  chemic  constituents, 
organic  and  inorganic,  of  the  muscle-cells  and  those  of  the  lymph  by 
which  they  are  surrounded. 


3o6  TEXT-BOOK  OF  PHYSIOLOGY 

According  to  the  first  view  the  stimulus  is  neurogenic,  according  to  the 
second  view  myogenic. 

The  presence  of  nerve-cells;  their  relation  to  the  muscle-cells;  the  pro- 
nounced rhythmic  activity  of  the  sinus  and  auricles  in  which  the  nerve-cells 
are  abundant;  the  feeble  activity  of  the  apex,  in  which  they  are  wanting — 
these  and  other  facts  lend  support  to  the  view  that  the  stimulus  originates  in 
the  nerve-cells.  To  them  have  been  attributed  the  power  of  automatic 
activity. 

The  absence  of  nerve-cells  in  portions  of  the  heart-muscle,  which  never- 
theless exhibit  rhythmic  contractions  for  quite  a  long  period  of  time;  the 
rhythmic  beat  of  the  embryonic  heart  before  the  migration  of  nerve-cells  to  its 
wails  shows  that  the  stimulus  does  not  necessarily  originate  in  nerve-cells. 
Moreover,  Porter  has  conclusively  shown  that  the  apex  of  the  dog's  heart, 
which  is  generally  believed  to  be  totally  devoid  of  nerve-cells,  can  be  made 
to  beat  for  hours  by  feeding  it  through  its  nutrient  artery  with  warm  defibrin- 
ated  blood.  Unless  it  be  assumed  that  the  heart-muscle  contracts  auto- 
matically, without  a  cause,  it  is  a  fair  assumption  that  the  exciting  cause  of 
the  contraction  arises  within  the  muscle-cells  themselves,  and  that  it  is  in 
all  probability  the  outcome  of  a  reaction  between  the  chemic  constituents 
of  the  blood  or  lymph  on  the  one  hand,  and  the  chemic  constituents  of  the 
muscle-cells  on  the  other.  The  discovery  that  some  of  the  inorganic  salts  of 
the  blood  have  a  specific  physiologic  action  on  the  heart-muscle  was  made  in 
1882  by  Ringer.  Since  then,  many  attempts  have  been  made  to  isolate 
these  constituents,  to  determine  not  only  their  individual,  but  also  their 
collective  action,  when  combined  in  proportions  approximating  those  in 
which  they  exist  in  the  blood. 

The  Action  of  Inorganic  Salts. — i.  On  the  Frog  and  Terrapin  Heart. — 
The  inorganic  salts  which  are  most  directly  concerned  in  exciting  and  sustain- 
ing the  heart-beat  are  sodium  chlorid,  calcium  phosphate  or  chlorid,  and  potas- 
sium chlorid.  A  combination  of  these  salts  in  the  proportions  in  which  they 
exist  in  the  blood  was  first  suggested  by  Ringer  and  is  made  by  saturating  a 
0.65  per  cent,  solution  of  sodium  chlorid  with  calcium  phosphate,  and  then 
adding  to  each  100  c.c,  2  c.c.  of  a  i  per  cent,  solution  of  potassium  chlorid. 
A  frog's  heart  immersed  in  this  solution  will  continue  to  beat  for  some  hours. 
A  combination  of  the  chlorids  of  sodium,  calcium,  and  potassium  in  amounts 
which  will  vary  for  different  animals  is  equally  efficient  in  maintaining  the 
heart-beat. 

The  collective  as  well  as  the  individual  actions  of  these  salts  have  been 
strikingly  brought  out  by  the  experiments  of  Profs.  Howell  and  Greene,  from 
whose  published  results  the  following  statements  are  derived.  Instead  of 
employing  the  entire  heart,  they  used  for  various  reasons  strips  from  the 
terminations  of  the  venae  cavae  and  from  the  ventricle  of  the  terrapin  heart. 
The  proportion  of  the  inorganic  salts  most  favorable  for  the  contraction  of 
the  vena  cava  strips  is  the  following:  viz.,  sodium  chlorid,  0.7  per  cent.; 
calcium  chlorid,  0.026  per  cent.;  potassium  chlorid,  0.03  per  cent.  When 
vena  cava  strips  are  immersed  in  this  solution,  they  begin  in  a  short  time  to 
exhibit  rhythmic  contractions  which  may  continue  for  several  days.  In  the 
same  strength  of  solution  the  ventricular  strips  remain  inactive  but  if  the 
percentage  of  the  calcium  chlorid  be  raised  from  0.026  per  cent,  to  0.04,  or 
0.05  per  cent.,  spontaneous  contractions  soon  develop  and  continue  for 


THE  CIRCULATION  OF  THE  BLOOD  307 

several  days  or  more.  In  the  foregoing  solution  when  the  calcium  chlorid 
is  present  only  to  the  extent  of  0.026  per  cent.,  though  the  ventricular  strip 
does  not  contract,  it  is  kept  in  good  condition  for  contraction,  for  even  after 
many  hours  the  raising  of  the  percentage  of  calcium  chlorid  to  0.04  or  0.05 
per  cent. will  call  forth  after  a  brief  latent  period,  rapid  and  energetic  con- 
tractions. From  this  fact  it  is  inferred  that  the  vena  cava  region  is  more 
sensitive  to  the  combined  action  of  the  salts  than  is  the  ventricle. 

The  action  of  the  individual  salts  is  also  best  shown  with  ventricular  strips. 
In  a  0.7  per  cent,  sodium  chlorid  solution  the  strip  beats  rhythmically  and 
energetically,  but  only  for  a  short  period  and  with  gradually  diminishing  force, 
and  a  loss  of  tonicity  until  it  entirely  ceases  to  beat.  A  reason  assigned  for 
this  is  the  removal  of  other  salts  necessary  to  the  excitation  of  the  contrac- 
tion. In  a  calcium  chlorid  solution — 0.9  per  cent. — i.e.,  isotonic  with  the 
sodium  chlorid — the  heart  strip  is  thrown  into  strong  tone,  but  does  not 
rhythmically  contract.  If,  however,  the  strip  is  placed  in  normal  saline, 
and  calcium  chlorid  added  in  amounts  equal  to  that  present  in  the  blood,  it 
will  after  a  very  short  period  begin  to  contract  rapidly  and  energetically  and 
for  a  longer  time  than  when  in  sodium  chlorid  solution  alone.  The  con- 
tractions not  infrequently  occur  before  relaxation  is  completed,  so  that  the 
strip  passes  into  the  condition  of  contracture. 

In  potassium  chlorid  solutions  isotonic — 0.9  per  cent. — with  sodium 
chlorid  solution  the  heart  strip  also  fails  to  contract.  This  is  the  case  also 
when  the  potassium  is  added  to  the  sodium  chlorid  in  amount  practically 
equal  to  that  found  in  the  blood. 

2.  On  the  Mammalian  Heart. — The  collective  action  of  the  inorganic 
salts  on  the  isolated  heart  of  all  members  of  this  class  of  animals  which  have 
been  made  the  subject  of  experimentation,  is  as  marked,  if  not  more  so,  than 
it  is  on  the  heart  of  the  frog  or  terrapin  especially  when  the  coronary  blood- 
vessels are  perfused  with  Ringer's  solution  or  the  modification  of  it  suggested 
by  Locke,  as  follows:  NaCl  0.90  per  cent.;  CaClj  0.024  per  cent.;  KCl  0.042 
per  cent.;  NaHCOg  0.02  per  cent.,  dextrose  o.i  per  cent.  The  reviving  and 
sustaining  power  of  this  solution  is  extraordinary.  Locke  and  Rosenheim 
were  able  to  revive  the  isolated  heart  of  a  rabbit  and  to  excite  it  to  active 
contraction,  for  several  hours  at  a  time,  on  four  consecutive  days  by  perfusing 
it  with  this  solution  saturated  with  oxygen  and  at  a  temperature  of  35°C. 
No  special  precautions  were  observed  other  than  keeping  it  cool  (io°C.)  and 
moist  during  the  intervals  of  experimentation.  The  duration  of  the  irrita- 
bihty  and  contractility  extended  over  a  period  of  95  hours.  Kuliabko 
revived  the  heart  of  a  rabbit  for  an  hour  nearly  three  days  after  removal  from 
the  body  of  the  animal.  It  was  then  placed  on  ice,  and  after  four  days  it 
was  again  revived  by  perfusing  it  with  Ringer's  solution.  Altogether  this 
heart  retained  its  irritability  for  seven  days.  Hering  revived  the  heart  of  a 
monkey  on  three  different  occasions,  the  first,  4^  hours,  the  second,  28 
hours,  and  the  third  54  hours  after  the  death  of  the  animal.  In  the  inter- 
vening periods  the  heart  was  also  kept  on  ice.  In  this  animal  it  was  even 
possible  to  increase  and  decrease  the  activity  of  the  heart  by  stimulation  of 
the  nerves  which  normally  control  the  rate  of  the  beat.  Kuliabko  was  also 
able  to  revive  the  isolated  heart  of  a  child  20  hours  after  death  from  a  double 
pneumonia.  It  was  made  to  beat  rhythmically  at  a  rate  varying  from  70  to  80 
per  minute  when  the  solution  had  a  temperature  of  39°C.,  and  at  a  rate  of 


3o8  TEXT-BOOK  OF  PHYSIOLOGY 

98  to  102  per  minute  when  it  had  a  temperature  of  4i°C.,  though  at  this  tem- 
perature the  beat  became  arrhythmic.  All  these  instances  demonstrate  the 
extreme  persistence  of  the  irritability  of  the  heart-muscle  under  appropriate 
conditions. 

The  action  of  individual  salts  has  been  shown  experimentally  on  the 
hearts  of  rabbits,  cats,  dogs,  monkeys,  by  Gross,  Howell  and  others.  Thus 
it  has  been  found  that  when  an  isolated  heart  is  rhythmically  beating  in 
response  to  the  perfusion  of  Ringer's  or  Locke's  solution,  the  addition  of 
potassium  chlorid  in  small  amounts  is  followed  by  a  decrease  in  the  rate  and 
force  of  the  contraction,  and  in  larger  amounts  by  a  complete  cessation  of  the 
contraction  and  a  standstill  in  diastole.  On  the  withdrawal  of  the  potassium, 
the  former  frequency  and  vigor  are  regained. 

Under  the  same  conditions,  the  addition  of  calcium  chlorid  in  sufficient 
amounts  is  followed  by  an  increase  in  the  rate  and  in  the  vigor  of  the 
contractions;  on  its  withdrawal  both  rate  and  force  return  to  the  previous 
condition.  Potassium  exerts  a  depressor  or  an  inhibitor  influence  on  the 
irritability  and  contractility  of  the  heart-muscle.  Calcium  exerts  an  accel- 
erator and  an  augmentor  influence  on  the  irritability  and  contractility  of  the 
heart. 

A  Theory  of  the  Heart-beat. — From  the  foregoing  facts  it  seems 
probable  that  the  heart-beat  is  connected  with  and  dependent  on  the  presence 
and  interaction  of  the  inorganic  salts  present  in  the  lymph,  though  as  to  the 
manner  in  which  they  interact  to  initiate  the  beat,  there  is  some  obscurity. 
A  very  plausible  theory  as  to  the  part  played  by  the  inorganic  salts  in  initiat- 
ing the  contraction  and  one  apparantly  in  accordance  with  the  facts  has  been 
presented  by  Howell  as  follows: 

The  heart-muscle,  it  is  assumed,  contains  a  stable  organic  energy-yielding 
compound  of  which  potassium  is  one  of  the  constituents  and  on  which  its 
stability  depends.  This  compound  must  be  present  in  relatively  large 
amounts  as  the  heart  will  continue  to  contract  and  expend  energy  for  many 
hours  after  the  blood-supply  has  been  withdrawn. 

During  the  diastole  a  reaction  takes  place  between  this  compound  and 
the  calcium  or  the  calcium  and  the  sodium  salts,  whereby  a  portion  of  the 
organic  compound  is  freed  from  potassium  and  is  then  combined  with  calcium 
or  with  calcium  and  sodium.  In  consequence,  this  portion  of  the  organic 
compound  in  combination  with  the  calcium  acquires  and  gradually  increases 
in  instability,  reaching  its  maximum  at  the  end  of  the  diastole,  when  it  under- 
goes a  dissociation  giving  rise  to  a  chain  of  events  that  culminate  in  a  con- 
traction. The  initial  step,  therefore,  is  a  dissociation  of  a  complex  unstable 
molecule  followed  by  an  oxidation  of  the  dissociated  products.  That  an 
active  dissociation  of  some  character  takes  place  is  evident  from  the  consump- 
tion of  oxygen,  the  production  of  carbon  dioxid,  the  liberation  of  heat, 
electricity,  and  mechanic  motion. 

Inasmuch  as  the  contraction  is  always  maximal  and  as  the  heart  is  refrac- 
tory to  a  stimulus  during  the  systole,  the  probabilities  are  that  all  of  the 
unstable  portion  of  the  energy-yielding  compound  is  dissociated  with  each 
contraction.  With  the  relaxation  there  is  a  renewal  of  the  unstable 
combination  of  calcium  with  the  organic  molecules,  which  increases 
in  amount  until  the  maximum  is  again  attained  when  another  dis- 
sociation   occurs    followed    by  another  contraction.     The  rhythmicity  of 


THE  CIRCULATION  OF  THE  BLOOD  309 

the  heart's  action,  the  appearance  of  a  refractory  condition  during  the 
systole  and  its  gradual  disappearance  during  the  diastole,  as  well  as  other 
phenomena,  are  readily  explained  by  the  foregoing  hypothesis. 

The  cause  of  the  dissociation  of  the  energy-\delding  material  is,  however, 
a  subject  of  discussion.  According  to  Howell  it  is  not  necessary  to  assume 
the  presence  of  any  cause  other  than  the  extreme  instabihty  of  the  organic 
compound  in  question.  According  to  Engelmann,  Langendorff  and  others, 
the  dissociation  is  not  spontaneous  but  is  the  result  of  the  action  of  a  specific 
stimulus,  an  "inner  stimulus,"  arising  within  the  muscle  elements  themselves 
through  metabolic  processes;  and  so  long  as  these  processes  are  chemically 
and  physically  conditioned  by  blood  or  tissue  fluids  containing  the  inorganic 
salts,  so  long  will  this  stimulus  be  produced.  As  to  the  nature  of  this  stimu- 
lus, whether  chemic,  electric  or  enzymic,  nothing  definite  can  be  stated  at 
present. 

The  Response  of  the  Heart  to  the  Action  of  an  Artificial  Stimulus. — 
The  heart  of  the  frog  as  well  as  of  some  other  animals  may  be  brought  to  a 
standstill  by  the  ligation  of  the  tissues  between  the  sinus  venosus  and  the 
auricle,  a  procedure  first  introduced  by  Stannius  and  now  known  as  the  first 
Stannius  ligature.  Under  such  circumstances  the  heart  may  be  made  to 
contract  by  stimulating  it  with  the  single  induced  current.  With  each 
passage  of  the  current  the  heart  contracts.  Contrary  to  what  is  observed 
in  skeletal  muscles,  the  heart  contraction,  if  it  occurs  at  all,  at  once  reaches 
its  maximal  value.  Any  increase  in  the  strength  of  the  stimulus  above  the 
threshold  value  has  no  greater  effect  on  the  extent  or  force  of  the  contraction 
than  the  minimal  stimulus.  A  conclusion  which  may  be  drawn  from  this 
fact,  according  to  Engelmann  is  as  follows:  By  reason  of  the  fact  that  the 
heart  contracts  at  its  maximal  value  to  the  action  of  any  strength  of  stimulus, 
under  given  conditions,  there  is  always  ensured  a  more  or  less  complete 
emptying  of  the  ventricular  contents  and  a  uniform  discharge  of  blood  into 
the  arteries,  which  would  not  be  the  case  if  the  extent  of  the  contraction 
varied  with  the  strength  of  the  stimulus;  and  there  are  reasons  for  believing 
that  the  normal  stimulus  for  the  contraction  varies  within  wide  limits  above 
the  threshold  value  both  in  normal  and  abnormal  conditions  of  the  heart. 
The  changes  in  the  extent  or  force  of  the  contraction  are  the  result,  not  of 
changes  in  the  intensity  of  the  stimulus,  but  of  changes  in  the  heart-muscle, 
caused  by  variations  in  mechanical  resistances. 

The  periodicity  of  the  heart's  action  or  its  rhythm  may  also  be  elucidated 
by  the  foregoing  facts.  There  are  reasons  for  believing  that  at  the  time  of 
the  contraction  practically  all  of  the  available  energy-yielding  material  is 
completely  utilized,  after  which  the  heart  relaxes  and  remains  at  rest  in  the 
diastolic  condition  for  a  given  period;  and  before  a  second  excitation  wave 
can  be  developed  and  pass  from  the  sinus  over  the  heart  there  must  be  a 
re-accumulation  of  energy-yielding  material,  and  a  restoration  of  the  irrita- 
bility. This  is  accomplished  during  the  diastole.  By  virtue  of  this  fact  the 
heart  cannot  act  otherwise  than  in  a  periodic  manner. 

Inasmuch  as  there  is  a  conversion  of  all  of  the  potential  energy  into 
kinetic  energy  during  the  systole,  there  is  of  necessity,  a  lowering  of  the 
irritability,  and  to  so  great  an  extent  is  this  the  case  that  the  heart  will  not 
respond  to  the  action  of  a  second  stimulus  either  physiologic  or  artificial 
during  the  systolic  period.     This  non-responsiveness  of  the  heart  may  be 


3IO 


TEXT-BOOK  OF  PHYSIOLOGY 


shown  by  throwing  into  it  a  second  stimulus  at  any  moment  during  the  systole. 
Whatever  the  moment  or  whatever  the  strength  of  the  stimulus  may  be  the 
extent  of  the  contraction  remains  the  same.  During  the  systolic  period  the 
heart  is  said,  therefore,  to  be  refractory  or  non-responsive  to  a  second 
stimulus.  If,  however,  a  second  stimulus  of  average  strength  be  thrown 
into  the  ventricle  at  any  moment  during  the  relaxation,  a  second  contraction 

will  be  developed,  which  is  known  as 
the  extra  systole  (Fig.  138). 

The  Extra  Systole.— The  extent 
of  this  extra  systole  will  be  propor- 
tional to  the  time  at  which  the  stimu- 
lus is  thrown  into  the  ventricle  as  it 
passes  from  the  beginning  to  the  end  of 
its  relaxation.  Whatever  the  extent  of 
the  extra  systole,  its  height  is  no 
greater  than  that  of  the  first  systole. 
For  this  reason  it  is  believed  a  tetanic 
contraction  cannot  be  developed.  If 
the  stimulus  be  thrown  into  the  heart 
just  as  the  relaxation  is  completed,  the 
extra  systole  attains  the  same  height  as 
the  preceding  systole.  In  passing  from 
the  beginning  to  the  end  of  the  relaxa- 
tion and  into  the  diastolic  or  resting 
period,  it  has  been  found  that  the 
extra  systole  can  be  evoked  by  a  stimu- 
lus which  is  steadily  decreased  in  in- 
tensity. It  is  evident  from  this  fact 
that  the  restoration  of  the  energy- 
yielding  material  and  the  return  of  the 
irritability  gradually  increases  from  the 
beginning  of  the  relaxation  to  the  end 
of  the  diastole  (Fig.  139).  For  this 
reason  weak  stimuli  are  more  effective 
in  the  later  than  in  the  earlier  period 
of  the  relaxation  and  the  diastole. 

After  the  development  and  disap- 
pearance of  the  extra  systole  a  consider- 
able pause  in  the  heart's  action  occurs 
to  which  the  term  compensatory  pause 
has  been  given  on  the  assumption  that 
it  was  necessary  on  the  part  of  the 
heart  to  compensate  for  the  disturbance 
of  the  rhythm  by  remaining  at  rest  until  the  time  of  the  next  beat  and  thus 
restore  the  rhythm.  This  was  thought  to  be  a  special  property  of  the  heart- 
muscle.  This  view,  however,  is  no  longer  entertained.  For  if  an  isolated 
ventricle  of  a  frog  heart  be  employed  and  made  to  contract  rhythmically  by 
an  artificial  stimulus,  or  if  a  spontaneously  beating  portion  of  the  dog's 
heart  be  employed  for  experimentation  instead  of  the  whole  heart,  the  re- 
sults of  the  same  methods  of  stimulation  are  different.     Though  an  extra 


Fig.  138. — Myograms  of  the  Frog's 
Ventricle  Showing  the  Effects  of  an 
Induced  Electric  Current  Sent  into 
the  Ventricle  at  Different  Times  of 
THE  Cycle. — {Marey.) 

00'  indicates  the  beginning  of  the  con- 
traction in  each  series.  The  break  in  the 
line  e,  indicates  the  time  the  stimulus  is 
sent  in.  In  i,  2,  3,  the  stimulus  falls  into 
the  ventricle  in  the  non-responsive  period 
or  the  refractory  period,  i.e.,  during  the  sys- 
tole when  the  irritability  has  practically 
disappeared.  From  4  to  8  the  stimulus 
falls  into  the  ventricle  in  the  responsive 
period  or  the  period  of  returning  and  in- 
creasing irritability,  i.e.,  at  the  end  of  the 
systole  and  during  the  diastole.  The  width 
of  the  shaded  lines  is  a  measure  of  the 
latent  period,  i.e.,  the  period  between  the 
time  of  stimulation  and  the  beginning  of 
the  resulting  contraction  which  diminishes 
as  the  diastole  progresses. 


THE  CIRCULATION  OF  THE  BLOOD 


311 


systole  is  called  forth  as  usual,  there  is  no  compensatory  pause;  indeed,  if 
anything  the  pause  is  shorter  than  the  regular  pause.  The  theory  that  a 
compensatory  pause  is  necessitated  for  the  restoration  of  the  normal  rhythm 
is  therefore  not  tenable. 

The  explai)ation  assigned  and  generally  accepted  at  present  for  the 
production  of  a  compensatory  pause  is  as  follows:  In  a  spontaneously 
beating  heart  the  ventricular  systole  is  evoked  by  the  arrival  of  an  excitation 
process  coming  from  the  auricles.  When  the  extra  systole  is  induced  by  an 
artificial  stimulus,  the  next  succeeding  excitation  from  the  auricle  falls  into 
the  refractory  period  and  hence  the  ventricle  is  not  stimulated.  It,  therefore, 
simply  waits  for  the  arrival  of  the  second  succeeding  excitation,  when  it 
responds  and  takes  up  the  regular  rhythm. 

This  fact  is  of  great  interest  clinically  for  it  frequently  happens  that 
extra  systoles  of  the  ventricle  arise  in  the  human  heart  in  conditions  of  the 
circulation  characterized  by  a  high  blood-pressure  and  especially  when  there 


Refracfori/ 


Period. 


Irritadle 


Fig.  139. — Diagram  Showing  the  Variations  of  Irritability  during  the  Systole  and  the 
Diastole. — {Modified  from  Waller.) 

is  coincidently  an  impairment  in  the  irritability  and  contractility  of  the  heart- 
muscle.  Extra  systoles,  however,  may  have  their  origin  in  the  auricular 
walls  as  well. 

If  a  series  of  successive  stimuli  be  thrown  into  the  heart-muscle  the 
effect  will  vary  in  accordance  with  their  time  intervals.  Should  this  be  less 
than  about  three  seconds  there  will  be  a  gradual  increase  in  the  height  for 
some  half  dozen  contractions,  a  result  to  which  the  term  "staircase"  or 
'Ureppe^'  has  been  given.  This  increase  in  the  height  of  the  contraction 
is  attributed  to  an  increase  in  the  irritability  and  contractility  of  the  muscle 
the  result  of  the  primary  stimulating  action  of  fatigue  products. 


THE  NERVE  MECHANISM  OF  THE  HEART 

By  this  term  is  meant  a  combination  of  nerves  and  nerve  centers  which 
cooperate  to  increase  or  decrease  either  the  rate  or  force  of  the  heart's  con- 
traction, and  hence  the  proper  distribution  of  the  blood,  in  accordance  with 
the  activities  of  different  organs  of  the  body.  That  the  heart  is  normally 
influenced  by  nerve  centers  in  response  to  nerve  impulses  transmitted  to 


312 


TEXT-BOOK  OF  PHYSIOLOGY 


them  in  consequence  of  cerebral  (psychic)  or  physiologic  activities  in  different 
parts  of  the  body  is  a  matter  of  personal  experience;  that  the  heart  is  abnor- 
mally influenced  by  the  same  nerve-centers  in  response  to  nerve  impulses 
transmitted  to  them  in  consequence  of  pathologic  and  traumatic  processes 
occurring  in  different  regions  of  the  body,  and  that  both  heart  and  nerves 

Emotional  Centers 

Exhilorating  fetue) 
J)epressing  f^eo) 


Carctio  ■Inhibitor  Center. 


Ganglion  Stellatum 


Inlra'CardlacNerve  Cei 


Canilo  -Accelerator  Center 


sNerve 

^ff^r,>,/\^"^"('"'    (blue) 

"^^'''"'[/nkibitor  (blue) 
Ifftnnt  [inhibitor  (red) 


Sympath  etic  Mroe.s  • 

Acctlerator  i.Auj/nentor 


Fig,  140. — Diagram  of  the  Nerve  Mechanism  of  the  Heart. — (G.  Bachman.) 

are  modified  in  different  ways  by  the  action  of  drugs  mtroduced  into  the 
body,  are  matters  of  daily  clinical  experience. 

The  nerves  comprising  this  mechanism  and  the  relation  they  bear  one  to 
another  are  represented  in  Fig.  140. 

It  was  stated  in  a  previous  paragraph,  page  297,  that  the  contraction  of 
the  heart-muscle  is  independent  of  its  connection  with  the  central  organs  of 


THE  CIRCULATION  OF  THE  BLOOD  313 

the  nerve  system,  and  that  it  will  continue  to  contract  in  a  rhythmic  manner 
for  a  variable  length  of  time  even  after  its  removal  from  the  body  of  the 
animal,  the  length  of  time  varying  w^ith  the  animal  and  the  conditions  to 
which  it  is  subjected;  that  the  stimulus  is  myogenic  and  chemic  in  character, 
the  result  of  a  reaction  between  the  chemic  constituents,  organic  and  in- 
organic, of  the  muscle-cells  and  those  in  the  lymph  by  which  they  are  sur- 
rounded. It  has  also  been  further  shown  that  even  in  the  living  animal 
the  heart  will  continue  to  beat  and  fulfil  its  functions  after  division  of  all 
nerves  in  connection  with  it.  A  dog  thus  experimented  on  lived  for  eleven 
months,  and  beyond  the  fact  of  becoming  fatigued  more  readily  upon  exer- 
tion than  formerly,  exhibited  no  striking  disturbance  of  its  functions. 
Nevertheless  groups  of  nerve-cells  are  present  in  certain  portions  of  the  heart 
in  all  classes  of  vertebrate  animals,  which  bear  an  anatomic  and  physiologic 
relation  to  the  heart-cells  on  the  one  hand,  and  to  the  nerves  connecting 
them  with  the  central  organs  of  the  nerve  system  on  the  other  hand. 

Intra-cardiac  Nerve-cells. — In  the  frog  heart  a  group  of  nerve-cells 
is  found  in  the  sinus  at  its  junction  with  the  auricle,  known  as  the  crescent 
or  ganglion  of  Remak;  a  second  group  is  found  at  the  base  of  the  ventricle 
on  its  anterior  aspect,  and  known  as  the  ganglion  of  Bidder;  a  third  group 
is  found  in  the  auricular  septum,  known  as  the  septal  ganglion,  or  the  gan- 
glion of  V.  Bezold  or  of  Ludwig.  The  majority  of  the  cells  are  situated  on 
the  surface  of  the  heart  just  beneath  the  pericardium.  From  the  cell- 
body  fine  non-medullated  fibers  pass  into  the  substance  of  the  heart,  to 
become  histologically  and  physiologically  related  with  the  muscle-fiber. 

In  the  dog  heart  and  in  the  mammalian  heart  generally,  though  nerve- 
cells  are  present,  they  are  not  arranged  in  such  definite  groups,  but  are  more 
widely  distributed  in  the  terminations  of  the  venae  cavae,  pulmonary  veins, 
the  walls  of  the  auricles,  and  in  the  neighborhood  of  the  base  of  the  ventricles. 

Bxtra-cardiac  Nerves. — The  extra-cardiac  nerves  which  connect  the 
heart  with  the  central  nerve  system  and  through  which  the  activities  of  the 
heart  are  influenced  are  two:  viz.,  the  sympathetic  and  the  vagus  or  pneumo- 
gastric.  Experimental  investigation  has  established  the  fact  that  the  sympa- 
thetic is  the  motor  nerve  to  the  heart,  the  nerve  which  accelerates  the  rate 
and  augments  the  force  of  the  normal  beat;  while  the  vagus  is  the  inhibitor 
nerve,  the  nerve  which  inhibits  or  controls  the  rate  and  the  force  of  the  beat 
in  accordance  with  the  necessities  of  blood  distribution.  For  this  reason 
these  two  nerves  will  be  considered  in  the  order  stated.  The  course  of 
the  fibers  composing  these  nerves,  from  their  origin  to  their  termination,  and 
the  relation  they  bear  to  one  another  and  to  neighboring  structures,  vary 
somewhat  in  different  animals. 

The  Origin  and  Distribution  of  the  Sympathetic  Nerves  in  Mammals. 
— The  sympathetic  nen^e-fibers  which  influence  the  action  of  the  heart,  are 
connected  on  the  one  hand  with  the  heart-muscle  itself  and  on  the  other 
hand  with  nerve-fibers  coming  from  the  central  nerve  system.  The  former 
are  non-medullated  and  post-ganglionic,  the  latter  medullated  and  pre- 
ganglionic. 

The  pre-ganglionic  fibers  have  their  origin  in  the  medulla  oblongata  and 
very  probably  from  nerve-cells  in  the  gray  matter  beneath  the  floor  of  the 
fourth  ventricle.  From  this  origin  they  descend  the  spinal  cord  as  far  as  the 
level  of  the  second,  third,  and  at  times  the  fourth  thoracic  nerves.     At  this 


314  TEXT-BOOK  OF  PHYSIOLOGY 

level  they  emerge  from  the  cord  in  company  with  the  nerve-fibers  composing 
the  anterior  roots  of  the  second  third,  and  fourth  thoracic  nerves.  After 
a  short  course,  they  enter  the  white  rami  communicantes,  then  the  sympa- 
thetic chain  and  pass  upward  to  the  ganghon  stellatum  (the  first  thoracic), 
and  by  way  of  the  annulus  of  Vicussens  (in  the  dog)  to  the  inferior  cervical 
ganglion  as  well,  around  the  nerve-cells  of  both  of  which  their  terminal 
branches  arborize.^  From  the  nerve-cells  of  both  the  stellate  and  inferior 
cervical  ganglia,  the  sympathetic  nerves  proper  arise,  which  after  emerging 
from  the  ganglia  i)ass  toward  the  heart  and  become  associated  with  the  fibers 
of  the  vagus  and  assist  in  the  formation  of  the  cardiac  plexuses.  On  reach- 
ing the  heart  they  may  terminate  directly  in  the  muscle-cell  or  indirectly 
through  the  intermediation  of  intra-cardiac  nerve-cells.  The  former  mode 
of  termination  is  the  more  probable.  Experiment  has  shown  that  both 
the  pre-  and  post-ganglionic  fibers  are  efferent  in  function. 

The  Origin  and  Distribution  of  the  Vagus  Nerve  in  Mammals. — 
The  vagus  nerve-fibers  which  influence  the  heart  are  connected  on  the  one 
hand  with  the  heart,  through  the  intermediation  of  the  intra-cardiac  cells,  and 
on  the  other  hand  with  the  central  nerve  system.  Histologic  investigation  has 
shown  that  the  vagus  nerve-trunk  of  man  and  mammals  generally,  contains 
medullated  fibers  of  large  and  small  size.  Experiment  has  shown  that  the 
large  fibers  are  afferent,  the  small  fibers  efferent  in  function. 

The  large  afferent  fibers  arise  in  the  ganglia  situated  on  the  trunk  of  the 
nerve.  From  their  contained  nerve-cells  a  short  axon  process  proceeds 
which  soon  divides  into  a  central  and  a  peripheral  branch.  The 
central  branch  passes  toward  and  into  the  gray  matter  beneath  the  floor  of 
the  fourth  ventricle,  where  its  end-tufts  arborize  around  nerve-cells;  the 
peripheral  branch  passes  toward  the  general  periphery  to  be  distributed  to 
the  mucous  membrane  of  the  lungs,  stomach,  intestine,  etc. 

The  small  efferent  fibers  are  the  peripherally  coursing  axons  of  nerve- 
cells  situated  in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle  at 
the  tip  of  the  calamus  scriptorius.  The  exact  course  of  these  fibers  from 
their  origin  into  the  trunk  of  the  vagus  is  not  positively  known.  According 
to  some  investigators,  they  leave  the  medulla  by  way  of  the  spinal  accessory 
nerve  and  enter  the  trunk  of  the  vagus  through  its  internal  or  anastomotic 
branch;,  according  to  recent  investigations  made  by  Schaternikoff  and 
Friedenthal,  they  leave  the  medulla  along  the  path  by  which  the  afferent 
fibers  enter  and  never  become  associated  with  the  spinal  accessory  nerve 
at  its  origin. 

In  the  neighborhood  of  the  inferior  or  recurrent  laryngeal  nerves,  branches 
containing  efferent  fibers  are  given  off,  which  pass  to  the  heart  by  way  of  the 
cardiac  plexus.  The  terminal  branches  of  these  fibers  are  not  distributed 
directly  to  the  heart-muscle,  but  to  the  intra-cardiac  nerve-cells,  around 
the  bodies  of  which  they  end  in  basket-like  formations.  The  fibers  in  the 
vagus  are  pre-ganglionic;  those  of  the  nerve-cells  post-ganglionic.  (See 
Fig.  140.) 

The  Origin  and  Distribution  of  the  Sympathetic  and  Vagus  Nerves 
in  the  Frog. — In  thtfrog  and  allied  animals  the  relation  of  these  two  sets  of 
nerve-fibers,  viz.,  the  efferent  sympathetic  fibers  and  the  efferent  vagus 

^In  man  the  annulus  of  Vieussens  connects  the  inferior  cervical  with  the  middle  cervical 
ganglion. 


THE  CIRCULATION  OF  THE  BLOOD  315 

fibers,  is  somewhat  different;  and  because  of  the  fact  that  these  nerves  in  this 
animal  are  largely  employed  for  determining  experimentally  their  respective 
actions  on  the  heart,  this  relation  should  be  clearly  understood. 

The  sympathetic  nerve-fibers  in  this  animal  are  also  in  connection  with 
the  heart  on  the  one  hand  and  with  nerve-fibers  coming  from  the  central 
nerve  system  on  the  other  hand.  The  pre-ganglionic  fibers  take  their  origin 
very  probably  in  ners'e-cells  in  the  medulla  oblongata.  From  this  origin 
they  descend  and  emerge  from  the  spinal  cord  in  the  anterior  roots  of  the 
third  spinal  nerve,  then  pass  through  the  white  rami  communicantes  to  the 
third  sympathetic  ganglion  around  the  nerve-cells  of  which  their  terminal 
fiber^  arborize. 

From  the  nerve-cells  of  this  ganglion,  the  sympathetic  nerves  proper, 
the  post-ganglionic,  non-medullated  fibers  arise.  From  this  origin  they 
ascend,  passing  successively  through  the  second  sympathetic  ganglion,  the 
annulus  of  Vieussens,  the  first  sympathetic  ganglion,  to  the  ganglion  on  the 
trunk  of  the  vagus,  at  which  point  they  enter  the  sheath  of  the  vagus  fibers 
and  in  company  with  them  pass  to  the  heart.  For  this  reason  the  common 
trunk  is  generally  spoken  of  as  the  vagosympathetic  nerve. 

The  vagus  nerve  is  connected  with  the  medulla  oblongata  by  a  series 
of  from  six  to  eight  roots.  A  short  distance  from  the  medulla,  the  nerve 
trunk  passes  through  a  large  opening  in  the  cranium  beyond  which  it  presents 
an  enlargement,  termed  the  vagus  ganglion.  The  peripheral  end  of  this 
ganglion  gives  off  two  trunks,  one  the  glossopharyngeal,  the  other  the  vagus 
proper. 

The  vagus  nerve  proper  in  the  frog  also  consists  of  both  afferent  and 
efferent  fibers  which  have  practically  the  same  origin,  distribution  and 
termination  as  the  corresponding  fibers  in  the  mammal. 

After  the  union  of  the  sympathetic  fibers  with  the  vagus  fibers,  the  com- 
mon trunk  passes  forward  to  the  angle  of  the  jaw,  winds  around  the  pharynx 
just  beneath  the  border  of  the  petro-hyoid  muscle  and  in  close  relation  with 
the  carotid  artery.  As  the  nerve  approaches  the  heart  it  divides  into  two 
branches,  the  pulmonary  and  the  cardiac.  At  the  sinus  venosus  some  of 
the  fibers  become  related,  histologically  and  physiologically,  with  the  ganglion 
cells,  while  others  plunge  into  the  heart,  course  along  the  auricular  septum 
on  the  left  side  and  finally  terminate  at  or  near  the  ganglion  cells  of  the  base 
of  the  ventricle.  The  mode  of  termination  of  both  the  vagus  and  sympa- 
thetic fibers  is  similar  to  that  obsen^d  in  the  mammals. 

The  Physiologic  Actions  of  the  Sympathetic  Nerves  in  the  Frog. — 
The  information  now  possessed  regarding  the  influence  which  the  central 
nerve  system  exerts  on  the  heart  through  these  nerves,  has  been  derived 
largely  from  experiments  made  on  the  nerA-es  of  the  frog,  toad,  and  turtle. 
Inasmuch  as  the  sympathetic  and  vagus  nerves  in  the  frog  and  related 
animals  are  bound  up  in  a  common  sheath,  it  is  necessary  in  order  to  demon- 
strate their  respective  functions  first  to  divide  the  nerves,  above  their  union 
at  the  vagus  ganglion,  and  then  stimulate  their  peripheral  ends.  The  heart 
should  be  exposed  and  attached  to  a  recording  lever  so  that  its  movements 
may  be  taken  up  and  recorded  on  a  moving  recording  surface. 

Stimulation  of  the  sympathetic  fibers  with  induced  electric  currents, 
prior  to  their  union  with  the  vagus,  is  followed  by  an  increase  in  the  rate, 
or  an  augmentation  in  the  force  of  the  heart-beat  or  both,  at  the  same  time. 


3i6 


TEXT-BOOK  OF  PHYSIOLOGY 


The  effects  of  such  a  stimulation  with  induced  currents  of  moderate  intensity 
are  graphically  shown  in  Fig.  141 .  The  upper  tracing  shows  that  the  heart  was 
first  accelerated,  the  beats  increasing  from  15  per  minute  before  stimulation, 
to  30  per  minute  during  stimulation.  On  the  cessation  of  the  stimulation, 
the  heart  slowly  returned  to  its  former  rate.  Coincidently  with  this  accelera- 
tion of  the  rate  there  was  an  augmentation  of  the  force  of  the  ventricular 
contraction  as  shown  by  an  increase  in  the  height  of  the  ventricular  con- 
traction which  before  stimulation  was  9  mm.,  but  during  stimulation  12  mm. 
In  addition  to  the  foregoing  changes  in  the  heart-beat  there  is  an  altera- 
tion in  the  sequence  of  the  beat.  The  natural  delay  in  the  conduction  of  the 
excitation  process  from  the  auricles  to  the  ventricle  is  increased,  in  conse- 
quence of  which  the  auricle  completely  relaxes  before  the  ventricular  con- 
traction begins.     Moreover,  the  auricular  contraction  again  occurs  before 


Fig.  141. — Tracings  Showing  the  Effects  on  the  Heart-beat  of  the  Frog  from  Stimu- 
lation OF  THE  Sympathetic  Nerves  Prior  to  Their  Union  with  the  Vagus  Nerve.  The 
upper  tracing  shows  an  mcrease  in  the  rate,  which  before  stimulation  was  15  per  minute  and 
during  stimulation  30  per  minute.  Before  stimulation  the  height  of  the  ventricular  beat  was 
9  mm.  and  during  the  stimulation  it  was  12  mm.  The  lowest  tracing  shows  a  similar  series  of 
effects,  the  differences  being  only  of  degree. — {Brodie.) 

the  ventricle  has  completely  relaxed.  After  the  effect  of  the  stimulation 
passes  away,  the  acceleration  diminishes,  the  augmentation  declines  and  a 
reverse  change  in  the  sequence  occurs.  The  lower  tracing  shows  a  similar 
series  of  effects.  If  the  stimulus  be  applied  to  the  pre-ganglionic  sympathetic 
nerves,  an  acceleration  or  augmentation  of  the  heart  follows,  similar  in  all 
respects  to  that  which  follows  stimulation  of  the  post-ganglionic  or  sympa- 
thetic fibers  proper;  and  the  inference  may  be  drawn  that  if  the  stimulus 
could  be  applied  directly  to  the  nerve-cells  in  the  medulla  oblongata  from 
which  the  fibers  take  their  origin,  the  same  acceleration  or  augmentation 
would  follow;  for  this  reason  this  collection  of  nerve-cells  is  known  as  the 
cardio-accelerator  or  augmentor  center.  Since  stimulation  of  the  nerve  in 
any  part  of  its  course,  which  in  all  probability  exaggerates  its  normal  function, 
is  followed  by  an  acceleration  or  an  augmentation,  the  sympathetic  is  said 


THE  CIRCULATION  OF  THE  BLOOD 


317 


to  have  an  accelerator  or  an  augmentor  influence  on  the  heart-beat;  with 
the  cessation  of  the  stimulation,  and  very  frequently  before,  the  heart  returns 
to  its  normal  condition. 

The  Physiologic  Action  of  the  Vagus  Nerve  in  the  Frog. — Stimulation 
of  the  intra-cranial  roots  of  the  vagus  with  very  weak  induced  electric  cur- 
rents is  followed  by  a  gradual  diminution  in  the  rate  and  a  diminution  in  the 
force  of  the  heart-beat.  If  the  induced  currents  are  moderate  in  strength, 
the  heart  will  at  once  come  to  a  standstill  in  diastole.  (Fig.  142.)  If  the 
stimulus  be  applied  to  the  trunk  or  the  peripheral  portion  of  the  vagus,  for 
example  close  to  the  sinu-auricular  junction,  an  inhibition  occurs  similar  in 


Fig.  142. — Tracing  showing  the  Effect  on  the  Heart-beat  of  the  Toad  of  Long 
Stimulation  of  tee  Intra-cranial  Roots  of  the  Vagus  with  Moderately  Strong  Electric 
Currents. — (Gaskell.) 

all  respects  to  that  which  follows  stimulation  of  the  intra-cranial  roots,  and 
judging  from  what  is  known  regarding  the  action  of  nerve-cells,  the  inference 
may  be  drawn  that  if  the  stimulus  could  be  applied  directly  to  the  group  of 
nerve-cells  from  which  the  efferent  libers  arise,  the  same  inhibition  would 
follow;  for  this  reason  this  collection  of  nerve-cells  is  known  as  the  cardio- 
inhihitor  center.  Since  stimulation  of  the  nerve,  either  at  its  center,  in  its 
course,  or  at  its  periphery,  which  in  all  probability  exaggerates  its  normal 
function,  is  followed  by  a  period  of  rest  or  inactivity,  the  vagus  is  said  to 
have  a  retarding  or  an  inhibitor  influence  on  the  beat  of  the  heart. 

During  the  continuance  of  the  inhibition,  the  heart-muscle  is  relaxed, 


Fig.  143.— Tracing  showing  the  Diminution  in  the  Rate  of  the  Heart-beat 
following  Weak  Tetanization  of  the  Vagus  Trunk. 

its  cavities  dilated  and  filled  with  blood.  The  dilatation  usually  exceeds 
that  observed  prior  to  the  vagus  stimulation,  from  which  it  is  inferred 
that  some  fibers  of  the  vagus  at  least  diminish  the  tonicity  of  the  heart- 
muscle. 

After  cessation  of  the  stimulation,  the  heart  resumes  its  activity.  At 
first  the  beat  usually  is  slow  and  feeble,  but  with  each  succeeding  beat  both  the 
rate  and  force  increase,  until  they  attain  or  exceed  that  observed  prior  to  the 
stimulation.  In  some  cases,  however,  the  heart  begins  to  beat  with  as  much 
and  even  more  vigor  than  it  did  prior  to  the  stimulation.  The  duration  of 
the  inhibitor  effect  varies  with  the  duration  of  the  stimulation.     Thus  during 


3i8  TEXT-BOOK  OF  PHYSIOLOGY 

and  after  a  stimulation  of  thirty-eight  seconds  the  heart  of  the  toad  remained 
at  rest  for  292  seconds  (Gaskell);  the  heart  of  a  snake  for  from  one-half  to 
one  hour  (Meyer);  the  heart  of  a  turtle  for  four  and  a  half  hours  (Mills). 
The  period  of  inhibition  will  depend  on  the  strength  of  the  electric  current 
employed,  the  nerve  stimulated,  the  season  of  the  year,  etc. 

The  effects  on  the  heart-beat  which  will  follow  stimulation  of  the  vago- 
sympathetic in  its  course  vary,  however,  because  of  the  antagonistic  action 
of  the  inhibitor  and  accelerator  nerve  impulses.  Thus  stimulation  of  the 
peripheral  end  of  the  divided  trunk  of  the  vagus  in  the  frog  or  the  toad  with 
weak  tetanizing  induced  electric  currents  is  followed  by  an  increase  in  the 
rate  of  the  heart-beat  because  of  the  stimulation  of  the  accelerator  fibers, 
which  apparently  respond  before  the  inhibitor  fibers;  stimulation  with  some- 
what stronger  currents  is  followed  by  a  diminution  in  the  rate  of  the  beat 
because  of  the  greater  effect  on  the  inhibitor  nerve-fibers  (Fig.  143).  Stimu- 
lation with  strong  tetanizing  currents  is  followed  by  complete  inhibition 
(Fig.  144). 

The  foregoing  facts  lead  to  the  inference  that  the  cardio-accelerator 
and  the  cardio-inhibitor  centers  have  as  their  function  the  discharge  of  nerve 
impulses  which  are  conducted  by  their  related  nerves,  the  efferent  sympathetic 


Fig.  144. — Tracing  showing  Complete  Inhibition  following  Strong  Tetan- 

IZATION  OF  THE  VaGUS  TrUNK. 

and  vagal  fibers,  to  the  heart,  and  which,  in  a  manner,  as  yet  unexplained 
accelerate  or  augment  or  inhibit,  the  action  of  the  heart.  The  relation  which 
these  two  centers  bear  one  to  the  other  and  the  manner  in  which  they  are 
influenced  in  their  activities  both  directly  and  reflexly  and  thus  regulate  the 
action  of  the  heart  from  moment  to  moment  will  be  considered  in  a  subse- 
quent paragraph. 

Changes  in  the  Conductivity  of  the  Heart. — In  addition  to  the 
changes  in  the  rate  and  force  of  the  heart  caused  by  stimulation  of  the  inhib- 
itor and  the  augmentor  nerves,  it  is  stated  by  Gaskell  that  there  is  also  during 
the  inhibition  a  decrease  in  the  conductivity  of  the  heart  at  both  the  sino- 
auricular  and  auriculo-ventricular  junctions,  and  an  increase  in  the  con- 
ductivity during  acceleration  of  the  beat.  The  decrease  in  conductivity 
may  be  so  pronounced  that  only  every  second  or  third  contraction  of  the 
auricle  will  be  followed  by  a  contraction  of  the  ventricle.  In  other  instances 
both  auricles  and  ventricles'  remain  at  rest  while  the  sinus  maintains  its  usual 
rate.  The  increase  in  conductivity  is  shown  by  first  artificially  blocking  the 
contraction  wave  at  the  auriculo-ventricular  junction  with  the  clamp,  until 
only  every  second  or  third  auricular  contraction  is  conducted  to  the  ventricle, 
and  then  stimulating  the  sympathetic.  At  once  the  auricular  contraction 
forces  the  block,  and  passes  to  the  ventricle,  calling  forth  a  normal  contraction. 


THE  CIRCULATION  OF  THE  BLOOD 


319 


The  Physiologic  Actions  of  the  Sympathetic  Nerves  in  Mammals. — 

In  the  mammal  the  actions  of  the  sympathetic  nerves  closely  resemble  their 
action  in  the  frog.  Thus  stimulation  of  the  nerves  in  any  part  of  their 
course,  either  through  the  rami  communicantes,  or  after  their  emergence 
from  either  the  stellate  or  the  inferior  cervical  ganglion  is  followed  by  effects 
similar  to  those  observed  in  the  frog:  viz.,  an  acceleration  or  augmentation, 
or  both,  of  the  heart-beat.  The  percentage  increase  in  the  acceleration 
varies  in  different  animals.  In  some  instances  the  increase  varies  from 
58  per  cent,  to  100  per  cent.  (Hunt).  If  the  heart  is  beating  slowly  before 
stimulation,  the  acceleration  is  more  marked  than  if  it  is  beating  rapidly. 


\j\J^J\J\^'^^wv^^ft^^^^^«v^nw/«>^^^ 


Ftg.  145. — Acceleration  of  the  Heart-be^t  following  Stimulation  of  the  Cardiac 
Branches  which  come  from  the  Annulus  of  Vieussens. 


The  effect  of  the  accelerator  impulses  is  apparently  a  change  in  the  inner 
mechanism  of  the  heart-muscle  itself  and  not  a  change  in  the  peripheral 
portion  of  the  inhibitor  apparatus.  This  is  indicated  by  the  fact  that 
acceleration  occurs  after  the  full  physiologic  action  of  atropin,  which  acts 
upon,  and  impairs  the  conductivity  of  the  intra-cardiac  nerve-cell  (post- 
ganglionic) terminals. 

A  peculiarity  of  the  sympathetic  nerve  is  that  it  does  not  respond  to 
stimulation  as  rapidly  as  do  many  nerves,  so  that  a  rather  long  latent  period 
intervenes  between  the  moment  of  stimulation  and  the  appearance  of  the 
acceleration  as  shown  in  Fig.  145.  A  further  peculiarity  is  that  the  accelera- 
tion sometimes  continues  after  the  stimulus  is 
withdrawn,  and  sometimes  cea;ses  before  it  is 
withdrawn. 

The  force  of  the  heart-beat  may  be  in- 
creased without  there  being  any  increase  in 
the  rate  (Fig.  146).  The  auricular  contraction 
is  probably  increased  whereby  the  ventricle  is 
more  completely  filled. 

Though  an  increase  in  both  the  rate  and 
force  frequently  occur  simultaneously,  there  is 
no  necessary  relation  or  connection  between 
the  two  as  they  can  and  do  occur  independ- 
ently of  each  other.  For  this  reason  it  is  generally  assumed  that  the  sym- 
pathetic nerves  contain  two  groups  of  fibers,  viz.,  accelerators  and  augmen- 
tors,  the  functions  of  which  are  respectively  to  accelerate  the  rate  and 
augment  the  force  of  the  heart-beat.  From  the  fact  that  both  auricles  and 
ventricles  exhibit  these  changes  it  is  assumed  that  the  nerve  impulses  stimu- 
late both  chambers.  This  is  rendered  probable  also  from  the  experiments 
of  Erlanger,  who  found  that  after  complete  heart-block,  stimulation  of 
the  sympathetic  caused  independent  acceleration  of  both  auricles  and 
ventricles. 

Division  of  the  sympathetic  nerves  is  at  once  followed  by  a  diminution 
in  the  rate,  the  degree  of  which  will  depend   to  some  extent  on  the  rate 


Fig.  146. — Increase  in  the 
Force  of  the  Ventricular  Con- 
traction (Curve  of  Pressure  in 
Right  Ventricle)  from  Stimu- 
lation OF  AUGMENTOR  FiBERS. 
There  is  little  or  no  change  in 
frequency. —  {Francis,  reduced.) 


320  TEXT-BOOK  OF  PHYSIOLOGY 

at  which  the  heart  was  beating  prior  to  the  division.  The  results  therefore 
that  follow  stimulation  and  division  of  these  nerves  indicate  that  they  are 
transmitting  nerve  impulses  from  the  centers  from  which  they  arise  to  the 
heart  upon  which  they  exert  a  stimulating  influence  on  the  rate  and  force 
'Of  the  beat. 

The  Physiologic  Action  of  the  Vagus  Nerve  in  Mammals. — In  the 
mammal  the  same  or  similar  effects  in  varying  degree  result  from  stimu- 
lation of  the  vagus  as  in  the  frog.  These  results  can  be  readily  shown  in 
the  dog  or  rabbit  in  the  following  way.  The  thorax  of  the  animal  is  opened 
and  artificial  respiration  maintained.  Under  these  circumstances  the  heart 
will  Continue  to  beat  in  a  practically  normal  manner  for  a  long  time.  The 
vagus  nerve  is  then  exposed  on  one  side  and  divided,  and  its  peripheral  end 
stimulated  with  induced  electric  currents  of  moderate  strength;  whereupon 
the  heart  will  come  to  a  standstill  almost  immediately  in  the  condition  of 
diastole,  and  may  be  so  kept  for  a  variable  period,  from  fifteen  to  thirty 
seconds  or  more,  during  which  its  walls  are  relaxed  and  its  cavities  filled 
with  blood.  On  cessation  of  the  stimulation  the  contractions  return  and  in 
a  very  short  time  the  former  rate  and  force  of  the  beat  are  regained.  If 
the  electric  currents  are  of  feeble  strength,  the  heart  will  come  to  rest 
gradually,  through  a  gradual  diminution  in  the  rate  and  force  of  the  con- 
traction. During  the  period  of  the  inhibition  the  heart  presents  an 
appearance  similar  to  that  presented  by  the  heart  of  the  cold-blooded 
animal,  viz. :  completely  relaxed  walls  and  the  cavities  filled  and  distended 
with  blood.  When  the  heart  of  an  animal  is  thus  exposed,  the  auricle 
and  the  ventricle  of  one  side  may  be  attached  by  threads  to  writing  levers 
and  their  contractions  registered  on  a  moving  recording  surface.  The 
effects  on  both  auricles  and  ventricles  which  follow  vagus  stimulation  will 
then  become  more  apparent.  Fig.  147  is  a  tracing  thus  obtained.  The 
animal  employed  for  the  experiment  was  a  rabbit. 

Division  of  one  vagus  is  followed  in  some  mammals,  e.g.,  dog  by  a  marked 
increase  in  the  rate  of  beat  and  if  both  vagi  are  divided  the  increase  may 
amount  to  from  50  to  75  per  cent.  The  results  of  stimulation  and  division 
of  the  vagus  nerves  indicate  that  they  are  continuously  transmitting  nerve 
impulses  from  the  centers  from  which  they  arise,  to  the  heart-muscle,  on 
the  activity  of  which  they  exert  a  restraining  or  inhibitor  influence. 

The  inhibitor  effect  of  the  vagus  varies  in  degree  and  duration  in 
different  animals.  In  the  dog  the  effect  of  vagus  stimulation  is  usually 
pronounced,  lasting  from  15  to  30  seconds;  in  the  rabbit  it  is  perhaps  equally 
well  pronounced  but  somewhat  less  in  duration ;  in  the  cat  it  is  frequently  want- 
ing. In  this  latter  animal  a  complete  standstill,  even  for  a  few  seconds,  is 
very  rarely  seen ;  usually  there  is  produced  merely  a  slight  diminution  in  the  rate 
of  the  beat  even  though  the  stimulus  employed  is  quite  strong.  In  all  these 
animals,  however,  after  a  very  short  time  the  nerve  impulses  lose  their 
inhibitor  influence  on  the  heart-muscle,  and  notwithstanding  continued 
stimulation  of  the  vagus,  the  heart  returns  to  its  former  rate  and  vigor. 
This  result  is  in  marked  contrast  to  that  observed  during  stimulation  of  the 
vagus  in  the  cold-blooded  animals,  in  which  the  heart  may  be  kept  at  rest 
for  relatively  very  long  periods  of  time.  No  satisfactory  explanation  for 
this  loss  of  vagus  control  or  escape  of  the  heart  from  the  vagus  control  has 
as  yet  been  offered. 


THE  CIRCULATION  OF  THE  BLOOD 


321 


Seat  of  Action  of  the  Vagus  Impulses. — In  the  foregoing  experiment  of 
which  Fig.  147  is  a  graphic  record,  stimulation  of  the  left  vagus  with  a  fairly 
strong  current  was  followed  by  a  diminution  in  both  the  rate  and  force  of  the 
contraction  of  both  auricles  and  ventricles,  though  the  effect  was  most 
marked  in  the  auricles.  From  this  and  similar  facts  it  has  come  to  be  the 
general  belief  that  the  inhibitor  nerve  impulses  exert  their  influence  mainly, 
if  not  exclusively,  on  the  auricle,  and  especially  on  the  sino-auricular  node, 


Fig.  147. — Result  of  the  Stimulation  of  the  Peripheral  End  of  the 
Divided  Left  Vagus  in  the  Rabbit. — (Brodie.) 

and  that  the  cessation  of  ventricular  action  is  a  secondary  effect  due  to  the 
non-arrival  across  the  conducting  apparatus  of  the  normal  excitation  process 
from  the  auricle.  This  is  the  case  undoubtedly  in  the  cold-blooded  animals, 
and  the  experiments  of  Erlanger  on  the  heart  of  the  dog  indicate  that  the 
same  holds  true  for  the  mammals.  This  investigator  has  found  that  when 
the  auriculo-ventricular  tissues  are  suddenly  clamped,  including  presumably 
the  muscle  bundle  of  His,  there  is  for  a  time  a  complete  cessation  of  ven- 


322  TEXT-BOOK  OF  PHYSIOLOGY 

tricular  activity,  but  after  a  variable  period  of  time,  fifty  seconds  or  more, 
the  ventricle  develops  an  independent  rhythm  which  gradually  increases  in 
frequency,  but  seldom,  if  ever,  attains  that  of  the  auricles.  Under  such 
circumstances  tetanic  stimulation  of  the  auriculo-ventricular  tissues  by 
means  of  the  clamp  now  transformed  into  stimulating  electrodes,  failed  to 
bring  about  a  stoppage  of  the  ventricles.  Moreover,  if  during  the  time  the 
clamp  is  applied  and  after  the  ventricle  has  developed  a  rhythm  of  its  own, 
the  vagus  is  stimulated,  the  auricles  will  cease  to  beat  as  usual,  but  the  ven- 
tricles will  continue  to  beat  at  their  usual  rate.  These  and  similar  facts 
lead  to  the  conclusion  that  vagal  inhibitor  action  is  limited  to  the  auricles. 

From  foregoing  facts  it  is  apparent  that  the  accelerator  and  augmentor 
effects  of  the  sympathetic  nerve  impulses,  and  the  inhibitor  effects  of  the 
vagus  nerve  impulses,  closely  resemble  on  the  one  hand,  the  accelerator  and 
augmentor  effects  of  increasing  amounts  of  diffusible  calcium  salts,  and  on 
the  other  hand,  the  inhibitor  effects  of  increasing  amounts  of  diffusible 
potassium  salts  in  the  blood  or  other  circulating  fluid;  and  so  closely  do 
these  two  sets  of  phenomena  resemble  each  other,  that  they  are  by  some 
observers  regarded  as  identical. 

Some  additional  facts  in  this  connection  have  been  presented  by  Howell, 
viz.,  that  an  increase  (within  limits)  and  a  decrease  in  the  percentage  of 
diffusible  calcium  salts  in  a  circulating  fluid  passing  through  the  cavities  of 
the  mammalian  (cat)  heart,  increases  on  the  one  hand,  and  decreases  on 
the  other  hand,  the  sensitiveness  of  the  heart  to  sympathetic  acceleration 
and  augmentation.  From  this  the  inference  is  deduced  that  the  acceleration 
and  augmentation  of  the  heart-beat  which  follow  stimulation  of  the  sympa- 
thetic nerves  are  due  to  the  presence  in  the  heart  tissue  of  a  certain  percentage 
of  diffusible  calcium  salts,  which  have  been  freed  from  combination  with 
organic  matter  by  the  action  of  the  sympathetic  nerve  impulses.  Again, 
that  an  increase  (within  limits)  and  a  gradual  decrease  in  the  percentage 
of  diffusible  potassium  salts  in  a  circulating  fluid  passing  through  the  cavities 
of  the  frog  and  the  cat  heart,  increases  on  the  one  hand  and  decreases  and 
finally  abolishes  on  the  other  hand  the  sensitiveness  of  the  heart  to  vagus 
inhibition.  From  this  the  inference  is  deduced  that  the  inhibition  of  the 
heart-beat  which  follows  stimulation  of  the  vagus  nerve  is  due  to  the  presence 
in  the  heart  tissue  of  a  certain  percentage  of  diffusible  potassium  salts, 
which  have  been  freed  from  combination  with  organic  matter  by  the  action 
of  the  vagus  nerve  impulses. 

The  foregoing  effects  of  the  sympathetic  and  vagus  nerves  on  the  heart 
muscle,  viz.:  changes  in  its  irritability,  conductivity,  rapidity,  and  the 
energy  of  the  beat,  have  been  termed  by  Engelmann  bathmo tropic,  dromo- 
tropic,  chronotropic,  and  inotropic.  Any  one  of  these  effects,  e.g.,  the  chrono- 
tropic, may  be  modified  in  a  positive  direction  by  the  sympathetic,  or  in  a 
negative  direction  by  the  vagus. 

The  Cardio-accelerator  Center. — The  collection  of  nerve-cells  from 
which  the  pre-ganglionic  fibers  of  the  sympathetic  system  arise  is  known  as 
the  cardio-accelerator  or  augmentor  center.  The  exact  location  of  this 
center  in  the  central  nerve  system  has  not  been  as  yet  accurately  determined. 
It  is  probably  located  in  the  medulla  oblongata. 

From  experiments  which  have  been  made  on  the  sympathetic  nerve 
apparatus  in  its  entirety,  it  is  believed  that  the  function  of  this  center  is  the 


THE  CIRCULATION  OF  THE  BLOOD  323 

discharge  of  nerve  impulses  which,  conducted  to  the  heart  by  the  pre- 
ganglionic and  post-ganglionic  sympathetic  fibers,  cause  an  acceleration  in 
the  rate  or  an  augmentation  in  the  force,  or  both,  of  the  heart-beat.  It  is 
also  generally  believed  since  the  publication  of  Hunt's  investigations  that 
this  center  is  in  a  state  of  tonic  activity.  This  is  shown  by  the  fact  that  after 
the  division  of  the  vagus  nerv^es  and  the  removal  of  all  possible  inhibitor 
influences,  division  of  the  sympathetic  nerves  or  extirpation  of  the  stellate  or 
inferior  cervical  ganglion,  is  yet  followed  by  a  decrease  in  the  rate  of  the  heart- 
beat. After  division  of  the  sympathetic  nerv^es  and  the  removal  of  accelerator 
influences  it  is  also  easier  to  bring  about  inhibition  through  vagus  stimulation. 

The  Factors  which  Determine  the  Activity  of  the  Cardio-accelera-' 
tor  Center. — The  question  has  been  raised  as  to  whether  the  tonic  activity 
of  this  center  is  maintained  by  central  or  peripheral  stimuli,  i.e.,  whether  it 
is  maintained  by  causes  within  itself,  the  result  of  an  interaction  between  the 
constituents  of  the  cell  substance  and  those  of  the  surrounding  lymph,  or 
whether  it  is  maintained  by  nerve  impulses  reflected  to  it  through  various 
afferent  or  sensor  nerves.  Inasmuch  as  there  is  no  way  of  determining 
whether  the  causes  are  central,  except  by  dividing  all  afferent  nerves,  it  is 
impossible  to  state  how  much  influence  is  to  be  attributed  to  this  factor. 
On  the  contrary,  though  it  is  readily  demonstrable  that  stimulation  of  many 
afferent  nerves  will  cause  an  acceleration  of  the  heart  it  cannot  be  stated 
positively  that  this  is  the  result  of  a  reflex  stimulation  of  the  accelerator  center. 
Though  earlier  investigators  believed  this  to  be  the  correct  interpretation, 
the  more  recent  experiments  of  Hunt  apparently  disprove  it;  for  this  investi- 
gator has  shown  that  if  the  vagus  nerves  are  divided  it  is  impossible  to  pro- 
duce reflex  acceleration  of  the  heart.  His  conclusion,  confirming  that  of 
others,  is  that  cardiac  acceleration  is  the  result  of  an  inhibition  of  the  cardio- 
inhibitor  center.  A  freer  play  to  the  tonic  activity  of  the  accelerator  center 
would  thus  be  made  possible. 

The  Cardio-inhibitor  Center. — The  collection  of  nerve-cells  from  which 
the  small  efferent  fibers  of  the  vagus  nerve  arise  is  known  as  the  cardio-in- 
hibitor center.^  It  is  situated  in  the  medulla  oblongata  or  more  exactly  in 
the  gray  matter  beneath  the  floor  of  the  fourth  ventricle  near  the  tip  of  the 
calamus  scriptorius.     It  is  in  all  probability  a  part  of  the  nucleus  ambiguus. 

From  the  experiments  which  have  been  made  on  the  vagus  inhibitor 
apparatus  in  its  entirety  it  is  believed  that  the  function  of  this  center  is  the 
discharge  of  nerve  impulses  which  conducted  to  the  heart  by  the  vagus 
fibers  cause  an  inhibition  of  its  beat  of  greater  or  less  extent.  In  the  dog, 
and  probably  in  many  other  mammals,  this  center  exerts  a  more  or  less 
constant  inhibitor  or  restraining  influence  on  the  heart's  activity.  This  is 
indicated  by  the  fact  that  the  rate  of  the  beat  is  very  much  increased  by 
simultaneous  division  of  both  vagi.  The  degree  of  the  inhibition  which  this 
center  exerts  varies  greatly,  however,  in  different  animals.  In  the  cat  and 
in  the  rabbit  the  inhibitor  control  is  normally  so  slight  that  there  is  but  a 
relatively  slight  increase  in  the  rate  of  the  beat  after  division  of  the  vagi. 
The  tone  of  the  vagus  in  these  animals  is,  therefore,  said  to  be  sUght  or 
feeble.     In  human  beings  the  tone  of  the  inhibitor  apparatus  is  poorly 

1  Inasmuch  as  the  nerve  system  is  bilaterally  arranged  there  are  of  course  two  nerve-centers 
here  as  elsewhere,  one  on  each  side  of  the  median  Une  though  they  are  associated  in  action  by 
either  commissural  fibers  or  bv  a  decussation  of  some  of  their  efferent  fibers. 


324  TEXT-BOOK  OF  PHYSIOLOGY 

developed  in  early  childhood,  as  shown  by  the  fact  that  the  administration 
of  atropin,  which  removes  temporarily  inhibitor  control,  is  not  followed  by  an 
increase  in  the  rate  of  the  beat.  It  develops  steadily  and  reaches  a  maximum 
at  from  the  twenty-fifth  to  the  thirtieth  year.  In  advanced  years  the  tone 
again  declines.  For  these  and  other  reasons  it  is  believed  that  this  center 
is  in  a  state  of  tonic  activity  in  many  if  not  all  mammals,  discharging  nerve 
impulses  which  exert  a  regulative  influence  on  the  cardiac  mechanism  in  ac- 
cordance with  its  needs  and  especially  in  reference  to  the  variable  resistances 
offered  to  the  flow  of  blood  which  the  heart  must  overcome. 

The  Factors  which  Determine  the  Activity  of  the  Cardio-inhibitor 
Center. — The  question  has  also  been  raised  as  to  whether  the  tonic  activity 
of  this  center  is  maintained  by  central  or  peripheral  stimuli,  i.e.,  whether  it  is 
maintained  by  causes  within  itself  the  result  of  an  interaction  between  the 
constituents  of  the  cell  substance  and  those  of  the  surrounding  lymph,  or 
whether  it  is  maintained  by  nerve  impulses  transmitted  to  it  through  various 
afferent  or  sensor  nerves.  Though  both  factors  play  an  important  part  in 
the  maintenance  of  its  activity,  it  is  difficult  to  state  which  is  the  more 
important  of  the  two.  That  nerve  impulses  transmitted  to  the  center 
through  afferent  nerves  lead  now  to  an  inhibition,  now  to  an  acceleration 
of  the  heart  has  been  abundantly  established  by  the  stimulation  of  afferent 
nerves  in  almost  any  region  of  the  body.  Thus  stimulation  of  the  dorsal 
roots  of  the  spinal  nerves,  the  trunks  of  the  cranial  sensor  nerves,  the 
splanchnic  nerves,  the  pulmonary  branches  of  the  vagus,  etc.,  gives  rise  to  a 
more  or  less  pronounced  inhibition  of  the  heart.  On  the  other  hand 
stimulation  of  afferent  nerves  sometimes  leads  to  the  opposite  result,  namely, 
acceleration  of  the  heart.  As  a  rule,  stimulation  of  the  peripheral  termina- 
tions of  these  nerves  is  more  effective  than  stimulation  of  their  trunks. 
From  facts  such  as  these,  it  has  come  to  be  believed  that  nerve  impulses 
developed  in  many  regions  of  the  body  are  being  constantly  transmitted 
to  this  center,  which  not  only  stimulate  it  to  acti\dty  but  modify  its  activity 
in  one  direction  or  another  from  moment  to  moment. 

The  first  explanation,  that  acceleration  of  the  heart,  the  result  of  a 
peripherally  acting  stimulus,  is  due  to  a  stimulation  of  the  cardio-accelerator 
center  by  the  arrival  of  nerve  impulses  coming  through  afferent  nerves, 
having  been  made  questionable  and  improbable  by  the  results  of  Hunt's 
experiments,  the  alternative  explanation  must  be  that  while  inhibition  of  the 
heart  is  due  to  an  excitation  of  the  normal  activity  of  the  cardio-inhibitor 
center,  the  acceleration  of  the  heart  is  due  to  an  inhibition  of  the  normal 
activity  of  the  cardio-inhibitor  center,  and  hence  there  follows  the  corollary 
that  afferent  nerves  contain  two  sets  of  nerve-fibers  which  are  in  physiologic 
relation  with  the  cardio-inhibitor  center,  one  of  which  when  stimulated 
peripherally  excites  or  augments  its  activity,  the  other  of  which  when  stimu- 
lated inhibits  or  depresses  its  activity,  thus  giving  rise  to  a  freer  play  of  the 
cardio-accelerator  center. 

The  extent  to  which  both  sets  of  fibers  are  present  in  any  one  afferent 
nerve  is  unknown.  In  the  trigeminus  it  is  believed  the  excitator  fibers 
preponderate  for  the  reason  that  peripheral  stimulation  of  this  nerve  is 
followed  by  inhibition  of  the  heart;  in  the  sciatic,  it  is  believed  the  inhibitor 
nerves  preponderate,  for  the  reason  that  stimulation  of  the  central  end  of  the 
divided  nerve  is  followed  generally  by  acceleration  of  the  heart. 


THE  CIRCULATION  OF  THE  BLOOD  325 

It  is  probable  from  the  effects  which  follow  gastro-intestinal  disorders, 
that  the  vagus  nerve  contains  both  classes  of  fibers  as  represented  in  Fig.  141, 
inasmuch  as  stimuli  of  a  pathologic  character  in  one  individual  may  reflexly 
excite  or  increase  the  activity  of  the  cardio-inhibitor  center,  to  be  followed 
by  an  inhibition  of  the  heart;  and  in  another  individual,  may  reflexly  inhibit 
the  activity  of  the  same  center  and  to  such  an  extent  that  thecardio-accelerator 
center  may  be  enabled  to  increase  either  the  rate  or  the  force  or  both,  of  the 
heart  movements.  Palpitation  of  the  heart  from  gastric  irritation  might 
thus  be  explained. 

From  the  results  of  stimulation  of  the  sympathetic  (accelerator)  and 
vagus  (inhibitor)  nerves  under  a  great  variety  of  conditions  it  has  been 
established  that  their  respective  centers  are  mutually  antagonistic;  that  the 
activity  of  the  accelerator  center  at  one  moment  limits  the  activity  of  the 
inhibitor  and  at  another  moment  is  limited  in  turn  by  it;  that  the  rate  of  the 
heart-beat  at  each  moment  is  the  resultant  of  the  relative  degree  of  activity 
of  the  two  centers. 

The  Influence  of  Psychic  States. — It  is  a  familiar  personal  experience 
that  emotional  states,  according  to  their  suddenness  or  intensity  may  in- 
crease or  decrease  the  activity  of  the  heart;  thus  depressing  emotions  may 
diminish  the  activity  almost  to  the  point  of  complete  inhibition,  while  joyous 
emotions  on  the  contrary  may  increase  both  rate  and  force  to  the  point  at 
which  the  action  becomes  tumultuous  and  even  irregular.  The  effects. in 
either  case  are  due  to  nerve  impulses  descending  from  the  cerebrum  to  the 
cardiac  center  in  the  medulla. 

The  nerve  impulses  discharged  by  cerebral  cells  during  the  occurrence 
of  both  the  depressing  and  joyous  emotions  exert  their  influence  in  all  prob- 
ability, on  the  cardio-inhibitor  center  alone.  In  the  case  of  depressing  emo- 
tions, the  nerve  impulses  excite  or  increase  the  normal  degree  of  activity  of 
this  center  in  consequence  of  which  its  inhibitor  influence  on  the  heart  is 
increased;  in  the  case  of  joyous  emotions,  the  nerve  impulses  inhibit  the 
normal  degree  of  activity  of  the  center  in  consequence  of  which  its  influence 
on  the  heart  is  decreased.  This  permits  of  a  freer  play  of  the  cardio-accel- 
erator  center  and  as  a  result  there  is  an  increased  activity  of  the  heart. 

The  Causes  of  the  Variations  in  the  Heart-beat. — It  has  been  stated 
elsewhere  in  the  text  (page  290),  that  the  rate  of  the  heart-beat  is  influenced 
by  age,  muscle  activity,  the  position  of  the  body,  meals,  variations  in  blood 
pressure,  etc.  The  manner  in  which  these  changes  are  brought  about  is 
not,  however,  always  apparent.  In  addition  to  variations  that  are  strictly 
physiological  in  character  there  is  abundant  evidence  that  other  factors,  e.g.^ 
the  action  of  peripheral  stimuli  of  a  physiologic  or  pathologic  character  in 
various  regions  of  the  body,  can  and  do  cause  reflexly  at  one  time  or  in  one 
individual  an  acceleration  of  a  marked  character,  and  at  another  time  or  in 
another  or  the  same  individual  an  inhibition  which  may  be  so  pronounced 
as  to  almost  lead  to  a  complete,  though  temporary  standstill  of  the  heart  in 
diastole.  The  records  of  clinical  medicine  contain  many  instances  which 
show  that  ocular,  dental,  gastric,  intestinal,  uterine  and  other  organic  dis- 
orders, as  well  as  various  operative  procedures  in  dift'erent  regions  of  the 
body  cause  now  an  acceleration,  now  an  inhibition  of  the  heart  which  may 
be  so  marked  and  pronounced  as  to  give  rise  to  serious  apprehensions. 

The  Depressor  Nerve. — The  vagus  trunk  also  contains  afferent  fibers 


326 


TEXT-BOOK  OF  PHYSIOLOGY 


stimulation  of  which  not  only  brings  about  a  reflex  inhibition  of  the  heart, 
but  also  a  dilatation  of  the  peripheral  arteries  and  a  fall  of  blood-pressure 
through  a  depressive  influence  on  the  vaso-motor  centers.  To  this  nerve 
the  term  depressor  has  been  given.  A  consideration  of  the  physiologic 
action  of  this  nerve  will  be  found  in  the  section  devoted  to  the  nerve  mechan- 
isms concerned  in  the  maintenance  of  the  blood-pressure. 

Modifications  of  the  Nerve  Mechanism  of  the  Heart  that  Follow 
Slightly  Toxic  Doses  of  Drugs. — ^The  functions  of  different  parts  of  the 
nerve  mechanism  of  the  heart  may  be  demonstrated  by  an  analysis  of  the 
effects  that  follow  the  administration  of  slightly  toxic  doses  of  the  alkaloids 
of  various  drugs.  The  effects  can  be  shown  to  be  due  to  a  stimulation  or  to 
a  depression  of  the  normal  activity  of  one  or  more  portions  of  the 
mechanism.  The  alkaloid  may  exert  its  specific  action  on  the  central 
portions  in  the  medulla,  or  on  the  peripheral  portions  in  the  heart,  or  on 
both  simultaneously.  The  heart-muscle  may  at  the  same  time  be  stimu- 
lated or  depressed  in  its  action  either  in  the  same  or  in  the  opposite 
direction  to  that  of  the  nerve  mechanism.  As  a  result  the  heart-beat  may 
be  increased  or  decreased  both  in  rate  and  force. 

The  following  examples  will  illustrate  the  action  of  alkaloids  in  general. 

Atropin. — After  the  administration  of  atropin  in  sufhcient  amounts  the 

heart-beat  increases  in  frequency  in  all  animals  in  which  the  cardio-inhibitor 

centers  exert  a  steady  inhibitor  influence  over 
the  heart.  This  is  especially  true  in  man  and 
the  dog.  In  animals  in  which  the  inhibitor 
control  is  slight,  as  the  rabbit  and  frog,  the  in- 
crease in  frequency  is  not  very  marked.  In  all 
animals  thus  far  experimented  on  after  the  ad- 
ministration of  atropin,  neither  stimulation  of 
the  trunk  of  the  vagus  nor  stimulation  of  the 


Seat  of  action 
of  Mcotin 


Ua^s  7ieri;e. 


Sympatketi^ 
neuron 


Seatofaction 
of  Atropin. 


f lose. 'I  f.,) 


intracardiac  ganglia  will  arrest  or  even  retard 
the  heart-beat.  The  inference,  therefore,  is 
that  the  alkaloid  exerts  its  action  upon  the  gan- 
glion cells  and  their  terminal  branches,  impair- 
ing their  chemic  integrity  and  abolishing  their 
normal  function,  that  of  conducting  nerve  im- 
pulses from  the  vagus  nerve  proper  to  the  heart- 
FiG.  148.— Diagram  showing  muscle.  Fig.  148.  In  consequence  of  this,  the 
Z  ^^^S.Z.'lll^:'"'  "  infl"»ce  of  the  cardio-inhibitor  center  is  cut  off 

and  the  cardio-accelerator  being  unopposed  in 
its  activity,  the  rate  of  the  beat  is  increased.  After  a  variable  period 
the  heart  returns  to  its  normal  rate.  Stimulation  of  the  vagus  is  again 
followed  by  the  usual  inhibition.  As  atropin  is  partly  oxidized,  and 
partly  excreted,  it  is  assumed  that  the  nerve  terminals  have  been  restored 
by  nutritive  forces  to  their  normal  condition  and  their  conductivity  regained. 
This  having  been  accomplished  the  vagus  nerve  impulses  can  again  reach 
the  heart-muscle  and  the  cardio-inhibitor  center  is,  therefore,  enabled  to 
re-establish  inhibitor  control  and  antagonize  the  activity  of  the  cardio- 
accelerator  center. 

Nicotin. — After  the  administration  of    nicotin    in    suflQcient    amounts 
the  heart-beat  is  primarily  decreased  in  frequency  even  to  the  point  of  stand- 


THE  CIRCULATION  OF  THE  BLOOD  327 

still  in  diastole  for  a  few  seconds,  and  secondarily  increased  both  in  fre- 
quency and  force  beyond  the  normal.  If  the  vagus  nerves  be  first  divided 
this  primary  decrease  is  not  so  marked  and  the  inference  is  that  the  alkaloid 
primarily  stimulates  the  cardio-inhibitor  center  and  increases  its  normal 
function  and  perhaps  the  terminal  branches  of  the  vagus  libers,  the  pre-gangli- 
onic,  as  well.  After  the  secondary  increase  in  the  rate  is  established  stimulation 
of  the  vagus  trunk  fails  to  inhibit  the  heart,  though  stimulation  of  the  intra- 
cardiac ganglia  is  at  once  followed  by  the  usual  inhibitor  phenomenon,  arrest 
of  the  heart  in  diastole.  For  this  reason  it  is  believed  that  nicotin  acts  on  the 
peripheral  terminations  of  the  pre-ganglionic  fibers  of  the  vagus  as  they 
arborize  around  the  intra-cardiac  ganglia,  depressing  them  and  suspending 
their  normal  function,  that  of  conducting  nerve  impulses  from  the  vagus  to 
the  ganglion  cells.  Since  stimulation  of  the  pre-ganglionic  fibers  of  the 
accelerator  apparatus  fails  to  accelerate  the  rate  of  the  heart-beat,  though 
stimulation  of  the  post-ganglionic  fibers  has  the  usual  accelerating  effect, 
the  inference  is  that  nicotin  acts  upon  and  suspends  the  conductivity  of  their 
terminal  branches  in  the  ganglia.  The  acceleration  of  the  heart  must 
therefore  be  attributed  either  to  a  stimulation  of  the  post-ganglionic  fibers 
or  of  the  cardiac  muscle  itself  (Cushny), 

Pilocarpin  and  Muscarin. — These  alkaloids,  whether  administered  in- 
ternally or  applied  locally  to  the  heart,  diminish  the  frequency  and  the 
force  of  the  beat  to  such  an  extent  that  it  very  shortly  comes  to  rest  in  dias- 
tole. For  the  reason  that  the  internal  administration  or  the  local  applica- 
tion of  atropin  in  proper  doses,  which  has  a  depressive  action  on  the  intra- 
cardiac cell  terminations,  removes  the  inhibition  and  restores  the  normal 
rhythm,  the  inference  is  drawn  that  both  these  alkaloids  either  increase  the 
irritability  of  the  nerve-cells  or  heighten  the  conductivity  of  their  terminal 
fibers.  The  return  of  the  heart-beat  is  attributed  to  a  decline  in  irritability 
to  the  normal  level  in  consequence  of  the  antagonistic  action  of  the  atropin. 

Digitalin. — The  administration  of  digitalin  gives  rise  to  effects  the 
character  and  extent  of  which  vary  in  different  animals.  In  the  frog,  as  a  rule, 
the  only  effect  produced  is  a  gradual  increase  in  the  duration  and  force  of  the 
ventricular  systole,  with  a  corresponding  decrease  in  the  duration  of  the  dias- 
tole, until  the  heart  comes  to  rest  in  the  systolic  state.  As  this  effect  is 
observed  after  division  of  the  vagus  trunk  and  also  after  the  suspension  of 
the  activity  of  the  intra-cardiac  cell-fibers  by  atropin,  it  is  evidently  due  to  a 
direct  stimulation  of  the  heart-muscle.  In  some  instances,  however,  the 
opposite  effect  is  produced,  viz. :  a  gradual  increase  in  the  length  of  the  diastole 
a  decrease  in  the  duration  of  the  systole,  until  the  heart  comes  to  rest  in  the 
diastolic  state.  As  this  effect  arises  only  when  the  vagus  nerve  is  intact  it  is 
very  probably  due  to  a  stimulation  of  th  e  cardio-inhibitor  center  and  a  con- 
sequent increase  of  its  functional  activity.  Though  either  effect  may  be 
produced  in  the  frog  the  predominant  effect  is  the  increase  in  the  contrac- 
tion of  the  heart-muscle  rather  than  an  inhibition  of  the  beat. 

In  mammals  both  effects  are  observed,  viz. :  a  diminution  in  the  rate  of 
the  beat,  a  lengthening  of  the  diastole  and  an  increase  in  the  vigor  of  the 
systole,  which  are  evidently  due  to  a  simultaneous  stimulation  of  the  cardio- 
inhibitor  center  and  of  the  cardiac  muscle.  Digitalin  thus  expends  itself  on 
two  opposing  mechanisms;  as  to  which  gains  the  ascendency  will  depend  on 
the  dosage  and  the  character  of  the  animal. 


CHAPTER  XIV 

THE  CIRCULATION  OF  THE  BLOOD  (Continued) 

THE  VASCULAR  APPARATUS :  ITS  STRUCTURE  AND  FUNCTIONS 

The  Vascular  Apparatus  in  its  entirety  consists  of  a  closed  system  of 
vessels  which  not  only  contain  the  blood,  but  under  the  driving  power  of  the 
heart,  transmit  it  to  and  from  all  regions  of  the  body.  It  is  usually  divided 
into  a  systemic  and  a  pulmonic  portion. 

The  Systemic  Vascular  Apparatus. — ^This  portion  of  the  general  vas- 
cular apparatus  includes  all  the  vessels  extending  from  the  left  ventricle  to 
the  right  auricle:  viz.,  the  arteries,  capillaries,  and  veins.  Though  serving 
as  a  whole  to  transmit  blood  from  the  one  side  of  the  heart  to  the  other, 
each  one  of  these  three  divisions  has  separate  but  related  functions,  which 
are  dependent  partly  on  differences  in  structure  and  physiologic  properties, 
and  partly  on  their  relation  to  the  heart  and  its  physiologic  activities. 

The  Structure,  Properties  and  Functions  of  the  Arteries. — The 
arteries  serve  to  transmit  the  blood  ejected  from  the  heart  to  the  capillaries; 
that  this  may  be  accomplished  they  divide  and  subdivide  and  ultimately 
penetrate  each  and  every  area  of  the  body.  Their  repeated  division  is 
attended  by  a  diminution  in  size,  a  decrease  in  the  thickness  and  a  change  in 
the  structure  of  their  walls. 

A  typical  artery  consists  of  three  coats:  an  internal,  the  tunica  intima;  a 
middle,  the  tunica  media;  an  external,  the  tunica  adventitia. 

The  internal  coat  consists  of  a  structureless  elastic  basement  membrane, 
the  inner  surface  of  which  is  covered  by  a  layer  of  elongated  spindle-shaped 
endothelial  cells.  The  middle  coat  consists  of  several  layers  of  circularly 
arranged,  non-striated  muscle- fibers,  between  which  are  networks  of  elastic 
fibers.  The  external  coat  consists  of  bundles  of  connective  tissue  of  the 
white  fibrous  and  yellow  elastic  varieties.  Between  the  external  and  middle 
coats  there  is  an  additional  elastic  membrane.  In  the  small  arteries  there  is 
but  a  single  layer  of  muscle-fibers.  In  the  large  arteries  the  elastic  tissue 
is  very  abundant,  exceeding  largely  in  amount  the  muscle-tissue.  It  is  also 
more  closely  and  compactly  arranged.  The -external  coat  is  well  developed 
in  the  large  arteries  (Fig.  149). 

The  presence  in  their  walls  of  both  elastic  and  contractile  elements, 
endows  the  arteries  with  the  two  properties  of  elasticity  and  contractility. 

Elasticity. — ^The  elasticity  is  best  developed  in  the  large  arteries,  though 
it  is  also  present  in  arteries  of  relatively  small  size.  By  virtue  of  the  elas- 
ticity, the  arteries  are  capable  of  being  distended  and  elongated  and  when 
the  distending  force  is  removed  of  recoiling  or  retracting  and  returning  to 
their  former  condition.  Thus  the  capacity  of  the  aorta  and  carotid  artery 
of  the  rabbit  can  be  increased  four  times  and  six  times  respectively  by  raising 
the  intra-arterial  pressure  from  o  to  200  mm.  of  mercury.  The  elasticity 
permits  of  a  wide  variation  in  the  amount  of  blood  the  arterial  system  can 
hold  between  its  minimum  and  maximum  distension.     The  arteries  thus 

328 


THE  CIRCULATION  OF  THE  BLOOD 


329 


adapt  themselves  to  the  variations  in  the  volume  of  blood  discharged  from 
the  ventricle  at  a  single  beat  or  in  a  unit  of  time.  The  elasticity  also  con- 
verts the  intermittent  movement  of  the  blood  imparted  to  it  by  the  heart 
as  it  is  ejected  from  the  ventricle,  into  a  remittent  movement  in  the  arteries 
and  linally  into  the  continuous  and  equable  movement  observed  in  the  capil- 
laries. This  is  accomplished  in  the  following  manner:  With  each  contrac- 
tion of  the  left  ventricle  more  blood  is  ejected  into  the  aorta  than  the  arteries 
can  discharge  into  the  capillaries  and  veins  during  the  time  of  the  contrac- 
tion, owing  to  the  small  caliber  and  friction  of  the  arterioles.  The  portion 
not  so  discharged  exerts  a  lateral  pressure  against  the  walls  of  the  arteries 
which  at  once  dilate  until  a  condition  of  ec|uilibrium  is  established  between 
the  pressure  from  within  and  the  elastic  reaction  of  the  arterial  walls  from 
without.  With  the  cessation  of  the  contrac- 
tion the  elastic  walls  recoil  and  propel  the 
blood  toward  the  capillaries.  The  intermit- 
tent action  of  the  heart  is  thus  succeeded  by 
the  continuous  reaction  of  the  arterial  wall. 

As  the  blood  advances  toward  the  periph- 
ery of  the  arterial  system  and  larger  amounts 
pass  into  the  capillaries,  both  the  distention 
and  the  elastic  recoil  diminish,  and  by  the  time 
the  blood  reaches  the  capillaries  its  intermit- 
tency  of  movement  has  been  so  far  obliterated 
by  the  elastic  recoil  that  as  it  enters  the  capil- 
laries the  movement  becomes  equable  and  con- 
tinuous. In  this  manner  the  arteries  modify 
and  change  the  character  of  the  blood  flow 
and  in  part  adapt  it  for  the  conditions  of  the 
blood  flow  in  the  capillary  vessels. 

In  youth  the  arterial  walls  are  highly  dis- 
tensible and  elastic;  in  advanced  years  they 
are  frequently  relatively  rigid  and  inelastic, 
and  in  consequence  the  flow  of  blood  toward 
and  into  the  capillaries  approximates  in  its 
characteristics  the  flow  of  a  fluid  through  a 
rigid  tube  under  the  intermittent  action  of  a 
pump;  that  is,  the  intermittent  movement  im- 
parted by  the  heart  is  not  so  completely  con- 
verted into  a  continuous  movement,  and  hence 
the  blood  flows  through  the  capillaries  during  the  systole  with  greater  velocity, 
and  during  the  diastole  with  less  velocity,  than  is  the  case  when  the  vessel 
is  normally  elastic.  For  these  and  other  reasons  the  tissues  are  not  so  well 
nourished  and  hence  their  nutrition  and  functional  activities  decline. 

Contractility. ^ — ^The  contractility  permits  of  a  variation  in  the  amount 
of  blood  passing  into  a  given  capillary  area  in  a  unit  of  time.  Normally  each 
artery  has  a  certain  average  caliber  due  to  a  given  contraction  of  the  muscle 
coat.  Beyond  this  average  condition  the  artery  can  pass  in  one  direction  or 
the  other  by  either  a  relaxation  or  increased  contraction  of  the  muscle  coat. 
During  the  functional  activity  of  any  organ  or  tissue  there  is  need  for  an  in- 
crease in  the  amount  of  blood  beyond  that  supplied  during  functional  inac- 


FiG.  149. — Coats  of  a  Small 
Artery,  a.  Endoth  elium.  b. 
Internal  elastic  lamina,  c.  Cir- 
cular muscular  fibers  of  the  middle 
coat.  d.  The  outer  coat. — {Lan- 
dois  and  Stirling.) 


330 


TEXT-BOOK  OF  PHYSIOLOGY 


tivity  or  rest.  This  is  accomplished  by  a  relaxation  of  the  muscle-fibers. 
With  the  cessation  of  activity  the  muscle-fibers  again  contract  and  reduce  the 
amount  of  blood  to  that  required  for  nutritive  purposes  only.  An  increased 
contraction  of  the  muscle-fibers  beyond  the  average,  diminishes  the  outflow 
of  blood,  and  if  sufficiently  great  may  give  rise  to  anemia  and  pallor.  The 
contractile  elements  at  the  periphery  of  the  arterial  system,  in  the  so-called 
arteriole  region,  therefore  regulate  the  supply  of  blood  to  the  tissues  in 
accordance  v^ith  their  functional  needs. 

Moreover,  as  w^ill  be  stated  in  subsequent  paragraphs  the  degree  of 
contraction  of  the  arteriole  muscle  influences  very  markedly  the  degree  of 
friction  which  the  blood  has  to  overcome  in  passing  from  the  arteries  into 
the  capillaries.  If  the  muscle  contracts  vigorously  the  caliber  of  the  arteriole 
is  diminished  and  the  friction  increases;  if  the  muscle  relaxes,  the  caliber 
of  the  arteriole  is  enlarged  and  the  friction  decreases.  By  virtue  of  its  tonic 
activity,  the  arteriole  muscle  at  the  periphery  of  the  arterial  system  offers 
considerable  resistance  to  the  outflow  of  the  blood  and  this  is  therefore 
spoken  of  generally,  as  the  peripheral  resistance,  though  there  is  included 

under  this  term  the  resistance 
offered  by  the  small  caliber  of 
the  capillary  blood-vessel  as  well. 
This  latter  factor  is  constant, 
the  former  variable. 

The  functions  of  the  arteries 
therefore  are  as  follows:  (i)  to 
distribute  the  blood  to  all  regions 
of  the  body;  (2)  to  accommodate 
varying  volumes  of  blood;  (3)  to 
convert  the  intermittent  flow  of 
blood  as  it  leaves  the  heart  into 
a  remittent  and  finally  into  a  con- 
tinuous and  equable  flow  as  it 
passes  into  the  capillaries;  (4)  to 
regulate  the  volume  of  blood  de- 
livered to  an  organ  in  accordance 
with  its  functional  activities,  and 
(5)  to  present  that  degree  of  re- 
sistance necessary  to  the  main- 
tenance of  the  normal  blood  pressure,  through  the  activities  of  the  muscle 
fibers  in  the  walls  of  the  arterioles. 

The  Structure,  Properties,  and  Functions  of  the  Capillaries. — ^The 
capillaries  are  small  vessels  continuous  with  the  arteries  on  the  one  hand 
and  with  the  veins  on  the  other  hand.  Though  different  in  structure  from 
a  small  artery  or  vein,  there  is  no  sharp  boundary  between  them,  as  their 
structures  pass  imperceptibly  one  into  the  other.  A  true  capillary,  however, 
is  of  uniform  size  in  any  given  tissue  and  does  not  undergo  any  noticeable 
decrease  in  size  from  repeated  branchings.  The  diameter  varies  in  different 
tissues  from  0.0045  mm.  to  0.0075  mm.,  just  sufficiently  large  to  permit  the 
easy  passage  of  a  single  red  corpuscle.  The  length  varies  from  0.5  mm.  to 
I  mm.  The  wall  of  the  capillary  (Fig.  150)  is  composed  of  a  single  layer  of 
nucleated  endothelial  cells  with  serrated  edges  united  by  a  cementing 'mate- 


FiG.  150. — Capillaries.  The  Outlines  of 
THE  Nucleated  Endothelial  Cells  with  the 
Cement  Blackened  by  the  Action  of  Silver 
Nitrate. — {Landois  and  Stirling.) 


THE  CIRCULATION  OF  THE  BLOOD  331 

rial.  Though  extremely  short,  the  capillaries  divade  and  subdivide  a  number 
of  times,  forming  meshes  or  networks,  the  closeness  and  general  arrange- 
ment of  which  vary  in  different  localities. 

As  the  endothelial  cells  are  living  structures  and  characterized  by  irrita- 
bility, contractility  and  tonicity,  it  may  be  assumed  that  the  capillary  wall  as 
a  whole  is  characterized  by  the  same  properties.  Upon  the  possession  of 
these  properties  the  functions  of  the  capillary  depend. 

The  functian  of  the  capillary  wall  is  to  permit  of  a  passage  of  the  nutritive 
materials  of  the  blood  into  the  surrounding  tissue  spaces  and  of  waste 
products  from  the  tissue  spaces  into  the  blood.  The  structure  of  the  capil- 
lary wall  is  well  adapted  for  this  purpose.  Composed  as  it  is  of  but  a  single 
layer  of  endothelial  cells,  the  thickness  of  which  defies  accurate  measurement, 
it  readily  permits,  under  certain  conditions,  of  the  necessary  exchange  of 
materials  between  the  blood  and  the  tissues.  The  forces  which  are  con- 
cerned in  the  passage  of  materials  across  the  capillary  wall  are  embraced 
under  the  terms  diffusion,  osmosis,  and  filtration.  As  a  result  of  the  inter- 
change of  materials  the  tissues  are  provided  with  nutritive  materials  and  re- 
lieved of  the  presence  of  the  products  of  metabolism.  As  the  blood  loses 
oxygen  and  gains  carbon  dioxid,  it  changes  in  color  from  a  scarlet  red  to  a 
bluish  red.  In  consequence  of  the  exchange  of  materials,  the  blood  under- 
goes a  change  in  composition,  the  extent  and  character  of  which  varies  in 
accordance  with  the  activities  and  character  of  the  organ  traversed  by  it. 

In  order  that  the  nutritive  materials  may  pass  through  the  capillary 
wall  in  amounts  sufficient  to  maintain  the  necessary  supply  of  lymph  in  the 
lymph  or  tissue  spaces,  it  is  essential  that  the  blood  shall  flow  into  and  out 
of  the  capillary  vessels  constantly  and  equably,  in  volumes  varying  with  the 
activities  of  the  tissues,  under  a  given  pressure  and  with  a  definite  velocity. 
These  conditions  are  made  possible  by  the  cooperation  of  the  physiologic 
properties  and  physiologic  actions  of  the  heart  and  vascular  apparatus,  the 
nature  of  which  will  be  explained  in  subsequent  pages. 

The  Structure,  Properties,  and  Functions  of  the  Veins. — The  veins 
arise  from  the  distal  side  of  the  capillary  vessels.  As  they  emerge  they  are 
quite  small  and  designated  venules.  By  their  convergence  and  union  the 
the  veins  gradually  increase  in  size  in  passing  from  the  periphery  toward 
the  heart.  Their  walls  at  the  same  time  correspondingly  increase  in  thick- 
ness. The  veins  from  the  lower  extremities,  the  trunk,  and.  abdominal 
organs  finally  terminate  in  the  inferior  vena  cava.  The  veins  from  the  head 
and  upper  extremities  terminate  in  the  superior  vena  cava.  Both  venas 
cavae  empty  into  the  right  auricle. 

A  typical  vein  consists  of  the  same  three  coats  as  the  artery:  viz.,  the 
tunica  intima,  the  tunica  media,  and  the  tunica  adventitia.  The  media, 
however,  does  not  possess  as  much  of  either  the  elastic  or  muscle  tissue  as 
the  artery,  but  a  larger  amount  of  the  fibrous  tissue.  Hence  they  readily 
collapse  when  empty.  In  virtue  of  their  structure  the  veins  also  possess 
both  elasticity  and  contractility,  though  in  a  far  less  degree  than  the  arteries. 
These  properties  come  into  play  and  are  of  value  in  furthering  the  movement 
of  the  blood  toward  the  heart,  especially  after  a  temporary  obstruction. 

Certain  veins,  especially  the  large  veins  of  the  extremities,  are  character- 
ized by  the  presence  of  valves  throughout  their  course.  These  are  arranged 
in  pairs  and  formed  by  a  reduplication  of  the  internal  coat,  strengthened 


332  TEXT-BOOK  OF  PHYSIOLOGY 

by  fibrous  tissue.  They  are  always  directed  toward  the  heart  and  in  close 
relation  to  the  walls  of  the  veins,  so  long  as  the  blood  is  flowing  forward. 
An  obstruction  to  the  flow  causes  the  valves  to  turn  backward  until  they 
meet  in  the  middle  line,  when  they  act  as  a  barrier  to  regurgitation.  Under 
these  circumstances  the  elastic  tissue  permits  the  veins  to  distend  and  accom- 
modate the  blood.  With  the  removal  of  the  obstruction  the  recoil  of  the 
elastic  tissue,  and  perhaps  the  contraction  of  the  muscle-tissue,  forces  the 
blood  quickly  onward.  The  veins  of  many  viscera,  of  the  bones,  of 
the  cranial  and  vertebral  cavities  are  devoid  of  valves.  The  function  of 
the  veins  is  therefore  to  collect  the  blood  from  the  capillary  areas  and  trans- 
mit it  to  the  heart. 

THE  FLOW  OF  THE  BLOOD  THROUGH  THE  VESSELS 
HYDRODYNAMIC  CONSIDERATIONS 

The  blood  flows  through  the  arteries,  capillaries  and  veins  in  accordance 
with  definite  laws.  During  its  transit  certain  phenomena  are  presented  by 
each  of  these  three  divisions  of  the  vascular  apparatus.  These  are  mainly 
velocity  and  pressure  and  in  the  arteries  alone  an  alternate  expansion  and 
recoil  of  the  arterial  wall  with  each  heart  beat,  termed  the  pulse.  Since  these 
phenomena,  as  well  as  the  laws  which  govern  them  are  similar  to,  though 
more  complex  than  the  phenomena  presented  by  relatively  simple  tubes  with 
rigid  or  elastic  walls  while  liquids  are  flowing  through  them  under  a  steadily 
acting  or  an  intermittently  acting  pressure,  it  will  be  conducive  to  clearness 
of  conception  of  the  mechanics  of  the  vascular  apparatus,  if  there  be  con- 
sidered : 

1.  The  flow  of  a  liquid  through  a  horizontal  tube  with  rigid  walls  and  of 
uniform  or  variable  diameter  under  a  steadily  acting  pressure. 

2.  The  flow  of  a  liquid  through  a  series  of  branching  and  again  uniting 
tubes  with  rigid  walls  under  a  steadily  acting  pressure. 

3.  The  flow  of  a  liquid  through  a  tube  with  elastic  walls  under  an  inter- 
mittently acting  pressure. 

THE  FLOW  OF  A  LIQUID  THROUGH  A  HORIZONTAL  TUBE  WITH  RIGID 
WALLS  UNDER  A  STEADILY  ACTING  PRESSURE 

The  phenomena  and  the  laws  which  govern  them,  that  attend  the  flow  of  a 
liquid  through  a  rigid  tube  of  uniform  diameter  under  a  steadily  acting  pressure 
may  be  readily  observed  in  an  apparatus  similar  to  that  represented  in  Fig.  151, 
which  consists  of  a  horizontal  tube  into  which  is  inserted  at  equal  distances  a  series 
of  vertical  tubes ,  and  which  is  connected  with  a  reservoir  or  pressure  vessel,  P.  If 
the  reservoir  be  filled  with  a  liquid,  water  for  example,  the  latter  under  certain  con- 
ditions will  exert  a  downward  pressure  and  act  as  a  propelling  or  driving  power,  the 
degree  of  which  will  depend  on  the  height  of  the  column  and  may  be  represented  by 
H.  If  the  stopcock  at  O  be  opened  the  column  of  water,  which  has  heretofore  been 
exerting  an  equal  pressure  in  all  directions,  will  now  show  its  downward  pressure, 
and  in  consequence  it  will  be  driven  into  and  through  the  horizontal  tube  and 
discharged  from  its  free  extremity  with  a  definite  velocity.  At  the  same  time  the 
fluid  will  rise  in  each  vertical  tube  to  a  height  directly  proportional  to  the  distance 
of  each  tube  from  the  free  extremity.  The  velocity  with  which  the  fluid  is  dis- 
charged can  be  determined  by  measuring  the  quantity,  q,  discharged  in  a  unit  of  time, 

(i  second)  and  dividing  it  by  the  area  of  the  tube,  nr"^;  viz.,  v=  -^.    Inasmuch  as 


THE  CIRCULATION  OF  THE  BLOOD 


333 


the  tube  is  of  uniform  diameter  the  velocity  through  each  cross-section  will  be  the 
same. 

As  the  water  flows  through  the  horizontal  tube  it  meets  with  resistance,  namely, 
the  cohesion  and  friction  of  its  molecules,  and  the  adhesion  between  the  walls  of 
the  tube  and  the  water  which  must  be  overcome  if  the  flow  is  to  continue.  Because 
of  the  fact  that  water  will  moisten  most  surfaces  with  which  it  comes  in  contact 
there  will  be  an  adhesion  between  the  walls  of  the  tube  and  the  outer  layer  of  the 
column  of  water,  in  consequence  of  which  it  will  become  more  or  less  retarded  in 
its  flow.  Between  the  outer  retarded  layer  and  the  axis  of  the  stream,  there  is  an 
infinite  number  of  layers  of  molecules,  the  cohesion  of  which  one  for  the  other  is 
more  and  more  overcome  by  the  pressure  in  the  vessel,  P.  The  force  of  adhesion 
between  wall  and  fluid,  together  with  the  force  of  cohesion  between  the  molecules 
of  the  fluid  give  rise  to  the  resistance  of  the  fluid  to  the  flow. 

As  a  result  of  the  resistance  the  forward  movement  of  the  water  under  the 
pressure  in  P,  is  somewhat  retarded,  and  as  a  consequence  it  will  exert  a  lateral  or 
radial  pressure  against  the  walls  of  the  tube.     That  such  a  pressure  exists  is  shown 


Fig.  151. — A  Pressure  Vessel,  P,  with  a  Horizontal  Outflow  Tube,  O-n,  into  which 
Vertical  Tubes  or  Manometers  are  Inserted. 


by  the  rise  of  the  fluid  in  each  of  the  vertical  tubes,  and  the  height  to  which  it  rises 
in  each  tube  is  a  measure  of  the  pressure  at  its  base.  In  the  tube  /,  the  fluid  rises 
to  but  a  slight  extent  for  the  reason  that  the  resistance  yet  to  be  overcome  is  slight 
in  amount.  It  is,  however,  a  measure  of  the  resistance  or  friction  between  the  base 
of  the  tube  and  the  orifice  of  outflow.  In  the  tube  e  the  fluid  rises  twice  as  high 
as  in  /  because  of  the  additional  friction  between  the  bases  of  the  tubes  e  and  /. 
What  is  true  of  these  two  points  is  equally  true  of  the  points  at  the  base  of  the  tubes 
d,  c,  b,  a.  Lines  drawn  to  the  pressure  vessel  from  the  top  of  the  fluid  in  each  tube 
and  parallel  to  the  horizontal  tube  will  show  how  much  of  the  pressure  force  is 
utilized  in  overcoming  the  friction  in  each  section  of  the  horizontal  tube.  The 
amount  of  the  lateral  pressure  at  any  given  point  is  therefore  indicated  and  measured 
by  the  height  to  which  the  water  rises  in  the  tubes.  For  this  reason  these  tubes  are 
termed  pressure  tubes  or  piezometers. 

Since  the  resistance  in  a  tube  of  uniform  diameter  is  proportional  to  its  length 
the  lateral  pressure  will  gradually  but  progressively  decrease  from  the  reservoir  to 
the  outlet.  Therefore  the  pressure  at  any  given  point  is  proportional  to  the  resist- 
ance yet  to  be  overcome  and  conversely  the  resistance  to  be  overcome  is  indicated 
by  the  amount  of  the  pressure.     (In  the  conduct  of  an  experiment  the  propelling 


334  ,  TEXT-BOOK  OF  PHYSIOLOGY 

power  should  be  kept  constant  by  permitting  fluid  to  flow  into  the  reservoir  as 
rapidly  as  it  flows  out  of  the  horizontal  tube.) 

The  power  or  force  which  overcomes  the  resistance  in  the  horizontal  tube  and 
imparts  velocity  to  the  fluid  is  the  downward  pressure  of  the  water  in  the  reservoir, 
represented  by  H.  The  amount  of  this  power  utilized  in  overcoming  the  resistance 
is  approximately  determined  by  drawing  a  line  from  the  outlet  to  the  reservoir, 
uniting  the  upper  levels  of  the  water  in  the  vertical  tubes.  The  height  of  the  fluid 
at  the  point  at  which  the  line  intersects  the  reservoir,  y,  is  a  measure,  therefore, 
not  only  of  the  resistance  but  also  an  indication  of  the  relative  amount  of  the  pres- 
sure used  in  overcoming  it  and  is  therefore  known  as  the  pressure  height  and 
indicated  by  the  letter,  h. 

The  amount  of  the  pressure  consumed  in  imparting  the  observed  velocity  is 
determined  by  ascertaining  the  height  from  which  a  particle  must  fall  in  empty 
space  to  acquire  this  velocity.  This  is  obtained  by  dividing  the  square  of  the  veloc- 
ity by  twice  the  accelerating  force  of  gravity,  980  centimeters,  as  expressed  in  the 

v2 
formula,       ;  the  quotient  is  the  height  and  is  known  as  the  velocity  height.     Con- 

2g  ^  _ 

versely  if  the  moving  fluid  were  discharged  into  empty  space  through  an  opening 
in  the  tube  at  n,  it  would  ascend  an  equal  distance.  If  now  this  height  is  repre- 
sented by  F,  and  a  line  be  drawn  from  it,  parallel  to  the  line  of  pressure  until  it 
meets  the  reservoir  at  x,  it  will  be  seen  what  percentage,  x  y,  or  h'  of  the  primary 
propelling  power  is  consumed  in  imparting  the  observed  velocity. 

Of  the  total  pressure  a  small  portion  is  left  over  which  is  utilized  in  forcing  into, 
and  overcoming  the  resistance  offered  by,  the  orifice  of  the  horizontal  tube.  The 
initial  pressure  in  P  therefore  divides  itself  mainly  into  two  portions;  one,  the  larger 
by  far,  h,  is  utilized  in  overcoming  the  resistance  to  the  flow  of  the  water;  the  other, 
the  smaller,  h'  in  imparting  velocity. 

Thus  the  two  phenomena  presented  by  the  flow  of  a  liquid  through  a  tube  with 
rigid  walls  and  of  uniform  diameter  are  velocity  and  pressure,  of  which  the  former 
is  the  same  for  each  cross-section,  and  the  latter  at  any  point  directly  proportional 
to  the  resistance  to  be  overcome. 

If,  instead  of  a  horizontal  tube  of  uniform  diameter,  there  be  substituted  a 
tube  the  middle  third  of  which  is  enlarged,  the  conditions  will  be  similar  to 
the  previous  case  until  the  fluid  flows  into  the  enlarged  portion,  when  the  velocity 
will  diminish,  being  inversely  proportional  to  the  area  of  the  cross-section.  The 
resistance  will  be  also  diminished  and  therefore  less  of  the  pressure  force  or 
driving  power  will  be  consumed  than  in  the  first  section  of  the  tube,  and  as  a  result, 
the  lateral  pressure  will  fall  less  rapidly  than  in  the  first  section.  When  the  liquid 
flows  into  the  narrow  or  third  section,  the  primary  velocity  returns.  Though  the 
resistance  again  increases  the  amount  to  be  overcome  is  small,  and  hence  there  is 
a  rapid  and  steady  fall  of  pressure. 

On  the  contrary,  if  a  tube  be  substituted  the  middle  third  of  which  is  narrowed, 
the  conditions  will  be  similar  to  the  previous  cases  until  the  liquid  flows  into 
the  narrowed  section,  when  at  once  the  velocity  increases  and  becomes  inversely 
proportional  to  the  area  of  the  cross-section;  the  resistance  being  increased  at  the 
same  time,  there  will  be  a  rapid  consumption  of  the  pressure  force  and  a  steep  fall 
of  lateral  pressure.  On  flowing  into  the  third  section,  the  velocity  again  diminishes 
and  the  pressure  falls  though  more  slowly  to  the  end  of  the  tube. 

THE  FLOW  OF  A  LIQUID  THROUGH  A  SERIES  OF  BRANCHING  AND 
AGAIN  UNITING  TUBES  WITH  RIGID  WALLS  UNDER  A  STEADILY 

ACTING  PRESSURE 

In  a  system  of  this  character,  such  as  represented  in  Fig.  152,  there  must  follow 
as  a  result  of  the  repeated  branchings,  a  progressive  increase  in  the  total  sectional 
area  of  the  collective  tubes  coincident  with  a  progressive  decrease  in  the  sectional 


THE  CIRCULATION  OF  THE  BLOOD 


335 


area  of  individual  tubes  in  the  section  b  c,  while  in  the  section  c  d,  there  must 
follow  a  progressive  decrease  in  the  total  sectional  area  of  the  collective  tubes  coin- 
cident with  a  progressive  increase  in  the  sectional  area  of  individual  tubes,  conse- 
quently there  will  be  a  combination  of  the  two  conditions  alluded  to  in  the  two 
preceding  paragraphs,  namely,  an  enlargement  of  the  stream  bed  coincident  with 
a  diminution  in  size  of  the  individual  tubes  composing  it,  in  the  middle  section. 
Moreover,  for  the  purpose  here  intended  it  may  be  assumed  that  the  tubes  com- 
posing the  middle  section  c  are  microscopic  in  size  and  that  their  total  sectional 
area  bears  to  the  sectional  area  of  tube  A  the  ratio  of  600  to  i. 

If  the  system  is  connected  with  a  pressure  vessel,  as  in  the  preceding  instance, 
and  the  stopcock  is  suddenly  opened,  the  column  of  water  will  exert  a  downward 


Fig.  152. — Pressure  Vessel  with  a  Series  of  Progressively  Branchimg  and  again  Uniting 

Tubes. 

a,  X,  a  series  of  stopcocks    by  which  the  peripheral  resistance  can   be  increased  or  decreased. 


pressure,  and  in  consequence  the  water  will  be  driven  into  and  through  the  system 
with  a  definite  velocity  and  pressure. 

The  velocity  of  the  fluid  will  gradually  decrease  from  b  to  c  in  a  ratio  inversely 
proportional  to  the  total  area  of  each  cross-section  until  at  c,  it  will  attain  its  mini- 
mal value;  the  velocity  will  again  increase  from  c  to  D  in  a  ratio  inversely  pro- 
portional to  the  total  area  of  each  cross-section  until  at  e,  when  if  will  attain  the 
value  it  had  in  A  if  the  entrance  and  exit  tubes  have  the  same  area. 

The  lateral  pressure  will  gradually  fall  from  the  beginning  to  the  end  of  the  sys- 
tem, though  the  fall  must  be  more  rapid  in  b-c  than  in  a-b  as  will  be  clear  from 
the  following  considerations. 

In  the  section  B-C  the  two  factors — viz.:  the  widening  of  the  stream  bed  which 
decreases  the  resistance,  and  the  narrowing  of  the  individual  tubes  which  increases 
the  resistance — exert  an  opposing  influence  on  the  pressure;  hence  the  fall  of 
pressure  will  be  proportional  to  the  ratio  between  these  two  factors.     As  the 


336  TEXT-BOOK:  OF  PHYSIOLOGY 

increase  in  the  resistance  due  to  the  progressive  decrease  in  the  size  of  the  individual 
tubes  preponderates  considerably  over  the  decrease  in  the  resistance  due  to  the 
widening  of  the  stream  bed,  there  must  be  an  increase  in  resistance  in  the  area 
B-c  and  therefore  a  more  rapid  fall  of  pressure  than  inA-B.  This  fall,  however, 
will  not  be  as  steep  as  it  might  be  for  the  reason  that  the  decrease  in  the  velocity 
is  attended  by  a  decrease  in  the  resistance  and  hence  a  lessened  consumption  of 
the  propelling  power.  In  the  section  c-d  the  two  factors,  viz.:  the  narrowing  of 
the  stream  bed  which  increases  the  resistance,  and  the  enlarging  of  the  individual 
tubes  which  decreases  the  resistance,  exert  an  opposing  influence  on  the  pressure, 
hence  the  fall  of  pressure  will  be  proportional  to  the  ratio  between  these  two  factors. 
As  the  decrease  in  the  resistance  due  to  the  progressive  enlargement  of  the  individual 
tubes  preponderates  considerably  over  the  increase  in  the  resistance  due  to  the 
narrowing  of  the  stream  bed,  there  should  theoretically  be  a  rapid  fall  of  pressure 
from  c  to  E.  This  rapid  fall,  however,  will  be  to  some  extent  prevented  for  the 
reason  that  the  increase  in  velocity  due  to  the  narrowing  of  the  stream  bed  in- 
creases the  resistance  to  a  high  value  and  hence  the  pressure  falls  less  rapidly  than 
it  otherwise  would. 

The  pressure  throughout  the  system  is  the  result  of  (i)  the  downward  pressure 
of  the  water  in  the  reservoir,  and  (2)  the  resistance  to  its  flow,  due  to  the  cohesion 
and  friction  of  its  molecules  and  its  adhesion  to  the  sides  of  the  tubes,  and  its 
extent  in  any  one  section  will  be  proportional  to  the  resistance  yet  to  be  overcome. 
It  will  naturally  be  higher  in  the  section  a-b  than  in  the  section  d-e,  though  the 
difference  in  the  level  of  the  pressure  between  these  two  points  will  not  be  as  great 
as  might  theoretically  be  supposed  from  the  small  size  of  the  tubes  in  c  for  the 
decrease  in  velocity  counterbalances  in  part  the  resistance  which  they  offer. 

The  general  curve  of  the  fall  of  pressure  in  this  system  is  indicted  by  the 
curved  line  extending  from  the  pressure  vessel  to  the  outlet  o*f  the  horizontal  tube. 

The  value  of  the  pressures  in  these  two  sections  and  their  relation  to  each  other 
could  be  varied  either  temporarily  or  permanently^by  the  insertion  of  a  series  of  stop- 
cocks a,  X,  along  the  course  of  the  tubes  between  b  and  c  in  the  neighborhood  of 
their  ultimate  brandlings  by  which  an  additional  resistance  could  be  superposed 
on  the  system  from  a  to  the  stopcocks.  If  the  lumen  of  each  stopcock  has  a  certain 
average  value,  so  as  to  permit  of  a  certain  outflow  of  water,  the  pressure  will 
have  a  certain  value  in  both  a-b  and  d-e.  But  if  the  lumen  of  each  stopcock  is 
decreased,  there  will  be  an  increase  in  the  resistance  and  hence  a  rise  of  pressure  in 
A-B  and  a  fall  of  pressure  in  d-e.  If,  on  the  contrary,  the  lumen  of  each  stopcock  is 
increased,  there  will  be  a  decrease  in  the  resistance  and  hence  a  fall  of  pressure 
in  A-B  and  a  rise  of  pressure  in  d-e.  The  stopcocks  may  be  spoken  of  as  a 
variable  peripheral  resistance. 

In  the  foregoing  exposition  it  has  been  assumed  that  in  all  instances  the  pressure 
in  the  pressure  vessel  was  steadily  acting.  If,  however,  the  pressure  be  made  to 
act  intermittently  as  it  can  be  by  alternately  opening  and  closing  the  stopcock,  at  A 
both  the  velocity  and  the  pressure  will  be  alternately  increased  and  decreased. 
The  outflow  of  the  fluid  during  the  moment  the  pressure  is  acting  will  be  rapid, 
and  during  the  moment  the  pressure  is  not  acting  the  outflow  will  cease.  It 
becomes  therefore  intermittent.  Coincidently  there  is  an  alternate  temporary 
increase  and  decrease  of  the  lateral  pressure. 

THE  FLOW  OF  A  LIQUID  THROUGH  A  TUBE  WITH  ELASTIC  WALLS 
UNDER  AN  INTERMITTENTLY  ACTING  PRESSURE 

When  a  tube  with  elastic  walls  is  connected  with  a  pressure  vessel,  the  con- 
ditions which  are  established  on  opening  the  stopcock  and  the  consequent  flow  of 
water,  will  soon  approximate  those  observed  in  a  tube  with  rigid  walls.  As  the 
water  moves  forward,  it  encounters  friction,  exerts  a  lateral  pressure  and  causes  a 


THE  CIRCULATION  OF  THE  BLOOD  337 

distention  of  the  tube.  This  latter  effect  continues  until  the  elastic  recoil  of  the 
walls  of  the  tube  exactly  counterbalances  the  pressure  of  the  water  from  within. 
When  this  condition  is  established  the  tube  becomes  practically  a  tube  with  rigid 
walls,  and  hence  so  long  as  the  primary  pressure  is  uniform,  the  velocity  and  lateral 
pressure  will  obey  the  laws  which  hold  true  for  rigid  tubes. 

If,  however,  the  primary  pressure  be  intermittently  applied  or  alternately 
increased  or  decreased,  and  the  water  forced  into  the  tube,  previously  filled  with 
water  but  under  no  particular  pressure,  it  will  be  forced  out  of  the  peripheral  end 
of  the  tube  more  rapidly  during  the  period  of  the  increase  of  pressure  and 
less  rapidly  during  the  period  of  the  decrease  of  pressure  or  it  may  cease  entirely. 
The  extent  to  which  the  outflow  becomes  merely  remittent,  or  entirely  intermittent, 
will  depend  on  the  amount  of  resistance,  whether  this  be  due  to  length  of  tube  or 
a  narrowed  outlet,  and  the  degree  of  elasticity. 

When  these  factors  are  of  such  a  nature  that  the  resistance  is  very  high  and  the 
elasticity  slight,  the  outflow  will  be  intermittent.  But  if  they  are  made  to  change 
gradually,  and  this  is  especially  the  case  with  the  resistance,  from  a  slight  to  a 
greater  value,  the  outflow  gradually  changes  from  an  intermittent  to  a  remittent 
and  finally  to  a  continuous  outflow  and  for  the  following  reasons: 

With  a  given  resistance  and  elasticity,  the  fluid  which  is  driven  into  the  tube  by 
the  action  of  the  primary  pressure  exerts  more  or  less  lateral  pressure,  gives  rise  to 
a  distention  of  the  tube,  and  acquires  a  certain  velocity  of  outflow.  In  consequence 
of  the  distention,  a  portion  of  the  fluid  accumulates.  With  the  cessation  in  the 
action  of  the  primary  pressure,  the  elastic  walls  recoil  and  force  the  accumulated 
fluid  forward  and  so  maintain  more  or  less  effectively  the  same  velocity  of  outflow 
until  there  is  a  return  of  the  pressure.  If  the  resistance  be  great  and  the  elasticity 
slight,  this  is  impossible  and  the  outflow  will  be  entirely  intermittent.  But  if  they 
are  made  to  increase  in  value,  the  proportionate  amount  of  the  fluid  which  accumu- 
lates during  the  action  of  the  primary  pressure  will  also  increase  in  amount  and 
hence  there  will  be  an  increase  in  the  distention  of  the  tube.  The  elastic  recoil 
will  therefore  be  greater  in  amount  and  longer  in  duration,  and  hence  the  outflow 
will  change  to  a  remittent  and  finally  to  a  continuous  outflow. 

Coincident  with  the  action  and  cessation  of  action  of  the  primary  pressure 
there  is  a  corresponding  increase  and  decrease  of  the  lateral  pressure  and  when  the 
intermittent  action  is  sufficiently  rapid,  the  excess  of  fluid  entering  the  tube  over 
that  discharged  becomes  sufficiently  great  to  maintain  a  certain  average  or  mean 
pressure,  which,  however,  undergoes  an  alternate  increase  and  decrease  with  each 
variation  in  the  primary  pressure. 

The  temporary  increase  and  decrease  of  the  pressure  and  the  consequent 
expansion  and  recoil  of  the  tube  in  the  neighborhood  of  the  pressure  vessel,  give 
rise  to  a  wave  on  the  surface  of  the  tube  which  is  propagated  with  more  or  less 
rapidity — though  with  decreasing  amplitude — from  the  beginning  to  the  end  of 
the  tube  and  causing  in  each  section  a  corresponding  expansion  and  recoil,  and 
which  is  known  as  the  expansion  wave. 

If  a  system  of  branching  and  again  uniting  tubes  with  elastic  walls  arranged 
in  a  manner  similar  to  that  represented  in  Fig.  153  is  connected  with  a  reservoir, 
and  the  primary  pressure  is  made  to  act  intermittently,  or  is  alternately  increased 
or  decreased,  phenomena  will  be  presented  which  closely  resemble  the  phenomena 
presented  by  the  circulatory  apparatus  at  the  time  of  the  heart  beat. 

THE  APPLICATION  OF  THE  FOREGOING  FACTS  TO  THE 
VASCULAR  APPARATUS 

The  systemic  vascular  apparatus  may  be  conceived  of  as  a  system  of  tubes 
which  have  symmetrically  divided  and  subdivided  and  afterward  again 
united  and  reunited  in  a  corresponding  manner.     The  arteries,  arterioles, 


338 


TEXT-BOOK  OF  PHYSIOLOGY 


capillaries,  venules,  and  veins  may  therefore  be  schematically  arranged 
(Fig.  153)  in  a  manner  identical  with  the  schematic  arrangement  of  tubes 
represented  on  page  335.  The  heart,  with  which  they  are  in  connection, 
when  filled  with  blood  may  be  compared  with  the  reservoir  filled  with  water, 
and  the  intra-ventricular  pressure  developed  during  the  contraction,  to 
the  downward  pressure  of  the  water  when  the  stopcock  at  A  is  opened. 

The  Stream-bed. — The  stream-bed,  the  path  along  which  the  blood 
flows,  varies  widely  in  its  total  sectional  area  in  different  parts  of  its  course, 
being  least  in  the  aorta  and  venae  cavae,  and  greatest  in  the  capillaries.  In 
passing  from  the  base  of  the  aorta  toward  the  capillaries  the  sectional  area  of 
individual  arteries,  in  consequence  of  repeated  branching,  diminishes, 
though  their  total  sectional  area  increases  and  in  direct  proportion  to  their 


Fig.  153. — Schematic  Arrangement  of  the  Vascular  Apparatus. 
a,  X,  the  arteriole  muscle  by  which  the  peripheral  resistance  can  be  increased  or  decreased. 


distance  from  the  heart.  In  the  capillary  system  the  sectional  area  of  an 
individual  capillary  attains  its  minimal  value,  though  the  total  sectional 
area  attains  its  maximal  value.  Comparing  one  with  the  other,  it  has  been 
estimated  that  the  total  sectional  area  of  the  aortic  bed  is  to  the  total  sectional 
area  of  the  capillary  bed  as  1  is  to  600  or  800.  This  statement  is  based  on 
the  estimated  values  of  the  sectional  area  of  the  aorta,  615  mm.,  the  velocity 
of  the  blood  in  the  aorta  300  mm.  per  second,  the  average  velocity  in  the 
capillaries  0.5  mm.  per  second.  If  these  estimates  are  accepted  as  approxi- 
mately correct,  then  the  sectional  area  of  the  capillary  system  or  bed  is 
obtained  by  the  following  formula: 

X  =  —  ■^      '^  or  ^6q,ooo  mm.,  a  ratio  of  the  aortic  sectional  area  to  the 
0.5  ^  ^ 


THE  CIRCULATION  OF  THE  BLOOD 


339 


capillary  sectional  area  of  i  to  600.  In  passing  from  the  capillary  into  the 
venous  system  the  sectional  area  of  individual  veins  increases,  though 
the  total  sectional  area  decreases  and  in  direct  proportion  to  their  distance 
from  the  capillaries. 

The  stream-bed  in  the  aorta  is  relatively  narrow,  but  widens  gradually  as 
it  approaches  the  capillaries,  where  it  attains  its  maximal  width;  it  again 
narrows  gradually  as  it  passes  into  the  veins,  until  in  the  vense  cavae  it  be- 
comes almost  as  narrow  as  in  the  aorta.  As  the  combined  sectional  areas  of 
the  venae  cavae  are  greater  than  the  sectional  area  of  the  aorta,  the  stream- 
bed  of  the  former  never  becomes  as  narrow  as  that  of  the  latter. 

The  gradual  increase  in  the  width  of  the  stream-bed  from  the  beginning 
of  the  aorta  to  the  middle  of  the  capillary  system,  and  the  gradual  decrease 
in  the  wudth  of  the  stream-bed  from  the  middle  of  the  capillary  system  to  the 


Fig.  154. — Diagram  Designed  to  Give  an  Idea  of  the  Aggregate  Sectional  Area  of 
THE  Different  Parts  of  the  Vascular  System.  A.  Aorta.  C.  Capillaries.  V.  Veins.  The 
transverse  measurement  of  the  shaded  part  may  be  taken  as  the  width  of  the  various  kinds  of 
vessels,  supposing  them  fused  together. — (Yeo.) 

terminations  of  the  venae  cavae,  which  result  from  the  repeated  branching 
and  subsequent  uniting,  as  well  as  its  relative  width  in  the  arteries,  capil- 
laries, and  veins,  are  shown  graphically  in  Fig.  154. 

The  Intra- ventricular  Pressure. — ^\Vhen  the  heart  contracts  and  the 
intra-ventricular  pressure  rises  above  the  pressure  in  the  aorta,  the  aortic 
valves  are  suddenly  forced  open  and  the  blood  is  driven  into  and  through  the 
arteries,  capillaries,  and  veins  to  the  right  side  of  the  heart  and  in  accordance 
with  foregoing  considerations  with  a  definite  velocity  and  against  a  certain 
resistance,  which  in  turn  gives  rise  to  a  side  pressure. 

The  Velocity. — ^The  velocity  of  the  blood  in  the  systemic  vascular  ap- 
paratus will  gradually  decrease  from  the  aorta  to  the  middle  of  the  capillary 
system  in  a  ratio  inversely  proportional  to  the  total  area  of  any  given  cross- 
section  of  the  stream-bed,  until  in  the  capillaries  it  will  attain  its  minimal 
value,  which  is  especially  small  because  the  resistance  to  the  flow  of  blood  in 
the  capillaries  increases  inversely  as  the  square  of  their  diameters,  while  in 


340  TEXT-BOOK  OF  PHYSIOLOGY 

the  larger  blood-vessels  the  increase  is  inversely  proportional  to  the  simple 
diameter;  the  velocity  will  again  increase  from  the  middle  of  the  capillary 
system  to  the  ends  of  the  venae  cavae  in  a  ratio  again  proportional  to  the 
total  area  of  each  cross-section  of  the  stream-bed  until  in  the  venae  cavae  it 
will  attain  its  maximal  value,  though  it  will  not  attain  its  initial  value  in  these 
vessels  because  their  combined  sectional  area  is  greater  than  that  of  the  aorta. 

The  Lateral  Pressure. — ^The  lateral  pressure  will  also  gradually  fall 
from  the  beginning  of  the  aorta  to  the  ends  of  the  venae  cavae,  though  the 
fall  will  be  most  rapid  at  the  periphery  of  the  arteries.  In  the  arterial  system 
the  fall  of  pressure  will  be  proportional  to  the  ratio  between  the  increase  in 
resistance  due  to  the  narrowing  of  individual  vessels,  and  the  decrease  in 
resistance  due  to  the  widening  of  the  stream-bed;  as  the  former  preponder- 
ates over  the  latter  there  must  be  an  increase  in  resistance  from  the  aorta 
to  the  capillaries  and  hence  a  sharper  fall  of  pressure  toward  the  termination 
of  the  arterioles,  which  is  very  steep  for  reasons  to  be  stated  later.  In  the 
venous  system  the  fall  of  pressure  will  continue  and  its  rate  will  be  propor- 
tional to  the  ratio  between  the  increase  in  resistance  due  to  the  narrowing  of 
the  stream-bed,  and  the  decrease  of  resistance  due  to  the  enlarging  of  indi- 
vidual vessels;  as  the  latter  preponderates  over  the  former  there  should  be 
a  rapid  fall  of  pressure  from  the  capillary  system  to  the  ends  of  the  venae 
cavae.  This,  however,  is  to  some  extent  prevented  for  the  reason  that  the 
increase  in  velocity  due  to  the  narrowing  of  the  stream-bed  increases  the 
resistance  to  a  relatively  high  value  and  hence  the  pressure  falls  less  rapidly 
than  it  otherwise  would. 

The  Resistance.— The  resistance  to  the  flow  of  blood  through  the  sys- 
temic vascular  apparatus  is  due  to  the  cohesion  and  friction  of  its  molecules 
as  well  as  that  of  the  corpuscles  and  the  adhesion  of  the  blood  to  the  sides  of 
the  vessels.  The  resistance  is  also  increased  by  the  small  diameter  of  the 
capillary  blood-vessels. 

The  high  pressure  characteristic  of  the  arterial  system  contrasted 
with  the  low  pressure  characteristic  of  the  venous  system  determined  by 
experiment,  cannot  be  accounted  for  alone  by  the  resistance  offered  by  the 
small  diameter  of  the  vessels  of  the  capillary  system.  This  in  itself  would  be 
insufficient  to  maintain  the  observed  differences  in  pressure  in  the  different 
sections  of  the  vascular  apparatus  necessary  for  physiologic  purposes.  To 
meet  this  necessity  there  has  been  developed  at  the  periphery  of  the  arterial 
system,  in  the  arteriole  wall,  a  special  muscle,  a,  x,  Fig.  152  which  by  con- 
tracting can  add  a  physiologic  resistance  to  what  might  be  termed  the  phys- 
ical resistance  of  the  system.  According  to  the  degree  of  its  contraction 
will  the  resistance  to  the  flow  of  blood  from  the  arteries  to  the  veins  at  the 
periphery  of  the  arterial  system  be  increased  and  the  arterial  pressure  be 
raised  and  the  venous  pressure  be  lowered.  According  to  the  degree  of  its 
relaxation  will  the  resistance  to  the  flow  of  blood  from  the  arteries  into  the 
veins  be  decreased  at  the  periphery  of  the  arterial  system  and  the  arterial 
pressure  be  lowered  and  the  venous  pressure  raised.  By  this  means  the 
extent  and  the  relation  of  the  pressure  in  the  two  main  sections  of  the 
systemic  vascular  apparatus  can  be  temporarily  or  permanently  changed  in 
one  direction  or  the  other.  The  effect  of  the  diminution  in  the  caliber  of 
the  arteriole  due  to  the  contraction  of  the  muscle  is  spoken  of  as  the 
peripheral  resistance. 


THE  CIRCULATION  OF  THE  BLOOD  341 

That  the  high  pressure  in  the  arteries  is  largely  due  to  this  physiologic  factor 
is  shown  by  the  rapid  and  pronounced  fall  of  pressure  that  occurs  when  this 
muscle  suddenly  relaxes  as  it  does  when  the  spinal  cord  is  transversely 
divided  in  the  cervical  region,  thus  cutting  off  from  the  arteriole  muscle 
those  nerve  influences  that  largely  determine  its  contraction.  Under 
such  circumstances  the  pressure  in  the  dog  may  fall  from  approximately 
140  mm.  to  40  mm.  of  mercury  or  even  less.  Stimulation  of  the  distal 
extremity  of  the  spinal  cord  will  be  followed  by  the  temporary  contraction  of 
the  arteriole  muscle  and  a  rise  of  pressure  to  its  former  value. 

The  Distribution  of  the  Intra-ventricular  Pressure. — The  pressure 
developed  during  the  ventricular  contraction  is  thus  expended  in  imparting 
velocity  to  the  blood  and  overcoming  the  cohesion  and  friction  of  its  mole- 
cules. The  percentage  of  the  pressure  utilized  in  overcoming  the  resistance 
could  be  approximately  determined  from  the  pressure  in  the  aorta  if  this  were 
accurately  known;  the  percentage  of  the  pressure  utilized  in  imparting 

velocity  could  be  determined  with  the  formula—  ,  if  the  actual  velocity  of  the 

2g  •' 

blood  in  the  aorta  could  be  experimentally  determined.  On  account  of 
the  difficulty  in  obtaining  this  latter  factor  at  least,  the  results  must  be  only 
approximative. 

An  idea  of  the  ratio  between  the  velocity  pressure  and  the  resistance  pressure, 
however,  may  be  obtained  from  the  distribution  of  the  aortic  pressure  in  the 
dog  in  reference  to  the  carotid  artery.     Thus,  if  it  be  assumed  that  the  aver- 

(35)2 
age  velocity  of  the  blood  is  35  cm.,  the  velocity  pressure  is  equal  to       ,    or 

0.62  centimeters  of  blood  or  0.046  centimeters  of  mercury,  and  if  the  average 
aortic  pressure  is  150  mm.  of  mercury,  the  ratio  of  the  velocity  pressure  to 
the  resistance  pressure  is  as  i  to  326. 

The  Pulse  Wave. — Inasmuch  as  the  heart's  action  is  intermittent  and 
the  walls  of  the  arteries  are  elastic,  there  is  a  temporary  increase  and  decrease 
of  the  pressure  with  each  beat  of  the  heart,  attended  by  a  corresponding 
expansion  and  recoil  of  the  arteries,  giving  rise  to  a  wave  on  the  surface  of 
the  arteries  which  is  propagated  with  more  or  less  rapidity — though  with 
decreasing  amplitude — from  the  beginning  of  the  aorta  to  the  capillaries, 
and  causing  in  each  successive  section  a  corresponding  expansion  and  recoil, 
and  which  is  known  as  the  expansion  wave  or  pulse  wave. 

The  general  statements  regarding  the  phenomena  attending  the  flow  of 
blood  through  the  systemic  vascular  apparatus  contained  in  the  foregoing 
paragraphs  will  be  further  elaborated  in  the  following  pages.  For  special 
reasons  it  is  convenient  to  consider  the  pressure  first. 

BLOOD-PRESSURE 

Blood-pressure  may  be  defined  as  the  pressure  exerted  radially  or  later- 
ally by  the  moving  blood-stream  against  the  sides  of  the  vessels.  This  pres- 
sure is  the  result  of  (i)  the  driving  power  of  the  heart,  and  (2)  of  the  resist- 
ance offered  to  the  forward  movement  of  the  blood — a  resistance  due  to  the 
cohesion  and  friction  of  the  molecules  of  the  blood,  of  the  blood  corpuscles, 
and  the  adhesion  of  the  blood  to  the  sides  of  the  blood-vessels.  That  there  is 
such  a  pressure  within  the  arteries,  capillaries,  and  veins,  different  in  amount 


342  TEXT-BOOK  OF  PHYSIOLOGY 

in  each  of  these  three  divisions  of  the  vascular  apparatus,  is  evident  from  the 
results  which  follow  division  of  an  artery  or  a  vein  of  corresponding  size. 
When  an  artery  is  divided,  the  blood  spurts  from  the  opening  for  a  consider- 
able distance  and  with  a  certain  velocity.  The  reason  for  this  lies  in  the 
fact  that  the  vessel  has  been  distended  by  the  pressure  from  within  and  its 
walls  thrown  into  a  condition  of  elastic  tension,  so  that  at  the  moment  there 
is  an  outlet,  the  vessel  suddenly  recoils  and  forces  the  blood  out  with  a  velocity 
and  to  a  height  proportional  to  the  distention.  When  a  vein  is  divided,  the 
blood  as  a  rule  merely  wells  out  of  the  opening  with  but  slight  momentum, 
and  for  the  reason  that  the  vessel  has  been  but  slightly,  if  at  all  distended  by 
the  pressure.  These  results  indicate  that  the  blood  in  the  arteries  stands 
under  a  pressure  considerably  higher  than  that  of  the  atmosphere,  while  that 
in  the  veins  stands  under  a  pressure  perhaps  but  slightly  above  that  of  the 
atmosphere.     Especially  true  is  this  of  the  larger  veins. 

The  difference  in  the  height  of  the  pressure  in  the  arterial  system  as 
contrasted  with  the  venous  system  is  due  to  the  progressive  diminution  of 
the  resistance  from  the  beginning  of  the  aorta  to  the  ends  of  the  venae  cavae, 
together  with  the  small  diameter  of  the  capillaries,  increased  to  a  variable 
extent,  by  the  tonic  contraction  of  the  arteriole  muscle. 

The  same  facts  may  be  demonstrated  in  another  and  more  striking  way. 
A  dog  or  cat  is  anesthetized  and  securely  fastened  in  an  appropriate  holder. 
The  carotid  artery  on  the  right  side  and  the  jugular  vein  on  the  left  side  are 
freely  exposed  and  clamped.  Into  the  artery  there  is  inserted  on  the  distal 
side  of  the  clamp  and  in  the  direction  of  the  heart  a  cannula  to  which  is 
connected  a  tall  glass  tube,  200  cm.  high  and  of  about  4  mm.  internal  di- 
ameter. Into  the  vein  there  is  passed  on  the  proximal  side  of  the  clamp  and 
in  the  direction  of  the  capillaries  a  second  cannula,  to  which  is  connected  a 
similar  tube,  though  of  less  height.  If  the  two  clamps  are  removed  at  the 
same  time,  the  blood  will  mount  in  both  tubes  simultaneously.  In  the 
arterial  tube  the  blood  will  ascend  by  leaps  corresponding  to  the  heart-beats 
until  a  certain  height  is  reached,  when  the  column  becomes  relatively 
stationary,  being  kept  in  equilibrium  by  the  blood-pressure  within  the 
vessel  and  the  atmospheric  pressure  without.  Though  stationary  in  a 
general  sense,  nevertheless  the  blood-column  oscillates,  rising  and  falling 
with  each  contraction  and  relaxation  of  the  heart.  Not  infrequently  larger 
excursions  of  the  column  are  seen  which  correspond  in  a  general  way  to  the 
respiratory  movements.  This  experiment  was  originally  performed  on  the 
horse,  by  the  Rev.  Stephen  Hales  (1732). 

In  the  venous  tube  the  blood  also  rises  to  a  certain  height,  after  which  it 
remains  quite  stationary,  as  the  effect  of  the  cardiac  contraction  is  not 
propagated  under  normal  conditions  beyond  the  arterial  system.  The 
height  to  which  it  rises  is  but  slight  as  compared  with  that  in  the  arterial 
tube.  The  pressure  in  both  vessels  is  thus  recorded  in  millimeters  of  blood. 
Strictly  speaking  the  pressure  thus  obtained  does  not  represent  the  lateral 
pressure  in  the  carotid  artery  but  in  the  vessel  from  which  it  arises.  The 
central  end  of  the  carotid  is,  under  the  circumstances,  but  a  continuation 
of  the  cannula  and  the  pressure  thus  obtained  is  the  lateral  pressure  of  either 
the  innominate  artery  or  the  aorta  as  the  case  may  be.  In  order  to  obtain  the 
lateral  pressure  in  the  carotid  or  any  other  artery  it  is  only  necessary  to  take 
the  end  pressure  of  any  one  of  its  branches  or  what  amounts  to  the  same 


THE  CIRCULATION  OF  THE  BLOOD 


343 


thing,  to  divide  the  vessel  and  insert  the  horizontal  portion  of  a  T-shaped 
tube  into  the  central  and  distal  ends  through  which  the  blood  can  con- 
tinue to  flow,  and  to  connect  the  vertical  portion  with  a  vertical  pressure 
tube  or  with  a  mercurial  manometer.  The  absolute  pressure  on  any  given 
unit  of  vessel  surface — e.g.,  i  sq.  mm. — is  obtained  by  multiplying  the  height 
of  the  column,  expressed  in  miUimeters,  by  the  unit  of  surface,  and  then 
determining  the  weight  of  this  mass  of  blood.  Thus  if  the  height  of  the 
column  of  blood  in  the  carotid  artery  tube  is  2000  mm.,  then  the  pressure  on 
I  sq.  mm.  is  the  weight  of  2000  mm.  of  blood.  The  weight  of  2000  c.mm. 
of  blood  is  equal  to  2.1  grams. 

The  Arterial  Blood-pressure. — For  accurate  and  long-continued  ob- 
servation the  arterial  blood-pressure  is  more  conveniently  studied  by  means 
of  a  U-shaped  tube  (a  manometer)  partially  filled  with  mercury.  One  limb 
of  the  manometer  is  connected  by  means  of  a  tube  and  a  cannula  with  an 


B.P.TRACING 
ABSCISSA- 


'V 


/^^ 


PRESSURE 
BOTTLE  . 


Fig.  155. — Diagram  to  Show  the  Relation  of  the  Meecltrial  Manometer  to  the  Artery, 
ON  One  Hand,  and  to  the  Recording  Cylinder,  on  the  Other  Hand,  when  Arranged  for 
Recording  Blood-pressure. 

artery  (Fig.  155).  For  the  purpose  of  retarding  coagulation  of  the  blood  and 
for  preventing  the  escape  of  a  large  volume  of  blood  from  the  vessels,  the 
system  is  filled  with  a  solution  of  carbonate  of  soda  of  sp.  gr.  1060,  55.8 
grams  per  1000  c.c,  or  a  25  per  cent,  solution  of  magnesium  sulphate  of 
sp.  gr.  1060,  and  under  a  pressure  approximately  equal  to  that  in  the  vessel 
of  the  animal  as  determined  in  previous  experiments.  When  communication 
is  established  between  the  vessel  and  the  cannula,  the  mercurial  column 
adjusts  itself  to  the  pressure  in  the  artery  and  at  once  exhibits  the  same 
cardiac  oscillations  and  respiratory  undulations  as  did  the  column  of  blood 
in  the  previous  experiment. 

The  height  of  the  mercurial  column  kept  in  equilibrium  by  the  pressure 
of  the  blood  within,  and  the  pressure  of  air  without  the  vessel  is  that 
between  the  lower  level  of  the  mercury  in  the  proximal,  and  the  higher  level 
in  the  distal  limb  of  the  manometer,  both  of  which  can  be  read  off  on  a  scale 
placed  between  the  two  limbs. 


344 


TEXT-BOOK  OF  PHYSIOLOGY 


The  height  of  the  mercury  as  well  as  its  oscillations  in  the  distal  limb 
may  be  recorded  by  placing  on  the  top  of  the  mercury  a  light  float,  the  upper 
end  of  which  carries  a  writing  point.  When  the  latter  is  placed  in  contact 
with  the  moving  blackened  surface  of  a  recording  cylinder  or  kymograph, 
the  height  and  the  oscillations  are  recorded  in  the  form  of  a  tracing  similar 
to  that  shown  in  Figs.  155  and  156,  in  which  the  smaller  oscillations  represent 
the  changes  in  pressure  due  to  the  systole  and  diastole  of  the  heart  and  the 
larger  oscillations  to  variations  in  the  average  pressure  due  to  the  respiratory 
movements.  The  height  of  the  mercurial  column  kept  in  equilibrium  at  any 
particular  moment  is  determined  by  measuring  the  distance  between  a  base- 
line or  abscissa,  which  represents  the  position  of  the  mercury  at  atmospheric 


r6o 

/v 

|gvVW'v/\^'^A./VVv% 

rj)5 

Tko 

T15 

^^o 

r25 

fao 

ps 

I 

fio  Tiwe  Record  in  Seconds 

rsi  1  I  (  1  f  1  1  (  1  1  1  1  1  (  1  1  1  1  II  1 

Line  of  AtmospKeric  Pressure 

Fig.  156. — A  Portion  of  a  Blood-pressure  Tracing  Obtained  from  the  Carotid  Artery 
OF  TEEE  Rabbit  with  a  Mercurial  Manometer.  The  small  oscillations  are  due  to  the  heart-beat; 
the  large  oscillations  are  due  to  the  respiratory  movements. 

pressure,  and  any  given  point  on  the  trace  above,  and  multiplying  it  by  2, 
for  the  reason  that  the  mercury  sinks  in  the  proximal  limb  as  high  as  it  rises 
in  the  distal  limb  of  the  manometer  and  hence  the  column  of  mercury  sup- 
ported is  that  observ^ed  between  the  upper  and  lower  levels  of  the  mercury 
in  the  distal  and  proximal  limbs  of  the  manometer. 

The  blood-pressure  as  revealed  by  the  tracing  may  be  resolved  into  two 
components:  viz.,  (i)  a  more  or  less  constant  element  represented  by  the 
pressure  in  the  arteries  during  the  period  of  the  cardiac  diastole,  which  is 
termed  the  diastolic  or  minimum  pressure;  and  (2)  a  variable  element 
represented  with  certain  limitations  by  that  additional  pressure  occurring 
at  the  time  of  the  cardiac  systole,  which  is  termed  the  systolic  or  maximum 
pressure.  The  diastolic  pressure  is  represented  by  the  distance  between  the 
base-line  and  the  points  of  the  curve  corresponding  to  the  diastolic  pause;  the 
systolic  pressure,  by  the  distance  between  the  base-line  and  the  apices  of 


THE  CIRCULATION  OF  THE  BLOOD 


345 


max  valve 


to  manometer 


mm  valve 


the  curves  following  the  cardiac  systole.  The  relation  of  these  two  com- 
ponents varies  in  different  animals  and  in  the  same  animal  at  different  times. 
If  the  diastolic  pressure  is  low,  the  systolic  increase  may  be  considerable;  if 
the  former  is  high,  the  latter  may  be  slight  in  extent. 

There  are  good  reasons  for  believing,  however,  that  this  record  does  not 
represent  either  the  true  diastolic  or  the  true  systolic  pressure  but  that  the 
limits  between  the  two  are  far  more  widely  apart  than  here  represented. 
For,  owing  to  its  inertia,  the  mercury  is  not  capable  of  following  the  rapid 
variations  of  the  pressure  throughout  their  extent,  that  occur  with  each 
heart-beat.  The  employment  of  one  of  the  various  forms  of  the  quickly 
responsive  spring  manometers  such  as  are  used  in  determining  the  rapid 
variations  of  intra-cardiac  pressure  will  show  a  much  greater  difference 
between  the  diastolic  and  systolic  pressures,  often  amounting  to  as  much  as 
40  millimeters. 

For  the  purpose  of  obtaining  the  maximum  systolic  and  the  minimum 
diastolic  pressures,  it  is  best,  however,  to  insert  between  the  cannula  and  the 
manometer  a  maximum  and  a  minimum 
valve  similar  in  principle  to  that  shown 
in  Fig.  157.  By  permitting  the  blood  to 
exert  its  pressure  first  through  the  maxi- 
mum valve  and  then  permitting  the  mer- 
curial column  to  exert  its  pressure  through 
the  minimum  valve  in  the  reverse  direction 
for  a  certain  length  of  time,  the  maximum 
systolic  and  the  minimum  diastolic  pres- 
sures will  be  recorded.  By  this  method 
Dawson  found  an  average  maximum  sys- 
tolic pressure  in  the  carotid  artery  of  the 
dog  of  162,  and  a  minimum  diastolic  pres- 
sure of  103  mm.  of  mercury,  a  difference  of 
59  mm.  Hg.  The  difference  between  these 
two  pressures  is  known  as  the  pulse  pres- 
sure. (A  diagram  showing  the  relation  of 
these  different  pressures  one  to  another  will 
be  found  on  page  348.) 

In  a  series  of  experiments  it  will  be  found  that  the  blood-pressure  in  the 
arteries,  recorded  with  the  mercurial  manometer,  though  rising  and  falling  a 
certain  number  of  millimeters,  yet  retains  a  fairly  constant  general  average, 
the  result  of  an  adjustment  between  the  number  of  heart-beats  per  minute 
and  the  amount  of  the  resistance  offered  to  the  escape  of  blood  into  the  capil- 
laries and  veins.  Though  the  tracing  fails  to  record  accurately  the  diastolic 
and  systolic  pressures  it  approximates  a  certain  average  or  mean  of  the  pres- 
sure thus  recorded,  which  represents  the  power  driving  the  blood  through 
the  vessels.  It  is  frequently  stated  that  in  a  tracing  in  which  the  respiratory 
undulations  are  absent,  the  mean  pressure  is  the  arithmetic  mean  of 
the  systolic  and  diastolic  pressures.  This  is,  however,  not  strictly  correct, 
for  if  the  pressure  is  recorded  by  means  of  a  spring  manometer  or  a  sphygmo- 
graph  applied  over  the  artery  of  man,  a  record  much  different  in  appearance 
and  similar  to  that  shown  in  Fig.  158  will  be  obtained.  In  such  a  record 
it  will  be  observed  that  the  passage  from  the  lowest  diastolic  pressure  {a)  to 


to  heart 

Fig.  157. — v.  Frank's  Valve, 
This  is  placed  in  the  course  of  the 
tube  between  heart  and  manometer, 
so  that  the  latter  may  be  used  as  a 
maximum,  minimum,  or  ordinary 
manometer  according  to  the  tap  which 
is  left  open. — {Starling.) 


346  TEXT-BOOK  OF  PHYSIOLOGY 

the  highest  systolic  pressure  (b)  takes  place  quickly,  occupying  about  o.i 
second;  and  that  the  change  from  the  highest  systolic  pressure  to  the  succeed- 
ing lowest  diastolic  pressure  takes  place  slowly,  in  about  0.7  second,  and  that 
the  line  of  descent  is  interrupted  by  a  secondary  rise  and  fall  of  pressure  be- 
fore the  former  diastolic  level  is  reached.  ^  If  a  horizontal  line  is  drawn  across 
the  record  parallel  to  a  line  uniting  the  lowest  diastolic  levels,  and  to  a  line 
uniting  the  highest  systolic  levels  and  in  the  position  of  the  arithmetic  mean, 
the  triangular  record  of  the  pressure  changes  will  be  divided  into  two  portions 
of  which  the  upper  has  a  smaller  area  than  the  lower  portion  from  which 
it  is  apparent  that  the  pressure  is  low  for  a  longer  period  [that  it  is  high 
and  hence  the  mean  pressure  cannot  be  the  arithmetic  mean  between  the 
diastolic  and  systolic  pressures.  The  mean  pressure,  however,  can  for  a 
given  period  at  least  be  experimentally  determined.  Thus,  if  at  some  one 
point  between  the  artery  and  the  manometer,  the  lumen  of  the  connecting 
tube  be  largely  obliterated  by  a  constriction,  the  variations  in  the  pressure 
following  the  systole  and  diastole  of  the  heart  will  be  largely,  if  not  entirely 
,  excluded,  and  the  mercury,  instead  of  rising  rapidly 

'Y  fsr  in  the  manometer  and  fluctuating  with  each  heart- 

XL  \^^^ beat,  will  rise  slowly  to  a  certain  level  and  then 

"vi^ ^N;^         -^?N^^-    remain  at  rest.     The  number  of  millimeters  of 

*■  ^  mercury  thus  supported  represents  the  mean  or  ab- 

solute pressure.  The  same  result  can  be  obtained 
by  employing  the  compensatory  manometer  of 
Fig.  158.— Spnygmogram  or  Marey  which  presents  a  constriction  of  this  char- 
pulse  curve.  acter.  From  many  experiments  made  by  Dawson 
it  has  been  learned  that  the  mean  pressure  lies  nearer  to  the  diastolic  than  to 
the  systolic  pressure  and  may  be  expressed  numerically  by  the  statement  that 
it  is  equal  in  millimeters  of  mercury  to  the  diastolic  pressure  plus  one-third 
of  the  pulse  pressure.  In  a  tracing  in  which  the  respiratory  undulations  are 
present  the  mean  pressure  can  be  calculated.  The  method  by  which  this 
is  done,  however,  is  rather  complicated  and  need  not  be  detailed  here.  In 
a  general  way  the  mean  pressure  in  such  a  tracing  may  be  represented  by  a 
line  drawn  horizontally  across  the  tracing  midway  between  the  apex  and 
trough  of  the  undulation. 

Estimates  of  the  Mean  Arterial  Pressure. — Because  of  the  difficulty 
in  obtaining  the  pressure  in  small  arteries,  the  experimental  determinations 
have  for  the  most  part  been  confined  to  large  arteries  such  as  the  carotid, 
brachial,  and  femoral,  and  hence  the  results  which  have  been  obtained  have 
reference  to  the  lateral  pressure  in  the  aorta  or  in  the  large  vessels  which 
immediately  arise  from  it.  The  pressure  obtained  in  the  usual  way  at  the 
central  end  of  a  divided  carotid  is  generally  known  as  the  "end  pressure" 
and  represents  the  mean  lateral  pressure  in  the  aorta  or  in  the  innominate 
artery.  Among  the  results  thus  obtained  in  different  experiments  from  the 
carotid  artery  of  different  animals  are  the  following:  In  the  horse,  from 
122  to  214  mm.  Hg.;  in  the  dog,  from  140  to  160  mm.;  in  the  cat,  150  mm.; 
in  the  rabbit,  from  90  to  100  mm,;  in  the  sheep,  170  mm,;  in  the  calf,  from 

1  It  must  be  remembered,  however,  that  the  cardiac  systole  endures  though  with  diminishing 
energy,  even  though  the  systoUc  pressure  in  the  arter}'  is  falling,  for  0.32  second,  when  a  rapid 
dilatation  sets  in  attended  by  the  closure  of  the  semilunar  valves ,  an  event  indicated  in  the  tracing 
by  the  notch  preceding  the  second  rise  of  pressure  at  c. 


THE  CIRCULATION  OF  THE  BLOOD  347 

133  to  165  mm.  In  two  observations  made  on  human  beings  previous 
to  the  amputation  of  a  hmb,  the  pressure  was  found  in  the  brachial 
artery  of  one  patient  to  vary  from  no  mm.  to  120  mm.  Hg.,  and  in  the 
anterior  tibial  artery  of  the  other  patient  from  no  mm.  to  160  mm.  Hg. 

The  investigations  made  in  different  parts  of  the  arterial  system  indicate 
that  the  mean  pressure  is  remarkably  constant  and  uniform  and  does  not 
show  any  noticeable  falling  off  until  near  the  arteriole  region  where  the  resist- 
ance suddenly  and  rapidly  increases.  Thus  Volkman  found  simultaneously 
in  the  carotid  artery  and  in  the  metatarsal  artery  of  the  sheep  a  mean  pressure 
of  165  and  146  mm.  Hg.  respectively  and  this  for  the  reason  that  the  resist- 
ance throughout  the  arterial  system  does  not  markedly  increase  until  the 
arteriole  region  is  reached.  The  careful  investigations  of  Dawson  show  that 
in  the  large  blood-vessels  of  the  dog  the  diastolic  pressure  is  as  constant 
as  the  mean  pressure  though  it  undergoes  slight  variations  in  different 
regions;  but  that  the  systolic  pressure,  as  shown  by  taking  the  end  pressure 
in  the  thyroid  and  similar  sized  arteries  in  different  parts  of  the  arterial  tree, 
undergoes  a  considerable  falling  off,  though  it,  too,  remains  high  in  large 
arteries. 

The  numerical  expressions  of  these  various  pressures  in  different  parts 
of  the  arterial  system  are  shown  in  the  following  table  abstracted  from  the 
more  extensive  tables  of  Dawson.  The  results  were  obtained  from  experi- 
ments made  on  dogs.  The  figures  represent  in  millimeters  of  mercury  certain 
average  end  pressures  in  the  arteries  named. 


Artery 

Systolic 

Mean 

Diastolic           Pulse  pressure 

Brachio-cephaiic 

Right  carotid 

Left  carotid 

163 
160 
160 
168 
160 
165 
152 
140 

121             1 

118             I 

123             [ 

123               ! 

118 

123 

118 

118 

103                           60 
no                          50 
lOI                                 ^0 

Left  subclavian 

Left  brachial 

105                          63 
no                          SO 
103                           62 
102                           50 
97                            43 

Left  renal 

Deep  femoral 

Thyroid 

The  Capillary  Pressure. — The  small  size  of  the  capillaries  precludes 
an  investigation  of  their  pressure  by  manometric  methods.  It  may  be  stated, 
however,  to  be  approximately  equal  to  the  pressure  required  to  obliterate 
their  lumina  and  to  whiten  the  skin.  The  apparatus  of  v.  Kries  is  based  on 
this  theory.  A  small  glass  plate,  from  2.5  to  5  sq.  mm.,  is  fastened  to  the 
under  surface  of  a  support  of  suitable  size  carrying  a  small  scale  pan.  The 
glass  plate  is  placed  on  the  skin  near  the  root  of  a  finger-nail  and  the  scale 
pan  gradually  weighted  until  the  vessels  are  obliterated,  as  shown  by  the 
blanching  of  the  skin.  From  results  obtained  with  this  apparatus  v.  Kries 
estimated  the  pressure  in  the  capillaries  of  the  hand  at  37  mm.  Hg.  and  in  the 
ear  at  20  mm. 

The  Venous  Pressure. — In  passing  from  the  capillaries  to  the  heart 
the  pressure  continues  to  fall.  The  increasing  size  of  the  veins  permits 
again  of  manometric  observations  in  different  regions.  In  the  crural  vein 
the  pressure  has  been  found  to  be  equal  to  14  mm.  Hg.,  and  in  the  brachial 
vein  9  mm.  of  Hg.     In  the  jugular  and  subclavian  and  other  vessels  near 


348 


TEXT-BOOK  OF  PHYSIOLOGY 


the  heart  it  is  zero  or  even  negative;  that  is,  less  than  atmospheric  pressure 
to  the  extent  of  from  i  to  lo  mm.  of  jnercury. 

The  amount  and  relation  of  the  different  pressures  in  the  three  divisions 
of  the  systemic  vascular  apparatus  are  approximately  shown  in  Fig.  159. 


160 


J20 


30 


JO 


f\    /r~"7r-\ 

Line  of 

Na/TTis'''' 

SYSTOUC   PRESSURE 

'  "m/]/)\" 

Line  of 

~  r/cA/y     r'ncJoUnt 

line  of          ^LiY\ 

DIASTOLIC vCl 

PULSE    PRESSURE 

PRESSURE           V^ 

The  difference  between 

DIASTOL/C 

and 

SYSTOLIC   PRESSURE 
\c 

H                  A 

C                        V      ""v -1 

Fig.  159. — A  Diagram  Designed  to  Show  the  Amount  and  the  Relation  of  the  Blood- 
pressure  IN  THE  three  Divisions  of  the  Vascular  Apparatus,  as  well  as  the  Relation  of 
THE  Diastolic,  the  Mean,  and  the  Systolic  Pressures  in  the  Arterial  System.  Based  on 
experiments  made  on  dogs.  H.  Heart.  A.  Arteries.  C.  Capillaries.  V.  Large  veins.  O,  O, 
being  the  zero  line  (  =  atmospheric  pressure),  the  pressure  is  indicated  by  the  height  of  the  curve. 
The  numbers  on  the  left  give  the  pressure  (approximately)  in  millimeters  of  mercury,  h.  Pressure 
in  heart,  a.  Arteriole  region  showing  sudden  fall  of  pressure,  c.  The  fall  of  pressure  in  the 
capllaries.     v.  The  negative  pressure  in  the  large  veins. 

RESUME  OF  THE  FACTS  OF  THE  BLOOD  PRESSURE  AND  OF  THE 
FACTORS  WHICH  CAUSE  IT 


From  a  consideration  of  the  foregoing  facts  and  statements  the  following 
r^sum^  may  be  made:  i.  The  blood  during  its  flow  exerts  a  pressure  against 
the  sides  of  the  blood-vessels.  2.  This  pressure  is  the  resultant  on  the  one 
hand  of  the  intra-ventricular  pressure  developed  at  the  time  of  the  contrac- 
tion, and  on  the  other  hand  of  the  resistance  to  the  forward  movement  of  the 
blood.  3.  The  resistance  is  to  be  sought  for  in  the  cohesion  and  friction  of  the 
molecules  of  the  blood  and  its  adhesion  to  the  walls  of  the  vessels.  4.  The 
resistance  is  inversely  proportional  to  the  diameter  of  the  vessel  and  is  there- 
fore least  in  the  large  arteries  and  veins  and  greatest  in  the  arterioles  and 
capillaries.  5.  The  pressure  is  highest  in  the  aorta  where  it  may  amount 
in  man  to  150  mm.  of  mercury  above  that  of  the  atmosphere,  and  lowest  at 
the  ends  of  the  venae  cavae  where  it  may  be  no  greater  than  that  of  the  atmos- 
phere or  may  be  even  10  mm.  Hg.  below  it.  6.  The  pressure  falls  from  the 
beginning  to  the  end  of  the  vascular  apparatus,  though  not  progressively, 
for  throughout  the  large  vessels  of  the  arterial  system  it  continues  relatively 
high.  7.  The  high  pressure  in  the  aorta  is  due  to  the  total  resistance  of  the 
vascular  apparatus  and  the  pressure  at  any  given  point  of  the  apparatus 
represents  the  resistance  yet  to  be  overcome.  8.  The  high  pressure  in  the 
arterial  system  and  its  marked  fall  at  the  periphery  is  more  especially  the 
result  of  the  very  great  resistance  at  this  point,  known  as  the  peripheral 


THE  CIRCULATION  OF  THE  BLOOD  349 

resistance,  the  result  of  a  rapid  diminution  in  the  diameter  of  the  arterioles 
and  the  capillary  vessels,  together  with  the  tonic  contraction  of  the  arteriole 
muscle.  9.  The  pressure  in  the  arterial  system  undergoes  considerable 
variation  both  above  and  below  the  mean  pressure  during  the  systole  and 
diastole  of  the  heart. 

The  Heart. — The  primary  factor  in  the  production  of  the  pressure  is  the 
pumping  action  of  the  heart.  Should  there  be  any  cessation  in  its  activity, 
the  elastic  walls  of  the  arteries  would  recoil  and  force  the  blood  into  the 
veins.  There  would  be  coincidently  a  fall  of  the  pressure  to  that  of  the 
atmosphere.  Even  under  normal  circumstances  this  condition  is  approxi- 
mated during  the  diastole.  The  recoil  of  the  arterial  wall  by  which  the  for- 
ward movement  of  the  blood  is  maintained  is  attended  by  a  fall  in  pressure. 
But  before  this  reaches  any  considerable  extent,  the  heart  again  contracts 
.  and  forces  its  contained  volume  of  blood  into  the  arteries. 

That  this  may  be  accomplished  it  is  essential  that  the  cardiac  energy 
be  sufl&cient  not  only  to  drive  a  portion  of  the  blood  through  the  capillaries 
into  the  veins,  but  to  oppose  the  recoiling  arteries,  and  to  distend  them  to 
their  previous  extent,  so  that  the  incoming  volume  of  blood  may  be  ac- 
commodated.    This  at  once  reestablishes  the  pressure  at  its  former  level. 

During  the  contraction  of  the  heart  the  kinetic  energy  is  transformed 
into  potential  energy,  represented  by  the  tense  distended  walls  of  the  arteries. 
With  the  relaxation  of  the  heart  and  the  closure  of  the  semilunar  valves  the 
potential  energy  of  the  arteries  is  again  transformed  into  kinetic  energy, 
represented  by  the  moving  blood.  The  artery  thus  continues  the  work 
of  the  heart  during  its  period  of  inactivity.  The  rapidity  with  which  the 
cardiac  contractions  succeed  each  other  prevents  the  pressure  from  sinking 
below  a  certain  average  level. 

The  Resistance. — The  secondary  factor  is  the  resistance  to  the  flow 
of  blood  through  the  vessels,  the  nature  of  which  has  been  previously  stated. 
So  long  as  the  resistance,  and  especially  that  variable  element  of  it  at  the 
periphery  of  the  arterial  system,  maintains  a  certain  average  value,  so  long  will 
the  pressure  in  each  division  of  the  vascular  apparatus  maintain  an  average  or 
a  physiologic  value.  Should  the  resistance  at  the  periphery  of  the  arterial 
system  vary  in  either  direction,  the  result  of  an  increase  or  a  decrease  in  the 
degree  of  the  contraction  of  the  arteriole  muscle,  there  will  arise  a  change 
in  the  relative  degree  of  pressure  in  each  of  the  three  divisions  of  the  vascular 
apparatus. 

The  Elasticity  of  the  Vessel  Walls. — A  tertiary  factor  is  the  elasticity 
of  the  arterial  wall.  While  it  can  hardly  be  said  that  the  elasticity  is  a  cause 
of  the  pressure,  there  can  be  attributed  to  it  the  capability  of  modifying 
and  assisting  in  the  maintenance  of  the  pressure  at  a  more  or  less  constant 
level;  for  were  it  not  for  this  property  of  the  vessel  wall  the  variations  in 
pressure  during  and  after  the  systole  would  be  far  more  extensive  than  they 
are,  and  would  approximate  the  variations  observed  in  tubes  with  rigid 
walls.  The  elasticity,  moreover,  assists  in  the  equalization  of  the  blood- 
stream, converting  the  intermittent  and  remittent  flow  characteristic  of  the 
large  arteries  into  the  continuous  equable  stream  characteristic  of  the  capil- 
laries. It  also  permits  of  wide  variations  in  the  amount  of  blood  the  arteries 
can  contain  between  their  minimum  and  maximum  distention. 


350  TEXT-BOOK  OF  PHYSIOLOGY 

VARIATIONS  IN  THE  BLOOD-PRESSURE 

A.  In  the  Arterial  Pressure. — It  is  apparent  from  the  preceding  state- 
ments that  the  arterial  blood-pressure  as  a  whole  may  be  increased  above  the 
normal,  by: 

1.  An  increase  in  the  rate  or  force  of  the  heart's  contraction. 

2.  An  increase  in  the  peripheral  resistance. 

3.  An  increase  in  both  the  force  of  the  heart  and  the  peripheral  resistance; 
and  it  is  further  apparent  that  if  the  pressure  is  higher  than  the  normal  it 
may  be  lowered  to  the  normal  by  a  decrease  in  either  one  or  both  of  these 
factors. 

It  is  also  apparent  that  the  arterial  blood-pressure  as  a  whole  may  be 
decreased  below  the  normal  by: 

1.  A  decrease  in  the  rate  and  force  of  the  heart's  contraction.    " 

2.  A  decrease  in  the  peripheral  resistance. 

3.  A  decrease  in  both  the  force  of  the  heart  and  the  peripheral  resistance; 
and  it  is  again  further  apparent,  that  if  the  pressure  is  lower  than  the  normal 
it  may  be  raised  to  the  normal  by  an  increase  in  either  one  or  both  of  these 
factors. 

1.  If  when  the  arterial  pressure  is  in  a  condition  of  equilibrium  the  heart 
ejects  into  the  arteries  in  a  given  period  of  time  an  increased  quantity  of 
blood  as  a  result  of  an  increased  rate  of  contraction,  there  will  be  an  accumu- 
lation of  blood  temporarily  in  the  arteries  and  a  rise  of  pressure  (the  peripheral 
resistance  remaining  the  same),  for  the  reason  that  the  pressure  is  only 
sufficient  to  force  into  the  capillaries  a  given  volume,  in  the  same  period  of 
time.  As  the  pressure  rises  the  velocity  and  the  outflow  will  be  increased 
until  equilibrium  is  restored  though  at  a  somewhat  higher  level.  A  rise 
of  pressure  from  an  increase  in  the  rate  of  the  beat  alone  has  been  questioned, 
for  it  has  apparently  been  demonstrated  that  there  is  a  definite  relation  be- 
tween the  normal  rate  and  the  volume  discharged  from  the  ventricle,  and 
that  when  the  rate  is  increased,  the  volume  discharged  diminishes  and  hence 
the  pressure  remains  normal  or  even  falls  below  the  normal.  Nevertheless, 
if  the  inhibition  of  the  heart  be  lessened,  or  entirely  removed,  as  by  division 
of  one  or  both  vagi,  there  is  generally  a  rise  in  the  blood-pressure  coincident 
with  the  increased  cardiac  rate. 

An  increase  in  the  pressure  is  readily  brought  about  by  an  increase  in  the 
force  or  power  of  the  contraction,  the  frequency  remaining  the  same.  An 
increase  in  the  volume  of  blood  ejected  at  each  contraction  will  necessarily 
lead  to  an  accumulation.  With  the  accumulation  there  goes  an  increased 
distention  of  the  artery  and  a  corresponding  increase  of  pressure.  In  a 
short  time,  therefore,  the  increased  pressure  will  force  out  of  the  arteries 
at  a  higher  rate  of  speed  this  excess  of  blood  until  the  outflow  again  equals  the 
inflow.  This  restores  the  equilibrium  but  establishes  the  mean  pressure  at  a 
higher  level. 

2.  If  the  peripheral  resistance  is  increased  by  a  contraction  of  the  muscle 
walls  of  the  arterioles,  the  frequency  and  force  of  the  heart  remaining  the 
same,  there  will  also  be  an  accumulation  of  blood  in  the  arteries,  an  increased 
distention  and  consequent  rise  of  pressure.  The  outflow  of  blood  will  at 
the  same  time  be  diminished. 

Arteriole  contraction  and  a  consequent  rise  of  blood-pressure  may  be 


THE  CIRCULATION  OF  THE  BLOOD 


351 


experimentally  brought  about  by  an  increase  in  the  activity  of  the  general 
vaso-motor  center  due  to  the  arrival  of  nerve  impulses  transmitted  to  it  through 
certain  afferent  nerves  as  when  after  division  of  the  sciatic  nerve  in  a  curarized 
animal  the  central  end  is  stimulated  with  induced  electric  currents  (Fig.  160), 
or  by  injecting  into  the  blood,  adrenalin,  which  it  is  g'nerally  believed  acts 
on  the  muscle  fibers  of  the  vessel  walls.     Fig.  161  Physiologic  causes  may  act 


Fig.  160. — A  Tracing  Showing  an  Increase  in  the  Blood-pressure  in  the  Carotid 
Artery  of  a  Rabbit  Due  to  an  Increase  in  the  Peripheral  Resistance  from  a  Contraction 
OF  the  Arterioles  Caused  by  Reflex  Stimulation  of  the  Vaso-motor  Center.  The  nerve 
stimulated  was  the  sciatic.  Stimulation  began  at  s.  The  rate  of  the  heart-beat  is  unchanged. 
With  the  cessation  of  the  stimulation  the  blood-pressure  falls  for  the  reverse  reasons. 

in  the  same  way  and  reflexly  cause  arteriole  contraction.  A  rise  of 
pressure  from  this  cause  much  beyond  the  normal  is  to  a  large  extent 
prevented  under  physiologic  conditions  by  a  simultaneous  decrease  in  the 
rate  and  force  of  the  heart-heat.  This  is  due  to  a  stimulation  of  the  peripheral 
ends  of  the  depressor  nerve,  and  a  consequent  reflex  stimulation  of  the  cardio- 
inhibitor  center,  and  not  to  a  f'irect  action  on  the  heart-muscle,  inasmuch 
as  the  effect  is  not  observed  after  division  of  the  vagi. 


Time  ^  Seconds 


Fig.  16 [. — Tracing  Showing  the  Rise  of  Blood  Pressure  in  a  Cat  after  the  intra- 
venous injection  of  minute  doses  of  adrenalin  injection  at  x.  The  abscissa  should  be  40 
millimeters  lower. 


3.  When  both  the  force  of  the  heart  and  the  peripheral  resistance  are  sim- 
ultaneously increased  there  is  a  rapid  increase  in  pressure,  the  former  factor 
tends  to  increase,  the  latter  factor,  to  decrease,  the  velocity  of  the  outflow. 
According  as  the  one  or  the  other  preponderates,  will  there  be  an  in- 
crease or  decrease  in  velocity.     If  they  balance  each   other,  there  will  be 


352 


TEXT-BOOK  OF  PHYSIOLOGY 


no  change.     A  rise  of  pressure  from  a  combination  of  these  factors  is  rather 
a  pathologic  than  a  physiologic  condition  and  is  observed  in  certain  diseases 

of  the  vascular  apparatus. 

The  converse  of  these  statements  also 
holds  true. 

T.  If  when  the  general  arterial  pres- 
sure is  in  a  condition  of  equilibrium  the 
heart  ejects  into  the  arteries  in  a  given 
period  of  time  a  lessened  quantity  of 
blood,  either  as  a  result  of  a  decrease  in 
the  rate  or  force,  there  will  soon  be  a 
diminution  of  the  arterial  distention  and 
a  consequent  fall  in  pressure.  The 
velocity  at  the  same  time  diminishes. 
This  continues  until  the  outflow  no  longer 
exceeds  the  inflow.  Equilibrium  will 
again  be  established,  but  the  pressure 
will  be  at  a  lower  level. 

The  decrease  in  the  rate  and  force  of 

■n      ,       .  rr^  ^  the  heart  may  be  brought  about  suddenly 

Fig.  162. — A  Tracing  OF  THE  Blood-  ,  1      ?i     1       i-       .     ,• 

PRESSURE  IN  THE  CAROTID  Artery  OF  A    ^r  morc  or  Icss  gradually  by  direct  stimu- 
Rabbit,  showing  a  sudden  decrease  in    lation  of  the  inhibitor  fibers  in  the  trunk 

of  the  vagus  nerve  with  strong  or  with 
weak  electric  currents  respectively  (Fig. 
162);  or,  by  a  stimulation  of  the  cardio- 
inhibitor  center  by  nerve  impulses  trans- 
mitted to  it  through  certain  afferent 
nerves,  e.g.,  the  afferent  fibers  in  the 
vagus,  when  stimulated  with  induced  electric  currents  (Fig.  163). 

2.  If  the  peripheral  resistance  is  diminished  by  a  dilatation  of  the  arteri- 


the  pressure  due  to  an  arrest  in  the  rate 
and  force  of  the  heart-beat  the  result  of 
stimulating  the  vagus  nerve  from  "  on  " 
to  "  off."  With  the  cessation  of  the  stimu- 
lation the  pressure  began  to  rise  as  the 
rate  and  the  force  of  the  heart-beat  re- 
turned. (The  abscissa  should  be  20  mm. 
lower.) 


Time  ~  Sl'cc 


Abscissa 


Fig. 


163. — Tracing  Showing  Reflex  Inhibition  of  the  Heart  and  Fall  of  Blood-press- 
ure IN  THE  Cat,  following  stimulation  of  the  central  end  of  the  vagus  nerve. 


oles,  the  heart's  contractions  remaining  the  same,  the  outflow  of  blood  at 
once  increases  and  the  existing  pressure  soon  diminishes. 

As  a  rule  a  diminution  in  peripheral  resistance  is  attended  by  an  increase 


THE  CIRCULATION  OF  THE  BLOOD 


353 


in  the  rate  or  force  of  the  heart,  and  this  is  especially  the  case  if  the  pressure 
has  been  above  the  normal. 

The  decrease  in  the  peripheral  resistance,  i.e.,  in  the  arteriole  contraction 
and  its  effect  on  the  blood-pressure  may  be  brought  by  a  depression  or  in- 
hibition of  the  activity  of  the  general  vaso-motor  center  due  to  the  arrival 
of  nerve  impulses  transmitted  to  it  by  certain  afferent  nerves,  e.g.,  the  de- 
pressor nerve  (see  page  386),  when,  after  division,  its  central  end  is  stimu- 
lated; or  by  the  inhalation  of  amyl  nitrite,  which  it  is  generally  believed, 
acts  locally  on  the  muscle  walls  of  the  arterioles  and  causes  their  relaxation 
(Fig.   164). 

3.  When  both  the  force  of  the  heart  and  the  peripheral  resistance  are  sim- 
ultaneously diminished,  there  will  be  a  rapid  fall  in  pressure.  The  former 
factor  tends  to  decrease,  the  latter  factor  to  increase  the  velocity  of  outflow. 


Time  ^Seconds 


Fig.  164. — Tracing    Showing  the  Fall  of  Blood-pressure  in  the  Cat,  following! the 
inhalation  01  amyl  nitrite.     Inhalation  begun  at  X. 

According  as  the  one  or  the  other  preponderates  will  there  be  a  decrease  or 
an  increase  in  velocity.  If  they  balance  each  other  there  will  be  no  change. 
This  condition  is  also  a  pathologic  rather  than  a  physiologic  condition  and 
observed  in  states  of  profound  depression  due  to  serious  injuries. 

Local  Variations  in  the  Arterial  Blood-supply. — The  variations  in 
pressure  and  velocity  from  variations  either  in  the  activity  of  the  heart  or  in 
the  peripheral  resistance  recorded  in  preceding  paragraphs,  have  reference  to 
the  arterial  system  in  its  entirety;  but  it  is  evident  from  many  facts  that 
similar  variations  take  place  in  special  regions  or  organs  of  the  body.  Thus, 
it  is  a  well-known  fact  that  for  the  exhibition  of  the  functional  activity  of 
every  organ  there  must  be  an  increase  in  the  volume  of  blood  supplied  to  it 
in  each  unit  of  time.  This  is  accomplished  by  an  active  dilatation  of  the 
arterioles  of  the  artery  of  supply,  and  unless  the  area  or  organ  supplied  is 
large,  as  the  splanchnic  area  for  example,  there  will  be  no  necessary  diminu- 
tion in  either  the  general  blood-pressure  or  the  average  velocity.  With  the 
cessation  of  functional  activity,  there  is  no  longer  any  need  for  so  large  a 
blood-supply  and  hence  the  arterioles  contract,  diminish  the  outflow,  and  raise 
23 


354  TEXT-BOOK  OF  PHYSIOLOGY 

the  pressure.  If,  on  the  other  hand,  the  area  to  be  supplied  be  large,  as  the 
splanchnic  area,  the  dilatation  of  the  intestinal  arteries  will  be  attended  by 
such  a  large  inflow  of  blood  that  not  only  will  there  be  a  fall  of  pressure  in 
these  vessels,  but  a  fall  of  pressure  in  other  arteries  as  well,  combined  with  a 
diminution  in  velocity  through  them.  With  the  contraction  of  the  intestinal 
arteries  the  reverse  conditions  at  once  arise.  By  constant  variations  in 
the  peripheral  resistance  of  individual  arteries  in  each  and  every  region  of  the 
body,  and  in  association  with  variations  in  the  rate  or  force  of  the  heart,  the 
blood  is  shunted  now  into  this,  now  into  that  organ  in  accordance  with  its 
functional  needs.  All  variations  in  peripheral  resistance  are  largely  brought 
about  reflexly  by  the  vaso-motor  nerves,  the  origin,  distribution,  and  mode  of 
action  of  which  will  be  considered  in  subsequent  paragraphs. 

B.  In  Capillary  Pressure. — The  pressure  in  the  capillaries,  though 
for  the  most  part  possessing  a  permanent  value,  is  subject  to  variations  in 
accordance  with  variations  in  the  pressure  in  either  the  arterial  or  venous 
systems  or  both.  The  marked  difference  in  the  pressure  in  the  large  arteries 
and  the  capillaries  is  partly  due  to  the  resistance  offered  by  the  narrow  arteri- 
oles. If  the  latter  dilate  in  any  given  area,  the  capillary  pressure  increases 
because  of  the  propagation  into  them  of  the  arterial  pressure.  The  reverse 
condition  would  decrease  the  pressure.  On  the  other  hand,  any  interference 
with  the  outflow  from  any  given  area,  due  to  venous  compression,  would 
likewise  increase  the  pressure;  any  factor  which  would,  on  the  contrary, 
favor  the  outflow  would  decrease  the  pressure.  Independent  of  any  change 
in  the  arteriole  resistance,  it  is  evident  that  a  rise  in  arterial  pressure  alone  would 
increase  the  capillary  pressure.  If  both  arterial  and  venous  pressures  rise, 
the  capillary  pressure  increases;  if  both  fall,  it  decreases. 

C.  In  Venous  Pressure. — Independent  of  any  change  in  the  venous 
pressure  in  a  given  area  from  local  or  temporarily  acting  causes — e.g.,  aspira- 
tion of  the  thorax  or  heart,  muscle  contractions,  change  of  position,  etc. — the 
general  venous  pressure  will  be  increased  by  a  decrease  in  the  value  of  those 
factors  which  produce  the  difference  of  pressure  between  the  arteries  and 
veins.  An  increase  in  the  value  of  these  factors  would  necessarily  decrease 
the  pressure. 

Variations  in  the  Relation  of  the  Arterial  and  Venous  Pressures. — 
So  long  as  the  heart  maintains  a  given  rate  and  force  and  the  resistance  at 
the  periphery  of  the  arterial  system  (due  to  the  contraction  of  the  arteriole 
muscle)  a  given  value,  will  the  usual  physiologic  difference  between  the  pres- 
sure in  the  arteries  and  veins  remain  unchanged.  If,  however,  either 
factor  changes  in  one  direction  or  another,  there  will  arise  a  change  in  the 
.  relative  degree  of  pressure  in  the  different  divisions  of  the  vascular  apparatus. 
Thus  if  the  heart  force  increases  and  a  larger  volume  of  blood  is  discharged 
into  the  arteries  in  a  unit  of  time,  the  amount  of  blood  in  the  venous  system 
diminishes,  and  the  result  is  a  rise  of  the  arterial  and  a  fall  of  the  venous 
pressures.  If,  on  the  contrary,  the  heart  force  decreases  or  the  mitral  valve 
permits  of  a  regurgitation,  a  smaller  volume  of  blood  is  ejected  into  the 
arteries  in  a  unit  of  time,  the  amount  of  blood  in  the  venous  system  increases, 
and  the  result  is  a  fall  of  the  arterial  and  a  rise  of  the  venous  pressure. 

Again  if  the  arteriole  muscle  relaxes  and  a  larger  volume  of  blood  flows 
from  the  arteries  into  the  veins  in  a  unit  of  time,  the  result  will  be  a  fall  of 
arterial  and  a  rise  of  venous  pressure.     If,  on  the  contrary,  the  arterial  muscle 


THE  CIRCULATION  OF  THE  BLOOD  355 

contracts  and  a  smaller  volume  of  blood  flows  into  the  veins,  the  reverse 
change  of  pressure  obtains. 

The  Determination  of  the  Arterial  Blood-pressure  in  Man. — Inas- 
much as  the  blood-pressure  undergoes  considerable  variation  in  both  physio- 
logic and  pathologic  conditions  as  well  as  in  response  to  the  action  of  drugs, 
it  seemed  desirable  to  possess  some  means  by  which  an  accurate  knowledge 
of  the  pressure  under  a  variety  of  conditions  could  be  obtained  both  for 
diagnostic  and  therapeutic  purposes.  The  foregoing  method  of  obtaining 
the  blood-pressure  not  being  of  general  application  to  human  beings  for 
obvious  reasons,  special  instruments  have  been  devised  by  which  the  pres- 
sures may  be  determined  at  least  approximately  without  resorting  to  any 
surgical  procedure. 

By  reason  of  the  fact  that  both  the  systolic  and  diastolic  pressures  are 
regarded  as  important  factors  in  clinical  conditions,  and  their  determination 
of  value  for  diagnostic  and  therapeutic  purposes  in  diseases  of  the  circulatory 
apparatus,  it  is  desirable  to  have  clear  ideas  of  what  is  meant  by  these  terms. 
If  the  changes  of  pressure  are  registered  by  a  sphygmograph,  a  curve  re- 
sembling that  shown  in  Fig.  173,  page  367,  will  be  recorded,  which  shows 
more  or  less  accurately  the  qualitative,  if  not  the  quantitative  variations,  of 
the  pressure  in  the  arteries  during  a  cardiac  cycle. 

Systolic  pressure  may  be  defined  as  the  highest  pressure  developed  in 
the  artery  during  the  systole  of  the  heart  and  occurs  in  the  first  half  of  the 
systole  and  is  therefore  of  very  short  duration;  after  this  the  pressure  begins 
to  fall,  but  endures  until  the  close  of  the  systole,  indicated  in  this  curve  by  the 
notch  preceding  the  elevation  c. 

Diastolic  pressure  may  be  defined  as  the  lowest  pressure  in  the  artery 
during  the  diastole  of  the  heart  and  occurs  at  the  end  of  the  diastolic  period 
just  before  the  time  of  the  succeeding  systole.  The  diastolic  pressure 
gradually  falls  from  the  time  of  closure  of  the  semilunar  valves  to  the  end 
of  the  diastolic  period. 

The  instruments  by  which  these  pressures  are  determined  are  called 
sphygmomanometers.  Some  of  the  many  forms  of  this  instrument  are 
adapted  for  obtaining  the  systolic  pressure  only,  while  others  are  adapted 
for  obtaining  either  the  systolic  or  the  diastolic  pressure,  or  both. 

The  principle  involved  in  the  first  group  is  the  application  of  a  hydrostatic 
pressure  to  an  artery,  e.g.,  the  temporal,  radial,  etc.,  until  the  lumen  is  com- 
pletely obliterated  as  indicated  by  the  disappearance  of  the  pulse  beyond  the 
point  of  compression,  and  at  the  same  time  the  registration  of  the  pressure 
applied,  by  means  of  a  mercurial  or  spring  manometer.  The  pressure  just 
sufficient  to  obliterate  the  pulse  or  to  allow  it  to  reappear  after  obliteration, 
is  taken  as  the  systolic  pressure. 

The  principle  involved  in  the  second  group  is  based  on  a  suggestion  of 
Marey,  that  the  maximum  pulsation  of  the  artery  or  the  maximum  distention 
and  recoil  following  a  heart-beat  would  be  most  likely  to  take  place  when 
an  elastic  pressure  applied  to  the  outside  of  an  artery  is  just  sufficient  to 
equalize  the  diastolic  pressure  within.  Inasmuch  as  these  pulsations  can 
be  transmitted  to,  taken  up  and  reproduced  by  a  mercurial  column  in  connec- 
tion with  the  pressure  appliances,  it  becomes  possible,  when  the  maximum 
oscillation  of  the  mercurial  column  is  attained,  to  read  off  the  diastolic 
pressure. 


356  TEXT-BOOK  OF  PHYSIOLOGY 

The  truth  of  this  suggestion  was  subsequently  demonstrated  by  Howell 
and  Brush.  These  experimenters  connected  the  right  carotid  artery  of  a 
dog  with  a  mercurial  manometer,  interposing  along  the  course  of  the  con- 
necting tube  a  maximum  and  a  minimum  valve.  The  left  carotid  artery 
was  surrounded  by  a  ])lethysmograph  which  was  connected,  with  both  a 
mercurial  and  a  spring  manometer,  the  former  for  the  purpose  of  indicating 
the  pressure  necessary  to  obtain  the  greatest  oscillation,  the  latter  for  the  pur- 
pose of  magnifying  and  recording  the  pulsation.  When  the  observations  were 
simultaneously  made  it  was  found  that  the  diastolic  pressure  in  the  right 
carotid  measured  by  the  minimum  manometer  was  almost  exactly  equal 
to  the  pressure  measured  by  the  manometer  in  connection  with  the  sphyg- 
momanometer surrounding  the  left  carotid  artery,  when  it  was  exhibiting 
its  maximum  excursions.  The  difference  in  the  results  of  the  two  sides 
scarcely  exceeded  more  than  one  or  two  millimerers  of  mercury.  It  was, 
therefore,  established  that  the  greatest  oscillations  record  diastolic  pressure. 

Many  forms  of  sphygmomanometers  adapted  for  clinic  purposes,  with 
which  both  the  systolic  and  the  diastolic  pressures  can  be  obtained,  have 
been  devised.  With  all  forms  however,  the  pressure  is  applied  to  the  arm 
by  a  rubber  armlet  which  is  at  least  8  centimeters  wide.  This  is  the  widest 
armlet  that  can  be  adjusted  to  the  average-sized  arm  and  presents  distinct 
advantages  over  narrower  armlets.  This  armlet  is  prevented  from  expand- 
ing outward  by  a  cuff  composed  of  some  unyielding  material  held  in  position 
by  straps  which  completely  encircle  the  cuff.  The  rubber  armlet  is  con- 
nected by  stiff-walled  rubber  tubes  with  a  mercurial  manometer  on  the  one 
hand  and  with  a  rubber  bulb  or  air  piston  on  the  other  hand.  In  using  the 
apparatus  the  pressure  is  raised  by  forcing  air  into  the  closed  system — dis- 
tending the  rubber  armlet  and  with  the  same  degree  of  force — changing  the 
levels  of  the  mercury  in  the  manometer,  until  the  pulse  is  no  longer  felt  at 
the  wrist.  By  special  devices  the  pressure  is  then  carefully  lowered  until 
the  mercurial  column  begins  to  fall.  At  a  given  level  it  exhibits  a  consider- 
able oscillation  which  may  be  mistaken  for  the  actual  systolic  pressure  but 
which  is  probably  due  to  the  impact  of  the  blood  against  the  upper  edge  of 
the  rubber  portion  of  the  cuff.  If  the  column  of  mercury  be  still  further 
lowered  so  that  the  pressure  indicated  is  a  trifle  lower  than  the  systolic  pres- 
sure the  blood  will  be  forced  through  the  compressed  artery  and  give  rise 
to  a  pulse  wave,  which  may  be  felt  at  the  wrist.  The  highest  excursion  of 
the  mercurial  column  noted  by  the  eye  at  the  moment  the  pulse  reappears 
is  regarded  as  the  systolic  pressure. 

The  pressure  is  then  lowered  5  millimeters  at  a  time  and  the  oscillations  of 
the  mercurial  column  noted.  As  the  pressure  is  thus  slowly  lowered  there 
will  come  a  moment  when  the  oscillations  will  attain  a  maximum  value  and 
beyond  which  the  oscillations  again  diminish.  The  lowest  level  of  the 
mercury  column  at  the  time  of  the  sudden  termination  of  the  greatest 
oscillation  is  taken  as  the  diastolic  pressure. 

Erlanger's  sphygmomanometer  is  a  most  valuable  instrument  for  obtain- 
ing both  systolic  and  diastolic  pressure.  It  possesses  an  advantage  in  that 
it  is  provided,  in  addition  to  the  mercurial  manometer,  with  a  tambour  and 
lever  by  which  changes  in  pressure  can  also  be  recorded  on  a  revolving 
cylinder  (Fig.  165).     A  complete  description  of  this  apparatus,  the  manner 


THE  CIRCULATION  OF  THE  BLOOD 


357 


of  using  it  and  the  results  that  can  be  obtained  with  it  will  be  found  in  the 
Johns  Hopkins  Hospital  Reports,  Vol.  XII. 

With  this  apparatus,  the  lever  often  exhibits  a  considerable  oscillation 
even  when  the  pressure  exerted  on  the  arm  exceeds  the  systolic  pressure. 
It  is  difficult,  therefore,  to  determine  at  times  the  moment  at  which  the  pres- 


FiG.  165.— Erlaxger's  Sphygmomanometer. 

sure  indicated  by  the  mercurial  column  just  falls  below  the  systolic  pressure 
and  allows  the  blood  to  pass  through.  A  new  criterion  for  this  determination 
has  been  furnished  by  Erlanger.  With  a  given  speed  of  the  drum,  the  up  and 
down  strokes  of  the  lever  practically  coincide.  But  if  the  speed  of  the  drum 
be  slightly  increased  "so  that  each  wave  subtends  about  i^  to  2  mm.  of  smoked 
paper  (this  speed  is  attained  merely  by  removing  the  governor) ,  the  change 


358  TEXT-BOOK  OF  PHYSIOLOGY 

in  form  of  the  successive  waves  manifests  itself  usually  as  a  more  or  less 
abrupt  separation  of  the  ascending  and  descending  strokes  of  the  pulse  record 
(Fig.  1 66).  The  phenomenon  may  vary  somewhat  with  the  form  of  the 
pulse  wave  and  may  even  be  obscured  by  fling,  but  there  has  been  no  great 
difl&culty  in  recognizing  it  in  every  case.  It  is  often  very  clear  when  the 
tracing  shows  no  abrupt  increase  in  amplitude  whatsoever.  It  is  just  as 
accurate  an  index  to  the  systolic  pressure  as  the  'sensory  criterion'  and  that 
of  V.  Recklinghausen.  The  change  in  form  occurs  because,  at  the  moment 
the  pressure  on  the  artery  falls  below  systolic,  blood  succeeds  in  making  its 
way  beneath  the  cuff.  This  must  be  squeezed  out  before  the  lever  can  re- 
turn to  the  base  line,  whereas  at  higher  pressures  the  lever  is  raised  only 
through  the  hydraulic  ram  action  of  the  pulse  wave  upon  the  upper  edge  of 
the  cuff." 

The  conclusions  of  Erlanger  regarding  the  results  of  his  investigations 
with  this  apparatus  may  be  partially  summed  up  in  the  following  statements, 

and  as  they  hold  true  for  other  forms  of 
apparatus  which  determine  both  systolic 
and  diastolic  pressures,  they  are  here  ap- 
pended: "The  pressure  that  is  deter- 
mined by  occluding  an  artery  is  probably 
the  maximum  end  pressure  of  the  artery 
occluded.  The  pressure  determined  by 
the  method  of  maximal  oscillations  is  the 
Fig.    i66.— Tracing    Showing   the    minimum  lateral  pressure  of  the  artery 

^IlJ^'^^I^Z.nlZTrrJt^^''^Atl  compressed,  and,  therefore,  as  the  mini- 
which  the  Systolic  Pressure  is  to  be  '^  '  '         .  '  •       1 1     r 

Noted.— (Erlanger.)  mum  lateral  pressure  is  the  same  m  all  of 

the  larger  arteries,  the  pulse  pressure, 
determined  when  the  pressures  in  the  brachial  artery  are  observed,  tends 
to  approximate  the  lateral  pulse  pressure  in  the  aorta." 

Any  positive  statement  as  to  the  numerical  values  of  the  different  pres- 
sures is  somewhat  difficult  to  make  inasmuch  as  they  will  vary  within  physio- 
logical limits  in  accordance  with  the  position  of  the  body,  exercise,  charac- 
ter of  psychic  states,  digestion,  temperature,  and  other  conditions.  For 
comparative  investigations  it  is  necessary,  therefore,  to  place  the  subject  of 
the  investigation  in  one  and  the  same  position,  to  apply  the  cuff  to  the  corre- 
sponding arm,  to  use  always  a  uniform  width  of  cuff  and  to  select  the  same 
time  of  day  with  reference  to  meals,  etc. 

It  may  be  stated,  however,  that  in  adult  life  the  systolic  pressure  in  the 
brachial  artery  ranges  from  no  to  135  millimeters  of  Hg.  in  men  and  about 
10  mm.  less  in  women;  the  diastolic  pressure  ranges  from  65  to  no  mm. 
Hg.;  the  pulse  pressure  ranges  from  25  to  40  mm.  Hg. 

The  Auscultatory  Method  of  Determining  the  Blood-pressure. — 
In  1905  a  new  method  was  introduced  and  described  by  Korotkow  for  the 
determination  of  both  the  systohc  and  diastolic  pressures,  which  in  the 
experience  of  clinicians  is  more  accurate  and  satisfactory  in  both  physiologic 
and  pathologic  conditions  than  any  of  the  other  clinical  methods.  (See 
papers  by  Goodman  and  Howell  in  the  Univ.  of  Pa.  Medical  Bulletin,  Nov., 
1 9 10  and  the  American  Journal  of  the  Medical  Sciences,  September, 
191 1).     It  consists  in  the  interpretation  of  certain  sounds  heard  with  the 


THE  CIRCLT.ATION  OF  THE  BLOOD  359 

stethoscope,  in  the  artery  under  observation  when  it  is  gradually  released  from  a 
pressure  that  has  obliterated  its  lumen  in  a  given  region. 

In  the  employment  of  this  method  the  brachial  artery  is  selected  and 
compressed  in  the  usual  manner  with  a  wide  cuff  in  connection  with  a 
graduated  mercurial  manometer. 

After  the  pulse  has  been  obliterated  the  stethoscope  is  placed  over  the 
artery  below  the  cuff,  care  being  taken  to  prevent  undue  pressure.  On 
releasing  the  pressure  in  the  cuff  very  gradually  as  in  the  employment  of 
other  methods  a  series  of  sounds  corresponding  with  each  arterial  pulsation 
is  heard  as  the  pressure  falls  from  the  systolic  to  the  diastolic  level.  The  series 
of  events  are  spoken  of  as  phases  of  which  five  are  recognized. 

The  first  phase  is  characterized  by  a  loud  clear-cut  snapping  sound: 
the  second  phase  is  characterized  by  a  series  of  murmurs;  the  third  phase 
by  a  succession  of  loud  clear  snapping  sounds  which  resemble  very  closely 
those  of  the  first  phase  but  are  less  loud;  the  fourth  phase  is  inaugurated  by 
a  sudden  decrease  in  the  intensity  of  the  murmurs  of  the  third  phase  giving 
rise  to  what  is  described  as  a  dull  tone  that  rapidly  becomes  weaker  and  soon 
fades  away;  the  fifth  phase  is  one  of  silence. 

These  phases  which  are  sharply  defined  and  easily  distinguishable  are 
believed  to  be  associated  with  vibrations  of  the  arterial  walls.  The  first 
sound  is  generally  believed  to  be  due  to  the  sudden  distention  of  the  artery, 
by  the  inrush  of  blood  beneath  the  cuff,  and  indicates  the  systolic  pressure 
which  can  be  at  once  observed  by  the  height  of  the  mercury  in  the  manome- 
ter. This  sound  lasts  until  the  pressure  falls  about  14  millimeters.  The 
second  sound,  a  succession  of  murmurs,  is  believed  to  be  caused  by  whirl- 
pool eddies  in  the  blood  stream  as  it  is  propelled  from  the  partially  con- 
stricted artery  into  the  non-constricted  region  below  the  cuff.  These  murmurs 
last  until  the  pressure  falls  about  20  millimeters.  The  third  sound  is  at- 
tributed to  the  vibration  of  the  arterial  wall  but  as  the  lumen  of  the  artery 
is  so  much  greater  than  that  of  the  compressed  portion  the  rapidity  of  the 
current  is  less  and  hence  the  sound  is  neither  so  sharp  nor  pronounced. 
It  lasts  until  the  pressure  falls  about  6  millimeters.  The  transition  from  the 
second  to  the  third  sound  involves  a  fall  of  about  5  millimeters.  The  disap- 
pearance of  the  sounds  is  coincident  with  the  return  of  the  artery  to  its  nor- 
mal size  and  hence  a  cessation  of  the  vibration.  It  therefore  indicates  the 
diastolic  pressure,  which  can  at  once  be  observed  by  the  height  of  the  mer- 
cury in  the  manometer. 

The  systolic  pressure  obtained  by  this  method  corresponds  to  the  first 
sound  that  is  heard  over  the  brachial  artery  and  is  about  130  millimeters; 
the  diastolic  pressure  corresponds  with  the  cessation  of  all  sounds  and  is 
about  85  millimeters.^     The  pulse  pressure  is  therefore  45  millimeters. 

In  pathologic  states  of  the  vascular  apparatus  the  duration  and  intensity 
of  the  sounds  undergo  considerable  modification.  In  some  diseases  they 
are  quite  characteristic  and  hence  have  both  a  diagnostic  and  therapeutic 
value, 

*  The  statement  that  the  fifth  phase  is  the  index  of  minimum  pressure  is  in  dispute.  Weysse 
and  Lutz,  using  the  Erlanger  sphygmomanometer  for  comparison  wdth  the  auscultatory  phe- 
nomena, have  showm  that  the  onset  of  the  fourth  phase  occurs  as  the  oscillations  recorded  by  the 
sphygmomanometer  begin  to  decrease  in  amplitude  and,  therefore,  this  phase  rather  than  the  fifth 
phase  should  be  taken  as  index  of  minimum  pressure. 


36o  TEXT-BOOK  OF  PHYSIOLOGY 

THE  VELOCITY  OF  THE  BLOOD 

From  the  number  of  heart-beats  per  minute,  72,  and  the  amount  of  blood 
discharged  from  the  left  ventricle  at  each  beat,  80  c.c,  it  is  evident  that  the 
blood  must  be  flowing  through  the  vascular  apparatus  with  a  certain  velocity, 
for  during  the  minute  the  entire  volume  of  blood,  3684  grams,  must  have 
passed  one  and  a  half  times  through  the  heart.  Direct  observation  of  the 
escape  of  blood  from  the  central  end  of  a  divided  artery,  and  from  the  pe- 
ripheral end  of  a  divided  vein,  as  well  as  of  the  flow  through  the  capillaries 
as  seen  with  the  microscope,  shows  that  the  velocity  of  the  flow  varies  in  dif- 
ferent parts  of  the  vascular  apparatus.  In  the  arteries,  moreover,  the  flow 
is  not  quite  uniform,  but  experiences  an  alternate  acceleration  and  retarda- 
tion with  each  heart-beat.  In  the  capillaries  and  veins  the  flow  is  continu- 
ous and  uniform,  as  the  conditions  of  the  arterial  walls  are  such  as  to 
completely  overcome  the  intermittency. 

If  the  systemic  vascular  apparatus  be  conceived  of  as  a  system  of  tubes 
which  have  symmetrically  divided  and  subdivided,  and  have  again  united 
and  reunited  in  a  corresponding  manner,  it  is  clear  that  the  total  sectional 
area  will  steadily  increase  from  the  beginning  to  the  middle  of  the  system,  and 
then  as  steadily  decrease  from  the  middle  to  the  end  of  the  system.  In 
such  a  system  the  same  volume  of  blood  must  pass  through  any  given  section 
in  a  unit  of  time  if  the  balance  of  the  circulation  is  to  be  maintained.  As  the 
velocity  of  a  fluid  is  inversely  as  the  sectional  area  of  the  tubes  through  which 
it  flows,  it  follows  that  the  initial  mean  velocity  of  the  blood  in  the  aorta  will 
steadily  decrease  as  it  flows  into  the  steadily  enlarging  stream-bed  until  it  reaches 
a  minimal  value  in  the  middle  of  the  capillary  system;  and  that  it  will  again 
steadily  increase  as  it  flows  into  the  narrowing  stream-bed  until  it  reaches  the 
heart.  The  initial  mean  velocity  of  the  blood  in  the  aorta  will  not  be  attained 
in  the  venae  cavae,  for  the  reason  that  the  total  sectional  area  of  the  latter  is 
somewhat  greater  than  that  of  the  former.  The  same  facts  hold  true  for  the 
pulmonic  vascular  system. 

The  Mean  Velocity  in  the  Aorta. — From  the  well-known  fact  that  the 
velocity  with  which  a  fluid  is  flowing  through  a  tube  may  be  determined 
by  dividing  its  sectional  area  into  the  quantity  discharged  in  a  unit  of  time, 
attempts  have  been  made  to  determine  the  mean  velocity  of  the  blood  at  the 
beginning  of  the  aorta.  If  it  be  assumed  that  the  volume  discharged  at  each 
contraction  is  80  c.c,  and  the  number  of  heart-beats  per  minute  is  72,  the 
total  volume  discharged  per  minute  would  be  5760  c.c,  or  96  c.c.  per 
second.  The  sectional  area  of  the  aorta  at  its  origin  is  6.15  sq.  cm.  On 
the  principle  above  stated,  these  two  factors  would  show  a  velocity  of  156 
mm.  per  second.  This  being  the  case  the  velocity  in  the  aortic  arch  at 
least  would  be  considerably  less  than  in  the  carotid  artery  as  will  be  stated 
later,  a  fact  which  may  however  be  explained  on  the  assumption  that  owing 
to  the  curvature  of  the  aorta  and  the  extensibility  of  its  walls  the  lateral 
pressure  becomes  very  great ;  as  a  result  the  sectional  area  is  increased  and 
the  velocity  diminished.  With  the  cessation  of  the  heart's  activity,  the  elastic 
recoil  gives  an  impetus  to  the  blood  and  increases  its  velocity. 

The  Mean  Velocity  in  the  Arteries. — The  mean  velocity  of  the  blood 
in  the  larger  and  more  superficially  lying  arteries  has  been  determined  by 
Volkmann  with  the  hemodromometer,  by  Ludwig  and  Dogiel  with  the 
Stromuhr,  and  by  other  investigators  with  different  forms  of  apparatus. 


THE  CIRCULATION  OF  THE  BLOOD 


361 


Since  neither  the  blood  nor  any  particle  placed  in  it  can  be  seen  through  the 
walls  of  the  artery,  it  occurred  to  Volkmann  to  intercalate  along  the  course 
of  a  vessel  a  U-shaped  glass  tube  about  one  meter  in  length  with  a  lumen 
the  diameter  of  that  of  the  selected  vessel,  into  and  through  which  the 
blood  could  be  made  to  flow.  The  mechanic  construction  of  the  apparatus 
is  such  (Fig.  167)  that  the  blood  can  be  made  to  flow  directly  into  the  distal 
portion  of  the  artery  across  the  base  or  indirectly 
by  way  of  the  glass  tube.     Previous  to  the  inter-  X 

calation  of  the  tube  it  is  filled  with  serum  or  nor- 
mal saline  solution.  With  the  turning  of  the  cocks 
as  B  the  blood  enters  the  glass  tube  and  drives  the 
serumi  ahead  of  it  into  the  arterial  system.  From 
the  difference  in  time  between  the  moment  the 
blood  enters  and  the  moment  it  leaves  the  tube  and 
from  the  capacity  of  the  tube  the  velocity  is 
determined. 

The  Stromuhr  or  rheometer  of  Ludwig  (Fig.  168) 
is  constructed  on  the  same  principle,  but  instead  of 
the    glass    tube    having    the   same   diameter   it  is 


Fig.  167. — Volkiiann's  Hemodromome- 
TER.     C,  C.  Arterial  cannulas. 


-i—> 


Fig.  168. — Ludwig  .4xd 
Dogiel's  Rheometer.  X,  Y. 
Axis  of  rotation.  A,  B.  Glass 
bulbs.  /?,  ^.  Cannulas  inserted 
in  the  divided  artery,  e,  Cj, 
rotates  on  g,  }.    c,  d.  Tubes. 


considerably  enlarged  on  its  two  sides.  The  bulbs  are  fastened  to  a  metallic 
disc  which  rotates  around  an  axis  in  the  metallic  base  which  carries  the  tubes 
to  be  inserted  into  the  arteries.  With  this  device  it  is  possible  to  place  either 
bulb  in  connection  with  the  proximal  end  of  the  artery.     Pre\'ious  to  the 


362 


TEDT-BOOK  OF  PHYSIOLOGY 


experiment  the  proximal  bulb  is  filled  with  oil,  the  distal  bulb  with  serum 
or  normal  saline.  On  removing  the  clips  on  the  artery  the  blood  flows  into 
the  proximal  bulb  and  drives  the  oil  into  the  distal  bulb.  As  soon  as  the 
former  is  filled  with  blood  the  bulbs  are  reversed  and  the  same  relative  condi- 
tions are  attained.  This  is  repeated  a  number  of  times.  Knowing  the 
capacity  of  the  bulbs,  and  the  number  of  times  they  are  filled  in  a  given 
period,  the  total  quantity  of  blood  discharged  is  obtained.  This  divided  by 
the  sectional  area  of  the  artery  gives  the  velocity.  The  following  values 
have  thus  been  obtained:  For  the  carotid  of  the  dog,  205  to  357  mm.  per 
second;  for  the  carotid  of  the  horse,  306  mm.;  for  the  metatarsal  artery  of  the 
horse,  56  mm.  (Volkmann).  For  the  carotid  of  rabbits,  94  to  226  mm.;  for 
the  carotid  of  the  dog,  349  to  733  mm.  (Dogiel). 

The  variations  in  the  velocity  of  the  blood  in  the  arteries  during  the  differ- 
ent phases  of  the  cardiac  cycle  have  been  determined  by  Chauveau  and 

Lortet    with   the    hematachometer 


(Fig.  169).  This  consists  of  a  me- 
tallic tube  carrying  a  graduated 
disc.  At  one  point  the  tube  is 
perforated  but  covered  with  a  rub- 
ber band  through  which  passes  an 
index.  When  the  tube  is  inserted 
into  the  divided  ends  of  an  artery, 
the  current  of  blood  strikes  the 
short  arm  of  the  index  and  gives  to 
the  outer  long  arm  a  movement  in 
the  opposite  direction.  The  extent 
of  the  excursion  indicates  the  veloc- 
ity. The  apparatus  is  first  gradu- 
ated with  currents  of  water  of 
known  velocity.  With  this  instru- 
ment  Chauveau  found  that  in  the 
'^'^'~*'  li  horse  the  velocity  during  the  systole 

Fig.  169.— The  Hemodromograph  or  Chau-  .  ner  second     at  the 

VEAU    AND    Lortet.     A,    B.   Tube    inserted    in  ^^^    520    mm.    per   secona,   ai   me 
artery.     C.  Lateral  tube  connected  with  a  manome-  begmnmg    of  the  diastole  220  mm. 

ter.    b.  Index  moving  in  a   caoutchouc   mem-  per  second,  and  during  the  pause 

brane,  a.     G.  Handle.  ^  j 

150  mm.  per  second. 

The  Velocity  in  the  Capillaries. — The  rate  of  flow  in  the  capillary 
vessels  cannot  be  experimentally  determined.  It  has  been  estimated  by 
Vierordt  at  0.5  mm.  per  second  in  his  own  retinal  capillaries;  by  Weber  at 
0.8  mm;  In  frogs  the  velocity  can  be  fairly  well  determined  by  observing 
the  time  required  for  a  corpuscle  to  pass  over  one  or  more  divisions  of  an 
ocular  micrometer.  Weber  calculated  in  this  way  that  the  velocity  is  0.5 
mm.  per  second. 

As  the  velocity  varies  inversely  with  the  sectional  area,  it  becomes  possible 
to  approximately  determine  the  relation  of  the  sectional  area  of  the  capillary 
system  to  that  of  the  aorta  from  the  above-mentioned  velocities.  If  it  be 
assumed  that  the  velocity  in  the  aorta  averages  300  mm.  and  in  the  capillaries 
0.5  mm.  per  second,  then  the  sectional  area  of  the  capillaries  is  to  that  of  the 
aorta  as  600  to  i. 


THE  CIRCULATION  OF  THE  BLOOD 


363 


The  Velocity  in  the  Veins. — In  the  venous  system  the  velocity  increases 
in  proportion  as  the  sectional  area  decreases.  In  the  jugular  vein  Volkmann 
found  the  velocity  225  mm.  per  second,  which  was  about  one-half  that  in 
the  aorta  of  the  same  animal.  The  reason  for  the  slow  rate  of  movement  in 
the  jugular  vein  is  to  be  found  in  the  fact  that  the  sectional  area  of  the  com- 
bined venae  cavse  is  about  twice  that  of  the  aorta;  hence  the  relation  of  the 
sectional  area  of  the  capillary  system  to  the  sectional  area  of  the  venae  cavae 
is  about  300  to  i. 

The  blood-pressure,  the  velocity  of  the  blood,  the  sectional  area  of  the 
vascular  apparatus,  and  their  relation  one  to  the  other  are  shown  in  Fig.  170. 

The  Relations  of  Blood-pressure  and  Velocity. — Though  the  pres- 
sure of  the  blood  bears  a  definite  relation  to  the  velocity  it  must  be  kept  in 
mind  that  it  is  rather  the  difference  in  pressure  between  the  beginning  and 


Arteries.                                     Capillaries. 
Fig.  170. ,  Blood-pressure.     ,  Velocity. 


Veins. 
3 — o,  Sectional  area. 


the  termination  of  the  arterial  system,  rather  than  the  mean  pressure  that 
influences  the  velocity.  Thus,  with  a  given  force  of  the  heart  and  a  given 
peripheral  resistance,  the  velocity  will  have  a  given  value,  and  so  long  as 
these  factors  remain  constant  will  the  velocity  remain  constant,  even  though 
the  mean  pressure  should  fall,  as  from  a  hemorrhage,  or  should  rise,  as  from 
an  injection  of  some  indifferent  fluid. 

If,  however,  the  primar}'  factors,  viz. :  the  cardiac  force  or  the  peripheral 
resistance,  change  their  values  in  either  the  same  or  opposite  directions, 
there  will  be  a  change  at  once  in  the  velocity.  The  variations  in  pressure 
and  velocity,  both  in  the  same  and  opposite  directions,  which  are  theoretically 
possible  from  a  change  in  the  force  of  the  heart,  or  in  the  peripheral  resistance 
or  both,  are  shown  in  the  following  table  arranged  by  Waller.  The  plus 
sign  indicates  increase,  the  minus  sign,  decrease,  in  efl'ect. 

The  statements  herein  embodied  have  been  established  by  Marey  with  an 
artificial  schema  of  the  circulatory  apparatus,  and  by  Chauveau  and  Lortet 
by  experiments  on  animals  with  the  hemodromograph,  a  specially  devised 
apparatus  for  this  purpose. 

Though  all  the  relations  between  pressure  and  velocity  in  the  table  are 
possible,  those  which  are  most  physiological  are  probably  5  and  6,  for  in  both 
instances  there  is  a  minimum  alteration  in  pressure,  but  a  maximum  altera- 


364 


TEXT-BOOK  OF  PHYSIOLOGY 


tion  in  blood  flow  or  velocity.  The  first  instance  is  the  condition  most 
favorable  for  the  functional  activity  of  organs,  for  the  reason  that  the  volume 
of  blood  which  the  organ  receives  in  a  unit  of  time  is  increased  without  any 
change  in  pressure;  and  it  is  an  established  fact  that  within  physiological 
limits  it  is  the  volume  of  blood  which  an  organ  receives  rather  than  the  pres- 
sure under  which  it  is  received,  that  determines  its  activity.  In  the  second 
instance,  on  the  cessation  of  activity  the  velocity  is  decreased  and  the  normal 
condition  restored  without  any  appreciable  change  in  pressure. 


No. 

Heart 

Arterioles 

Blood-pressure 

Blood  flow 

I 
2 

3 

4 

5 
6 

7 
8 

(  Force  constant Resistance  increased. . . 

\  Force  constant Resistance  diminished . 

/  Force  increased  ....     Resistance  constant. . .  . 
\  Force  diminished .  .  .     Resistance  constant. . .  . 

/  Force  increased Resistance  diminished .  . 

\  Force  diminished ....    Resistance  increased . . .  . 

(  Force  increased Resistance  increased. .  . 

\  Force  diminished .  . .     Resistance  diminished . 

1  +       1  + 

1  +       1  + 
1  +      +1 

1  +       1  + 

1  -f-      +1 
+  1        1  + 

THE  PULSE 

The  Arterial  Pulse. — The  pulse  may  be  defined  as  a  periodic  expansion 
and  recoil  of  the  walls  of  the  arterial  system.  The  expansion  is  caused  by 
the  discharge  from  the  heart  into  the  arteries  of  a  volume  of  blood,  approxi- 
mately 80  c.c,  during  the  systole;  the  recoil  is  due  to  the  elastic  reaction 
of  the  arterial  walls  on  the  blood,  driving  it  forward,  into,  and  through  the 
capillaries,  during  the  diastole. 

At  the  close  of  the  cardiac  diastole  the  arterial  system  is  full  of  blood  and 
considerably  distended.  During  the  occurrence  of  the  succeeding  systole, 
a  definite  volume  of  blood  is  again  discharged  into  the  aorta.  The  incoming 
volume  of  blood  is  now  accommodated  by  the  discharge  of  a  portion  of  the 
general  blood  volume  into  the  capillaries  and  by  the  expansion  of  the  arteries 
both  in  a  transverse  and  longitudinal  direction.  The  expansion  naturally 
begins  at  the  root  of  the  aorta  and  at  the  beginning  of  the  systole.  As  the 
blood  continues  to  be  discharged  from  the  heart,  the  expansion  increases  in 
extent;  at  the  same  time  adjoining  segments  of  the  aorta  and  its  branches 
expand  in  quick  succession,  and  by  the  time  the  systole  is  completed  the 
expansion  has  traveled  over  the  entire  arterial  system  as  far  as  the  capillaries. 
With  the  cessation  of  the  systole  and  perhaps  even  before,  the  recoil  of  the 
arterial  walls  at  once  occurs,  beginning  at  the  root  of  the  aorta  and  rapidly 
passing  over  the  arteries  to  the  capillaries. 

The  mode  of  development  as  well  as  the  propagation  of  the  expansion 
and  recoil  movement  of  the  arterial  wall,  which  together  constitute  the  pulse, 
are  illustrated  in  Fig.  171  in  which  A  B  represent  the  artery  subdivided  into 
six  equal  parts  indicated  by  the  letters  a  to  g.  In  accordance  with  this 
subdivision  of  the  artery  the  systole  of  the  heart  may  be  also  divided  into  six 
parts,  during  the  first  three  of  which  the  heart  increases  in  power,  and  during 
the  last  three  of  which  it  decreases  in  power,  gradually  falling  to  zero.     The 


THE  CIRCULATION  OF  THE  BLOOD 


36- 


effect  on  the  arterial  wall  of  the  discharge  of  blood  from  the  ventricle  is 
illustrated  in  the  figure.  During  the  first  one-sixth  of  the  systole  a  certain 
volume  of  blood  is  forced  into  the  artery,  which  at  this  moment  is  already 
full  of  blood.  Of  this  volume  a  portion  moves  forward  while  another 
portion  moves  sideways  as  the  arterial  wall  begins  to  expand  under  the 
pressure  of  the  heart.  At  the  end  of  the  first  one-sixth  of  the  systole  the 
condition  of  the  arterial  wall  may  be  represented  by  the  lines  ib.  During 
the  second  one-sixth  the  artery  expands  still  more  as  the  volume  of  blood 
increases  under  the  increasing  force  of  the  heart,  so  that  at  the  end  of  the 
second  period  the  expansion  of  the  arterial  wall  is  not  only  greater  at  the 
point  a  but  in  addition  has  extended  over  a  greater  length  of  the  artery  so 
that  the  condition  of  the  artery  may  be  represented  by  the  lines  2C.  Dur- 
ing the  third  sixth  the  same  process  continues;  the  incoming  volume  of  blood 
still  further  expands  the  artery  at  a,  as  well  as  successive  portions  further  on 
as  far  as  d,  so  that  at  the  height  of  the  systolic  power  the  condition  of  the 
artery  may  be  represented  by  the  lines  2,d. 


Fig.  171. — DUGILA.M  Showing  the  Development  of  a  Pulse  Wave.     {Rollet.) 

The  force  of  the  heart  now  begins  to  decline  and  from  this  moment  on,  the 
elastic  force  of  the  artery  preponderates  and  in  consequence  the  arterial  wall 
begins  to  recoil  at  the  point  a.  At  the  end  of  the  fourth  sixth  of  the  systole, 
therefore,  the  arterial  wall  at  a,  has  recoiled  to  2,  while  the  expansion  at  a 
has  advanced  to  63  where  the  present  force  of  the  heart  plus  the  elastic  recoil 
of  the  arterial  wall  at  a,  are  equal  to  the  elastic  force  of  the  arterial  wall  at  b. 
At  this  moment  the  condition  of  the  artery  may  be  represented  by  the  lines 
2,  &3,  e.  During  the  two  remaining  sixths  of  the  cardiac  systole,  the  same 
process  continues  until,  through  elastic  recoil,  the  artery  has  returned  to  its 
original  condition  at  a,  and  the  expansion  has  extended  as  far  as  g,  while  the 
height  of  the  expansion  has  advanced  to  d^  where  the  force  of  the  systole 
and  the  force  of  the  elastic  recoil  balance  each  other.  At  the  end  of  the 
systole  the  condition  of  the  arterial  wall  may  be  represented  by  the  lines  a, 
d^,  g,  which  indicates  that  the  expansion  and  recoil  of  the  artery,  which 
together  constitute  the  pulse,  partake  of  the  form  of  a  wave  the  length  of 
which  is  represented  by  the  line  0,  0,  and  the  height  by  the  distance  c?3. 

This  expansion  and  recoil  which  thus  pass  from  the  beginning  to  the 
end  of  the  arterial  system  assumes  the  form  of  a  wave  and  therefore  is  known 
as  the  pulse-wave  or  pulse.  Preceding  and  causing  the  expansion  of  the 
arterial  system  there  is  an  increase  of  the  general  blood-pressure;  preceding 
and  leading  to  the  recoil  of  the  arterial  system,  there  is  a  decrease  of  the 
general  blood-pressure,  both  of  which  facts  are  shown  by  the  small  curves  on 
a  blood-pressure  tracing,  and  for  this  reason  the  pressure  which  causes  the 
expansion  and  recoil  is  termed  the  pulse  pressure.     It  is  defined  as  the  rhyth- 


366  TEXT-BOOK  OF  PHYSIOLOGY 

mic  change  in  pressure  at  any  given  point  of  the  arterial  system;  and  in 
amount,  is  the  difference  between  the  diastoHc  and  the  systolic  pressures,  at 
the  corresponding  points.  The  volume  of  blood  ejected  from  the  ventricle 
is  frequently  termed  the  pulse  volume. 

The  Velocity  of  Propagation  of  the  Pulse-wave. — The  propagation 
of  the  pulse-wave  from  its  origin  at  the  root  of  the  aorta  to  any  given  point 
of  the  arterial  system  occupies  an  appreciable  period  of  time.  The  difference 
in  time  between  the  systole  and  the  appearance  of  the  pulse-wave  at  the 
dorsal  artery  of  the  foot  can  be  appreciated  by  the  sense  of  touch.  The 
absolute  time  occupied  by  the  wave  in  reaching  this  point  was  determined 
by  Czermak  to  be  0.193  second.  The  rate  at  which  the  wave  is  propagated 
over  the  vessels  of  the  lower  extremity  has  been  estimated  by  the  same  ob- 
server at  II. 16  meters  per  second,  and  for  the  upper  extremities  at  but  6.7 
meters  per  second.  Other  experimenters  have  obtained  for  the  lower  ex- 
tremities somewhat  different  results,  varying  from  6.5  to  11  meters  per  second. 
Weber's  original  estimate  was  from  7.92  to  9.24  meters  per  second.  The 
slower  rate  of  movement  in  the  vessels  of  the  upper  extremities  has  been 
attributed  to  a  greater  distensibility  of  their  walls,  a  condition  unfavorable 
to  rapid  propagation.  For  this  reason  a  low  arterial  pressure  will  occasion 
a  delay  in  the  appearance  of  the  pulse- wave  in  any  portion  of  the  body;  a 
high  arterial  pressure  will  of  course  have  the  opposite  effect.  The  difference 
in  the  speed  of  the  pulse-wave  and  the  blood-current  shows  that  they  are  not 
identical  and  must  not  be  confounded  with  each  other. 

The  pulse-wave  which  thus  spreads  itself  over  the  entire  arterial  system 
with  each  systole  of  the  heart  can  be  perceived  in  certain  localities  by  the  eye, 
by  the  sense  of  touch,  and  investigated  with  various  forms  of  apparatus  or 
instrumental  means.  The  pulse-wave,  or  at  least  the  elevation  of  the  soft 
tissues  overlying  it,  can  be  seen  in  the  radial  artery,  where  it  passes  across 
the  wrist-joint,  in  the  carotid  artery,  in  the  temporal  artery,  and  in  the  arteries 
of  the  retina  under  certain  conditions,  with  the  ophthalmoscope. 

The  Radial  Pulse. — If  the  ends  of  the  fingers  are  firmly  placed  over  the 
radial  artery,  not  only  the  increase  and  decrease  of  pressure,  but  also  many 
of  the  peculiarities  of  the  pulse-wave,  may  be  perceived.  Without  much 
difficulty  it  may  be  perceived  that  the  expansion  takes  place  quickly,  the  re- 
coil relatively  slowly;  that  the  waves  succeed  one  another  with  a  certain  fre- 
quency, corresponding  to  the  heart-beat;  that  the  pulsations  are  rhythmic  in 
character,  etc. 

Inasmuch  as  the  individuality  of  the  pulse-wave  varies  at  different  periods 
of  life  and  under  different  physiologic  and  pathologic  conditions,  various 
terms  more  or  less  expressive,  have  been  suggested  for  its  varying  qualities. 
Thus  the  pulse  is  said  to  be  frequent  or  infrequent  according  as  it  exceeds  or 
falls  short  of  a  certain  average  number — 72  per  minute;  strong  or  weak 
according  to  the  energy  with  which  the  vessel  expands;  quick  or  slow,  accord- 
ing to  the  suddenness  with  which  the  expansion  takes  place  or  strikes  the 
fingers;  hard  or  soft,  tense  or  easily  compressible,  according  to  the  resistance 
which  the  vessel  offers  to  its  compression  by  the  fingers;  large,  full  or  small, 
according  to  the  volume  of  blood  ejected  into  the  aorta,  or,  in  other  words, 
the  degree  of  fullness  of  the  arterial  system. 

The  three  qualities  which  are  of  most  value  to  the  clinician  are  rate, 
strength  or  force,  and  volume. 


THE  CIRCULATION  OF  THE  BLOOD 


367 


Frequency  of  the  Pulse. — As  the  pulse  or  the  arterial  expansion  and 
recoil  is  the  direct  result  of  the  heart's  action,  its  frequency  must,  under 
physiologic  conditions,  coincide  with  that  of  the  heart.  (See  page  290.)  All 
conditions  which  modify  the  rate  of  the  heart  will  modify  at  the  same  time 
the  rate  of  the  pulse. 

For  the  purpose  of  accurately  studying  and  analyzing  the  pulse-wave  and 
its  characteristic  features,  it  is  necessary  to  obtain  graphic  records  of  the 
alternate  expansion  and  recoil  of  the  artery  under  normal  and  abnormal 
conditions.     This  is  accomplished  by  means  of — 

The  Sphygmo graph. — ^The  sphygmograph  is,  therefore,  an  apparatus 
designed  to  take  up,  reproduce,  and  record  the  alternate  expansion  and  recoil 
of  an  artery  caused  by  the  temporary  increase  and  decrease  of  pressure  fol- 
lowing each  heart-beat.  The  tracing  or  record  obtained  with  it  is  termed  the 
pulse-curve  or  the  sphygmogram.     Different  forms  of  this  apparatus  have  been 


Fig.  172. — ^VoN  Feey's  Sphygmograph.      GS.  Metal  framework.      P.  Button  attached  to  spring. 
^F.  Vertical  rod.     U.  Clock-work  which  turns  the  recording  cylinder.     VI.  Time  marker. 

devised  by  Marey,  Dudgeon,  v.  Frey,  and  many  others.  The  instrument 
of  V.  Frey  is  shown  in  Fig.  172.  This  consists,  first,  of  a  metal  framework 
GS  by  which  the  apparatus  is  fastened  to  the  arm  and  support  given  to  the 
lever,  recording  surface,  etc.  The  essential  part  is  the  spring  carrying  a  but- 
ton P,  which  is  placed  over  the  artery,  usually  the  radial,  before  it  crosses  the 
wrist-joint.  A  vertical  rod  F  transmits  the 
movement  of  the  spring  to  the  recording  lever; 
the  movements  of  the  latter  are  recorded  on  a 
small  cylinder  inclined  slightly  so  that  the 
upstroke  may  be  vertical.  A  small  electro- 
magnet serv'esto  record  the  time  relations  of  the 
changes  in  the  blood-pressure.  The  artery 
usually  selected  for  obtaining  a  sphygmogram  is 
the  radial.  This  artery  lies  quite  superficially, 
is  covered  only  by  connective  tissue  and  skin  and  is  supported  by  the  flat 
surface  of  the  radial  bone,  conditions  most  favorable  to  technical  investiga- 
tion.    When  the  apparatus  is  properly  adjusted  a  tracing  similar  to  that 


Fig.  173. — The  Pulse-curve 
OR  Sphygmogram  of  the  Radial 
Artery. 


368  TEXT-BOOK  OF  PHYSIOLOGY 

shown  in  Fig.  173  is  obtained.  This,  however,  is  not  a  tracing  of  the  pulse 
wave,  but  rather  a  record  of  the  changes  in  pressure,  their  succession  and 
time  relations,  which  follow  each  beat  of  the  heart. 

The  sphygmogram  or  pulse-curve  may  be  divided  into  two  portions: 
viz.,  a  line  of  ascent  from  a  to  b,  the  anacrotic,  and  a  line  of  descent  from  b 
to  d,  the  catacrotic  (Fig.  170).  In  normal  tracings  the  former  is  almost 
vertical  and  caused  by  the  sudden  expansion  of  the  artery  immediately 
following  the  ventricular  contraction;  the  latter  is  in  general  oblique,  occupies 
a  longer  period  of  time,  due  to  the  slow  recoil  of  the  arterial  walls,  and  is 
marked  by  several  elevations  and  depressions,  both  of  which  indicate  that 
the  restoration  to  equilibrium  is  neither  immediate  nor  uncomplicated.  One 
of  these  elevations  is  quite  constant  and  known  as  the  dicrotic  wave,  c;  the 
depression  or  notch  just  preceding  it  is  known  as  the  dicrotic  notch.  Pre- 
and  post-dicrotic  waves  are  not  infrequently  present.  The  summit  is  gener- 
ally sharp  and  pointed. 

The  vertical  direction  of  the  line  of  ascent  is  taken  as  an  indication  that 
the  arterial  walls  expand  readily,  that  the  blood  is  discharged  quickly,  and 
that  the  ventricular  action  is  not  impeded.  An  oblique  direction  of  the 
line  of  ascent  is  an  indication  that  the  reverse  conditions  obtain.  The  height 
varies  inversely  as  the  arterial  pressure,  other  things  being  equal;  being 
high  with  a  low  pressure,  and  low  with  a  high  pressure. 

The  dicrotic  elevation  shows  that  a  second  expansion  wave  is  developed 
which  interrupts  temporarily  the  recoil  of  the  arterial  walls.  The  origin  of 
this  second  expansion  has  been  the  subject  of  much  investigation,  and  at 
present  it  may  be  said  that  the  question  is  not  fully  decided.  It  is  asserted 
by  some  investigators  that  it  is  central  in  origin,  beginning  at  the  base  of  the 
aorta  and  passing  to  the  periphery;  by  others,  that  it  is  peripheral  in  origin, 
beginning  near  the  capillary  region  and  reflected  to  the  heart.  The  former 
view  is  the  one  more  generally  accepted.  According  to  it,  the  expansion  is 
the  result  of  the  sudden  closure  of  the  aortic  valves,  and  a  backward  surge 
of  the  blood  column  against  them.  The  sudden  arrest  of  the  blood  and 
its  accumulations  again  expands  the  aorta. 

The  dicrotic  notch  is,  therefore,  taken  as  the  moment  at  which  the  ven- 
tricular systole  ceases  and  the  aortic  valves  close.  From  this  fact  it  is  evi- 
dent that  immediately  after  the  first  expansion  the  pressure  begins  to  fall, 
even  though  the  ventricular  systole  continues,  owing  to  the  discharge  of  blood 
from  the  arterial  into  the  capillary  and  venous  systems.  The  height  of  the 
dicrotic  wave  or  the  depth  of  the  dicrotic  notch  is  increased  by  low  arterial 
pressure  and  highly  elastic  arteries.  Both  features  are  diminished  by  the 
reverse  conditions.  The  apex  is  sometimes  rounded  and  even  flat,  indica- 
tive of  a  great  diminution  in  arterial  elasticity.  The  sphygmogram  not 
infrequently  varies  considerably  from  the  normal  type  in  different  pathologic 
conditions  of  the  circulatory  apparatus.  A  consideration  of  these  varia- 
tions does  not  fall  within  the  scope  of  this  work. 

The  Carotid  Pulse. — The  carotid  pulse  can  be  readily  recorded  by 
applying  over  the  carotid  artery,  anterior  to  the  sternocleidomastoid  muscle, 
on  a  level  with  the  thyroid  cartilage,  a  funnel-shaped  tambour  in  con- 
nection with  a  suitable  recording  tambour  and  lever.  The  sphygmogram 
thus  obtained  resembles  in  all  essential  respects  that  obtained  from  the 
radial  artery.     It  is  often  of  advantage  in   the   investigation  of   certain 


THE  CIRCULATION  OF  THE  BLOOD  369 

problems  of  the  heart,  both  physiologic  and  pathologic,  to  record  the 
carotid  pulse  and  the  cardiac  impulse  simultaneously. 

The  Venous  Pulse. — By  this  term  is  meant  a  pulsation  of  the  large  veins 
in  the  neighborhood  of  the  heart  but  more  especially  in  the  jugular  veins. 
It  is  caused  by  variations  of  pressure  transmitted  backward  into  the  veins 
during  and  after  the  systole  of  the  auricle.  Though  the  venous  pulsation  is 
not  very  marked  in  physiologic  conditions  it  frequently  becomes  pronounced 
in  certain  pathologic  conditions  of  the  heart. 

The  pressure  variations  in  the  jugular  vein  can  be  recorded  by  applying 
over  the  vein  a  properly  constructed  tambour,  a  glass  funnel  or  a  Mackenzie 
metal  tambour  connected  with  a  suitable  recording  tambour.  A  graphic 
record  of  a  normal  venous  pulse  thus  obtained,  shown  in  Fig.  174,  is  rather 
complicated,  consisting  of  three  positive  and  three  negative  waves  which 
are  related  to  variations  of  pressure  in  the  right  auricle,  the  result  of  the 
successive  contractions  of  the  auricular  and  ventricular  walls  and  the  action 
of  intra-ventricular  structures. 


Fig.  174. — Simultaneous  Tracings  of  the  Jugular  Pulse,  the  Carotid  Pulse,  and  the 
Apex  Beat. — (Bachmami.)  .\t  the  bottom  of  the  tracing  the  time  is  given  in  the  fiftieths  of  a 
second.  The  vertical  lines  o,  i,  2,  3,  etc.,  mark  synchronous  points  on  the  curves.  A,  The 
auricular  wave;  5,  the  so-called  c  wave  caused  by  the  systole  of  the  ventricle;  v,  the  stagnation  wave 
caused  by  the  filling  of  the  auricle.  It  will  be  noticed  that  the  c  wave  (marked  5  in  the  tracing) 
occurs  at  the  beginning  of  the  ventricular  systole  as  marked  on  the  apex  beat,  and  shortly  before 
the  pulse  in  the  carotid  artery.  The  height  of  the  v  wave  is  reached  just  after  the  occurrence  of 
the  dicrotic  notch  on  the  carotid  wave,  and  coincides  with  the  opening  of  the  auriculo-ventricular 
valves;  Af,  the  negative  wave  caused  by  the  effect  of  the  ventricular  systole;  Vf,  the  negative  wave 
following  the  opening  of  the  auriculo-ventricular  valves. 

As  the  venous  pulse  is  a  very  evident  symptom  in  some  pathologic  condi- 
tions of  the  heart,  and  as  its  proper  interpretation  assists  in  the  diagnosis  of 
these  conditions,  it  has  become  of  much  significance  in  modern  clinical  medi- 
cine. For  purposes  of  interpretation  it  is  desirable  to  obtain  simultaneously 
graphic  records  not  only  of  the  venous  pulse,  but  of  the  carotid  or  radial 
pulse,  and  of  the  cardiac  impulse  as  well.  In  the  accompanying  Fig.  171 
these  three  records  are  represented. 

The  generally  accepted  interpretation  of  these  waves  is  as  follows: 
24 


370  .  TEXT-BOOK  OF  PHYSIOLOGY 

The  first  positive  wave,  a,  is  due  to  an  expansion  of  the  vein,  the  result 
of  a  sudden  rise  of  pressure.  As  it  occurs  before  the  ventricular  systole,  it 
is  pre-systolic  in  time  and  caused  by  the  contraction  of  the  auricle,  the 
effect  of  which  is  to  cause  a  temporary  retardation  of  the  blood-stream 
flowing  toward  the  auricle  and  hence  a  backward  wave  of  pressure. 

The  first  negative  wave  is  due  to  a  recoil  of  the  veins  following  a  diminu- 
tion of  the  pressure  as  the  blood  again  moves  forward  in  consequence  of  the 
relaxation  of  the  auricular  walls. 

The  second  positive  wave,  c  or  s,  is  also  caused  by  a  wave  of  positive  pres- 
sure in  the  vein,  reflected  from  the  auricle,  though  it  is  not  of  auricular  origin. 
As  it  begins  with  the  ventricular  contraction  and  develops  during  the  closed 
period,  the  protosystolic  period,  i.e.,  between  the  closure  of  the  tricuspid 
valve  and  the  opening  of  the  semilunar  valves  (see  page  287),  it  is  believed 
to  be  due  to  the  bulging  of  the  auriculo-ventricular  valve  into  the  auricular 
cavity,  by  the  still  higher  intra-ventricular  pressure  thus  diminishing  its  size 
and  raising  its  pressure. 

The  second  negative  wave,  Af,  is  due  to  a  marked  fall  of  pressure,  a  col- 
lapse of  the  walls  of  the  vein  and  a  rapid  flow  of  blood  to  the  auricular  cavity. 
These  phenomena  begin  with  the  opening  of  the  semilunar  valves  and  are 
due  in  part  to  the  relaxation  of  the  auricular  walls,  but  more  especially  to  a 
descent  of  the  more  central  portions  of  the  auriculo-ventricular  valve  or 
septum,  into  the  ventricular  cavity  in  consequence  of  the  contraction  of  the 
papillary  muscles.  The  hollow  cone  thus  formed,  enlarges  the  auricular 
cavity,  withdraws  some  of  its  contained  blood,  and  hence  lowers  the  pressure, 
which  leads  to  the  inflow  of  blood  from  the  veins  and  hastens  the  auricular 
filling. 

The  third  positive  wave,  v,  is  caused  by  a  third  wave  of  pressure  reflected 
from  the  auricle.  It  occurs  toward  the  end  of  the  ventricular  systole  and 
is  probably  due  to  a  slight  retardation  of  the  blood  flow  in  consequence  of 
the  return  of  the  auriculo-ventricular  septum  to  its  normal  position,  the 
result  of  a  relaxation  of  the  papillary  muscles,  when  the  intra-ventricular 
pressure  is  still  higher  than  the  intra-auricular  pressure. 

The  third  negative  wave,  Vf,  is  caused  by  a  third  fall  of  pressure  in  the 
vein  and  appears  very  shortly  after  the  beginning  of  the  ventricular  relaxa- 
tion, and  the  closure  of  the  semilunar  valves.  It  develops  during  the  common 
pause  of  auricles  and  ventricles.  The  fall  of  the  venous  pressure  follows 
the  passage  of  the  blood  from  the  auricle  into  the  ventricle.  It  continues 
during  the  ventricular  filling  but  disappears  on  the  return  of  the  auricular 
contraction. 

The  Volume  Pulse. — If  an  individual  artery  expands  with  each  systole 
and  recoils  with  each  diastole  of  the  heart,  the  same  is  true  of  all  arterie, 
and  as  a  result  the  volume  of  any  organ  or  part  of  the  body  must  undergo 
similar  changes.  To  such  alternate  changes  in  volume  the  term  volume 
pulse  is  given.  The  extent  to  which  an  organ  will  increase  in  volume  will 
depend  to  some  extent  on  its  elasticity.  The  reason  for  the  increase  in 
volume  is  the  resistance  offered  to  the  flow  of  blood  into  and  through  the 
capillaries;  the  decrease  in  volume  to  the  overcoming  of  the  resistance  through 
the  arterial  recoil. 

The  variations  in  volume  may  be  recorded  by  enclosing  the  organ  in  a 
rigid  glass  or  metal  vessel,   which  at  one  point  is  in  communication  with  a 


THE  CIRCULATION  OF  THE  BLOOD 


371 


recording  apparatus,  e.g.,  a  tambour  with  a  lever  or  a  piston  recorder  with 
float  and  writing  point.  The  space  between  the  organ  and  vessel  is  filled 
with  normal  saline,  air,  or  oil.  Such  an  apparatus  is  known  as  a  plethysmo- 
graph.  Fig.  175.  Many  forms  of  this  apparatus  have  been  devised  in 
accordance  with  the  character  of  the  organ — spleen,  kidney,  etc. — to  be 
investigated,  though  the  principle  underlying  them  is  essentially  the  same. 
In  addition  to  changes  in  volume  due  to  the  heart's  action,  most  organs 
undergo  additional  changes  in  volume  from  vaso-motor  and  respiratory 
causes. 

Indeed  the  plethysmographic  is  the  most  generally  employed  method 
of  showing  the  action  of  vaso-motor  nerves  in  changing  the  contraction  of  the 
arterioles  and  hence  the  outflow  of  blood.     Thus  when  an  organ  is  enclosed 


Fig.  175. — A  Plethysmograph  for  the  Arm. 

in  a  plethysmograph  and  the  arterial  contraction  increased  by  either  a  direct 
or  reflex  stimulation  of  the  vaso-motor  center  there  will  be  a  rise  in  the 
pressure,  a  diminution  in  the  outflow  of  blood  and  a  decrease  in  the 
volume  of  the  organ  under  observation;  and  on  the  contrary,  if  the  arteriole 
contraction  is  diminished  by  a  direct  or  reflex  inhibition  of  the  vaso-motor 
center  there  will  be  a  fall  of  pressure,  an  increased  outflow  of  blood  and  an 
increase  in  the  volume  of  the  organ.  From  this  it  is  learned  that  the  func- 
tional activity  of  an  organ  which  is  attended  and  conditioned  by  an  increased 
blood-supply  is  alwaj'^s  associated  with  an  increase  in  volume.  On  plethys- 
mographic records  large  undulations  are  frequently  observed  which  are 
regarded  as  of  respiratory  origin. 


THE  CAPILLARY  CIRCULATION 

In  certain  regions  of  the  body  of  many  animals  it  is  possible,  on  account 
of  the  delicacy  and  transparency  of  the  tissues,  to  observe  not  only  the  flow 


372 


TEXT-BOOK  OF  PHYSIOLOGY 


of  blood  through  the  smaller  arteries,  capillaries,  and  veins,  but  many  of 
the  phenomena  connected  with  it,  to  which  reference  has  already  been  made; 
The  structures  usually  selected  for  the  observation  of  these  phenomena  are 
the  interdigital  membranes  (Fig.  176),  the  tongue,  the  lung,  the  bladder, 
and  the  mesentery  of  the  frog.  Though  any  one  of  these  structures  will 
afford  an  admirable  view  of  the  blood  flow,  the  mesentery  for  many  reasons 
is  the  most  satisfactory.  For  a  comparison  of  the  phenomena  observed  in 
the  cold-blooded  animals  with  those  in  the  warm-blooded  animals  the  omen- 
tum of  the  guinea-pig  may  be  employed.  If  the  frog  is  the  subject  of  ex- 
periment, it  should  be  slightly  curarized  and  the  brain  destroyed  by  pithing. 
The  animal  is-  then  placed  on  a  small  board  capable  of  adjustment  to  the 

stage  of  the  microscope.  The  abdo- 
men is  then  opened  along  the  side 
and  a  loop  of  intestine  withdrawn 
and  placed  around  a  cork  ring  which 
surrounds  an  opening  in  the  side  of 
the  frog  board.  The  loop  of  the  in- 
testine should  be  so  placed  that  it  will 
lie  between  the  observer  and  the  body 
of  the  frog.  The  mesentery  thus  ex- 
posed must  be  kept  moist  with  nor- 
mal saline  solution. 

When  examined  with  low  powers 
of  the  microscope,  arteries,  veins,  and 
capillaries  will  be  found  occupying  the 
field  of  vision.  Their  general  arrange- 
ment, their  size  and  connections,  can 
be  readily  determined.  After  a  few 
preliminary  adjustments  a  region  will 
be  found  in  which  the  blood  is  flowing 
in  opposite  directions.  The  vessel  ap- 
parently carrying  blood  away  from  the 
observer  is  an  artery;  the  vessel  appar- 
ently carrying  blood  toward  the  ob- 
server is  a  vein ;  the  smallest  vessels  are  capillaries.  The  blood  in  the  artery 
is  of  a  brighter  color  than  the  blood  in  the  vein;  the  blood  in  the  capillaries 
is  almost  colorless.  The  arterial  blood-stream  not  infrequently  shows  remit- 
tency,  an  alternate  acceleration  and  retardation,  corresponding  to  each 
heart-beat;  the  capillary  and  venous  streams  are  uniform  and  continuous. 
The  relative  velocities  in  the  three  sets  of  vessels  are  indicated  by  the  move- 
ment of  the  red  corpuscles.  In  the  arteries  they  pass  before  the  eye  so  rapidly 
that  they  cannot  be  distinguished;  in  the  capillaries  they  pass  so  slowly  that 
both  form  and  structure  may  be  determined;  in  the  veins,  though  again 
moving  rapidly,  they  can  often  be  distinguished. 

The.  relative  positions  of  the  red  and  white  corpuscles  in  the  blood- 
stream are  also  apparent;  the  former  occupy  the  central,  the  latter  the  per- 
ipheral portion,  at  the  same  time  adhering  to  the  sides  of  the  vessel.  Be- 
tween the  axial  portion  of  the  stream  occupied  by  the  red  corpuscles  and  the 
wall  of  the  vessel  there  is  a  clear  still  layer  of  plasma,  the  result  of  an  adhe- 
sion of  the  plasma  to  the  wall.     It  is  this  feature  which  gives  rise  to  the 


Fig.  176. — The  Vessels  of  the  Frog's 
Web.  a.  Trunk  of  vein,  and  {b,  b)  its 
tributaries  passing  across  the  capillary  net- 
work. The  dark  spots  are  pigment  cells. — 
( Yeo's  ''Physiology.") 


THE  CIRCULATION  OF  THE  BLOOD 


373 


friction  between  successive  layers  of  the  blood-stream,  the  resistance  of  the 
blood  flow,  and  the  development  of  the  blood-pressure.  The  relative 
breadth  of  the  still  layer  and  amount  of  friction  are  greater  in  small  than  in 
large  vessels. 

The  volume  of  blood  passing  into  any  given  capillary  area  is  determined 
by  the  degree  of  contraction  of  the  arterioles.  Thus  on  the  application  of 
warm  saline  solution,  which  relaxes  the  arterioles,  there  is  a  large  increase 
in  the  inflow  of  blood;  vessels  previously  invisible  suddenly  come  into  view 
as  the  blood  with  its  corpuscles  passes  into  them.  On  the  application  of 
cold  water,  w^hich  contracts  the  arterioles  and  di- 
minishes the  inflow,  many  of  the  smaller  vessels 
entirely  disappear  from  view.  The  contraction  and 
relaxation  of  the  arterioles  will  therefore  deter- 
mine the  quantity  of  blood  flowing  into  and 
through  the  capillary  system. 

Migration  of  the  White  Corpuscles. — A 
phenomenon  frequently  observed  in  the  capillary 
vessels  of  the  mesentery  or  of  the  bladder  of  the 
frog  is  the  passage  of  the  white  corpuscles  through 
the  walls  into  the  surrounding  lymph-spaces.  To 
this  process  the  term  migration  or  diapedesis  is 
given.  After  the  tissues  have  been  exposed  to  the 
air  for  some  time  or  subjected  to  an  irritant,  the 
vessels  dilate  and  become  distended  with  blood. 
In  a  short  time  the  blood-stream  slows,  and  finally 
comes  to  rest.  The  condition  of  stasis  is  then 
established.  During  the  development  of  this  condi- 
tion the  white  corpuscles  accumulate  in  large  num- 
bers along  the  inner  surface  of  the  vessels  and  soon 
begin  to  pass  through  the  vessel-walls.  This  they 
do  by  protruding  a  portion  of  their  substance  and 
inserting  it  into  and  through  the  vessel-wall.  This 
once  accomplished,  the  remainder  of  the  cell  in  due 
time  follows  until  it  has  entirely  passed  out  into  the 
tissue-space.  The  opening  in  the  vessel-wall  now 
closes.  The  successive  steps  in  this  process  are 
shown  in  Fig.  177.  As  this  migration  occurs 
mainly  after  the  circulation  has  ceased  or  when 
the  tissues  present  the  phenomena  of  approaching 
inflammation,  it  is  difficult  to  state  in  how  far  it  is  strictly  a  physiologic 
process. 

The  Venous  Circulation. — The  blood,  having  passed  through  the 
capillary  vessels,  is  gathered  up  by  the  veins  and  conveyed  to  the  right  side 
of  the  heart.  As  the  veins  converge  and  unite  to  form  larger  and  larger 
trunks  the  sectional  area  gradually  diminishes,  and  hence  the  velocity  of  the 
blood-flow  increases,  though  it  never  attains  the  velocity,  even  in  the  vense 
cavae,  that  it  had  in  the  aorta,  for  the  reason  that  the  sectional  area  of  the 
venae  cavse  is  considerably  larger  than  that  of  the  aorta.  The  pressure  also 
is  very  low  in  the  larger  veins  because  the  friction  still  to  be  overcome  is 
relatively  very  slight. 


Fig.  177.—  Diagram  to 
SHOW  Various  Stages  in 

THE  DIAPEDESIS  OR  MI- 
GRATION OF  White  Cor- 
puscles. 


374  TEXT-BOOK  OF  PHYSIOLOGY 

The  capacity  of  the  venous  system  is  considerably  greater  than  that  of 
the  arterial  system,  as  there  are  usually  two  and  even  three  veins  accom- 
panying each  artery.  This,  taken  in  connection  with  its  greater  disten- 
sibility,  makes  of  the  venous  system  a  reservoir  in  which  blood  can  be  stored. 
On  this  reservoir  the  arterial  system  can  call  for  that  amount  of  blood 
necessary  for  the  maintenance  of  its  normal  volume  and  pressure,  and  into 
it  any  excess  can  be  discharged.  The  relative  amounts  of  blood  contained 
in  the  two  systems  are  regulated  by  the  degree  of  contraction  of  the  arteriole 
muscles  and  this  in  turn  by  the  vaso-motor  nerves.  The  movement  of  the 
blood  through  the  veins  is  accomplished  by  the  cooperation  of  several 
forces,  reference  to  which  will  be  made  in  a  following  paragraph. 

The  Pulmonic  Vascular  Apparatus. — ^The  pulmonic  vascular  appara- 
tus consists  of  a  closed  system  of  vessels  extending  from  the  right  ventricle 
to  the  left  auricle,  and  includes  the  pulmonic  artery,  capillaries,  and  pul- 
monic veins.  In  its  anatomic  structure  and  physiologic  properties  it  closely 
resembles,  with,  the  systemic  apparatus. 

The  stream-bed  widens  from  the  beginning  of  the  pulmonic  artery  to 
the  middle  of  the  capillary  system;  it  again  narrows  from  this  point  to  the 
terminations  of  the  pulmonic  veins. 

The  movement  of  the  blood  from  the  beginning  to  the  end  of  the  system 
is  due  to  a  difference  of  pressure  between  these  two  points,  the  result  of  the 
friction  between  the  blood  and  the  vascular  walls.  The  pressure  in  the  pul- 
monic artery  of  the  dog  has  been  shown  by  Beutner  to  be  about  one-third 
that  in  the  aorta;  by  Bradford  and  Dean  to  be  one-fifth.  Wiggers  has 
recently  shown  that  in  normally  breathing  dogs  with  arterial  pressures  ran- 
ging from  no  to  112  mm.  of  mercury,  the  maximal  or  systolic  pressure  in  the 
pulmonic  artery  averaged  36  mm.,  and  the  minimal  or  diastolic  averaged 
5  mm.  The  reason  for  the  low  pressure  may  be  found  in  the  large  size  and 
rich  development  of  the  pulmonic  capillaries  and  the  imperfect  development 
of  an  arteriole  muscle  at  the  periphery  of  the  pulmonic  artery,  the  result  of 
which  is  a  diminution  in  the  friction.  Inasmuch  as  the  friction  is  relatively 
low,  the  work  of  the  right  heart  is  less  than  that  of  the  left  heart  and  hence 
its  walls  are  not  so  well  developed.  The  pulmonic  pressure  being  low  the 
intraventricular  pressure  of  the  right  heart  is  relatively  low  as  compared 
with  that  of  the  left  heart.  The  velocity  of  the  blood-stream  in  each  of  the 
three  divisions  of  the  system  cannot  well  be  determined.  The  time  occupied 
by  a  particle  of  blood  in  passing  from  the  right  to  the  left  ventricle  has  been 
estimated  at  one-fourth  the  time  required  to  pass  from  the  left  to  the  right 
ventricle.  Assuming  the  latter  to  be  thirty  seconds,  the  former  would  be 
seven  and  one-half  seconds. 

The  presence  of  vaso-motor  nerves  in  the  walls  of  the  arterioles  of  the 
pulmonic  artery  has  not  been  definitely  determined.  Adrenalin  which  con- 
stricts blood  vessels  supplied  with  vaso-motor  nerves  is  stated  by  some 
investigators  to  have  this  effect,  but  by  others  to  be  without  it.  If  vaso-motor 
nerves  are  present  their  action  is  relatively  slight. 

The  capillary  vessels  are  spread  out  in  a  very  elaborate  manner  just 
beneath  the  inner  surface  of  the  pulmonic  air-cells,  and  form,  by  their 
close  relation  to  it,  a  mechanism  for  the  excretion  of  carbon  dioxid  and  the 
absorption  of  oxygen.  The  extent  of  the  capillary  surface  is  very  great. 
It  has  been  estimated  at  200  square  meters.     The  amount  of  blood  flowing 


THE  CIRCULATION  OF  THE  BLOOD  375 

through  this  system  hourly  and  exposed  to  the  respiratory  surface  is  about 
430  liters.  The  reason  for  the  existence  of  the  pulmonary  circulation  is 
the  renewal  of  the  oxygen  in  the  blood  and  the  elimination  of  the  carbon 
dioxid;  for  the  accomplishment  of  both  objects  ample  provision  is  here 
made.  The  flow  of  blood  through  the  cardio-pulmonary  vessels  is  subject 
to  variation  during  both  inspiration  and  expiration  in  consequence  of  their 
relation  to  the  respiratory  apparatus.  The  mechanism  by  which  these 
variations  are  produced  will  be  considered  in  the  chapter  devoted  to 
Respiration. 

FORCES  CONCERNED  IN  THE  CIRCULATION  OF  THE  BLOOD 

1.  The   Contraction   of  the  Heart. — The  primary  forces  which   keep 

the  blood  flowing  from  the  beginning  of  the  aorta  to  the  right  side  of  the 
heart  and  from  the  beginning  of  the  pulmonary  artery  to  the  left  side  are 
the  contractions  of  the  left  and  right  ventricles  respectively.  This 
is  evident  from  the  fact  that  each  ventricle  at  each  contraction  not 
only  overcomes  the  pressure  in  the  aorta  and  pulmonary  artery,  the 
sum  of  all  resistances,  but  imparts  a  given  velocity  to  the  blood.  Since 
the  pressure  continuously  falls  from  the  beginning  to  the  end  of  each 
system,  it  follows  that  the  blood  must  flow  from  the  point  of  high  to 
the  point  of  low  pressure.  During  the  interval  of  the  heart's  activity, 
the  walls  of  the  arteries,  to  which  the  heart's  energy  was  largely  trans- 
ferred, now  take  up  and  continue  the  work  of  the  heart,  and  by  recoil- 
ing drive  the  blood  forward  and  into  the  venous  system.  Though 
the  heart's  energy  is  probably  sufi&cient  to  drive  the  blood  into  the 
opposite  side  of  the  heart,  it  is  supplemented  by  other  forces — e.g.: 

2.  Muscle  Contraction. — As  a  result  of  the  relation  which  the  veins  bear 

to  the  muscles  in  all  parts  of  the  body  it  is  clear  that  with  the  contrac- 
tion and  relaxation  of  the  muscles  there  will  be  exerted  an  intermittent 
pressure  on  the  veins.  With  each  contraction,  the  blood  on  the  proxi- 
mal side  will  at  once  be  driven  forward  with  increased  velocity,  while 
that  on  the  distal  side  will  be  retarded,  will  accumulate  and  distend 
the  veins,  owing  to  the  closure  of  the  valves;  with  the  relaxation  of  the 
muscle  the  elastic  and  contractile  tissues  in  the  walls  of  the  veins  will 
come  into  play  and  force  the  blood  forward. 

3.  Thoracic  Aspiration. — The  inspiratory  movement  aids   the  flow  of 

blood  through  the  vense  cavge  and  their  tributaries.  With  each  inspira- 
tion the  pressure  within  the  thorax  but  outside  the  lungs  undergoes  a 
diminution  more  or  less  pronounced  in  accordance  with  the  extent  of 
the  movement.  As  a  result,  the  blood  in  the  large  veins  outside  of  the 
thorax,  being  subjected  to  a  pressure  greater  than  that  in  the  thorax, 
flows  more  rapidly  toward  the  heart.  With  each  expiration  the  re- 
verse obtains. 

4.  Action  of  the  Valves. — It  is  quite  probable  that  gravity  opposes  to 

some  extent  the  flow  of  blood  through  the  veins  below  the  level  of  the 
heart.  This  opposition  to  the  upward  flow  is  largely  prevented  by 
the  valves,  for  each  retardation  is  immediately  checked  by  their  closure 
and  support  given  to  the  column  of  blood.  The  influence  of  gravity 
is  shown  when  the  relation  of  the  arm  to  the  heart  is  changed.     Thus, 


376  TEXT-BOOK  OF  PHYSIOLOGY 

if  the  arm  be  allowed  to  hang  passively  by  the  side  of  the  body,  the 
veins,  as  seen  on  the  back  of  the  hand,  will  become  distended  with 
blood.     If  now  the  arm  be  raised,  the  blood  will  flow  rapidly  toward 
the  heart,  as  shown  by  the  rapid  emptying  of  the  veins. 
Work  Done  by  the  Heart. — The  work  which  the  left  ventricle  per- 
forms at  each  contraction  when  it  discharges  its  contained  volume  of  blood 
into  the  aorta  is: 

1.  To  overcome  the  total  resistance  of  the  systemic  vascular  apparatus. 

2.  To  impart  velocity  to  the  blood. 

The  resistance  may  be  expressed  in  terms  of  aortic  pressure.  The 
pressure  in  the  aorta  has  not  been  absolutely  determined,  though  for  many 
reasons  it  may  be  assumed  to  be  about  150  mm.  Hg.,  or  its  equivalent,  a 
column  of  blood  1.93  meters  in  height.  If  the  volume  of  blood  which  the 
heart  discharges  is  assumed  to  be  83  grams,  the  work  done  may  be  calculated 
by  multiplying  the  weight  by  the  height  to  which  it  is  raised:  viz., 
0.083X1.93  =  0.16019  kilogrammeter. 

The  velocity  of  the  blood  in  the  aorta  has  been  approximately  estimated 
at  0.5  meter  per  second.  The  work  done  in  imparting  this  velocity  to  83 
grams  is  estimated  by  squaring  the  velocity  and  dividing  by  the  accelerating 
force  of  gravity  (j^'^)  and  multiplying  the  quotient  by  0.083.  The 
value  of  the  fraction  given  above  represents  the  distance  a  body  would  have 
to  fall  to  acquire  this  velocity:  viz.,  0.0127  meter.  The  work  done  is  there- 
fore 0.083X0.0127,  or  0.01054  kilogrammeter. 

The  entire  work  of  the  left  ventricle  is  the  sum  of  these  two  amounts, 
or  0.17073  kilogrammeter.  Assuming  that  the  heart  beats  72  times  per 
minute,  the  work  done  daily  would  be  0.17073X72X60X24,  or  17701.3 
kilogrammeters.  The  right  ventricle  approximately  performs  one-third  of 
this  amount  of  work  in  overcoming  the  resistance  offered  by  the  pulmonary 
system  and  in  imparting  velocity  to  its  contained  volume  of  blood.  The 
work  of  the  entire  heart  would  therefore  be  for  the  twenty-four  hours  about 
23,600  kilogrammeters. 

THE  NERVE  MECHANISM  OF  THE  VASCULAR  APPARATUS 

By  this  expression  is  meant  a  combination  of  nerves  and  nerve-centers 
by  which  the  rate  and  force  of  the  heart  contractions  and  the  contraction 
of  the  arteriole  muscles  are  maintained.  It  includes  the  cardiac  nerves 
(the  cardio-accelerator  and  the  cardio-inhibitor)  and  the  vascular  or  vaso- 
motor nerves  (the  vaso-augmentor  or  constrictor  and  the  vaso-inhibitor  or 
dilatator  nerves).  The  function  of  this  mechanism  is  to  maintain  the 
high  blood-pressure  characteristic  of  the  arterial  system,  and  to  regulate 
from  moment  to  moment,  the  quantity  of  blood  flowing  into  and  out  of  organs 
in  accordance  with  their  functional  activities.  The  cardiac  nerves  have 
been  considered  on  pages  312-313. 

Arterial  Tonus. — The  arteries,  especially  those  in  the  peripheral  region 
of  the  arterial  system,  possess  a  well-defined  layer  of  non-striated  muscle- 
fibers  arranged  in  a  circular  direction  or  at  right  angles  to  the  long  axis  of 
the  vessel.  In  the  physiologic  condition  these  arterioles  are  distended 
beyond  the  natural  condition  by  the  side  pressure  of  the  blood  flowing 
through  them,  at  the  same  time  the  muscle-fibers  are  in  a  state  of  tonic  con- 


THE  CIRCULATION  OF  THE  BLOOD  377 

traction,  thus  giving  to  the  arteries  a  certain  average  caliber  which  permits 
a  definite  volume  of  blood  to  flow  through  them  in  a  given  unit  of  time.  To 
this  condition  of  the  arterial  wall  the  term  tonus  is  applied. 

The  cause  of  this  tonic  contraction  is  not  definitely  known.  It  has 
been  attributed  to  the  action  of  local  nerve-ganglia,  to  the  pressure  of  blood 
from  within,  to  the  influence  of  organic  substances  in  the  blood,  the  prod- 
ucts of  gland  activity:  e.g.,  adrenalin  or  epinephrin. 

This  tonic  contraction  of  the  vascular  muscle  is  subject  to  increase  or  de- 
crease, to  augmentation  or  inhibition,  in  accordance  with  the  action  of  various 
agents.  An  augmentation  of  the  contraction  will  result  in  a  decrease  of 
the  caliber  and  a  reduction  in  the  outflow  of  blood.  An  inhibition  of  the 
contraction  or  relaxation  will  result  in  an  increase  both  of  the  caliber  and 
outflow  of  blood.  The  small  arteries  thus  determine  the  volume  of  blood 
passing  to  any  given  area  or  organ  in  accordance  with  its  functional 
activities. 

The  Vaso-motor  Nerves. — The  activities  of  the  vascular  muscle  are 
regulated  by  the  central  nerve  system  through  the  intermediation  of  nerve- 
fibers,  termed  vaso-motor  nerves.  Of  these  there  are  two  kinds,  one  which 
increases  or  augments  the  contraction,  the  vaso-constrictors  or  vaso-aug- 
mentors;  and  another  which  decreases  or  inhibits  the  contraction,  the  vaso- 
dilatators or  vaso-inhihitors. 

The  Vaso-constrictor  Nerves. — The  vaso-constrictor  nerves  take  their 
origin  from  nerv'e-cells  located  in  the  anterior  horns  and  lateral  gray  matter 
of  the  spinal  cord.  They  emerge  from  the  cord  in  company  with  the  fibers 
that  compose  the  ventral  roots  of  the  spinal  nerves  from  the  second  thoracic 
to  the  second  or  third  lumbar  nerves  inclusive.  A  short  distance  from  the 
cord  they  leave  the  ventral  roots  as  the  white  rami  communicantes  and  enter 
for  the  most  part  the  vertebral  or  lateral  sympathetic  ganglia.  From  the 
results  of  many  observations  and  experiments  it  is  probable  that  the  great 
majority  of  the  vaso-constrictor  nerves  terminate  in  these  ganglia;  that  is  to 
say,  it  is  here  that  the  pre-ganglionic  fibers  arborize  around  the  contained 
nerve-cells.  From  the  nerve-cells  new  fibers  arise,  the  post-ganglionic, 
which  pass  to  the  blood-vessels  of  (i)  the  body  walls;  (2)  the  fore  limbs;  (3) 
the  head,  neck  and  face;  (4)  the  hind  limbs;  and  (5)  the  abdominal  viscera. 

1.  The  vaso-constrictors  for  the  blood-vessels  of  the  walls  of  the  trunk  of 

the  body  emerge  from  the  spinal  cord  in  the  ventral  roots  of  the  spinal 
nerves  from  the  second  thoracic  to  the  third  lumbar  nerves  inclusive. 
After  a  short  course  they  leave  the  ventral  roots  and  pass  by  way  of  the 
white  rami  into  the  corresponding  thoracic  and  lumbar  ganglia  around 
the  nerve -cells  of  which  they  arborize.  From  these  ganglia  new  nerve- 
fibers  arise  which  pass  back  by  way  of  the  gray  rami,  into  the  thoracic 
and  lumbar  nerves  and  in  association  with  them  pass  directly  to  the 
blood-vessels  of  the  skin.     (Fig.  178  A.) 

2.  The  vaso-constrictors  for  the  blood-vessels  of  the  fore  limbs  emerge  from 

the  spinal  cord  in  the  ventral  roots  of  the  fourth  to  the  tenth  thoracic 
nerves  inclusive.  After  a  short  course  they  leave  the  ventral  roots,  pass 
into  the  white  rami,  thence  into  the  sympathetic  chain,  after  which  they 
take  an  upward  direction  and  terminate  around  the  cells  of  the  ganglion 
stellatum.  From  this  ganglion  the  new  nerve-fibers  enter  by  way  of 
the  gray  rami,  the  trunks  of  the  cervical  nerves  which  unite  to  form  the 


378 


TEXT-BOOK  OF  PHYSIOLOGY 


brachial  plexus  and  by  this  route  pass  to  the  blood-vessels  of  the  skin 
and  possibly  of  the  muscles  of  the  fore  limb. 
The  vaso-constrictors  for  the  blood-vessels  of  the  head,  face  and  neck 
emerge  from  the  spinal  cord  in  the  ventral  roots  of  the  first  four  thoracic 
nerves.  After  a  short  course  they  leave  the  ventral  roots  by  way  of  the 
white  rami  and  pass  into  the  sympathetic  chain  after  which  they  pass 
upward  through  the  ganglia  and  cord  to  the  superior  cervical  ganglion, 
around  the  cells  of  which  these  fibers  arborize.  From  this  ganglion 
new  fibers  arise  which  follow  the  carotid  artery  and  its'  branches  to  their 
superficial  terminations  at  least  (Fig.  lySB). 


Balbarvasoco?tstriitor  center 


Bi/Jbar  vasoconstrictor  cmter 


£7ood  pesieh  of  ?iead  &  face 

Sup.  cervical  ^u^^ff. 


Spinal  vasoconsti:  cm. 
..SpIoJicJmic  mrve 

Ih-^a/t^Iianic  f.. 

SemiJu?tar  ^arifflio/i. 
Post-ganffUonic  fibers. 

Bloodvessels  of  alciewiual  viscera 


Spin  at  vasocoiistiictor  center 


C  B 

Fig.  178. — Diagrams  showing  the  course  of  the  pre-  and  post-ganglionic  vaso-motor  fibers 
for  the  blood-vessels;  of  A,  the  walls  of  the  trunk  of  the  body;  of  B,  the  head  and  face;  and  of 
C,  the  abdominal  viscera. 

4.  The  vaso-constrictors  for  the  blood-vessels  of  the  hind  limbs  emerge  from 

the  spinal  cord  in  the  ventral  roots  of  the  tenth  thoracic  to  the  second  or 
third  lumbar  nerves  inclusive.  They  then  pass  by  way  of  the  white 
rami  into  the  corresponding  ganglia  around  the  nerve-cells  of  which 
some  of  the  fibers  arborize.  Others,  however,  descend  the  sympathetic 
chaui  to  terminate  in  successive  ganglia  as  far  as  the  third  sacral  ganglion. 
From  all  these  ganglia  new  nerve-fibers  arise  which  pass  backward  into 
the  corresponding  nerves  which  enter  into  the  formation  of  the  lumbar 
and  sacral  plexuses  and  by  this  route  reach  the  blood-vessels  of  the  skin 
of  the  lower  portions  of  the  trunk  and  hind  limbs. 

5.  The  vaso-constrictors  for  the  blood-vessels  of  the  viscera  of  the  abdominal 

cavity  emerge  from  the  spinal  cord  in  the  ventral  roots  from  the  fifth 
thoracic  downward.  After  leaving  these  roots  by  way  of  the  white 
rami  they  pass  into  and  across  the  ganglia  composing  the  thoracic  por- 


THE  CIRCULATION  OF  THE  BLOOD  379 

tion  of  the  sympathetic  chain  and  by  their  union  subsequently  assist  in 
the  formation  of   the  splanchnic  nerves  the  terminal  fibers  of  which 
arborize  around  the  cells  of  the  collateral  ganglia,  viz. :  the  semilunar, 
superior  mesenteric  renal,  etc.     From  these  various  ganglia  an  elaborate 
network  of  non-medullated  fibers  passes  to  the  blood-vessels  of  the 
stomach,  intestines,  and  other  viscera.     The  great  splanchnic  nerye  is 
one  of  the  most  important  vaso-constrictor  trunks  of  the  body,  on  ac- 
count of  the  large  vascular  area  it  controls  (Fig.  178  C). 
The  existence,  course,  distribution,  and  functions  of  the  vaso-constrictor 
nerves  have  been  determined  by  a  variety  of  methods,  physiologic  and  ana- 
tomic.    Stimulation   of   the   nerve   trunks   under  appropriate    conditions 
gives  rise  to  a  contraction,  division  to  a  dilatation  of  the  blood-vessels. 
The  physiologic  continuity  of  the  pre-ganglionic  fibers  with  the  nerve-cells 
of  the  sympathetic  ganglia  has  been  shown  by  the  intra-vascular  injection 
or  the  local  application  of  nicotin.     This  agent,  as  shown  by  Langley,  has 
a  selective  action  on  the  arborizations  of  the  pre-ganglionic  fibers,  and  when 
given  in  sufficient  doses  suspends  their  conductivity;  hence  stimulation  of 
pre-ganglionic  fibers  is  without  effect,  though   stimulation   of   the   post- 
ganglionic fibers  is  followed  by  the  usual  contraction. 

The  following  facts  will  serve  as  illustrations  of  the  functions  of  vaso- 
constrictor nerves.  Division  of  the  great  splanchnic  is  followed  by  a  marked 
dilatation  of  the  blood-vessels  of  the  intestinal  tract  and  a  decided  fall  in 
blood-pressure;  stimulation  of  the  peripheral  end  by  their  contraction  and  a 
marked  rise  in  blood-pressure.  Division  of  the  cervical  cord  of  the  sym- 
pathetic is  followed  by  dilatation  of  the  blood-vessels  of  the  side  of  the  head; 
stimulation  of  the  peripheral  end  by  their  contraction. 

The  Vaso-dilatator  Nerves. — ^The  vaso-dilatator  nerves  have  been 
found  in  some  of  the  cranial  nerves  and  in  association  with  vaso-constrictor 
fibers  in  most  of  the  nerve  trunks  of  the  body.  Thus  they  have  been 
found  in  the  nerve  of  Wrisberg,  the  glosso-pharyngeal,  in  the  nerve  trunks  of 
the  upper  and  lower  limbs,  of  the  body  walls,  of  the  abdominal  viscera  (the 
splanchnic  nerve)  and  of  the  pelvic  viscera. 

The  vaso-dilatator  nerve-fibers  that  are  found  in  some  of  the  cranial 
nerves  have  their  origin  in  nerve-cells  in  the  gray  matter  of  the  medulla 
oblongata.  From  these  cells  they  pass  out  in  the  trunk  of  the  pars  intermedia 
or  nerve  of  Wrisberg  and  in  the  trunk  of  the  glosso-pharyngeal  nerve. 
Those  fibers  which  are  contained  in  the  nerve  of  Wrisberg,  enter,  after  a 
short  course,  the  trunk  of  the  facial  nerve  and  through  its  branches  the 
great  petrosal  and  the  chorda  tympani,  are  ultimately  distributed  as  pre- 
ganglionic fibers  to  the  spheno-palatine  and  submaxillary  ganglia  re- 
spectively. From  the  spheno-palatine  ganglion  cells  post-ganglionic  fibers 
are  distributed  to  the  blood-vessels  of  the  mucous  membrane  of  the  nasal 
chambers  posteriorly,  and  to  adjacent  regions.  From  the  submaxillary 
ganglion  post-ganglionic  fibers  pass  to  the  blood-vessels  of  the  submaxillary 
and  sublingual  glands. 

The  vaso-dilatator  fibers  that  are  contained  in  the  glosso-pharyngeal 
nerve  pass  through  the  tympanic  plexus  by  way  of  Jacobson's  nerve  to  the 
otic  ganglion,  around  the  cells  of  which  their  end  branches  arborize;  from 
the  cells  of  this  ganglion  post-ganglionic  fibers  pass  to  the  walls  of  the 
blood-vessels  of  the  parotid  gland  and  of  the  cheek  and  gums. 


38o  TEXT-BOOK  OF  PHYSIOLOGY 

The  vaso-dilatator  nerve-fibers  that  are  found  in  some  of  the  sacral 
nerves  have  their  origin  in  nerv-e-cells  in  the  gray  matter  of  the  lumbar 
or  sacral  region  of  the  spinal  cord.  From  these  cells  they  pass  into  the  ventral 
roots  of  the  second  and  third  sacral  nerves  to  be  ultimately  distributed  by 
way  of  the  pelvic  nerve  to  sympathetic  gangha  in  the  pelvic  region  around 
the  cells  of  which  their  terminal  branches  arborize.  From  these  ganglia 
post-ganglionic  fibers  emerge  which  pass  to  the  blood-vessels  of  the  organs  of 
generation  and  adjacent  structures. 

Antidromic  Vaso-dilatator  Nerve-fibers. — Vasodilatator  nerve-fibers 
are  associated  with  the  vaso-constrictor  fibers  and  are  present  in  the  trunks  of 
the  spinal  nerves  and  are  distributed  to  the  blood-vessels  of  the  skin  of  the 
limbs  and  trunk.  Though  it  has  been  generally  believed  that  these  vaso- 
dilatator fibers  have  their  origin  in  nerv^e-cells  in  the  ventral  horns  of  the 
gray  matter,  that  they  pass  outward  through  the  ventral  roots  of  the  thoracic 
and  lumbar  nerves,  that  they  belong  to  the  efferent  system  of  nerves,  yet 
these  facts  have  never  been  positively  determined.  While  this  may  be  the 
correct  interpretation  doubt  has  been  thrown  upon  it  by  the  investigations 
of  Bayliss.  From  the  results  of  a  long  series  of  experiments  this  investigator 
concludes  that  vaso-dilatator  nerves  for  the  regions  of  the  body  just  men- 
tioned, do  not  leave  the  spinal  cord  in  the  ventral  roots;  that  the  vaso-dilata- 
tion  observed  on  stimulation  of  the  mixed  spinal  nerve  is  due  to  the  presence 
of  nerv' e-fibers  that  do  not  differ  from  the  ordinary  afferent  or  sensor,  posterior 
or  dorsal  root  fibers;  that  these  nerve -fibers,  moreover,  have  their  origin  in  the 
nerve-cells  of  the  ga;nglia  of  the  dorsal  roots.  From  the  fact  that  they  trans- 
mit nerve  impulses  to  blood-vessels  in  a  direction  contrary  to  that  of  other 
afferent  nerve-fibers,  the  term  antidromic  has  been  given  to  them.  The 
centers  from  which  they  arise  are  capable  apparently  of  being  aroused  to 
activity  by  impulses  transmitted  to  them  from  other  regions  of  the  body. 

These  statements  are  based  on  the  following  facts:  Stimulation  of  the 
peripheral  ends  of  the  divided  dorsal  roots  of  the  upper  thoracic  and  lumbo- 
sacral nerves  gives  rise  to  vascular  dilatation  in  the  upper  and  lower  limbs; 
separation  from  the  cord  is  not  followed  by  their  degeneration,  hence  they 
are  not  efferent  nerves;  extirpation  of  the  ganglia  of  the  dorsal  roots  is, 
however,  followed  by  their  degeneration,  hence  their  trophic  centers  are 
in  these  ganglia.  Whether  the  blood-vessels  of  the  abdominal  viscera  which 
apparently  receive  vaso-dilatator  nerve  impulses  are  supplied  by  nerves 
having  the  foregoing  origin  and  action,  is  a  subject  for  further  investigation. 

The  course,  distribution,  and  functions  of  the  vaso-dilatator  nerves  have 
been  determined  by  the  same  methods  as  those  employed  in  the  investigation 
of  the  vaso-constrictor  nerves.  Thus  division  and  stimulation  of  the  pe- 
ripheral branches  of  the  nerve  of  Wrisberg,  e.g.,  the  great  petrosal  and  the 
chorda  tympani,  is  followed  by  an  active  dilatation  of  the  blood-vessels  of 
the  nasal  chambers  and  palate,  and  of  the  blood-vessels  surrounding  the  sub- 
maxillary and  sublingual  gland.  The  inflow  of  blood  is  so  great  that  the 
submaxillary  gland  becomes  bright  red  in  color.  Its  tissues  being  unable 
to  consume  all  the  oxygen,  the  blood  emerges  in  the  veins  almost  arterial 
in  color.  Stimulation  of  Jacobson's  nerve  has  the  same  effect  on  the  blood- 
vessels of  the  parotid  gland.  Stimulation  of  the  branches  of  the  sacral 
nerves  which  collectively  constitute  the  nervus  erigens  is  followed  by  a  dilata- 
tion of  the  blood-vessels  of  the  sexual  organs. 


THE  CIRCULATION  OF  THE  BLOOD 


381 


Slow  stimulation  of  the  peripheral  ends  of  various  spinal  nerves  is  followed 
by  dilatation  of  the  blood-vessels  in  the  areas  to  which  they  are  distributed. 

From  these  and  many  other  facts  of  a  similar  character  it  is  probable 
that  the  blood-vessels  of  some  of  the  organs  at  least  are  under  the  control 
of  two  antagonistic  classes  of  nerve-fibers,  one  augmenting  the  degree  of 
their  contraction  (the  vaso-constrictors),  the  other  diminishing  it  through 
inhibition  (the  vaso-inhibitors).  Through  the  cooperative  antagonism  of 
these  two  classes  of  nerves  the  caliber  of  the  blood-vessels  and  thereby  the 
volume  of  the  blood  is  accurately  adapted  to  the  needs  of  each  organ  both 
during  rest  and  during  activity.  It  is  also  to  the  alternate  activity  of  these 
nerves  'that  the  variations  occurring  from  time  to  time  in  the  volume  of 
organs  are  to  be  attributed. 

A  general  vaso-dilatator  center  has  never  been  located  and  there  are 
many  reasons  for  thinking  that  such  a  center  has  no  anatomic  existence. 
There  are,  however,  special  or  local  vaso-dilatator  centers  in  the  medulla 
oblongata  and  in  various  regions  of  the  spinal  cord  especially  in  the  sacral 
region. 

Physiologic  Properties  of  Vaso-motor  Nerves. — ^The  vasoconstrictor 
and  the  vaso-dilatator  nerve-fibers  differ  somewhat  in  their  physiologic 
properties,  as  shown  by  the  results  of  experiment.  Thus,  when  a  mixed 
nerve,  i.e.,  one  containing  both  classes  of  fibers — e.g.,  the  sciatic — is  stimu- 
lated with  frequently  repeated  induced  currents,  the  constrictor  eft'ect  is 


A  B 

Fig.  179. — ^Plethysmogiiams  of  the  Hind-leg  of  the  Cat  following  Stimulation  of 
THE  Sciatic  Nerve.  In  A  the  rate  of  stimulation  was  sixteen  per  second,  in  B  one  per  second 
for  fifteen  seconds.     To  be  read  from  right  to  left.     (Boroditch  and  Warren.) 

the  more  pronounced,  the  dilatator  effect  being  wanting  or  prevented;  when 
stimulated  v/ith  slowly  repeated  induced  currents,  the  dilatator  effect  is 
the  more  pronounced.  These  dift'erent  eft'ects  are  strikingly  shown  in 
Fig.  179,  A  and  B. 

In  the  experiment  of  which  these  tracings  are  the  result  the  leg  of  a  cat  was 
enclosed  in  a  plethysmograph  and  the  variations  in  volume  due  to  dilatation 
or  contraction  of  the  vessels,  following  stimulation  of  the  sciatic  nerve,  were 
recorded  by  means  of  tambour  and  lever  on  a  slowly  revolving  cylinder.  In 
A  the  fall  of  the  curve  indicates  a  diminution  of  volume,  from  contraction  of 
blood-vessels  following  a  rate  of  stimulation  of  the  sciatic  nerve  of  16  per 
second  for  fifteen  seconds.  In  B  the  rise  of  the  curve  indicates  an  increase 
in  volume  from  dilatation  of  the  vessels  following  a  rate  of  stimulation  of  i 


382  TEXT-BOOK  OF  PHYSIOLOGY 

per  second  for  fifteen  seconds.     With  different  rates  of  stimulation  some- 
what different  results  are  obtained. 

After  division  of  a  mixed  nerve  the  vaso-constrictors  degenerate  and  lose 
their  influence  over  the  blood-vessel  in  from  four  to  five  days,  the  vaso- 
dilatators in  from  seven  to  ten  days,  as  shown  by  the  response  to  electrical 
stimulation. 

When  a  ner\'e  is  cooled,  the  vaso-constrictors  lose  their  irritability  before 
the  vaso-dilatators. 

Vase -constrictor  Centers. — ^The  nerve-cells  throughout  the  spinal 
cord  from  which  the  vaso-constrictor  nerves  take  their  origin  may  be  re- 
garded as  nerve-centers  which  through  their  related  nerve-fibers  cause  a 
varying  degree  of  contraction  of  the  arteriole  muscle.  In  how  far  these 
centers  are  independent  in  their  activity  it  is  difficult  to  state.  From  the 
results  of  experiments  that  have  been  made  with  a  view  of  isolating  these 
centers,  such  as  division  of  the  cord  at  different  levels,  it  is  fairly  well  proven 
that  they  respond  to  nerve  impulses  transmitted  to  them  from  different  regions 
of  the  body,  as  shown  by  the  contraction  of  blood-vessels.  This  is  especially 
true  of  lower  animals  such  as  the  frog  and  it  may  possibly  be  true  of 
mammals.  Though  it  is  probable  that  the  spinal  vaso-constrictor  cells 
possess  a  certain  degree  of  tonicity,  nevertheless  they  are  subordinate  in 
their  activity  to,  and  dominated  by,  a  group  of  nerve-cells  in  the  upper  part 
of  the  floor  of  the  fourth  ventricle,  and  termed  for  this  reason  the  medullary 
bulbar  vaso-constrictor  center. 

Though  the  blood-pressure  falls  to  a  very  low  level  after  the  separa- 
tion of  the  medulla  from  the  spinal  cord,  the  animal,  if  properly  cared  for, 
may  survive  the  operation  and  live  for  a  considerable  time.  Under  these 
circumstances  the  arteries  gradually  recover  their  former  degree  of  con- 
traction. This  is  accepted  as  evidence  that  the  nerve-cells  in  the  spinal 
cord  have  acquired  an  independent  activity,  or  developed  an  activity  that 
had  hitherto  been  dormant.  After  this,  these  nerve-centers  can  be  excited 
to  activity  by  nerve  impulses  transmitted  from  the  periphery. 

The  Medullary  or  Bulbar  Vaso-constrictor  Center. — The  existence  of 
such  a  dominating  center  has  been  determined  experimentally:  thus  if  a 
definite  region  of  the  medulla  oblongata  is  punctured  or  in  anyway  destroyed 
there  is  an  immediate  dilatation  of  the  blood-vessels  throughout  the  body 
and  a  fall  of  blood-pressure  below  one-half  or  one-third  of  the  normal  value. 
This  region  has  a  width  of  one  and  a  half  millimeters  and  extends  longitudi- 
nally for  a  distance  of  four  or  five  millimeters,  terminating  at  a  point  four 
millimeters  above  the  tip  of  the  calamus  scriptorius.  Because  of  the  effects 
that  follow  the  destruction  of  this  area  the  anatomic  existence  of  a  general 
vaso-constrictor  center  has  been  assumed. 

A  transection  of  the  medulla  above  the  upper  limit  of  this  area  is  without 
effect  on  the  blood-pressure.  A  similar  section  below  it,  however,  is  at  once 
followed  by  vascular  dilatation,  a  loss  of  vascular  tone,  and  a  general  fall  of 
blood-pressure.  Subsequent  stimulation  of  the  peripheral  end  of  the  divided 
medulla,  the  animal  being  curarized  and  artificial  respiration  maintained,  will 
give  rise  to  a  marked  contraction  of  the  blood-vessels  and  a  rise  of  blood- 
pressure  up  to  and  far  beyond  the  normal  value. 

If  the  experimental  lesion  is  limited  to  the  area  mentioned  in  the  foregoing 
paragraph,  the  vascular  dilatation  also  passes  away  after  a  time,  the  blood- 


THE  CIRCULATION  OF  THE  BLOOD  383 

vessels  regain  their  normal  tone,  and  the  pressure  again  rises.  These  and 
the  foregoing  facts  indicate  that  there  is  in  the  gray  matter  beneath  the  floor 
of  the  fourth  ventricle  a  restricted  area  composed  of  nerve-cells,  which  main- 
tains through  efferent  nerve-fibers  the  tonus  of  the  blood-vessels  by  virtue 
of  its  dominating  influence  over  the  vaso-motor  centers  in  the  cord,  and 
which  is  therefore  to  be  regarded  as  the  general  vaso-motor  {constrictor) 
center.  The  vaso-motor  centers  throughout  the  cord  are  to  be  regarded  as 
subsidiary  centers.  The  nerve-fibers  which  transmit  the  regulative  nerve 
impulses  from  the  general  to  the  subsidiary  centers  are  to  be  found  in  the 
lateral  columns  of  the  spinal  cord. 

The  Tonic  Activity  of  the  General  Vaso-constrictor  Center. — 
Since  the  blood-vessels  maintain  a  more  or  less  constant  tone  in  the  physio- 
logic condition,  it  is  assumed  that  the  vaso-motor  center  is  in  a  state  of 
continuous  or  tonic  activity  or  tonus,  and  as  a  result  continuously  discharging 
nerve  impulses  through  vaso-constrictor  nerves  to  the  blood-vessels.  The 
causes  of  this  activity  or  tonicity  have  been  difficult  to  formulate.  It  may  be 
accepted,  however,  from  the  results  of  experimentation  that  the  activity  of 
the  center  is  maintained  (i)  by  the  chemic  character  of  the  blood  and 
lymph  by  which  it  is  surrounded  and  (2)  by  a  continuous  inflow  of  nerve  im- 
pulses transmitted  from  all  regions  of  the  body,  or  to  both.  The  follow- 
ing facts  will  show  that  both  factors  are  probably  involved. 

The  group  of  nerve  cells  which  collectively  constitute  the  vaso-motor 
center  has  been  shown  by  the  experiments  of  Porter  to  consist  of  two  groups 
or  centers,  viz.,  a  vaso-tonic  center  which  maintains  the  vascular  tonus,  and  a 
vaso-reflex  center  which  permit  of  various  vaso-motor  reflexes.  After  the 
administration  of  curara  the  depressor  and  sciatic  reflex  changes  in  blood- 
pressure  are  more  than  double,  while  the  arterial  tonus  remains  substantially 
unchanged.  Alcohol  on  the  contrary  diminishes  or  removes  reflex  activity, 
but  leaves  the  tonus  unchanged. 

Central  Stimulation  of  the  Vaso-tonic  Center. — The  general  vaso- 
motor (constrictor)  center  at  least  is  markedly  influenced,  b}  the  quantity  and 
quality  of  blood  and  lymph  circulating  around  and  through  it.  If  the  blood- 
supply  to  the  medulla  and  associated  structures  be  diminished  by  compression 
of  the  carotid  arteries,  the  activity  of  the  center  is  at  once  increased,  as  shown 
by  a  rise  of  pressure,  the  result  of  increased  vascular  contraction.  Restora- 
tion of  the  blood-suply  is  followed  by  a  return  of  the  center  to  its  normal 
degree  of  activity.  An  increase  of  the  percentage  of  carbon  dioxid  in  the 
blood  or  a  decrease  in  the  percentage  of  oxygen  increases  the  activity  of  the 
center.  In  asphyxia,  a  condition  in  which  there  is  a  deficiency  in  the  elimina- 
tion of  carbon  dioxid  and  an  insufficiency  in  the  absorption  of  oxygen  the 
center  becomes  extremely  excitable  as  shown  by  the  rapid  rise  of  the  arterial 
pressure.  By  reason  of  these  and  other  facts  it  is  assumed  that  the  presence 
and  the  normal  pressure  of  CO2  in  and  around  the  vaso-motor  center  is 
the  efficient  stimulus  to  its  activtiy.  In  compression  of  the  carotid  arteries 
as  well  as  in  asphyxia,  the  CO2  is  not  removed  from  the  center  sufficiently 
rapidly  and  hence  its  pressure  increases  and  unduly  excites  the  center  as 
shown  by  the  rapid  rise  of  the  blood-pressure.  The  subsidiary  centers  in 
the  spinal  cord  are  influenced  by  corresponding  conditions. 

Reflex  Stimulation  of  the  Vaso-reflex  Center. — The  results  of  experi- 
mentation render  it  certain  that  the  vaso-reflex  center  may  be  augmented 


384  TEXT-BOOK  OF  PHYSIOLOGY 

or  inhibited  in  its  activity,  and  hence  followed  by  increased  or  decreased 
vascular  contraction,  by  nerve  impulses  transmitted  to  it  from  the  brain,  or 
from  the  periphery  through  afferent  nerves.  The  effects  may  be  local  and 
confined  to  the  area  in  which  the  stimulus  is  acting  or  they  may  be  general. 
The  following  experiments  may  be  cited  in  illustration: 

Psychic  states  of  an  affective  or  emotional  character  influence  the  ac- 
tivity of  this  center,  doubtless  as  a  result  of  the  arrival  of  nerve  impulses 
from  the  cortex  of  the  cerebrum.  Thus  it  is  well  known  that  fear  causes  a 
contraction  of  the  blood-vessels  of  the  head  and  face  and  that  shame  causes 
a  dilatation  of  the  same  vessels.  With  the  cessation  or  the  disappearance 
of  the  emotional  state,  the  blood-vessels  return  to  their  former  degree  of 
contraction. 

Stimulation  of  the  central  end  of  a  divided  posterior  root  of  a  spinal  nerve 
gives  rise  to  increased  vascular  contraction,  as  shown  by  the  rise  of  blood- 
pressure.  Stimulation  of  the  central  end  of  the  divided  sciatic  will  give  rise 
to  opposite  results,  according  to  the  strength  of  the  stimulus,  weak  stimuli 
producing  dilatation,  strong  stimuli  producing  contraction  of  the  vessels. 
Stimulation  of  the  central  end  of  the  divided  vagus  gives  rise  to  dilatation  of 
the  vessels  of  the  lips,  cheeks,  and  nasal  and  palatal  mucous  membranes. 
Stimulation  of  the  tongue  is  followed  by  dilatation  of  the  vessels  of  the  submax- 
illary gland.  Stimulation  of  certain  branches  of  the  vagus  nerve  is  followed 
by  a  passive  dilatation  of  blood-vessels  and  a  marked  fall  of  pressure. 

The  preceding  statements  as  to  the  effects  on  the  degree  of  vascular  con- 
traction, and  hence  on  the  blood-pressure  which  follow  stimulation  of  differ- 
ent afferent  nerves,  has  lead  to  the  assumption  that  there  are  in  most  afferent 
nerves  two  classes  of  nerve-fibers,  though  perhaps  in  varying  proportions,  one 
of  which  when  in  activity  augments,  the  other  of  which  when  in  activity  inhibits 
the  activity  of  the  vaso-constrictor  center.  The  former  class  is  generally 
termed  pressor  or  excitator,  the  latter  depressor  or  inhibitor  fibers. 

Theories  of  the  Mode  of  Action  of  the  Vaso-motor  Centers. — ^A 
satisfactory  explanation  of  the  manner  in  which  peripheral  stimuli  influence 
the  activity  of  the  vaso-motor  centers  and  thus  bring  about  the  opposite 
results  mentioned  in  the  foregoing  paragraphs  is  difficult  to  present.  Several 
interpretations  have  been  offered,  viz.: 

1.  The  peripheral  stimuli  according  to  their  qualities  act  on  either  the  ter- 

minals of  the  pressor  or  the  depressor  fibers;  according  as  they  do,  will 
the  vaso-constrictor  center  be  augmented  or  inhibited  in  its  activity 
and  followed  by  either  an  increase  or  decrease  in  the  degree  of  the 
previous  vascular  contraction. 

2.  The   peripheral   stimuli  act   simultaneously  on   the  terminals   of  both 

pressor  and  depressor  fibers,  and  hence  the  vaso-constrictor  center  is 
acted  on  simultaneously  by  two  antagonistic  influences.  The  resulting 
effect  on  the  blood-vessels,  viz.,  increased  or  decreased  contraction  will 
be  the  resultant  of  the  action  of  these  two  influences  on  the  vaso-con- 
strictor center.  If  the  stimuli  act  preponderantly  on  the  pressor  fibers, 
the  vaso-constrictor  center  will  still  be  excited;  if  they  act  proponder- 
antly  on  the  depressor  fibers  the  center  will  be  inhibited,  though  in  either 
case  not  to  the  same  degree  as  in  the  first  instance. 

3.  The  peripheral  stimuli  may  act  on  the  terminals  of  the  pressor  fibers  alone. 
The  nerve  impulses  thus  developed  are  transmitted  to  both  centers  but 


THE  CIRCULATION  OF  THE  BLOOD 


38- 


their  effect  will  be  to  excite  the  activity  of  the  vaso-constrictor  centers 
and  to  inhibit  the  activity  of  the  vaso-inhibitor  centers;  or  the  peripheral 
stimuli  may  act  on  the  terminals  of  the  depressor  fibers  alone.  The 
nerve  impulses  thus  developed  are  likewise  transmitted  to  both  centers 
but  their  effect  will  be  to  inhibit  the  activity  of  the  vaso-constrictor  centers 
and  to  excite  the  activity  of  the  vaso-inhibitor  centers.  In  this  view  it  is 
assumed  that  both  centers  are 
continuously  active  and  in  a 
state  of  tonus,  and  that  for 
the  necessary  vascular  reac- 
tion one  center  must  be 
stimulated,  and  the  other  be 
inhibited.  As  to  which  cen- 
ter will  be  stimulated  or  in- 
hibited, will  probably  depend 
on  the  character  of  the  stim- 
ulus. Hence,  the  general 
vascular  tonus  as  well  as  its 
variations  from  time  to  time, 
in  whole  or  in  part,  is  the 
resultant  of  the  simultaneous 
activity  and  variations  in 
activity  of  these  two  mutually 
antagonistic  but  cooperative 
centers. 
Local  Special  Vaso-dilata- 

tor  Centers. — ^The  vaso-dilatator 

centers  in  the  medulla  oblongata 

and  in  the  sacral  region  of  the 

spinal  cord,  which  give  origin  to 

nerve-fibers     that    regulate    the 

blood-supply     to     the     salivary 

glands    on  the  one  hand  and  to 

the    generative    organs    on    the 

other  hand  as  previously  stated 

(page  379)   are  ordinarily  in   a 

condition    of    relative   inactivity 

and  hence  the  blood-vessels  as- 
sume a  caliber  just  sufficient  to 

supply    the   materials    necessary 

to  maintain  the  nutritive  activi- 
ties of   the  organs  concerned. 


car.a. 


Fig.  180. — DiAGR.\M  showing  the  Origin 
AND  Relation  of  the  Depressor  Nerve  ix  the 
Rabbit.  Depr.  n.,  depressor  nerve;  vag.  n.,  vagus 
nerve;  sup.  1.  n.,  superior  laryngeal  nerve;  inf.  c.  g., 
inferior  cervical  ganglion;  sym.  n.,  sympathetic  nerve; 
car.  a.,  carotid  artery;  dig.  m.,  digastric  muscle; 
hvp.  n.,  hypoglossal  nerve;  sup.  c.  g.,  superior  cer- 
vical ganglion;  inf.  1.  n.,  inferior  laryngeal  nerve. 


If,  however,  these  centers  are  aroused  to 
activity,  either  by  nerve  impulses  descending  from  the  cerebrum  in  conse- 
quence of  psychic  states  of  an  affective  or  emotional  character,  or  by  nerve 
impulses  transmitted  to  them  through  afferent  nerves,  the  blood-vessels  of 
the  salivary  glands  and  of  the  organs  of  generation,  at  once  actively  dilate 
and  the  amount  of  blood  they  transmit  so  great  that  on  emerging  from  the 
capillaries  it  still  retains  its  arterial  hue.  The  same  phenomena  arise  if 
the  efferent  pre-ganglionic  fibers,  e.g.,  the  chorda  tympani  and  the  nerve  of 
Jacobson,  and  the  nervus  erigens  or  pelvic  nerve  are  stimulated  with  in- 

25 


386  TEXT-BOOK  OF  PHYSIOLOGY 

duced,  electric  currents.  The  organs  of  generation  in  consequence  of  the 
increased  blood  flow  exhibit  changes  in  volume  and  rigidity  similar  to  those 
exhibited  previous  to  the  act  of  coition. 

In  the  particular  instances  in  which  stimulation  of  the  peripheral  termina- 
tions of  afferent  nerves,  associated  with  the  nervus  erigens  and  chorda 
tympani,  is  followed  by  active  dilatation  of  the  blood-vessels,  it  has  been 
assumed  that  there  are  afferent  nerve-fibers  which  directly  stimulate  or 
augment  the  activity  of  the  special  vaso-dilatator  centers  and  for  this  reason 
should  be  termed  "reflex  vaso-dilatator  nerves"  (Hunt). 

The  Depressor  Nerve. — A  striking  illustration  of  the  depressor  or  inhibitor 
action  of  afferent  nerves  upon  the  vaso-constrictor  (reflex)  center  is  furnished 
by  the  result  of  stimulation  of  a  branch  of  the  vagus,  the  so-called  "depressor 
nerve."  In  the  rabbit.  Fig.  i8o,  there  is  a  small  nerve  formed  by  the  union 
of  a  branch  from  the  trunk  of  the  vagus  with  a  branch  from  the  superior 


Fig.  i8i. — Fall  of  Blood-pressure  from  Excitation  of  the  Depressor  Nerve.  The 
cylinder  was  stopped  in  the  middle  of  the  curve  and  the  excitation  maintained  for  seventeen  min- 
utes.    The  line  of  zero  pressure  (0,0)  should  be  30  mm.  lower  than  here  shovra. — (Bayliss.) 

laryngeal.  The  peripheral  distribution  of  this  nerve  is  over  the  wall  of  the 
ventricle  and  perhaps  to  some  extent  to  the  structures  of  the  aorta  near  its 
origin.  A  similar  anatomic  arrangement  is  met  with  in  the  horse,  pig,  and 
hedge-hog.  In  some  other  animals,  as  the  dog,  it  is  bound  up  in  the  vago- 
sympathetic. In  man  it  is  also  present,  though  shortly  after  its  origin  it 
enters  the  trunk  of  the  vagus.  Division  of  this  nerve  is  without  effect 
on  either  the  heart  or  the  vessels.  Stimulation  of  the  peripheral  end  has 
neither  an  accelerator  nor  an  inhibitor  action  on  the  heart.  Stimulation  of 
the  central  end  is  followed  by  a  fall  in  blood-pressure,  frequently  to  a  level 
below  one-half  the  normal  value;  at  the  same  time  there  is  a  diminution, 
brought  about  reflexly,  in  the  rate  of  the  heart-beat  (Fig.  181).  The  fall  in 
pressure,  however,  is  due  mainly  to  the  dilatation  of  the  arterioles,  the  result 
of  an  inhibition  of  the  general  vaso-reflex  center,  and  partly  to  the  diminu- 
tion in  the  rate  and  force  of  the  heart,  the  result  of  a  simultaneous  stimula- 
tion of  the  inhibitor  center.  This  latter  factor  is  of  less  importance  than  the 
former  for  the  fall  of  pressure  occurs  equally  well  after  division  of  all  the 
cardiac  nerves.  For  this  reason  the  nerve  was  termed  the  depressor  nerve 
of  the  vaso-motor  center. 

On  exposure  of  the  abdominal  cavity,  it  is  observed  during  stimulation  of 
the  depressor  that  there  is  a  notable  dilatation  of  the  intestinal  vessels. 


THE  CIRCULATION  OF  THE  BLOOD  387 

From  this  fact  it  was  assumed  that  the  action  of  the  depressor  nerve  was  to 
lower  the  general  pressure  through  reflex  dilatation  of  these  vessels.  It  has 
been  shown  by  Porter  and  Beyer  that  if  the  splanchnics  are  divided  and  the 
peripheral  end  stimulated  so  as  to  maintain  the  tonus  of  the  intestinal  vessels, 
and  hence  the  general  pressure,  stimulation  of  the  depressor  nerve  will 
nevertheless  be  followed  by  a  fall  of  the  blood-pressure  almost  as  great  as 
when  the  splanchnics  are  intact.  From  this  it  is  evident  that  the  depressor 
nerve  is  related  to  centers  which  influence  the  vascular  apparatus  in  its 
entirety.  It  has  been  supposed  that  through  it  the  heart  can  protect  itself 
from  injurious  results  of  an  excessive  rise  of  arterial  pressure. 

Thus,  when  the  intra-cardiac  pressure  or  the  intra-aortic  pressure 
rises  beyond  a  normal  amount  from  increased  resistance,-  the  peripheral 
terminations  of  this  nerve  are  stimulated  with  the  result  that  the  vaso-motor 
center  is  inhibited  and  the  arterioles  relaxed.  Through  this  means  the 
pressure  falls  and  the  work  of  the  heart  is  lessened. 

The  Traube-Hering  Waves. — Under  certain  experimental  conditions 
the  arterial  blood-pressure  tracing  exhibits,  in  addition  to  the  usual  respira- 
tory variations,  certain  longer  rhythmic  variations  more  or  less  wave-like  in 
character,  which  are  known  as  Traube-Hering  waves.  They  can  be  devel- 
oped on  a  blood-pressure  tracing  by  injecting  magnesium  sulphate  or  mor- 
phine into  the  circulation,  by  tying  the  cerebral  arteries,  etc.  These  waves 
indicate  a  periodic  contraction  and  dilatation  of  the  blood-vessels,  the  result 
of  a  periodic  stimulation  of  the  vaso-motor  center. 


CHAPTER  XV 

RESPIRATION 

Respiration  is  a  process  by  which  oxygen  is  introduced  into,  and  carbon 
dioxid  removed  from,  the  body.  The  assimilation  of  the  former  and  the 
evolution  of  the  latter  take  place  in  the  tissues  as  a  part  of  the  general 
process  of  nutrition.  Without  a  constant  supply  of  oxygen  and  an  equally 
constant  removal  of  the  carbon  dioxid,  those  chemic  changes  which  under- 
lie and  condition  all  life  phenomena  could  not  be  maintained. 

The  general  process  of  respiration  may  be  considered  under  the  following 
headings,  viz.: 

1.  The  anatomy  and  general  arrangement  of  the  respiratory  apparatus. 

2.  The  mechanic  movements  of  the  thorax  by  which  an  interchange  of 

atmospheric  and  intra-pulmonary  air  is  accomplished. 

3.  The  chemistry  of  respiration;  the  changes  in  composition  undergone  by 

the  air,  blood,  and  tissues. 

4.  The  nerve  mechanism  by  which  the  respiratory  movements  are  main- 

tained and  coordinated. 

THE  RESPIRATORY  APPARATUS 

The  respiratory  apparatus  consists  essentially  of: 

1.  The  lungs  and  the  air-passages  leading  into  them:  viz.,  the  nasal  chambers, 

mouth,  pharynx,  larynx,  and  trachea. 

2.  The  thorax  and  its  associated  structures. 

The  Nasal  Chambers. — ^The  nasal  chambers  are  the  natural  entrances 
for  the  inspired  air.  Their  complicated  structure  slightly  retards  the 
movement  of  the  air,  in  consequence  of  which  its  temperature  and  moisture 
are  adjusted  to  the  physiologic  conditions  of  the  lower  respiratory  passages. 
The  mouth,  though  frequently  serving  as  an  entrance  for  air,  is  not  primarily 
a  respiratory  passage.  Both  the  nasal  chambers  and  the  mouth  com- 
municate posteriorly  with  the  pharynx,  in  which  the  respiratory  and  the 
deglutitory  passages  cross  each  other,  the  former  leading  directly  into  the 
larynx. 

The  Larynx. — ^The  larynx  is  a  complicated  mechanism  serving  the 
widely  different  though  related  functions  of  respiration  and  phonation. 
It  consists  of  a  framework  of  cartilages,  articulating  one  with  another, 
united  by  ligaments  and  capable  of  being  moved,  one  on  the  other,  by  the 
action  of  muscles;  it  is  covered  externally  with  fibrous  tissue  and  lined  with 
mucous  membrane.  The  superior  laryngeal  aperature  is  triangular  in 
shape,  the  base  being  directed  upward  and  slightly  forward,  the  apex 
downward  and  backward.  The  inclination  of  the  laryngeal  aperature  is 
almost  vertical. 

The  cavity  of  the  larynx  is  partially  subdivided  by  the  interposition  of 
the  vocal  bands  into  a  superior  and  an  inferior  portion.     The  opening, 


RESPIRATION 


389 


bounded  by  the  vocal  bands,  is  also  triangular  in  shape,  though  in  this  case 
the  base  is  directed  backward,  and  the  apex  forward.  (See  chapter  on  Voice 
and  Speech.) 

The  introduction  of  the  vocal  bands  narrows  at  this  level  the  air-passage 
and  to  some  extent  interferes  with  the  free  entrance  of  air.  According  to 
the  investigations  of  Semon,  the  area  of  the  air-passage  above  and  below 
the  phonatory  apparatus  is  about  200  sq.  mm.;  while  the  area  bounded 
by  the  vocal  apparatus  is  but  155  sq.  mm.  during  quiet  respiration. 


Fig.  182.- 


-Human  larynx,  trachea,  bronchi,  and  lungs;  showing  the  ramification  of  the  bronchi, 
and  the  division  of  the  lungs  into  lobules. 


The  Trachea. — The  trachea  is  a  tube,  some  12  centimeters  in  length, 
from  two  to  two  and  a  half  centimeters  in  breadth,  extending  from  the  lower 
border  of  the  larjmx  to  a  point  opposite  the  fifth  thoracic  vertebra.  It 
consists  of  an  external  fibrous  and  an  internal  mucous  membrane,  between 
which  is  a  series  of  superposed  C-shaped  arches  or  rings  of  elastic  cartilage, 
some  18  or  20  in  number.  Between  the  fibrous  and  mucous  coats  pos- 
teriorly, and  occupying  the  space  between  and  attached  to  the  free  ends  of 
the  cartilages,  there  is  a  layer  of  transversely  arranged  non-striated  muscle- 
fibers,  known  as  the  tracheal  muscle.  The  alternate  contraction  and  relaxa- 
tion of  this  muscle  would  by  varying  the  distance  between  the  ends  of  the 
cartilages,  either  diminish  or  increase  the  caliber  of  the  trachea.  The 
surface  of  the  mucous  membrane  is  covered  by  a  layer  of  stratified  columnar 
ciliated  epithelium.  In  the  submucous  tissue  there  are  a  number  of  glands 
the  ducts  of  which  open  on  the  free  surface. 

Opposite  the  fifth  thoracic  vertebra  the  trachea  divides  into  a  right 
and  a  left  bronchus.  Each  bronchus  again  subdivides  into  two  or  three 
branches,  which  penetrate  the  corresponding  lung. 

The  Lungs. — The  lungs,  in  the  physiologic  condition,  occupy  the  greater 
part  of  the  cavity  of  the  thorax.     They  are  separated  from  each  other  by  the 


390  TEXT-BOOK  OF  PHYSIOLOGY 

contents  of  the  mediastinal  space:  viz.,  the  heart,  the  large  blood-vessels, 
the  esophagus,  etc.  Each  lung  is  somewhat  pyramidal  in  shape  with  the 
apex  directed  upward.  The  outer  surface  is  convex  and  corresponds  to  the 
general  conformation  of  the  thorax.  The  inner  surface  is  concave  and  accom- 
modates the  contents  of  the  mediastinal  space.  The  under  surface  of  the 
lung  is  concave  and  rests  on  the  diaphragm.  The  posterior  border  is  con- 
vex; the  anterior  border  is  thin.  At  about  the  middle  of  the  inner  surface 
of  the  lung  the  blood-vessels  which  connect  the  heart  with  the  interior  of 
the  lung  enter  and  leave  in  company  with  the  branches  of  the  bronchi, 
bronchial  arteries,  veins,  nerves,  and  lymphatics. 

A  histologic  analysis  of  the  lung  shows  it  to  consist  of  the  branches  of 

the  bronchi,  their  subdivisions  and  ultimate  terminations,  blood-vessels, 

lymphatics  and  nerves,  imbedded  in  a  stroma  of  fibrous  and  elastic  tissue. 

The  anatomic  relations  which  these  structures  bear 

one  to  another  is  as  follows : — 

Within  the  substance  of  the  lung  the  bronchi 
divide  and  subdivide,  giving  origin  to  a  large  num- 
ber of  smaller  branches,  the  bronchial  tubes,  which 
penetrate  the  lung  in  all  directions.  With  this  re- 
peated subdivision  the  tubes  become  narrower, 
their  walls  thinner,  their  structure  simpler.  In 
passing  from  the  larger  to  the  smaller  tubes  the 
cartilaginous  arches  become  shorter  and  thinner, 
and  finally  are  represented  by  small  angular  and 
irregularly  disposed  plates.  In  the  smallest  tubes 
the  cartilage  entirely  disappears.  With  the  diminu- 
FiG  1 8^  —Single  Lob-  ^^^^  ^^  ^^^  caliber  of  the  tube  and  a  decrease  in  the 
uLE  OF  HTJM.W  Lung.  a.  thickness  of  its  walls,  there  appears  a  layer  of  non- 
Alveolar  passage.  _  b.  Cav-     striated  muscle-fibers,  the  so-called  bronchial  muscle, 

ity   of  lobule  or  infundibu-      i     ,  .1  j        i  .  •  i  •  i 

lum.  c.  Pulmonary  sacs,  between  the  mucous  and  submucous  tissues,  which 
— (DaUon.)  Completely  surrounds  the  tube  and  becomes  especi- 

ally well  developed  in  those  tubes  devoid  of  cartilage. 
The  fibrous  and  mucous  coats  at  the  same  time  diminish  in  thickness. 

Bronchial  Innervation. — The  bronchial  muscles  are  presumably  in  a 
state  of  tonic  contraction  and  impart  to  the  bronchial  tubes  a  certain  average 
caliber  best  adapted  for  respiratory  purposes.  Experimental  investigations 
indicate  that  they  are  innervated  by  efferent  fibers  of  the  vagus  nerve  (broncho- 
constrictors  and  possibly  broncho-dilatators)  inasmuch  as  stimulation  of  this 
nerve  is  usually  followed  by  a  contraction  of  the  muscles  and  a  narrowing  of 
the  lumen  of  the  bronchial  system.  These  muscles  may  also  be  thrown  into 
increased  activity  by  the  inhalation  of  irritating  gases  and  into  a  tetanus  by 
pathologic  causes  as  seen  in  the  various  forms  of  asthma. 

When  the  bronchial  tube  has  been  reduced  to  the  diameter  of  about  one 
millimeter,  it  is  known  as  a  bronchiole  or  a  terminal  bronchus.  From  the 
sides  of  the  terminal  bronchus  and  from  its  final  termination  there  is  given 
off  a  series  of  short  branches  which  soon  expand  to  form  lobules-  or  alveoli. 
The  cavity  of  the  alveolus  is  termed  the  infundibulum.  From  the  inner  sur- 
face of  the  alveolus  and  of  the  passageway  leading  into  it,  there  project  thin 
partitions  which  subdivide  the  outer  portion  of  the  general  cavity  or  infundib- 
ulum into  small  spaces,  the  so-called  air-sacs  or  air-cells  (Fig.  183).     The 


RESPIRATION  391 

wall  of  the  alveolus  is  extremely  thin  and  consists  of  fibro-elastic  tissue,  sup- 
porting a  very  elaborate  capillary  network  of  blood-vessels.  The  bronchial 
system  as  far  as  the  alveolar  passages  is  lined  by  ciliated  epithelium.  The 
air-sacs  are  lined  by  flat  epithelial  plates  of  irregular  shape,  termed  the  re- 
spiratory epithelium.  The  alveoli  are  united  one  to  another  by  fibro-elastic 
tissue. 

The  bronchial  arteries  which  supply  nutritive  material  to  the  pulmonary 
structures  arise  from  the  aorta  as  a  rule,  though  sometimes  from  an  inter- 
costal artery.  Each  lung  receives  two  arteries  which  accompany  the  bronchi 
as  far  as  the  distal  ends  of  the  alveolar  passages.  From  the  capillary  net- 
work formed  out  of  the  terminals  of  these  arteries,  two  systems  of  veins  arise, 
one  of  which  returns  the  blood  from  the  larger  tubes  and  empties  it  into  the 
azygos  vein;  the  other  of  which  returns  the  blood  from  the  smaller  tubes  and 
the  alveolar  passages,  and  empties  it  into  the  pulmonic  veins.  The  blood 
in  the  pulmonic  veins,  though  largely  arterialized,  nevertheless  contains 
some  venous  blood  derived  from  the  veins  arising  from  the  capillary  network 
of  the  bronchial  arterioles. 

The  nerv-es  distributed  to  the  muscle-fibers  of  the  bronchial  arteries,  and 
of  the  bronchial  tubes  and  to  the  mucous  membrane,  are  derived  from  the 
vagus  and  the  sympathetic  and  enter  the  substance  of  the  lung  at  and  around 
its  root. 

In  consequence  of  the  presence  of  the  elastic  tissue,  the  lungs  are  disten- 
sible and  elastic.     After  removal  from  the  body  the  elastic  tissue  at  once 
recoils,  forcing  out  a  portion  of  the  con- 
tained air.     The  condition  of  the  lung  P\©® 
is  now  one  of  collapse.     Under  pressure,               pv— —*•©»■--— pa 
however,   the  lung   can  be  readily  dis-  y^^ 
tended    or    inflated.     These   properties              _,^^  //iWvv-^^*,??*. 
endure  for  a  long  period  after  death,           ^^^wA/i^^^^^^^^ 
if  not  indefinitely,  if  the  lungs  are  prop-       ^^^^       ^^^^m  ^^m,^ 
erly    preser\-ed.     The    capacity   of   the      #^^       ^^^  ^^  ^^ 
lungs  can  be  made  to  vary  within  rather      ^     a          M     C^      ^      ^^%i 
wide  limits  in  virtue  of  the  presence  of     ^^         ^^^     ^  a  (« 
the  elastic  tissue.                                              ^^          ^P       ^W^        ,er\>^ 

The     Puhnonic     Blood-vessels. —      ^A,,  ^^         ^  ^  ^W 

The  pulmonic  artery  which  conducts  the         ^^^^P^  w^^?^^ 

venous  blood  from  the  heart  to  the  lungs        ^^^     184  .-The    Relation    of   the 

divides  beneath  the  arch  of  the  aorta  into  Pulmonic  Artery,  pa,  and  the  Pol- 

a  right  and  a  left  branch.     Each  branch  ^o^l,^^^^'  P^'  ^°  ^™  Lobules,  aa. 

.^P  .,  1    ,.   .  .  ^         .11  ^   B.  The  Bronchiole. 

with  its  subdivisions  enters  the  lung  at 

the  hilum  in  company  with  the  larger  divisions  of  the  bronchi.  Within 
the  lung  the  arteries  divide  and  subdivide  in  a  manner  corresponding 
to  that  of  the  bronchial  tubes,  which  they  follow  to  their  ultimate  ter- 
minations. As  the  pulmonic  lobules  are  approached,  a  small  arterial 
branch  plunges  into  the  wall  of  the  lobule  (Fig.  184),  in  which  it  forms 
an  elaborate  capillary  network  which  surrounds  and  embraces  the  air- 
sacs  on  all  sides.  As  this  network  is  to  subserve  the  respiratory  exchange 
of  gases  it  lies  nearer  the  inner  than  the  outer  surface  of  the  lobule  and  in 
close  relation  to  the  respiratory  epithelium.  The  air  and  blood  are  thus 
brought  into  intimate  relationship,  being  separated  only  by  the  respiratory 


392 


TEXT-BOOK  OF  PHYSIOLOGY 


epithelium  and  the  wall  of  the  capillary  vessel.  The  blood  emerging  from 
the  capillary  vessels  is  conducted  by  a  corresponding  converging  system  of 
vessels,  the  pulmonic  veins,  out  of  the  lungs  and  into  the  left  auricle  of  the 
heart.  The  main  function  of  the  pulmonic  apparatus  and  the  pulmonic 
division  of  the  circulatory  apparatus  is  to  afford  a  ready  means  for  the 
exhalation  of  the  carbon  dioxid  and  the  absorption  of  oxygen.  In  conse- 
quence of  this  exchange  of  gases  the  blood  changes  in  color  from  dark  bluish- 
red  to  scarlet  red.  The  relations  of  the  heart  and  its  vessels  to  the  lungs  and 
bronchial  tubes  are  shown  in  Fig.  184. 

The  Thorax. — The  thorax,  in  which  the  respiratory  organs  and  their 
associated  structures  are  lodged,  is  conic  in  shape,  though  somewhat  com- 


FiG.  185. — Bronchi  and  Lungs,  Posterior  View,  i,  i.  hiummit  of  lungs.  2,  2.  Base  of 
lungs.  3.  Trachea.  4.  Right  bronchus.  5.  Division  to  upper  lobe  of  lung.  q.  Division  to  lower  lobe. 
10.  Left  branch  of  pulmonic  artery.  11.  Right  branch.  12.  Left  auricle  of  heart.  13.  Left 
superior  pulmonic  vein.  14.  Left  inferior  pulmonic  vein.  15.  Right  superior  pulmonic  vein 
16.  Right  inferior  pulmonic  vein.  17.  Inferior  vena  cava.  18.  Left  ventrice  of  heart.  19. 
Right  ventricle. — (Sappey.) 


pressed  from  before  backward.  Its  apex  is  directed  upward,  its  base  down- 
ward. The  walls  of  the  thorax  are  composed,  first,  of  a  bony  framework 
or  skeleton  and,  second,  of  muscles  and  fascia. 

The  bony  framework  is  formed  posteriorly  by  the  thoracic  vertebrae 
and  the  posterior  extremities  of  the  ribs,  laterally  by  the  ribs,  and  anteriorly 
by  the  costal  cartilages  and  the  sternum.  The  superior  opening,  through 
which  pass  the  trachea,  esophagus,  and  blood-vessels,  is  oval  in  outline  and 
measures  from  side  to  side  about  12.5  cm.,  and  from  before  backward  about 
6.25  cm.  The  inferior  opening  is  of  large  size,  but  irregular  in  its  boundaries 
from  the  upward  inclination  of  the  ribs  and  the  downward  projection  of 
the  sternum. 

The  ribs,  which  form  a  large  part  of  the  thoracic  walls,  constitute  a  series 


RESPIRATION 


393 


of  bony  arches  attached  posteriorly  to  the  vertebrae  and  anteriorly  to  the 
sternum  through  the  intermediation  of  their  cartilages.  The  last  two  form 
an  exception.  The  ribs  are  somewhat  twisted  upon  themselves  and  pursue 
an  oblique  direction  from  above  dow-nward  and  forward.  As  a  result  the 
anterior  extremity  lies  at  a  lower  level  than  the  posterior.  The  costal 
cartilages  are  directed  upward  and  forward,  with  the  exception  of  the  upper 
three,  which  are  almost  horizontal. 

The  costo-vertebral  and  costo-chondral  and  the  chondro-sternal  articula- 
tions are  diarthrodial  in  character  and  endow  the  thoracic  walls  with  a  con- 
siderable degree  of  mobility.  The  costo-vertebral  joints  are  two  in  number, 
the  first  being  formed  by  the  beveled  head  of  the  rib  and  the  bodies  of  the 
two  adjoining  vertebrae;  the  second,  by  the  tubercle  of  the  rib  and  the  trans- 
verse process.  The  costo-chondral  and  the  chondro-sternal  articulations, 
as  their  names  imply,  are  formed  by  the  ribs,  cartilages,  and  sternum. 

The  muscles  which  complete  the  formation  of   the   thoracic  walls  are 


Ext. Intercostal 

':  Tntereostal 
Intercafiiluffine 


Fig.  1 86. — Showing  the  Situation,  the 
Points  of  Attachment,  and  Direction  of 
THE  Intercostal  Muscles,  i.  The  inter- 
cos  tales  externi.  2.  The  intercostales  in- 
terni.     3.  The  intercartilaginei. 


IntJrtte/rastal 


Fig.  187. — View  FROM  behind  of  Four 
Dorsal  Vertebr.e  and  Three  Attached 
Ribs,  showing  the  Attachment  of  the 
Elevator  Muscles  of  the  Ribs  and  the 
InterCostals. — {After  Allen  Thomson.) 


as  follows:  the  diaphram,  the  intercostales  externi  and  interni,  the  levatores 
costarum,  the  triangularis  sterni,  and  the  infra-costales. 

The  diaphragm  is  the  musculo-membranous  sheet  which  closes  the  in- 
ferior opening  of  the  thorax  and  completely  separates  its  cavity  from  that  of 
the  abdomen.  It  consists  of  two  muscles  which  arise  from  the  bodies  of 
the  first  three  or  four  lumbar  vertebrae  and  neighboring  fascia,  from  the 
border  of  the  six  lower  ribs,  and  from  the  ensiform  cartilage.  From  this 
extensive  origin  the  muscle-fibers  pass  centrally  to  be  inserted  into  a  com- 
mon tendon.  As  the  direction  of  the  fibers  is  from  below  upward  and  in- 
ward, the  diaphragm  is  somewhat  dome-shaped.  Its  inferior  border  is  for 
a  short  distance  in  contact  with  the  sides  of  the  thorax. 

The  intercostales  externi,  eleven  in  number  on  each  side,  occupy  the 
spaces  between  the  ribs  to  which  they  are  attached  from  the  tubercle  to  the 
anterior  extremity  (Figs.  186  and  187).     Their  fibers,  which  are  arranged 


394  TEXT-BOOK  OF  PHYSIOLOGY 

in  parallel  bundles,  are  directed  from  above  downward  and  from  behind 
forward.  The  point  of  attachment,  therefore,  of  any  given  bundle  of  fibers 
to  the  rib  above,  lies  nearer  the  vertebral  column,  nearer  the  fulcrum,  than 
the  point  of  attachment  below. 

The  intercostales  interni,  eleven  in  number,  occupy  the  spaces  between, 
and  are  attached  to  the  ribs  from  the  tubercle  to  the  anterior  extremity  of 
the  cartilages.  Their  fibers,  which  are  also  arranged  in  parallel  bundles, 
are  directed  from  above  downward  and  backward  (Figs.  i86  and  187).  The 
portions  of  the  internal  intercostals  between  the  cartilages  are  frequently 
termed  inter  car  tilaginei. 

The  levaiores  costarum  are  twelve  in  number  on  either  side.  They  arise 
from  the  tips  of  the  transverse  processes  of  the  last  cervical  and  the  thoracic 
vertebrae  with  the  exception  of  the  last.  From  the  point  of  origin  the  fibers 
pass  downward  and  outward  in  a  diverging  manner  to  be  inserted  into  the 
ribs  between  the  tubercle  and  the  angle.  Their  action,  as  their  name  im- 
plies, is  to  elevate  the  posterior  portion  of  the  ribs. 

The  triangularis  sterni  arises  from  the  side  of  the  posterior  surface  of  the 
lower  third  of  the  sternum  and  is  inserted  by  fleshy  slips  into  the  cartilages 
of  the  ribs  from  the  second  to  the  sixth. 

From  the  fact  that  the  inferior  opening  of  the  thorax  as  well  as  the  inter- 
costal spaces  are  completely  closed  by  the  foregoing  muscles,  and  from  the 
further  fact  that  the  superior  is  closed  by  fascia  except  at  those  points  through 
which  pass  the  trachea,  blood-vessels  and  esophagus,  the  cavity  of  the  thorax 
is  absolutely  air-tight. 

The  Pleurae. — Each  lung  is  surrounded  by  a  closed  invaginated  serous 
sac,  the  pleura,  of  which  the  inner  portion  is  reflected  over  and  is  closely 
adherent  to  the  surface  of  the  entire  lung  as  far  as  its  root;  the  outer  portion 
is  reflected  over  the  inner  wall  of  the  thorax,  the  superior  surface  of  the  dia- 
phragm, and  the  viscera  of  the  mediastinum.  Under  normal  conditions 
these  two  layers  of  the  pleura,  the  pulmonic  and  parietal  or  thoracic  are  in 
contact,  or  at  most  separated  only  by  a  thin  capillary  layer  of  lymph.  The 
presence  of  this  fluid  prevents  appreciable  friction  as  the  two  surfaces  play 
against  each  other  in  consequence  of  the  movement  of  the  lungs. 

THE  MECHANIC  MOVEMENTS  OF  THE  THORAX 

As  the  blood  flows  through  the  pulmonic  capillaries  it  yields  carbon  dioxid 
to,  and  receives  oxygen  from,  the  air  in  the  pulmonic  alveoli.  As  a  re- 
sult, the  intra-pulmonic  air  changes  in  composition  which  interferes  to  a 
greater  or  less  extent  with  the  further  exchange  of  gases.  That  this  ex- 
change may  continue,  it  is  of  primary  importance  that  the  air  within  the 
alveoli  be  renewed  as  rapidly  as  it  is  vitiated.  This  is  accomplished  by  an 
alternate  increase  and  decrease  in  the  capacity  of  the  thorax,  accompanied 
by  corresponding  changes  in  the  capacity  of  the  lungs.  During  the  former 
there  is  an  inflow  of  atmospheric  air  (inspiration),  during  the  latter  an  out- 
flow of  intra-pulmonic  air  (expiration) .  The  continuous  recurrence  of  these 
two  movements  brings  about  that  degree  of  pulmonic  ventilation  necessary 
to  the  normal  exchange  of  gases  between  the  blood  and  the  air.  The 
two  movements  together  constitute  a  respiratory  act  or  cycle. 

In  the  course  of  the  respiratory  cycles  the  thorax  presents  alternately  a 


RESPIRATION  395 

short  period  of  rest — viz. :  between  the  end  of  an  expiration  and  the  beginning 
of  an  inspiration — and  a  relatively  long  period  of  activity,  including  both 
inspiration  and  expiration.  The  former  may  be  regarded  as  the  static,  the 
latter  as  the  dynamic  condition  of  the  thorax.  In  the  static  condition,  the 
thorax  and  its  contained  and  associated  organs  sustain  a  definite  relation 
one  to  another;  in  the  dynamic  conditions  these  relations  undergo  a  change 
the  extent  of  which  is  proportional  to  the  extent  of  the  movements/ 

THE  STATIC  CONDITION 

Relation  of  the  Thoracic  Organs. — Intra-puhnonic  Pressure :  Intra- 
thoracic Pressure. — In  the  static  condition  of  the  thorax  the  lungs,  by 
virtue  of  their  distensibility,  completely  fill  all  parts  of  the  thorax  not 
occupied  by  the  heart  and  great  blood-vessels  (Fig.  188).  This  condition  is 
maintained  by  the  pressure  of  the  air  v^^ithin  the  lungs,  the  intra-ptdmonic 
pressure,  which  with  the  respiratory  passages  open,  is  that  of  the  atmosphere, 
760  mm.  Hg.  This  relation  persists  so  long  as  the  thorax  remains  air- 
tight. If  the  skin  and  muscles  covering  an  intercostal  space  be  removed  the 
lung  can  be  seen  in  close  contact  with  the  parietal  layer  of  the  pleura  gliding 
by  with  each  inspiration  and  expiration.  If,  however,  an  opening  be  now 
made  in  the  pleura  sufl&cient  to  admit  air,  the  lung  immediately  collapses  and 
a  pleural  cavity  is  established.  A  pleural  cavity,  therefore,  does  not  exist  in 
physiological  conditions,  it  is  potential  only.  The  pressure  of  air  within 
and  without  the  lung  counterbalancing,  at  the  moment  the  air  is  admitted, 
the  elastic  tissue  at  once  recoils  and  forces  a  large  part  of  the  air  out  of  the 
lung.  This  is  a  proof  that  in  the  normal  condition,  the  lungs,  distended  by 
atmospheric  pressure  from  within,  are  in  a  state  of  elastic  tension  and  ever 
endeavoring  to  pull  the  pulmonic  layer  of  the  pleura  away  from  the  parietal 
layer.  That  they  do  not  succeed  in  doing  so  is  due  to  the  fact  that  the 
atmospheric  pressure  from  without  is  prevented  from  acting  on  the  lung  by 
the  firm  unyielding  walls  of  the  thorax. 

Intra-thoracic  Pressure. — As  a  result  of  the  elastic  tension  of  the  lungs  a 
fractional  part  of  the  intra-pulmonic  pressure,  760  mm.  Hg.,  is  counter- 
balanced or  opposed,  so  that  the  heart  and  great  vessels  and  other  intra-thoracic 
viscera  are  subjected  to  a  pressure  somewhat  less  than  that  of  the  atmosphere; 
the  amount  of  this  pressure  will  be  that  of  the  atmosphere  less  that  exerted  by 
the  elastic  tissue  of  the  lung  in  the  opposite  direction,  expressed  in  terms  of 
millimeters  of  mercury.  In  the  thorax  but  outside  the  lungs,  there  then 
prevails  a  pressure,  negative  to  the  pressure  inside  the  lungs  and  which  is 
known  as  the  intra-thoracic  pressure. 

The  amount  of  this  intra-thoracic  pressure  can  be  approximately  deter- 
mined in  several  ways.  Thus,  if  shortly  after  death  a  mercurial  manometer 
be  inserted  air-tight  into  the  trachea  of  a  human  being  and  the  thorax  opened, 
the  lungs  will  recoil  and  compress  their  contained  air.  The  mercurial 
manometer  will  at  once  show  an  excess  of  pressure  in  the  trachea  of  about 

^  It  is  a  matter  of  discussion  as  to  whether  or  not  there  is  an  absolute  cessation  of  movement 
of  the  thoracic  walls  at  the  end  of  expiration.  A  graphic  record  of  the  movement  shows  that 
if  there  is  no  absolute  cessation,  the  movement  is  so  slight  that,  for  the  purposes  here  intended, 
a  pause  may  be  admitted.  With  this  admission  it  is,  however,  recognized  that  the  forces,  both 
elastic  and  muscular,  which  are  always  acting  on  the  thoracic  walls,  though  in  opposite  directions, 
have  not  ceased  to  act,  but  have  become  so  nearly  equal  that  for  a  brief  period  they  are  practically 
in  a  condition  of  equilibrium,  during  which  the  thoracic  walls  are  stationary. 


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6  mm.  This  was  taken  by  Bonders  as  a  measure  of  the  force  with  which 
the  lungs  endeavor  to  recoil.  The  intra-thoracic  pressure  would  be,  there- 
fore, atmospheric  pressure,  760  mm.,  less  6  mm.,  or  754  mm.  Hg.  Another 
method  is  to  insert  a  rubber  catheter  through  a  small  opening  in  an  intercostal 
space  into  the  thoracic  cavity.  The  air  which  enters  through  the  open  ex- 
tremities of  the  catheter  and  leads  to  a  collapse  of  the  lungs  may  be  subse- 
quently aspirated,  when  the  lung  returns  to  its  normal  position.  The 
catheter  is  then  placed  in  connection  with  a  water  manometer.  On 
establishing  a  communication  between  them,  by  the  turning  of  a  stopcock, 
the  water  will  rise  in  the  proximal  and  fall  in  the  distal  limb  of  the  manometer, 
indicating  a  pressure  in  the  thorax  negative  to  that  in  the  lung.  The  differ- 
ence in  the  level  of  the  water  in  the  two  limbs  of  the  manometer,  expressed  in 
millimeters  of  mercury,  would  also  represent  the  force  with  which  the  elastic 


m LCA 


Fig.  188. — Section  of  Thorax  with  the  Lungs,  Heart,  and  Principal  Vessels.  5. 
Catheter  introduced  into  the  pleural  space  and  connected  with  a  manometer. — {After  Moral  and 
Doyen.) 

tissue  strives  to  recoil,  and  the  extent  to  which  it  opposes  the  atmospheric 
pressure.  This  subtracted  from  the  atmospheric  pressure  would  give  the 
intra-thoracic  pressure.  In  the  living  dog  this  latter  is  less  than  the  former, 
to  the  extent  of  from  3.5  to  5.5  mm.  For  the  same  reason  the  superior  surface 
of  the  diaphragm  also  experiences  a  pressure  less  than  that  of  the  atmosphere. 
Owing  to  the  soft  and  yielding  character  of  the  abdominal  walls  the  atmospheric 
pressure  is  transmitted  through  the  abdominal  organs  to  the  inferior  surface 
of  the  diaphragm.  The  pressure  being  greater  from  below  than  above,  the 
diaphragm  is  forced  upward  until  it  assumes  the  dome-like  appearance  it 
usually  presents.     (These  relations  are  shown  in  Fig.  188.) 

The  cause  of  the  negativity  of  the  intra-thoracic  pressure  is  connected 
with  the  change  in  the  relation  of  the  lungs  to  the  thorax  attending  the  first 
inspiration.  Previous  to  birth  the  walls  of  the  alveoli  and  bronchioles  are 
collapsed  and  in  apposition.     The  larger  bronchial  tubes  in  all  probability 


RESPIRATION  397 

contain  fluid.  The  lungs  therefore  are  devoid  of  air  (atelectatic),  and, 
having  a  specific  gravity  greater  than  water,  readily  sink  when  placed  in  this 
fluid.  The  capacity  of  the  thorax  does  not  exceed  the  volume  of  the  lungs. 
With  the  first  inspiration,  however,  the  thoracic  walls  take  a  new  position. 
The  air  at  once  rushes  into  the  lungs  and  distends  them.  But  as  the 
capacity  of  the  thorax  even  at  the  end  of  the  expiration  is  now  greater  than 
the  volume  which  the  lungs  would  assume  unless  distended,  there  at  once 
arises  the  elastic  recoil  in  the  opposite  direction,  the  condition  which  gives  rise 
to  the  negativity  of  the  pressure  in  the  thoracic  cavity.  It  is  also  probable 
that  as  the  child  develops,  the  thorax  grows  more  rapidly  than  the  lungs, 
giving  rise  to  a  condition  which  would  increase  and  accentuate  the  elastic 
tension  and  thus  increase  the  negativity  of  the  intra-thoracic  pressure. 

THE  DYNAMIC  CONDITION 

In  the  dynamic  condition,  the  thorax  and  its  contained  organs  undergo  a 
series  of  movements  in  consequence  of  which  the  relations  among  them 
characteristic  of  the  static  condition,  are  temporarily  changed.  To  these 
movements  the  term  respiratory  has  been  given,  as  a  result  of  which,  the 
ventilation  of  the  lungs  is  accomplished. 

The  Respiratory  Movements. — The  respiratory  movements  consist  of 
an  alternate  increase  and  decrease  in  the  capacity  of  the  thorax,  accompanied 
by  corresponding  changes  in  the  capacity  of  the  lungs,  the  two  movements 
being  known  as  inspiration  and  expiration  respectively.  During  the  increase 
in  the  thoracic  capacity,  the  air  passively  flows  into  the  lungs;  during  the 
decrease  in  the  thoracic  capacity,  the  air  passively  flows  out  of  the  lungs. 
In  both  movements  the  lungs  play  an  entirely  passive  part,  their  movements 
being  determined  by  the  pressure  of  air  within  them  and  by  the  outward 
movement  of  the  thoracic  walls,  with  which  they  are  in  close  contact, 

1.  Inspiration  is  an  active  process,  the  result  of  muscle  activity. 

2.  Expiration  is  a  passive  process,  the  result  mainly  of  the  recoil  of  the 

elastic  tissue  of  the  walls  of  the  thorax  and  abdomen  and  of  the  elastic 
tissue  of  the  lungs. 

In  inspiration  the  thorax  is  enlarged  in  all  its  diameters:  viz.,  vertical, 
transverse,  and  antero-posterior.  In  expiration  the  thorax  is  diminished  in 
all  its  diameters  as  it  returns  to  its  former  condition. 

Inspiratory  Muscles. — The  muscles  which  from  their  origin,  direction, 
and  insertion  contribute  to  the  enlargement  or  expansion  of  the  thorax  are 
quite  numerous,  and  include  those  muscles  which  enter  into  the  formation 
of  the  thoracic  walls  (intrinsic  muscles),  as  well  as  certain  muscles  which, 
having  their  origin  elsewhere,  are  attached  to  the  thoracic  walls  at  different 
points  (extrinsic  muscles),  though  the  extent  to  which  they  are  called  into 
activity  depends  on  the  necessity  for  either  tranquil  or  energetic  inspirations. 
The  gradations  between  a  minimum  and  a  maximum  inspiration  are  very 
slight,  and  it  is  difficult  to  state  at  what  particular  instant  any  given  muscle 
begins  to  act.  It  is  customary,  however,  to  divide  the  muscles  into  two  groups : 
(i)  Those  active  in  the  average  or  ordinary  inspirations,  and  (2)  those  active 
in  maximum  or  extraordinary  inspirations.  Among  the  muscles  active 
in  ordinary  inspirations  may  be  mentioned  the  diaphragm,  the  intercostales 
externi,  the  inter cartilaginei,  the  levatores  costarnm,  the  scaleni,  and  the  ser- 


398 


TEXT-BOOK  OF  PHYSIOLOGY 


ratus  posticus  superior.  Among  the  muscles  active  in  extraordinary  inspira- 
tions may  be  mentioned,  in  addition  to  the  foregoing,  the  sterno-cleido- 
mastoideus,  the  trapezius,  and  the  pectorales  minor  and  major. 

The  vertical  diameter  is  increased  by  the  contraction  and  descent  of  the 
diaphragm,  and  more  especially  of  its  lateral  muscular  portions.  At  the 
end  of  an  expiration  the  diaphragm  is  relaxed,  and  the  lower  portion  closely 
applied  to  the  walls  of  the  thorax.  At  the  beginning  of  an  inspiration  the 
muscle-fibers  contract,  shorten,  and  approximate  a  straight  line,  whereby 
not  only  is  the  convexity  of  the  diaphragm  diminished,  but  that  portion  in 
contact  with  the  thorax  is  drawn  away,  thus  making  a  large  free  space 
triangular  in  shape,  termed  the  complementary  pleural  space,  into  which 
the  lateral  and  posterior  portions  of  the  lungs  at  once  descend.  The  attach- 
ment of  the  central  tendon  of  the  diaphragm  to  the  pericardium  prevents 

any  marked  descent  of  this  portion 
except  in  forcible  inspiratory  efforts. 
The  vertical  diameters  are  thus  en- 
larged, though  unequally  in  different 
regions  of  the  thorax. 

As  the  diaphragm  descends  it 
displaces  the  abdominal  viscera, 
forcing  them  downward  against  the 
abdominal  walls,  which  advance  and 
become  more  convex.  In  forcible 
inspiration  the  diaphragm,  acting 
from  the  central  tendon  as  the  more 
fixed  point,  would  draw  the  lower 
portion  of  the  thorax  inward  were 
this  not  prevented  by  the  outward 
pressure  of  the  displaced  viscera. 
Coincidently  with  the  descent  of  the  diaphragm  and  the  partial  removal 
of  the  pressure  on  the  under  surface  of  the  lung,  the  intra-pulmonic  air  ex- 
pands. As  it  expands  it  distends  the  lungs  in  the  vertical  direction,  causing 
them  to  follow  the  diaphragm  and  to  occupy  the  so-called  complementary 
pleural  space.  With  the  expansion  of  the  intra-pulmonic  air  there  is  a  fall 
in  its  pressure  below  the  atmospheric  pressure,  to  be  followed  immediately 
by  an  inflow  of  air  until  atmospheric  pressure  is  again  established.  This 
occurs  at  the  end  of  the  inspiration. 

The  antero-posterior  and  transverse  diameters  are  increased  by  the  eleva- 
tion and  outward  rotation  of  the  ribs  and  an  advance  of  the  sternum,  both 
movements  made  possible  by  the  construction  and  arrangement  of  the  costo- 
vertebral and  costo-chondral  and  chondro-sternal  articulations.  The 
construction  of  these  articulations  is  such  as  to  permit  at  the  first  a  slight 
elevation  and  depression  of  the  head  of  the  rib,  and  at  the  second  a  glid- 
ing of  the  tubercle  on  the  transverse  process.  The  axis  around  which  the 
rib  rotates  practically  coincides  with  the  axis  of  the  rib  neck,  which  in  the 
upper  part  of  the  thorax  is  almost  horizontal,  in  the  lower  part  some- 
what sagittal  in  direction.  Hence  when  the  ribs  are  elevated  the  upper 
part  of  the  thorax  increases  in  its  antero-posterior,  the  lower  part  in  its 
transverse  diameters.  At  the  same  time,  the  lower  portion  of  the  sternum 
is  pushed  forward  and  upward  by  the  elevation  of  the  anterior  extremity 


Fig.  189. — Diagram  Showing  the  Position 
AND  Shape  of  the  Duphragm  at  Rest  a  and 
During  Inspiration  a'  and  b. — {Borutlau.) 


RESPIRATION 


399 


of  the  ribs  and  the  widening  of  the  angle  of  the  costo-chondral  articulation. 
With  the  elevation  of  the  ribs  there  goes  an  eversion  or  outward  rotation 
which  still  further  increases  the  transverse  diameters.  Coincidently  with 
the  increase  in  the  transverse  and  antero-posterior  diameters  of  the  thorax, 
and  the  partial  removal  of  the  pressure  on  the  lateral  surfaces  of  the  lungs 
there  is  also  an  additional  expansion  of  the  intra-pulmonic  air.  As  it  expands 
it  distends  the  lungs,  causing  them  to  occupy  the  available  space  thus  estab- 
lished. With  the  expansion  of  the  intra-pulmonic  air  there  is  a  still  further 
fall  of  pressure  and  an  additional  inrush  of  air.  Between  the  descent  of  the 
diaphragm  and  the  elevation  of  the  ribs  and  the  advance  of  the  sternum  the 
volume  of  air  necessary  for  the  ordinary  respiratory  needs  is  introduced  into 
the  lungs. 

This  elevation  and  outward  rotation  of  the  ribs  is  the  resultant  of  the  coop 
oration  of  the  following  muscles,  viz.:  the  intercostales  externi,  the  intercar- 
tilaginei,  the  levatores  costarum,  the  scaleni  and  the  serratus  posticus  superior. 

The  action  of  the  external  intercostal  muscles,  as  well  as  the  action  of 
the  intercartilaginei  muscles,  has  been  a  subject  of  much  discussion.     Some 


Fig.  190. — ^Diagrams  Illustrating  the  Action  of  the  External  Intercostal  and  In- 
tercartilaginei Muscles. 

investigators  have  maintained  that  they  are  elevators  of  the  ribs,  and  there- 
fore inspiratory;  others  that  they  are  depressors  of  the  ribs,  and  therefore 
expiratory  in  function.  At  the  present  time  the  general  consensus  of  opinion 
is  that  the  former  view  is  the  one  most  in  accordance  with  the  facts.  In 
the  following  explanation  as  to  their  action,  their  relation  to  the  ribs  and  to 
the  cartilages,  must  be  recalled  to  mind.  The  relation  of  the  external  inter- 
costals  is  such  that  the  point  of  attachment  of  any  given  bundle  of  fibers 
to  the  rib  above  lies  nearer  the  vertebral  column,  nearer  the  fulcrum, 
than  the  point  of  attachment  to  the  rib  below.  The  relation  of  the  inter- 
cartilaginei to  the  cartilages  is  such  that  the  point  of  attachment  of  any 
given  bundle  of  fibers  to  the  cartilage  above  lies  nearer  the  sternum,  nearer 
the  fulcrum,  than  the  point  of  attachment  to  the  cartilage  below.  The 
situation  of  the  muscles  and  the  shortness  of  their  fibers  render  it  extremely 
difi&cult  to  obtain  myographic  tracings  which  would  elucidate  their  action 
in  elevating  the  ribs  and  cartilages. 

A  clear  conception  of  their  action,  however,  may  be  arrived  at  by  the 
study  of  the  schematic  model  first  presented  by  Hamberger.     Fig.  190.     In 


400  TEXT-BOOK  OF  PHYSIOLOGY 

this  model  v-v'  is  a  vertical  support  carrying  two  freely  movable  parallel 
bars  r  r' ,  united  at  their  opposite  ends  with  two  other  freely  movable 
and  parallel  bars  cc,  cc'  carried  by  a  second  vertical  support,  s,  representing 
respectively  the  vertebral  column,  two  adjoining  ribs,  two  adjacent  cartilages, 
and  the  sternum.  Diagram  A  shows  the  position  of  the  different  parts  at 
the  end  of  expiration  and  B  their  position  at  the  end  of  inspiration.  The 
parallel  bars  are  joined  to  each  other  by  elastic  bands  ei  and  ic  having  the 
direction  of  and  representing  the  external  intercostal  and  intercartilaginei 
muscles,  respectively.  The  bars  are  depressed  to  sufficiently  elongate  and 
tense  the  elastic  bands  and  thus  imitate  the  condition  of  the  muscles  in  so 
far  as  tension  is  concerned  prior  to  their  contraction.  On  releasing  the 
bars  the  elastic  bands  at  once  recoil  and  the  bars  representing  ribs  and 
cartilages  are  raised.  Although  the  elastic  forces,  acting  in  opposite  direc- 
tions as  indicated  by  the  arrows  are  equal,  the  bars  are  yet  raised  for  the 
reason  that  in  accordance  with  the  parallelogram  of  forces,  the  components 
acting  upward  on  the  long  arms  of  the  levers  preponderate  over  the  com- 
ponents acting  downward  on  the  short  arms  of  the  levers.  This,  taken  in  con- 
nection with  the  fact  that  the  distances  between  the  adjoining  bars  are  fixed, 
leads  not  only  to  an  elevation  of  the  bars,  but  to  a  widening  of  the  angle 
between  them  and  an  advance  of  the  second  vertical  support.  The  actions 
of  these  bands  thus  disclose  and  illustrate  the  actions  of  both  the  external 
intercostal  and  intercartilaginei  muscles.  It  must  therefore  be  concluded 
that  these  muscles  are  the  elevators  of  the  ribs  and  cartilages  and  hence, 
inspiratory  in  function.  Of  late  the  correctness  of  Hamberger's  view  has 
been  confirmed  by  experiments  on  living  animals. 

The  levatores  costarum,  as  is  evident  from  their  points  of  origin  and  in- 
sertion, elevate  the  ribs  posteriorly. 

The  scalenus  muscles,  anticus,  medius,  and  posticus,  arise  from  the  trans- 
verse processes  of  the  cervical  vertebrae,  and  after  pursuing  a  downward  and 
forward  direction  are  inserted  into  the  sternal  end  of  the  first  and  second  ribs. 
The  action  of  the  first  two,  at  least,  is  to  elevate  the  first  rib  and  thus  establish 
a  fixed  point  from  which  the  intercostal  muscles  can  act.  The  posticus  has 
doubtless  a  similar  action  on  the  second  rib. 

The  serratus  posticus  superior,  sl  quadrilateral  sheet  of  muscle-fibers, 
arises  mainly  from  the  spines  of  the  last  cervical  and  first  and  second  thoracic 
vertebras.  The  anterior  extremity  is  serrated  and  attached  to  the  outer 
surfaces  of  the  second,  third,  fourth,  and  fifth  ribs  beyond  the  angle.  The 
action  of  the  muscle  is  the  elevation  of  the  ribs  to  which  it  is  attached. 

Forced  Inspiration.— In /or ce^/  or  extraordinary  inspirations,  whereby 
the  capacity  of  the  thorax  is  still  further  increased,  the  foregoing  muscles 
are  reinforced  by  the  sterno-cleido-mastoideus,  the  trapezius,  and  the  pectorales 
minor  and  major.  Their  functions  will  become  apparent  from  a  considera- 
tion of  their  origins  and  insertions. 

Expiratory  Forces  and  Muscles. — Expiration,  as  previously  stated,  is 
a  passive  process  brought  about  by  the  recoil  of  the  elastic  tissues  of  the 
thoracic  and  abdominal  walls,  and  of  the  lungs,  all  of  which  have  been 
stretched  and  made  tense  during  inspiration.  With  the  cessation  of  the  in- 
spiratory effort  the  elastic  forces,  assisted  by  the  weight  of  the  ribs,  sternum, 
and  soft  tissues,  return  the  thorax  to  its  former  condition.  The  result  is  a 
diminution  of  all  the  diameters  of  the  thorax.     The  vertical  diameter  is 


RESPIRATION  401 

diminished  by  the  recoil  of  the  tense  abdominal  walls,  the  replacement  of  the 
abdominal  organs  and  the  consequent  ascent  of  the  diaphragm  to  its  former 
position.  The  transverse  and  antero-posterior  diameters  are  diminished  by 
the  descent  of  the  ribs,  sternum,  and  lungs.  Coincident  with  the  return  of  the 
thoracic  walls  to  their  former  condition  there  is  a  recoil  of  the  elastic  tissue 
of  the  lungs,  in  consequence  of  which  there  is  a  compression  of  the  intra- 
pulmonic  air.  With  its  compression  there  is  a  rise  of  pressure  above  atmos- 
pheric and  at  once  there  is  an  outflow  of  intra-pulmonic  air  until  atmospheric 
pressure  is  again  established  at  the  end  of  expiration. 

It  is  somewhat  uncertain  if  a  normal  expiratory  movement  necessitates 
active  muscle  contraction.  If,  however,  there  is  any  impairment  of  the 
elasticity  of  the  lungs  or  ribs,  or  any  interference  with  the  free  exit  of  the 
intra-pulmonic  air,  it  is  highly  probable  that  the  elastic  forces  are  assisted 
by  the  internal  intercostal  and  triangularis  sterni  muscles.  It  has  been  in- 
sisted upon  also  that  while  the  recoil  of  the  elastic  tissues  is  effective  in  the 
early  stages  of  an  expiration,  it  is  ineffective  in  the  later  stages.  Hence  there 
arises  a  necessity  for  muscle  assistance. 

The  action  of  the  internal  intercostals  is  less  clearly  understood  than  that 
of  the  external  intercostals.  If,  however,  the  direction  of  these  muscles  as 
indicated  in  Fig.  190,  diagram  A,  by  the  dotted  line  it,  ii,  be  considered  it 
would  seem  that  their  action  would  be  the  opposite  of  that  of  the  external 
intercostals — that  is,  it  would  be  to  depress  the  ribs.  Employing  the  Bam- 
berger model  to  elucidate  the  functions  of  these  muscles  it  is  apparent 
that  if  the  elastic  band,  ii,  ii,  recoils,  the  elastic  force  of  the  two  halves,  though 
equal,  will  act  in  opposite  directions,  but  as  the  component  acting  on  the 
long  arm  of  the  level  preponderates  over  that  acting  on  the  short  arm  of 
the  lever,  the  ribs  will  be  depressed.  The  action  of  the  band  is  supposed 
to  disclose  and  illustrate  the  action  of  the  muscle.  If  this  is  the  case  the 
internal  intercostal  muscles  must  therefore  be  expiratory  in  function. 

The  triangularis  sterni  muscle,  judging  from  its  anatomic  relations,  in  all 
probability  assists  in  expiration  by  depressing  the  cartilages  to  which  it  is 
attached  and  as  a  further  result  depressing  the  anterior  extremities  of 
the  ribs. 

Forced  Expiration. — After  the  elastic  forces  have  ceased  to  act  and  the 
normal  expiratory  movement  has  been  brought  to  a  close,  the  thorax  can  be, 
to  a  considerable  extent,  still  further  diminished  in  all  its  diameters  by  the 
contraction,  through  volitional  effort,  of  abdominal  and  thoracic  muscles. 
To  this  decrease  in  the  capacity  of  the  thorax,  as  a  result  of  which  a  much 
larger  volume  of  air  is  expelled  from  the  lungs  than  during  passive  expiration, 
the  term  forced  expiration  has  been  given.  With  the  cessation  of  muscle 
activity  the  elastic  forces  of  the  now-compressed  thoracic  walls,  aided  by  the 
return  of  the  upwardly  displaced  abdominal  organs,  at  once  restore  the 
thoracic  walls  to  the  position  they  had  attained  at  the  end  of  passive  expira- 
tion. Of  the  muscles  active  in  forced  expiration  in  addition  to  the  inter- 
costales  interni  and  the  triangularis  sterni,  the  following  may  be  mentioned, 
viz.:  the  abdominales,  the  serratiis  posticus  inferior,  and  the  qiiadratus 
luniborum. 

The  conjoint  action  of  these  muscles  is  to  diminish  the  convexity  of  the 
abdominal  walls  and  to  exert  a  pressure  on  the  abdominal  organs.  These, 
taking  the  line  of  least  resistance,  are  forced  upward  against  the  inferior 
26 


402  TEXT-BOOK  OF  PHYSIOLOGY 

surface  of  the  diaphragm,  which  in  consequence  becomes  more  strongly 
curved  and  ascends  higher  into  the  thorax.  The  vertical  diameter  of  the 
thorax  is  thus  diminished.  Acting  from  the  pelvis  as  a  fixed  point,  these 
muscles  will  also  draw  downward  and  inward  the  lower  end  of  the  ster- 
num and  the  lower  ribs  and  diminish  the  antero-posterior  and  transverse 
diameters. 

Movements  of  the  Lungs. — As  the  thorax  is  enlarging  in  all  its  diame- 
ters during  inspiration,  through  muscle  activity,  the  lungs  are  correspondingly 
enlarging  in  all  their  diameters,  by  virtue  of  their  distensibility,  through  the 
pressure  of  the  air  within  them.  The  lungs  must  therefore  move  downward, 
outward  and  forward.  That  this  is  the  case  is  made  evident  both  by  an 
examination  of  the  lungs  through  an  intercostal  space  after  removal  of  the 
skin  and  intercostal  muscles,  and  by  the  methods  of  percussion.  The  inferior 
border  of  each  lung  descends  from  the  lower  border  of  the  sixth  to  the 
eleventh  rib,  inserting  itself  into  the  space  developed  between  the  thorax 
and  diaphragm  as  the  latter  contracts  and  is  drawn  away  from  the  former. 
In  consequence  of  the  lateral  expansion  the  anterior  border  of  each  lung 
advances  toward  the  middle  line  until  the  heart  is  almost  covered.  With 
the  beginning  and  continuance  of  expiration  the  lungs  exhibit  a  reverse 
movement  which  continues  until  they  reach  their  original  position.  At  all 
times,  however,  the  movements  of  the  lungs  are  entirely  passive  and  deter- 
mined by  the  movements  of  the  thorax. 

The  Changes  in  the  Relation  of  the  Thoracic  Organs,  and  in  the  In- 
tra-pulmonic  and  Intra-thoracic  Pressures. — In  the  dynamic  condition,  as 
previously  stated,  the  relations  of  the  thoracic  organs  undergo  a  change  as 
well  as  the  intra-pulmonic  and  intra-thoracic  pressures.  Thus  during 
inspiration  the  diaphragm  descends,  the  ribs  ascend  and  outwardly  rotate 
and  the  sternum  advances,  the  result  of  which  is  an  enlargement  in  the 
diameters  of  the  thorax.  Coincidently  with  the  enlargement  of  the  thorax 
through  muscle  activity  there  goes  a  corresponding  increase  in  the  size  and 
capacity  of  the  lungs,  in  consequence  of  the  expansion  and  pressure  of  the 
air  in  the  pulmonary  alveoli. 

During  expiration  the  diaphragm  ascends  in  consequence  of  the  return 
of  the  displaced  abdominal  viscera,  the  ribs  descend  and  inwardly  rotate 
and  the  sternum  recedes  from  the  recoil  of  the  elastic  tissues,  the  result  of 
which  is  a  diminution  in  the  diameters  of  the  thorax.  Coincidently  with  the 
diminution  of  the  thorax  there  goes  a  decrease  in  the  size  and  capacity  of  the 
lungs  in  consequence  of  the  recoil  of  their  elastic  tissue  whereby  the  air  in 
the  lungs  is  compressed. 

The  intra-pulmonic  pressure  in  consequence  of  the  alternate  expansion 
and  compression  of  the  intra-pulmonic  air  also  undergoes  a  considerable 
variation. 

During  inspiration  the  intra-pulmonic  air  expands.  "With  the  expansion 
its  pressure  falls;  but  though  it  is  now  less  than  atmospheric  pressure  it  is 
yet  much  greater  than  the  opposing  force  of  the  lung  tissue.  As  a  result  of 
the  fall  of  intra-pulmonic  pressure,  there  is  a  rapid  inflow  of  air  which  con- 
tinues until  atmospheric  pressure  is  restored;  that  is,  at  the  end  of  the 
inspiration. 

During  expiration  the  intra-pulmonic  air  becomes  compressed.  With 
the  compression  its  pressure  rises  above  that  of  the  atmosphere  and  in  conse- 


RESPIRATION  403 

quence  there  is  a  rapid  outflow  of  air,  which  continues  until  atmospheric 
pressure  is  again  restored;  that  is,  at  the  end  of  the  expiration.     (Fig.  ig2,"'A .) 

The  cause  for  the  fall  of  intra-pulmonic  pressure  during  inspiration  and 
the  rise  during  expiration  is  to  be  found  in  the  resistance  offered  by  the  air- 
passages  to  the  movement  of  the  air,  throughout  their  entire  extent,  and 
especially  at  the  level  of  the  vocal  bands.  The  greater  the  resistance,  from 
whatever  cause,  physiologic  or  pathologic,  the  greater  the  variations  of 
the  pressure.  If  the  inspiratory  and  expiratory  movements  take  place  slowly 
the  intra-pulmonic  pressure  may  scarcely  vary  in  either  direction. 

In  quiet  inspiration  the  fall  of  pressure,  as  indicated  by  a  manometer 
inserted  into  one  nostril,  seldom  amounts  to  more  than  1.5  mm.  of  Hg.,  the 
rise  in  expiration,  2.5  to  3  mm.  of  Hg.  In  forcible  inspiratory  and  expiratory 
efforts  these  limits  may  be  largely  exceeded.  Thus  it  was  found  by  Bonders 
that  with  one  nostril  closed  and  a  mercurial  manometer  inserted  into  the 
other  the  pressure  by  voluntary  efforts  could  be  made  to  fall  57  mm.  during 
inspiration  and  to  rise  87  mm.  during  expiration.  The  changes  in  intra- 
pulmonic  pressure  are  graphically  represented  in  the  upper  half  of  Fig.  191. 


<—  l7ispira//.on  -> 
760m7?i. 

.:li^^^-'^^  <r-  Expiration 

7S8 
A.  Intea-pulmonic  Pressures. 

760  mm,  "^  ^'^-^P^^^^^o^^  -^>^EzpiraMon 


7SI 

B.  Intea-thoracic  Pressure. 

Fig.  191. — Representing  the  Changes,  i,  in  the  Intra-pulmonic,  and  2,  in  the  Intr.\-tho- 
EACic  Pressures  during  Inspiration  and  Expiration. 

The  intra-thoracic  pressure  also  varies  during  both  inspiration  and  expira- 
tion. As  the  thorax  enlarges  and  the  intra-pulmonic  pressure  falls,  the 
recoil  of  the  elastic  tissue  increases,  with  the  result  of  still  further  dimin- 
ishing the  intra-thoracic  pressure,  until  its  maximum  is  reached  near 
the  end  of  the  inspiration.  The  fall  of  intra-thoracic  pressure  at 
the  end  of  a  quiet  inspiration  reaches  to  about  9  mm.  Hg.  In 
forcible  inspiratory  efforts  this  fall  in  intra-thoracic  pressure  may 
amount  to  30  or  40  mm.  of  Hg.  As  the  thorax  again  diminishes  and  the 
intra-pulmonic  pressure  rises  above  the  atmospheric  pressure,  the  recoil 
of  the  elastic  tissue  is  again  opposed,  with  the  result  of  increasing  the 
intra-thoracic  pressure,  until  the  former  condition  of  pressure  has 
been  regained  at  the  end  of  the  expiration.  Neither  the  fall  nor 
the  subsequent  rise  of  the  intra-thoracic  pressure  takes  place,  however, 
in  a  steadily  progressive  manner  for  the  following  reasons:  If  a  tracing 
were  made  of  the  variations  in  the  circumference  of  the  thorax  during  a 
respiratory  movement  it  would  resemble  in  its  main  features  the  tracing  in 
Fig.  191,  and  variations  in  any  linear  dimension  of  the  lung  would  be  of 
course  in  the  same  proportion.     This  amount  of  elongation  of  elastic  tissue 


404  TEXT-BOOK  OF  PHYSIOLOGY 

in  any  direction  would  likewise  be  proportional  to  the  force  of  elastic  recoil. 
Therefore  the  intra-thoracic  pressure  would  vary  from  a  uniform  decrease 
and  increase  just  as  the  curve  of  Fig.  191  varies  from  uniform  straight  lines. 
The  changes  in  intra-thoracic  pressure  are  graphically  represented  in  Fig. 
191,  B. 

The  intra-thoracic  pressure  and  its  variations  influence  favorably  the 
flow  of  lymph  through  the  thoracic  duct  (see  page  225),  as  well  as  the  flow 
of  blood  from  the  extra-thoracic  veins  into  the  intra-thoracic  veins,  the  right 
side  of  the  heart,  and  the  cardio-pulmonary  vessels.  (See  paragraphs  at  the 
end  of  this  chapter.) 

The  succession  of  events  in  the  thorax  at  the  time  of  a  respiratory  act 
may  be  summarized  as  follows: 

During  Inspiration. 

1.  Enlargement  of  the  thoracic  diameters  by  muscle  action. 

2.  Increase  in  the  negativity  of  the  intra-thoracic  pressure. 

3.  Expansion  of  intra-pulmonic  (alveolar)  air. 

4.  Expansion  of  the  lungs. 

5.  Lowering  of  the  intra-pulmonic  air  pressure  below  the  atmospheric 

air  pressure. 

6.  Inflow  of  atmospheric  air,  in  consequence  of  its  higher  pressure,  until 

the  intra-pulmonic  air  pressure  rises  to  that  of  the  atmosphere. 
During  Expiration. 

1.  Diminution  of  the  thoracic  diameters  by  the  action  of  elastic  forces. 

2.  Decrease  in  the  negativity  of  the  intra-thoracic  pressure. 

3.  Recoil  of  the  lungs, 

4.  Compression  of  the  intra-pulmonic  (alveolar)  air. 

5.  Rise  of   intra-pulmonic  air    pressure    above    the   atmospheric  air 

pressure. 

6.  Outflow  of  intra-pulmonic  air,  in  consequence  of  its  higher  pres- 

sure, until  the  intra-pulmonic  air  pressure  falls  to  that  of  the 
atmosphere. 

Respiratory  Movements  of  the  Upper  Air-passages. — The  resistance 
to  the  entrance  of  air  into  and  through  the  respiratory  tract  is  much  dimin- 
ished by  respiratory  movements  of  the  nares  and  larynx  which  are  associated 
and  occur  synchronously  with  the  movement  of  the  thorax. 

The  nares  at  each  inspiration  are  dilated  by  the  outward  movement  of 
their  alae  or  wings,  the  result  of  muscle  activity.  At  each  expiration  they 
are  diminished  by  the  return  of  their  cartilages  through  the  play  of  elastic 
forces.  The  larynx,  as  shown  by  observation  with  the  laryngoscope,  exhibits 
corresponding  movements  of  the  vocal  membranes.  Their  introduction  at 
this  level  naturally  narrows  the  tract,  and  would  interfere  with  both  the 
entrance  and  the  exit  of  air  were  they  not  kept  widely  asunder  during  the 
time  they  are  not  required  for  purposes  of  phonation.  This  is  accomplished 
by  the  tonic  contraction  of  the  posterior  crico-arytenoid  muscles,  which  are 
entirely  respiratory  in  function. 

It  is  not  infrequently  stated  that  these  membranes  exhibit  considerable 
oscillations,  outward  and  inward,  corresponding  to  the  movements  of 
inspiration  and  expiration.  The  statements  of  the  majority  of  laryngologists 
do  not  favor  this  view.  During  tranquil  breathing  the  membranes  are 
widely  separated  and  almost  stationary,  seldom  moving  in  either  direction 


RESPIRATION  405 

more  than  a  few  millimeters.  In  labored  respirations  these  movements  are 
naturally  increased  in  extent.  The  irregular  movements  of  the  membranes 
occasioned  by  the  unskilful  use  of  the  laryngoscope,  especially  with  nervous 
patients,  are  not  to  be  regarded  as  strictly  physiologic.  The  respiratory 
space  in  quiet  breathing  is  an  isosceles  triangle,  with  a  length  of  20  mm.  and 
a  width  at  the  base  of  15.5  mm.  with  an  area  of  155  mm. 

Respiratory  Types. — Observation  of  the  respiratory  movements  in  the 
two  sexes  shows  that  while  the  enlargement  of  the  thoracic  cavity  is  accom- 
plished both  by  the  descent  of  the  diaphragm  (as  shown  by  the  protrusion  of 
the  abdomen)  and  the  elevation  of  the  thoracic  walls,  the  former  movement 
preponderates  in  the  male,  the  latter  in  the  female,  giving  rise  to  what  has  been 
termed  in  the  one  case  the  diaphragmatic  or  abdominal  type  and  in  the 
other  the  thoracic  or  costal  type  of  respiration.  The  cause  of  this  greater 
mobility  and  activity  of  the  thorax  in  the  female  has  been  a  subject  of  much 
discussion.  It  has  been  attributed,  on  the  one  hand,  to  the  necessity  for  a 
physiologic  adjustment  between  respiration  and  child-bearing,  and  therefore 
a  specific  sex  peculiarity;  on  the  other  hand,  it  has  been  attributed  to  per- 
sistent constriction  of  the  waist,  in  consequence  of  which  the  full  play  of  the 
diaphragm  is  prevented  and  the  burden  of  inspiration  is  thrown  on  the  thora- 
cic muscles.  It  has  been  assumed  that  if  inspiration  were  confined  in  women 
to  the  diaphragm,  there  would  arise  in  the  latter  stages  of  gestation  such  an 
increase  in  intra-abdominal  pressure  that  not  only  would  respiratory  ex- 
changes be  interfered  with,  but  fetal  life  might  be  unfavorably  influenced, 
if  not  endangered.  Modern  investigations  have  not  confirmed  this  assump- 
tion, but,  on  the  contrary,  have  corroborated  the  view  that  the  preponderance 
of  thoracic  movement  is  due  to  the  influences  of  dress  restrictions,  for  with 
their  removal  the  so-called  costal  type  of  breathing  entirely  disappears. 
While  gestation  may  lead  to  a  greater  activity  of  the  thorax,  this  is  but  tem- 
porary, for  with  its  termination  there  is  a  return  to  the  diaphragmatic  type 
of  breathing. 

Number  of  Respirations  per  Minute. — The  number  of  respirations 
which  occur  in  a  unit  of  time  varies  with  a  variety  of  conditions,  the  most 
important  of  which  is  age.  The  results  of  the  observations  of  Quetelet  on 
this  point,  which  are  generally  accepted,  are  as  follows: 

Age.  Respirations  per  Minute.  Age.  Respirations  per  Minute. 

o-  I  year 44  20-25  years 18.7 

5  years 26  25-30  years 15 -O 

15-20  years 20  30-50  years 17.0 

From  these  observations  it  may  be  assumed  that  the  average  number  of 
respirations  in  the  adult  is  eighteen  per  minute,  though  varying  from  moment 
to  moment  from  sixteen  to  twenty.  During  sleep,  however,  the  respiratory 
movements  often  diminish  in  number  as  much  as  30  per  cent.,  at  the  same 
time  diminishing  in  depth. 

Rhythm. — Each  respiratory  act  takes  place  normally  in  a  regular 
methodic  manner,  each  event  occurring  in  a  definite  sequence  and  occupy- 
ing the  same  relative  period  of  time.  This  rhythm,  however,  is  not  infre- 
quently temporarily  disturbed  by  emotions,  volitional  acts,  muscle  activity, 
phonation,  changes  in  the  composition  of  the  blood,  etc.;  with  the  removal 
of  these  disturbing  factors,  the  respiratory  mechanism  soon  returns  to  its 
normal  condition. 


4o6 


TEXT-BOOK  OF  PHYSIOLOGY 


A  graphic  representation  of  the  excursions  of  the  thoracic  walls,  rhythmic 
or  otherwise,  is  obtained  by  fastening  to  the  thorax  an  apparatus,  a  stethome- 
ter  or  a  pneumograph,  which  by  means  of  a  tambour  takes  up  and  trans- 
mits the  movement  to  a  second  tambour  provided  with  a  recording  lever. 
A  simple  form  of  pneumograph,  suggested  by  Fitz  (Fig.  192),  consists  of  a 
coil  of  wire  two  and  a  half  centimeters  in  diameter  and  about  40  centimeters  in 
length,  enclosed  by  thin  rubber  tubing,  one  end  of  which  is  closed,  the  other 


Fig.  192. — Pneumograph. — (Fitz.) 

placed  in  communication  either  with  a  tambour  and  lever  or  with  a  piston 
recorder.  By  means  of  an  inelastic  cord  or  chain  the  apparatus  is  securely 
fastened  to  the  chest.  With  each  inspiration  the  spring  is  elongated,  the  air 
within  the  system  is  rarefied,  and  as  a  result  the  lever  falls;  with  each  expira- 
tion the  reverse  conditions  obtain  and  the  lever  rises.  If  the  lever  be  ap- 
plied to  the  recording  surface  of  a  moving  cylinder,  a  curve  of  the  thoracic 
movement,  a  pneumatogram,  is  obtained  (Fig. 
193),  from  which  it  is  apparent  that  inspira- 
tion takes  place  more  abruptly  and  occupies 
a  shorter  period  of  time  than  expiration;  that 
expiration  immediately  follows  inspiration, 
but  that  there  is  a  slight  pause  between  the 
end  of  the  expiration  and  the  beginning  of  the 
inspiration.  The  time  relations  of  the  two 
movements  can  be  obtained  by  a  magnet-signal 
actuated  by  an  electric  current  interrupted, 
once  a  second.  The  ratio  of  inspiration  to 
expiration  has  been  represented  as  5  to  6,  or 
6  to!8. 


<NSP. 


Fig.  193.  —  A  Pneumatogram.  —  (After 
Marey.) 


Fig.  194. — A  Spirometer.- 
(Boruttau.) 


Volumes  of  Air  Breathed. — The  volumes  of  air  which  enter  and  leave 
the  lungs  with  each  inspiration  and  expiration  naturally  vary  with  the  ex- 
tent of  the  movement,  though  four  volumes  at  least,  may  be  determined: 
(i)  that  of  an  ordinary  inspiration;  (2)  that  of  an  ordinary  expiration;  (3) 
that  of  a  forced  inspiration;  (4)  that  of  a  forced  expiration. 

The  apparatus  employed  for  the  determination  of  these  different  volumes 


RESPIRATION  407 

is  the  spirometer,  a  modification  of  the  gasometer.  The  form  introduced  by 
Jonatlian  Hutchinson,  of  which  Fig.  194  is  a  modification,  consists  of  two 
metallic  cylinders,  one  containing  water,  the  other  containing  air,  the  latter 
being  inserted  into  the  former.  The  air  cylinder  is  balanced  by  a  weight  so 
accurately  that  it  remains  stationary  in  any  position.  A  tube,  penetrating 
the  base  of  the  water  cylinder,  is  continued  upward  through  and  above  the 
level  of  the  water.  The  air-space  above  is  thus  placed  in  free  communica- 
tion with  the  atmospheric  air.  A  stopcock  at  the  outer  end  of  this  tube 
prevents  the  escape  of  the  air  when  this  is  not  desirable.  To  the  free  end  of 
the  tube  a  rubber  tube  provided  with  a  suitable  mouthpiece  is  attached, 
through  which  air  can  be  breathed  into  or  out  of  the  air  cylinder.  With 
each  inspiration  the  air  cylinder  descends;  with  each  expiration  it  ascends. 
A  scale,  on  the  side  support,  graduated  in  cubic  inches  or  centimeters,  in- 
dicates the  volume  of  air  inspired  or  expired. 

With  an  apparatus  of  this  character  Hutchinson,  from  a  long  series  of 
observations,  defined  and  determined  the  above-mentioned  four  volumes  as 
follows : 

1.  The  tidal  volume,  that  which  flows  into  and  out  of  the  lungs  with  each 

inspiration  and  expiration,  which  varies  from  20  to  30  cubic  inches  (330 
to  500  c.c). 

2.  The  complemental  volume,  that  which  flows  into  the  lungs,  in  addition  to 

the  tidal  volume,  as  a  result  of  a.  forcible  inspiration,  and  which  amounts 
to  about  no  cubic  inches  (1800  c.c). 

3.  The  reserve  volume,  that  which  flows  out  of  the  lungs,  in  addition  to  the 

tidal  volume,  as  a  result  of  2.  forcible  expiration,  and  which  amounts  to 
about  100  cubic  inches  (1650  c.c). 

After  the  expulsion  of  the  reserve  volume  there  yet  remains  in  the 
lungs  an  unknown  volume  of  air  which  serves  the  mechanic  function  of 
distending  the  air-cells  and  alveolar  passages,  thus  maintaining  the 
conditions  essential  to  the  free  movement  of  blood  through  the  capil- 
laries and  to  the  exchanges  of  gases  between  the  blood  and  alveolar  air. 
As  this  volume  of  air  cannot  be  displaced  by  volitional  effort,  but 
resides  permanently  in  the  alveoli  and  bronchial  tubes  though  constantly 
undergoing  renewal,  it  was  termed — 

4.  The  residual  volume,  the  amount  of  which  is  difficult  of  determination, 

but  has  been  estimated  by  different  observers  at  914  c.c,  1562  c.c, 
1980  c.c. 
The  Vital  Capacity  of  the  Lungs. — From  foregoing  statements  it  is 
clear  that  the  thorax  and  lungs  are  capable  of  a  maximum  degree  of  expan- 
sion, at  which  moment  the  lungs  contain  their  maximum  volume  of  air. 
This  volume,  whatever  it  may  be,  represents  the  entire  capacity  of  the  lungs 
in  the  physiologic  condition,  and  includes  the  tidal,  the  complemental,  the 
reserve,  and  the  residual  volumes.  Mr.  Hutchinson,  however,  defined  the 
vital  or  respiratory  capacity  of  the  lungs  as  the  amount  of  air  which  can  be 
expelled  by  the  most  forcible  expiration  after  the  most  forcible  inspira- 
tion, this  therefore  excludes  the  residual  volume.  The  vital  capacity 
was  supposed  to  be  an  indication  of  an  individual's  respiratory  power,  not 
only  in  physiologic  but  also  in  pathologic  conditions.  Though  averaging 
about  230  cubic  inches  (3770  c.c)  for  an  individual  5  feet  7  inches  in  height, 
the  vital  capacity  varies  with  a  number  of  conditions,  the  most  important 


4o8 


TEXT-BOOK  OF  PHYSIOLOGY 


which  is  stature.     It  is  found  that  between  5  and  6  feet  the  capacity  in- 
creases 8  inches  (130  c.c.)  for  each  inch  increase  in  height. 

The  Total  Volume  of  Air  Breathed  Daily. — For  the  solution  of  certain 
problems  connected  with  ventilation  it  is  necessary  to  determine  the  total 
volume  of  air  taken  into  the  lungs  in  the  course  of  24  hours.  This  can 
be  determined  approximately  if  the  two  factors,  the  average  volume  of  air 
taken  into  the  lungs  at  each  inspiration,  and  the  average  number  of  res- 


FiG.  195. — Gad's  Pneumatograph. 

pirations  per  minute  be  known.  If  it  be  accepted  that  the  inspired  volume 
varies  from  328  to  492  c.c.  and  that  the  respiratory  frequency  averages  18  per 
minute,  then  the  total  volume  breathed  would  amount  to  from  8500  to 
12,752  liters. 

The  volume  changes  of  the  thorax  indicated  by  the  volumes  of  air  en- 
tering and  leaving  the  lungs  can  be  not  only  determined  but  graphically 
represented  by  means  of  an  apparatus  similar  in  principle  to  the  spirometer, 
devised  by  Gad  and  known  as  the  pneumatograph  or  aeropleihysmograph 

4000 

ssoo 

3000 

2000 
JSOO 

1000 

SCO 

o 

Fig.  196. — Diagram  Representing  the  Volume  Changes  of  the  Thorax  and  Lungs. — 

{Modified  jrom  Boruttau.) 

(Fig.  195).  This  consists  of  a  quadrangular  box  with  double  walls,  the 
space  between  which  is  filled  with  water.  The  center  of  the  box  is  an  air 
chamber.  A  thin  walled  mica  box  sinks  into  the  water.  Posteriorly  it  is 
attached  to  and  rotates  around  an  axis,  which  permits  of  an  elevation  or 
depression  of  the  anterior  portion.  It  is  also  carefully  counterpoised.  A 
light  lever  attached  to  the  mica  box  records  its  movements.  The  interior 
of  the  box  communicates  by  a  tube  with  a  large  reservoir  into  which  the 
individual  breathes,  the  object  being  to  prevent  a  too  rapid  vitiation  of  the 


A                                      ^ 

/\ 

z  \ 

/    Vol.    \ 

Vital 

=A  A  r — - — '\  A              /  / 

> 

Tidal 
Vol. 

\tompie/ 
\?m/iM/ 

Ca//ac(ti/. 

r/ 

V       J 

V          ) 

\ 

RESPIRATION  409 

air.  Inspiration  causes  the  lever  to  descend,  expiration  to  ascend.  Previous 
graduation  of  the  apparatus  is  necessary  to  determine  the  volumes  breathed. 
A  graphic  record  of  the  volume  changes  is  shown  in  Fig.  196. 

Respiratory  Sounds. — On  applying  the  ear  over  the  trachea  and  bronchi 
there  is  heard  during  both  inspiration  and  expiration  a  well-defined  sound, 
which  is  loud,  harsh,  and  blowing  in  character,  and  which  from  its  situation  is 
known  as  the  bronchial  sound.  It  is  especially  well  heard  between  the 
scapulae  above  the  level  of  the  fourth  thoracic  vertebra.  This  sound  is 
produced  in  the  larynx,  tor  with  its  separation  from  the  trachea  the  sound 
disappears.  The  cause  of  the  sound  is  to  be  found  in  the  narrowing  of  the 
air-passage  at  the  level  of  the  vocal  membranes,  though  the  mechanism  of 
its  production  is  uncertain.  On  applying  the  ear  to  almost  any  portion  of 
the  chest-wall,  but  especially  to  the  infrascapular  area,  there  is  heard  during 
both  inspiration  and  expiration  a  deHcate,  sighing,  rustling  sound,  which 
from  its  supposed  seat  of  origin,  the  air-vesicles  or  air-cells,  is  known  as  the 
vesicular  sound.  This  sound  is  supposed  to  be  due  to  the  sudden  expansion 
of  the  air-cells  during  inspiration  and  to  the  friction  of  the  air  in  the  alveolar 
passages. 

THE  CHEMISTRY  OF  RESPIRATION 

The  general  metabolic  process  as  it  takes  place  in  the  tissues  involves 
the  assimilation  of  oxygen  and  the  evolution  of  carbon  dioxid.  The  former 
is  the  first,  the  latter  the  last,  of  a  series  of  chemic  changes  the  continuance  of 
which  is  essential  to  the  maintenance  of  all  life  phenomena.  A  constant 
supply  of  oxygen  and  an  equally  constant  removal  of  carbon  dioxid  are 
necessary  conditions  for  tissue  activity.  The  blood  is  the  medium  by  which 
the  oxygen  is  transported  from  the  lungs  to  the  tissues  and  the  carbon  dioxid 
from  the  tissues  to  the  lungs.  The  respiratory  movements  constitute  the 
means  by  which  the  oxygen  of  the  air  is  brought  into,  and  the  carbon  dioxid 
expelled  from,  the  lungs  into  the  surrounding  air. 

The  exchanges  between  the  blood  and  the  tissues  constitute  mternal 
respiration,  while  the  exchanges  between  the  blood,  the  intra-pulmonic  and 
the  atmospheric  air,  the  result  of  the  mechanic  movements  of  the  thorax, 
constitute  external  respiration.  The  transfer  of  the  oxygen  by  the  blood 
from  the  interior  of  the  lungs  to  the  tissues,  and  of  the  carbon  dioxid  from 
the  tissues  to  the  interior  of  the  lungs,  is  the  outcome  of  a  series  of  physical 
and  chemic  changes  which  are  related  to  the  exchange  of  gases  between  the 
air  in  the  lungs  and  the  blood,  on  the  one  hand,  and  the  exchange  of  gases 
between  the  blood  and  tissues,  on  the  other  hand. 

In  consequence  of  the  many  and  complex  chemic  changes  which  attend 
these  gaseous  exchanges,  there  arise  changes  in  composition  of: 

1.  The  air  breathed. 

2.  The  blood,  both  arterial  and  venous. 

3.  The  tissue  elem^ents  and  the  lymph  by  which  they  are  surrounded. 

The  investigation  of  the  nature  of  these  changes,  the  mechanism  of  their 
production,  and  their  quantitative  relations  constitute  the  subject-matter  of 
the  chemistry  of  respiration. 

CHANGES  IN  THE  COMPOSITION  OF  THE  AIR 

Experience  teaches  that  the  air  during  its  sojourn  in  the  lungs  undergoes 
such  a  change  in  composition  that  it  is  rendered  unfit  for  further  breathing. 


4IO  TEXT-BOOK  OF  PHYSIOLOGY 

Chemic  analysis  has  shown  that  this  change  involves  a  loss  of  oxygen,  a  gain 
in  carbon  dioxid,  watery  vapor,  and  organic  matter.  For  the  correct  under- 
standing of  the  phenomena  of  respiration  it  is  essential  that  not  only  the 
character  but  the  extent  of  these  changes  be  known.  This  necessitates  an 
analysis  of  both  the  inspired  and  expired  airs,  from  a  comparison  of  which 
certain  deductions  can  be  made. 

The  results  which  have  been  obtained  are  represented  in  the  following 
table : 

Inspired  Air.  Expired  Air. 

(Oxygen 20.80.  f  Oxygen 16.02. 
Carbon  dioxid traces.  Carbon  dioxid . .  4.38. 
Nifxogen 79 .20.  ^  ,  ■  Nitrogen 79 .60. 
Watery  vapor variable.                            *     Watery  vapor.,  saturated. 

[  Organic  matter .  a  trace. 

These  analyses  indicate  that  under  ordinary  conditions  the  air  loses 
oxygen  to  the  extent  of  4.78  per  cent,  and  gains  carbon  dioxid  to  the  extent 
of  4.38  per  cent. ;  that  it  gains  in  nitrogen  to  the  extent  of  0.4  per  cent,  and  in 
watery  vapor  from  its  initial  amount  to  the  point  of  saturation,  as  well  as  in 
organic  matter.  It  is  to  these  changes  in  their  totality  that  those  disturbances 
of  physiologic  activity  are  to  be  attributed  which  arise  when  expired  air  is 
re-breathed  for  any  length  of  time  without  having  undergone  renovation. 

Special  forms  of  apparatus  have  been  devised  for  the  collection  and  analy- 
sis of  gases.  Their  construction  as  well  as  the  methods  of  analysis  involved 
are  complicated  and  need  not  be  described  in  this  connection.  The  presence 
of  the  carbon  dioxid,  however,  may  be  readily  shown  by  breathing  through 
a  glass  tube  into  a  vessel  containing  barium  or  calcium  hydrate  solution.  The 
turbidity  which  immediately  follows  is  due  to  the  formation  of  barium  or 
calcium  carbonate,  which  can  be  due  only  to  the  presence  of  carbon  dioxid. 
That  this  turbidity  is  not  due  to  the  carbon  dioxid  normally  present  in  the 
air  is  shown  by  the  fact  that  the  solution  remains  clear  until  the  passage  of 
the  atmospheric  air  has  been  maintained  for  some  time.  From  the  percent- 
age loss  of  oxygen  and  gain  in  carbon  dioxid,  the  total  oxygen  absorbed  and 
carbon  dioxid  exhaled  may  be  approximately  calculated.  Thus,  if  the 
volume  of  air  breathed  daily  be  accepted  at  either  8,500  or  12,752  liters,  and 
the  percentage  loss  of  oxygen  be  4.78,  the  total  oxygen  absorbed  may  be 
obtained  by  the  rule  of  simple  proportion,  e.g. : 

100  :  4.78  ::    8,500  :  a;=4o6  liters  or  580  grams' 
Or 

100  :  4.78  ::  12,752  :  »  =  6o9  liters  or  870  grams. 

By  the  same  method  the  total  carbon  dioxid  exhaled  is  found  to  be  either 
372  liters  or  735  grams,  or  558  liters  or  1103  grams;  volumes  in  both  instances 
which  agree  very  well  with  volumes  obtained  by  other  methods. 

From  the  fact  that  only  558  liters  of  carbon  dioxid  are  exhaled  as  compared 
with  609  liters  of  oxygen  absorbed,  it  is  evident  that  not  all  of  the  oxygen 
unites  with  carbon  to  form  carbon  dioxid  and  that  the  remainder  of  the 
oxygen  must  imite  with  some  other  element.  As  there  is  usually  an  excess 
of  water  eliminated  over  that  introduced  into  the  body,  it  is  highly  probable 
that  the  oxygen  combines  with  free  hydrogen  to  form  water.  The  relative 
amounts  of  the  oxygen  so  utilized  are  not  fixed  but  variable,  and  depend  on 
the  quality  and  quantity  of  the  foods,  exercise,  etc.  The  ratio  of  the 
'  I  liter  of  oxygen  weighs  1.4298  grams;  i  liter  of  carbon  dioxid  weighs  1.977  grams. 


RESPIRATION  411 

volume  of  the  carbon  dioxid  exhaled  to  the  volume  of  oxygen  absorbed  is 
known  as  the  respiratory  quotient,  and  is  usually  represented  by  the  symbol 

CO, 

p.    .    Thus  in  the  foregoing  analysis  the  respiratory  quotient  is  0.916. 

The  gain  in  nitrogen  is  a  variable  factor,  ranging  from  zero  to  0.9  per 
cent.  This  gain  is  probably  of  accidental  occurrence,  due  to  absorption 
from  the  large  intestine,  in  which  decomposition  of  nitrogen-holding  com- 
pounds is  taking  place.  It  is  generally  believed  that  free  nitrogen  plays  no 
part  in  any  phenomenon  of  combination  or  decomposition  within  the  body. 

The  gain  in  watery  vapor  will  depend  on  the  amount  previously  present 
in  the  air.  This  is  conditioned  by  the  temperature.  With  a  rise  in  tempera- 
ture the  percentage  of  water  increases;  with  a  fall,  it  decreases.  By  breath- 
ing into  a  vessel  containing  pumice  stone  saturated  with  sulphuric  acid,  the 
vapor  may  be  collected.  The  difference  observed  between  the  weight  before 
and  after  breathing  is  an  indication  of  the  amount  by  weight  of  water  exhaled 
during  the  time  of  breathing.  It  has  been  calculated  that  the  amount  of 
water  exhaled  daily  varies  between  300  and  500  grams.  Though  invisible 
at  ordinary  temperatures,  it  becomes  visible  at  low  temperature  as  soon  as 
it  emerges  from  the  respiratory  tract.  The  loss  of  heat  is  followed  by  a  con- 
densation of  the  vapor,  which  appears  at  once  as  a  cloudy   precipitate. 

The  gain  in  organic  matter  is  also  variable.  The  amount  present  is  not 
sufficient  to  permit  of  a  thorough  chemic  analysis,  but  there  are  reasons  for 
believing  that  it  belongs  to  the  proteid  group  of  bodies.  If  it  accumulates 
in  the  air,  especially  at  high  temperatures,  it  readily  undergoes  decomposition, 
with  the  production  of  offensive  odors.  Traces  of  free  ammonia  have  also 
been  found  in  the  expired  air.  In  addition  to  these  chemic  changes,  the 
air  experiences  physical  changes;  e.g.,  a  rise  in  temperature  and  an  increase 
in  volume.  The  rise  in  temperature  can  be  shown  by  breathing  through  a 
suitable  mouthpiece  into  a  glass  tube  containing  a  thermometer.  By  this 
means  it  has  been  shown  that  inspired  air  at  2o°C.  rises  in  temperature  to 
37°C.;  at  6.3°  to  29.8^0.  The  increase  in  the  temperature  w411  depend  upon 
that  of  the  air  inspired  and  the  time  it  remains  in  the  lungs.  If  retained  a 
sufficient  length  of  time  it  will  always  become  that  of  the  body.  As  a  result 
of  the  heat  absorption  the  expired  air  increases  in  volume  about  one-ninth 
of  that  of  the  inspired  air.  When  corrected  for  temperature  and  pressure 
and  freed  from  aqueous  vapor,  the  volume  of  the  expired  air  is  less  than 
that  of  the  inspired  air  by  about  one  two-hundred  and  fiftieth. 

The  Composition  of  the  Alveolar  Air. — The  foregoing  statement  of  the 
composition  of  the  expired  air,  derived  in  part  from  the  upper  air-passages, 
trachea,  and  bronchi,  does  not  necessarily  represent  the  composition  of  the 
alveolar  air.  It  is  very  probable  that  the  percentage  of  carbon  dioxid  is 
greater,  the  percentage  of  oxygen  less,  in  the  latter  than  in  the  former.  This 
is  made  evident  by  collecting  in  several  portions  the  expired  air  as  it  escapes 
from  the  respiratory  tract  and  subjecting  it  to  analysis.  The  last  portion 
always  contains  a  larger  amount  of  carbon  dioxid  and  a  smaller  amount  of 
oxygen  than  the  first  portion.  The  determination  of  the  composition  of  the 
alveolar  air  is  extremely  difficult.  It  has  been  estimated  to  contain  from 
5  to  6  per  cent,  of  carbon  dioxid  and  from  14  to  18  per  cent,  of  oxygen. 

Pulmonary  Ventilation. — It  is  owing  largely  to  this  inequality  of 
volumes  and  consequently  of  the  "partial  pressures"  of  these  two  gases  in 


412  TEXT-BOOK  OF  PHYSIOLOGY 

the  trachea  and  alveoH  that  the  degree  of  ventilation  necessary  for  the  ex- 
change of  gases  between  lungs  and  air  is  maintained.  Though  the  respira- 
tory movements  doubtless  create  currents  in  the  air-passages  which  carry, 
on  the  one  hand,  a  portion  of  the  inspired  air  directly  into  the  alveoli,  and, 
on  the  other  hand,  carry  a  portion  of  the  alveolar  air  directly  out  of  the  body, 
other  portions  find  their  way  into  and  out  of  the  alveoli  in  accordance  with  the 
laws  of  diffusion.  If  the  pressure  of  the  oxygen  in  the  trachea  is  158  mm. 
Hg.  and  in  the  alveoli  approximately  122  mm.  Hg.,  diffusion  downward 
will  take  place.  Equilibrium,  however,  is  never  established,  as  the  oxygen 
is  continually  disappearing  by  passing  into  the  blood.  On  the  contrary,  if 
the  carbon  dioxid  pressure  in  the  alveoli  is  approximately  28  to  40  mm.  Hg., 
and  in  the  trachea  0.3  mm.  Hg.,diffusion  will  take  place  upward.  Equilib- 
rium will  never  be  established,  however,  as  the  carbon  dioxid  is  constantly 
coming  out  of  the  blood.  Pulmonary  ventilation  may  also  be  aided  by 
those  alternate  changes  in  volume  of  the  heart,  great  vessels,  and  lungs 
occurring  as  the  result  of  the  heart-beat  and  producing  the  so-called  cardio- 
pneumatic  movements. 

CHANGES  IN  THE  COMPOSITION  OF  THE  BLOOD 

The  blood  which  flows  through  the  pulmonic  artery  into  the  lungs  is 
dark  bluish-red,  while  that  which  flows  from  the  lungs  into  the  pulmonic 
veins  is  scarlet  red,  in  color.  The  blood  is  changed,  while  flowing  through 
the  lung  capillaries,  from  the  venous  to  the  arterial  condition.  As  the  air 
in  the  lungs  gains  carbon  dioxid,  and  loses  oxygen  it  is  fair  to  assume  that 
what  the  air  gains  the  blood  loses,  and  what  the  air  loses  the  blood  gains. 
In  other  words,  the  blood,  while  passing  through  the  lungs,  is  changed 
from  venous  to  arterial  by  the  loss  of  carbon  dioxid  and  the  gain  of  oxygen. 
The  change  in  color  of  venous  blood  from  dark  bluish,  to  scarlet  red  is 
strikingly  shown  by  shaking  it  in  a  test-tube  with  oxygen  or  atmospheric  air. 

The  blood  which  flows  through  the  arteries  into  the  tissues  is  scarlet  red, 
while  that  which  flows  from  the  tissues  into  the  veins  is  bluish-red  in  color. 
The  blood  while  flowing  through  the  tissue  capillaries  is  changed  from  the 
arterial  to  the  venous  condition.  Since  arterial  blood  when  deprived  of 
oxygen  becomes  bluish-red,  the  indication  is  that  the  change  in  color  is 
associated  with,  if  not  entirely  due  to,  the  escape  of  oxygen  into  the  tissues. 
The  constant  elimination  of  carbon  dioxid  from  the  blood  into  the  lungs 
indicates  that  the  carbon  dioxid  is  as  constantly  passing  from  the  tissues 
across  the  capillary  walls  into  the  blood. 

These  considerations  are  confirmed  by  the  results  of  analyses  which  have 
been  made  of  both  arterial  and  venous  blood.  The  presence  of  gas  in  the 
blood  is  demonstrated  by  subjecting  it  under  appropriate  conditions  to  the 
vacuum  of  the  mercurial  air-pump,  into  which  it  at  once  escapes.  From 
100  volumes,  an  average  of  60  volumes  of  gas  at  standard  pressure,  760  mm. 
Hg.  and  temperature  o°C.,  can  thus  be  obtained. 

Gases  of  the  Blood. — An  analysis  of  the  volumes  of  gas  removed  from 
both  arterial  and  venous  blood  shows  that  each  consists  of  oxygen,  carbon 
dioxid,  and  nitrogen,  though  in  different  amounts.  Average  composi- 
tions of  the  gases  extracted  from  dog's  blood  obtained  from  the  carotid 
artery  and  right  ventricle  respectively  are  given  in  the  following  table: 


RESPIRATION  41 


Arterial  blood   f  ^^S^'^' ,:  ••.-,••     '°  ^°^-        Venous  blood  J  ^xygen    .    . 12  vols. 

100  vols.         S^f  b°'^  <^^°'^^-  •     40  vo  s.  ^^^  ^^j^  Carbon  dioxid 45  vo  s. 

L  Nitrogen 1-2  vols.  [  Nitrogen 1-2  vols. 

The  changes  produced  in  the  blood  by  respiration,  both  external  and  in- 
ternal, become  apparent  from  a  comparison  of  these  analyses.  The  arterial 
blood  while  passing  through  the  capillaries  of  the  tissues  loses  eight  volumes 
per  cent,  of  oxygen  and  gains  five  volumes  per  cent,  of  carbon  dioxid. 
The  venous  blood  while  passing  through  the  capillaries  of  the  lungs  gains 
oxygen  and  loses  carbon  dioxid  in  corresponding  amounts.  These  amounts 
will  vary  somewhat  in  the  analyses  of  the  blood  of  different  animals  and 
under  different  physiologic  conditions.  The  volume  of  nitrogen  is  not 
appreciably  changed. 

The  Relation  of  the  Gases  in  the  Blood. — The  mechanism  by  which 
the  gases  become  associated  with  the  blood  at  the  moment  of  their  entrance 
into  it,  and  again  become  dissociated  just  prior  to  their  exit  from  it,  as  well 
as  their  relation  to  the  blood  while  in  transit,  will  be  more  readily  understood 
after  reference  to  a  few  elementary  facts  relative  to  the  absorption  of  gases 
by  liquids  in  general  and  the  conditions  of  temperature  and  pressure  by 
which  it  is  influenced. 

It  is  well  known  that  liquids  will  absorb  or  dissolve  at  any  constant 
pressure  unequal  volumes  of  difi"erent  gases  in  accordance  with  their  solubili- 
ties and  with  variations  in  temperature.  Water,  for  example,  will  absorb, 
in  accordance  with  the  foregoing  conditions,  oxygen,  carbon  dioxid,  and 
nitrogen,  as  well  as  many  other  gases.  The  volume  of  any  gas  thus  absorbed 
is  known  as  the  coefficient  of  absorption,  and  may  be  defined  as  the  number 
of  cubic  centimeters  of  the  gas  which  one  cubic  centimeter  of  water  will 
absorb  when  the  gas,  in  contact  with  the  water,  stands  under  a  pressure  of 
one  atmosphere  or  760  mm.  of  mxercury  and  at  a  temperature  of  o°C.  The 
volume  absorbed,  however,  varies  inversely  as  the  temperature.  Thus  at 
o°C.  the  volume  of  oxygen  absorbed  by  one  volume  of  water  is  0.0489  c.c; 
of  carbon  dioxid  1.7 13  c.c;  of  nitrogen  0.0234  c.c.  With  a  rise  of  tempera- 
ture, the  pressure  remaining  constant,  the  absorptive  power  of  water  for  each 
of  these  gases  diminishes-  Thus  at  i5°C.,  the  volumes  of  oxygen,  carbon 
dioxid  and  nitrogen  absorbed  are  0.0310  c.c,  1.0025  c.c  and  0.0168  c.c 
respectively.  Though  the  volume  of  the  gas  absorbed  diminishes  as  the 
temperature  rises,  it  is  independent  of  pressure,  for  no  matter  to  what  extent 
the  pressure  may  vary  the  volume  absorbed  is  always  the  same.  (Law  of 
Henry.) 

If  the  weight  of  the  gas  absorbed  be  considered  rather  than  the  volume 
(that  is  the  product  of  the  volume  and  the  density  or  the  number  of  molecules 
in  the  volume),  then  the  temperature  remaining  constant,  the  weight  of  the 
volume  absorbed  increases  and  decreases  proportionately  as  the  pressure 
rises  and  falls.  Thus  at  a  pressure  of  760  mm.  of  mercury  and  at  a  tempera- 
ture of  o°C.,  the  volume  of  oxygen  absorbed  by  one  volume  of  water  is 
0.0489  c.c;  at  1520  mm.  of  mercury,  the  same  volume  is  absorbed  but  its 
weight  is  doubled.  If  the  pressure  falls  below  760  mm.  of  mercury  the 
same  volume  is  absorbed  but  its  weight  is  diminished.  (Law  of  Dalton.) 
Because  of  the  foregoing  facts,  it  is  necessary  in  all  gaseous  determina- 
tions to  reduce  for  purposes  of  comparison  the  obtained  volumes  to  standard 
temperature  (o°C.)  and  pressure  (760  mm.  of  mercury). 


414  TEXT-BOOK  OF  PHYSIOLOGY 

When  the  liquid  is  once  saturated  with  a  gas  at  a  constant  pressure 
and  temperature,  there  is  coincidently  with  the  entrance  of  the  gas  into  the 
liquid,  an  equivalent  exit  of  the  gas  from  it,  though  the  volume  retained  in  the 
liquid  remains  constant.  The  reason  for  this  fact  is,  that  under  the  condi- 
tions, the  volume  of  the  gas  dissolved  by  the  liquid  though  small  in  amount 
exerts  a  pressure  in  the  opposite  direction  equivalent  to  the  pressure  acting 
upon  the  liquid.  If  one  cubic  centimeter  of  water  absorbs  0.0489  c.c.  of 
oxygen  at  760  mm.  and  o°C.,  this  volume  will  exert  a  pressure  opposite 
in  direction  of  760  mm.  of  mercury.  For  this  reason  the  entrance  and  exit  of 
the  gas  are  equal  and  opposite. 

If  water  be  exposed  to  atmospheric  air  consisting  of  oxygen,  carbon 
dioxid,  and  nitrogen  in  the  ordinary  proportions,  at  any  given  temperature 
and  pressure,  the  water  will  absorb  unequal  volumes  of  each  of  the  three 
gases.  The  pressure  under  which  each  gas  is  absorbed  is  a  part  only, 
however,  of  the  total  atmospheric  pressure  at  the  time.  The  pressure 
exerted  by  any  one  of  these  gases  is  known  as  its  "partial  pressure," 
and  depends  on  the  percentage  volume  of  the  gas  present.  If  atmospheric 
air  contains  at  standard  pressure  and  temperature  79.15  volumes  percent. 
of  nitrogen,  its  partial  pressure  will  be  "^^-  of  760,  or  601.54  mm.  Hg.; 
if  the  air  contains  0.04  volume  per  cent,  of  carbon  dioxid  and  20.85  volumes 
per  cent,  of  oxygen,  the  partial  pressure  of  each  will  be  0.30  mm.  Hg.  and 
158.46  mm.  Hg.  respectively.  The  absorption  of  each  gas  is  independent  of 
all  the  rest,  and  is  the  same  for  nitrogen,  for  example,  as  if  it  alone  were 
present  at  a  pressure  of  601.54  mm.  Hg. 

Again,  if  water  holding  in  solution  a  certain  volume  of  a  gas — carbon 
dioxid,  for  example — be  exposed  to  an  atmosphere  containing  but  0.04 
volume  per  cent,  of  carbon  dioxid,  and  having  therefore  a  pressure  of  but 
0.3  mm.  Hg.,  the  gas  will  at  once  begin  to  leave  the  water,  and  continue  to 
do  so  until  the  pressure  of  the  carbon  dioxid  in  the  atmosphere  balances  the 
pressure  of  the  gas  in  the  water,  at  which  moment  the  escape  of  the  gas 
ceases.  The  pressure  of  a  gas  in  a  liquid  is  equal  to  that  pressure  in  milli- 
meters of  mercury  of  the  same  gas  in  the  atmosphere  which  is  required  to  keep 
it  in  solution.  What  is  true  for  the  carbon  dioxid  is  true  for  any  other  gas 
that  may  be  in  solution.  If  a  liquid  has  a  greater  density  than  water,  as 
from  the  presence  of  inorganic  salts,  the  absorptive  power  under  standard 
conditions  of  temperature  and  pressure  becomes  less.  It  is  for  this  reason 
that  blood-plasma  contains  less  oxygen,  nitrogen,  and  carbon  dioxid  than 
water. 

It  will  be  recalled  that  the  blood  yields  up  its  gases  when  subjected  to  the 
vacuum  of  the  mercurial  pump;  that  is,  to  a  diminution  or  complete  removal 
of  the  atmospheric  pressure.  From  this  it  might  be  inferred  that  the  gases 
are  merely  held  in  solution  by  pressure,  and  at  once  escape  the  moment 
they  are  exposed  to  a  space  in  which  there  is  a  very  slight  or  a  total  absence 
of  pressure.  In  other  words,  that  the  absorption  of  gases  by  the  blood  and 
their  escape  from  it  follow  the  law  of  pressure  as  stated  in  foregoing  para- 
graphs. It  is  therefore  necessary  to  test  this  supposed  condition  of  the  gases 
in  the  blood  by  subjecting  the  latter  to  gradually  diminishing  pressures, 
with  a  view  of  determining  in  how  far  the  discharge  of  the  gases  follows  the 
law  of  falling  pressures.  For  convenience  the  conditions  of  each  gas  will 
be  considered  separately. 


RESPIRATION  415 

Oxygen. — If  blood  is  subjected  to  a  succession  of  pressures  progressively 
less  than  the  standard,  it  is  found  that  though  oxygen  is  evolved,  its  evolution 
is  not  in  accordance  with  the  law  of  partial  pressures;  that  is,  in  proportion 
to  the  diminution  of  pressure.  Within  wide  limits — e.g.,  from  760  to  332 
mm.  atmospheric  pressure,  to  which  correspond  oxygen  pressures  of  159 
and  70  mm.  respectively — there  is  but  a  slight  increase  in  the  amount  of 
oxygen  evolved;  and  it  is  not  until  the  pressure  of  the  oxygen  falls  below 
the  latter,  i.e.,  70  mm.  that  it  begins  to  be  liberated  in  large  amounts.  From 
this  on,  the  oxygen  condnues  to  be  liberated  with  decreasing  pressures, 
until  the  zero  point  is  reached,  when  all  gaseous  discharge  ceases,  Coin- 
cidently  the  blood  changes  in  color  from  a  bright  red  to  a  deep  bluish-red. 
It  is  evident  from  the  results  of  this  procedure  that  the  condition  of  the  oxygen 
in  the  blood  is  but  to  a  slight  extent  one  of  physical  absorption.  The 
indications  are  that  the  union  is  of  the  nature  of  a  chemic  combination. 

If  the  red  corpuscles  are  removed  from  the  blood  and  the  plasma  alone 
treated  in  the  manner  above  described,  it  will  be  found  that  the  oxygen 
liberated  now  follows  the  law  of  partial  pressure.  The  amount  so  liberated, 
however,  is  small,  scarcely  amounting  to  more  than  0.36  c.c.  per  100  c.c. 
of  blood.  The  agent  therefore  which  holds  the  oxygen  in  combination  is 
the  red  corpuscle,  or  more  exactly,  the  hemoglobin,  which  constitutes  about 
32  per  cent,  of  its  volume.  This  is  proved  by  the  fact  that  a  solution  of 
gas-free  hemoglobin  of  a  strength  equivalent  to  that  of  the  blood  (14  per 
cent.),  exposed  to  oxygen  under  a  gradually  increasing  pressure  from  zero 
up  to  50  to  70  mm.  pressure,  will  absorb  large  quantities  of  oxygen;  beyond 
this  point  the  amount  absorbed  is  again  small  in  comparison.  At  70  mm. 
pressure  the  hemoglobin  is  almost  saturated.  Coincidently  with  this  absorp- 
tion the  hemoglobin  changes  in  color  from  bluish-red  to  scarlet-red  and 
changes  from  hemoglobin  to  oxyhemoglobin.  The  reverse  method,  that  of 
subjecting  oxyhemoglobin  to  gradually  diminishing  pressures,  yields  opposite 
results,  that  is,  the  oxygen  becomes  dissociated  and  the  force  by  which  this  is 
accomplished  is  known  as  the  force  of  dissociation.  As  one  gram  of  hemo- 
globin combines  with  1.34  c.c.  of  oxygen,  and  as  the  percentage  of  hemoglobin 
is  13.50  to  14,  it  is  e\ddent  that  there  is  sufl&cient  hemoglobin  to  combine 
with  practically  all  the  oxygen  usually  present  in  the  blood.  Thus  the 
hemoglobin  in  100  c.c.  of  blood  would  hold  in  combination  18.76  c.c.  of 
ox}'gen.  This,  together  with  the  one  c.c.  held  in  solution  in  plasma,  would 
equal  the  volume  obtained  in  the  vacuum  of  the  air-pump. 

The  union  of  the  oxygen  with  the  hemoglobin  is  therefore  largely  chemic 
in  character,  dependent  however  on  pressure.  About  one  per  cent,  is 
physically  absorbed  by  or  dissolved  in  the  plasma;  the  remainder  is  chem- 
ically combined  with  the  hemoglobin. 

The  association  or  combination  of  oxygen  is  favored  by  a  pressure  of  at 
least  30  to  50  mm.  Hg.  and  upward;  the  dissociation,  by  diminution  of 
pressure.  In  the  conversion  of  hemoglobin  into  oxyhemoglobin  two  an- 
tagonistic forces  are  at  work,  heat  and  chemic  affinity.  The  former 
tends  to  prevent,  the  latter  to  favor,  the  union.  Chemic  affinity  increases 
with  the  influence  of  mass,  that  is,  in  proportion  to  the  number  of  atoms  in  a 
unit  of  volume,  with  the  density,  and  with  the  partial  pressure  of  the  oxygen. 
Diminution  of  pressure  reduces  the  mass  influence  and  permits  the  heat  to 
bring  about  dissociation  (Bunge).     The  following  table  by  Hiifner  shows 


41 6  TEXT-BOOK  OF  PHYSIOLOGY 

the  relative  proportion  of  hemoglobin  and  oxyhemoglobin  in  blood  contain- 
ing 14  per  cent,  hemoglobin  and  exposed  to  air  at  gradually  diminishing 
pressures : 


Atmospheric  Pressure 
in  mm.  Hg. 

Partial  P 
in 

ressure  of  Oxygen 
mm.  Hg. 

Hemoglobin 
Percentage. 

Oxyhemoglobin 
Percentage. 

760 

159-3 

1.49 

98.51 

524.8 
357-8 
238.5 

no 
75 
50 

2.14 

3-II 
4.60 

97.86 
96.89 
95-4° 

"93 
47-7 
23-8 

25 

10 

5 

8.79 
19.36 
32-51 

91.21 
80.64 
67-49 

0.0 

0.0 

100.00 

0.00 

Carbon  Dioxid. — The  blood  yields  up  its  contained  carbon  dioxid  to  the 
vacuum  of  the  gas-pump  as  completely  as  it  does  its  oxygen.  The  same  is 
not  the  case,  however,  if  the  red  corpuscles  are  first  removed  and  the  ex- 
periment made  with  either  plasma  or  serum.  Even  at  zero  pressure  the  fluid 
contains  carbon  dioxid,  as  shown  by  its  liberation  on  the  addition  of  some 
weak  acid,  as  tartaric  or  phosphoric,  an  indication  that  it  exists  in  a  state  of 
firm  combination.  The  same  result  follows  the  addition  of  the  red  blood- 
corpuscles,  which  act  in  a  manner  similar  to  the  acids  just  mentioned. 
This  property  of  the  corpuscles  has  been  attributed  to  hemoglobin,  and 
especially  when  in  the  state  of  oxyhemoglobin.  It  is  for  this  reason  that 
blood  yields  all  its  carbon  dioxid  to  the  vacuum  of  the  gas-pump. 

The  limit  of  pressure  at  which  the  plasma  ceases  to  absorb  carbon 
dioxid  physically  and  begins  to  combine  it  chemically  is  not  very  clearly 
defined.  It  has  been  estimated  that  of  the  entire  amount,  38  to  45  volumes, 
only  about  2.5  volumes  are  so  absorbed,  the  remainder  being  in  a  condition 
of  both  loose  and  stable  combination. 

An  analysis  of  the  serum,  and  presumably  of  the  plasma,  shows  the 
presence  of  sodium  salts,  with  which  the  carbon  dioxid  could  enter  into  com- 
bination, viz.:  sodium  carbonate  and  dibasic  sodium  phosphate.  The 
sodium  is  thus  partly  divided  between  carbonic  acid  and  phosphoric  acid. 
The  amount  of  the  sodium  which  falls  to  carbon  dioxid  will  depend  on  the 
mass  influence  of  the  latter;  that  is,  its  partial  pressure. 

At  its  origin  in  the  tissues  the  carbon  dioxid  acquires  a  considerable 
tension,  and  its  mass  influence  is  correspondingly  large.  On  entering  the 
blood  it  combines  with  sodium  carbonate,  with  the  formation  of  sodium 
bicarbonate,  as  shown  in  the  following  equation: 

NajC03-f-COj  +  H20  =  2NaHC03. 

At  the  same  time,  having  a  greater  mass  influence  than  the  phosphoric  acid, 
it  will  withdraw  from  the  dibasic  sodium  phosphate  one-half  of  its  sodium, 
with  the  formation  of  sodium  bicarbonate  and  monobasic  sodium  phosphate, 
as  shown  in  the  following  equation: 

Na2HPO,  +  COi,  +  H20  =  NaHC03-l-NaH2PO«. 

In  addition  to  the  amount  of  CO2  physically  absorbed  by  the  plasma  and 
chemically  combined  with  sodium,  a  third  portion  is  more  or  less  loosely 
combined  with  the  protein  constituent,  globin,  of  the  red  corpuscle.  As 
this  is  the  case  the  corpuscles  must  be  regarded  as  carriers  of  CO2  from  the 
tissues  to  the  lungs  as  well  as  carriers  of  oxygen.     It  has  also  been  suggested 


RESPIRATION  417 

that  the  presence  of  the  CO2  in  the  corpuscle,  assists  in  the  dissociation  of 
the  ox}^gen  as  the  blood  passes  through  the  capillaries.  With  the  diffusion 
of  the  carbon  dioxld  from  the  blood  into  the  alveoli  its  tension  in  the  venous 
blood  falls,  its  mass  influence  diminishes,  while  that  of  the  phosphoric 
acid  relatively  increases.  As  a  result,  the  sodium  is  withdrav^n  from  the 
sodium  bicarbonate,  an  additional  liberation  of  carbon  dioxid  takes  place 
and  dibasic  sodium  phosphate  is  re-formed.  The  association  or  com- 
bination of  the  carbon  dioxid  with  the  basic  salts  depends  on  its  partial 
pressure;  dissociation  in  the  lungs,  on  a  diminution  of  pressure. 

Nitrogen. — This  gas  exists  in  both  arterial  and  venous  blood  in  a  state 
of  solution.  There  is  no  evidence  that  it  enters  into  combination  with  any 
other  element. 

Tension  of  the  Gases  in  the  Blood. — It  will  be  recalled  that  a  liquid 
holding  in  solution  one  or  more  gases  will  on  exposure  to  an  atmosphere 
composed  of  the  same  gases  either  give  up  or  absorb  volumes  varying  in 
amount  and  in  accordance  with  their  partial  pressures  until  equilibrium  is 
established.  If  the  pressure  of  any  one  gas  in  the  atmosphere  is  greater  than 
the  pressure  of  the  same  gas  in  the  liquid,  it  is  absorbed;  if  the  pressure  is 
less  the  gas  is  discharged.  Knowing  the  pressure  of  the  gases  in  percent- 
ages of  an  atmosphere,  at  the  beginning  and  the  end  of  an  experiment,  the 
original  tension  or  pressure  of  the  gases  in  the  liquid  can  be  easily  calculated. 
On  this  principle  various  forms  of  apparatus  known  as  aerotonometers  have 
been  devised  by  which  the  tension  of  the  gases  in  the  blood  can  be  determined. 

These  appliances  consist  essentially  of  a  glass  tube  containing  oxygen, 
carbon  dioxid,  and  nitrogen  in  known  amounts  and  tensions.  The  blood 
from  an  animal  is  then  allowed  to  flow  directly  from  an  artery  or  vein  into  the 
tube.  As  it  flows  down  its  sides  in  a  thin  layer  it  presents  a  large  surface 
to  the  action  of  the  contained  gases.  In  the  aerotonometer  of  Fredericq 
the  blood,  made  non-coagulable  by  the  injection  of  peptone,  is  returned 
from  the  opposite  extremity  of  the  tube  to  the  animal.  This  enables  the 
experiment  to  be  continued  for  an  hour  or  more.  A  knowledge  of  the 
tensions  of  the  blood  gases  is  of  interest,  as  it  affords  a  clue  to  the  mechanism 
by  which  the  interchange  takes  place  between  the  lungs  and  the  blood,  on  the 
one  hand,  and  the  blood  and  tissues,  on  the  other.  The  results,  however,  of 
different  observers  are  not  sufficiently  in  accord  to  permit  of  positive 
deductions. 

In  the  well-known  experiments  of  Strassburger,  the  tension  of  the 
oxygen  in  the  arterial  blood  of  the  dog  was  found  to  be  29.64  mm.  Hg.,  or 
3.9  per  cent,  of  an  atmosphere,  and  in  the  venous  blood  22.04  nim.  Hg.,  or 
2.9  per  cent.  The  tension  of  the  carbon  dioxid  in  the  venous  blood  was 
found  to  be  41.14  mm.  Hg.,  or  5.4  per  cent,  of  an  atmosphere,  and  in  the 
arterial  blood  21.8  mm.  Hg.,  or  2.8  per  cent.  Very  different  results  have 
been  obtained  by  Fredericq  with  the  aerotonometer  devised  by  him  and  by 
the  employment  of  a  method  different  from  that  of  Strassburger.  Thus  he 
states  that  the  oxygen  tension  in  the  pulmonic  alveoli  is  136  mm.  Hg.,  or 
18  per  cent,  of  an  atmosphere  while  in  the  arterial  blood  it  is  106  mm.  Hg., 
or  14  per  cent.;  while  the  carbon-dioxid  tension  in  the  tissues  varies  from 
45  to  68  mm.  Hg.,  or  from  6  to  9  per  cent,  of  an  atmosphere;  while  in  the 
venous  blood  it  varies  from  30  to  41  mm.  Hg.,  or  from  3.8  to  5.4  per  cent. 
and  in  the  pulmonic  alveoli  it  is  about  21  mm.  or  2.8  per  cent. 
27 


4i8  TEXT-BOOK  OF  PHYSIOLOGY 

CHANGES  IN  THE  COMPOSITION  OF   THE  TISSUES  AND  LYMPH 

From  previous  statements  the  inferences  can  be  drawn  that  the  oxygen 
leaves  the  blood  as  the  latter  flows  through  the  capillaries;  that  it  passes 
through  the  capillary  wall  into  the  surrounding  lymph  and  so  to  the  tissue- 
cells;  that  it  oxidizes  food  materials  in  the  tissue-cells  whereby  the  potential 
energy  of  the  former  is  liberated  as  kinetic  energy;  that  the  carbon  dioxid 
so  evolved  passes  into  the  lymph  and  through  the  wall  of  the  capillary  into 
the  blood. 

While  this  is  doubtless  the  case,  the  presence  of  free  oxygen  in  the  tissues 
cannot  be  demonstrated  by  the  usual  methods  of  gas  analysis.  Only  in  the 
saliva  and  in  the  blood  of  the  placental  umbilical  vein  can  it  be  shown 
that  oxygen  has  direcdy  passed  through  the  capillary  wall.  For  this  reason 
it  has  been  claimed  by  a  few  investigators  that  oxygen  does  not  leave  the 
blood,  but  that  the  field  of  its  activity  as  an  oxidizing  agent  is  limited  to  the 
blood-current,  where  it  meets  with  and  oxidizes  easily  reducible  substances 
entering  from  the  tissues.  On  this  view  the  potential  energy  of  the  food 
would  be  liberated  by  mere  decomposition  or  cleavage  in  consequence  of 
cell  activity. 

Nevertheless  many  facts  from  the  fields  of  comparative  physiology  and 
physiologic  chemistry  combine  to  support  the  view  that  oxygen  is  absolutely 
necessary  to  the  maintenance  of  the  life  of  all  tissue  cells.  Though  they 
will  continue  to  manifest  their  characteristic  activities^e.^.,  contraction  on 
the  part  of  a  muscle,  secretion  by  a  gland,  the  conduction  of  a  nerve  impulse 
by  the  nerve,  etc. — for  a  variable  length  of  time  after  oxygen  is  prevented 
from  gaining  access  to  them,  nevertheless  they  will  in  due  time  die. 

The  necessity  for  oxygen  on  the  part  of  the  tissues  and  the  avidity  with 
which  they  absorb  it,  is  shown  by  their  power  of  reducing  pigments  such  as 
alizarine  blue.  If  this  pigment  be  injected  into  the  blood-vessels  of  an 
animal  and  the  animal  killed  in  about  ten  minutes,  it  will  be  found  that  while 
the  blood  exhibits  a  deep  blue  color  the  tissues  present  their  usual  colors. 
But  after  exposure  to  the  air  or  to  free  oxygen  the  latter  also  acquire  the 
characteristic  blue  color.  The  explanation  offered  for  this  fact  is  that  the 
tissues  in  their  need  for  oxygen  absolutely  extract  it  from  the  pigment, 
reducing  it  to  a  colorless  compound,  which,  however,  on  exposure  recom- 
bines  with  oxygen  and  regains  the  original  color. 

Though  free  oxygen  cannot  be  shown  to  be  present  in  the  tissues,  there 
are  many  reasons  for  believing  that  it  is  continually  passing  into  them  by  way 
of  the  lymph-stream.  Its  rapid  disappearance  would  indicate  that  it  is 
immediately  utilized  for  the  production  of  carbon  dioxid  (which  is  improb 
able  on  other  grounds) ,  or  that  the  tissues  possess  a  capacity  for  oxygen  storage, 
of  placing  it  in  reserve  under  some  combination  or  other,  by  which  it  can  be 
securely  retained  until  required  for  oxidation  purposes.  This  is  rendered 
probable  from  the  fact  that  the  carbon  dioxid  evolved  at  any  given  moment 
is  not  necessarily  dependent  on  the  oxygen  just  absorbed,  for  if  oxygen  be 
withheld  from  a  nutritive  fluid  which  is  being  artificially  circulated  through 
a  recently  isolated  organ,  carbon  dioxid  will  continue  to  be  discharged  for 
some  time.  A  muscle,  or  even  a  living  animal — e.g.,  a  frog — placed  in  an 
atmosphere  of  pure  nitrogen  will  remain  active  and  evolve  CO 2  even  for 
several  hours. 


RESPIRATION 


419 


Naturally  the  absorption  of  oxygen  and  the  discharge  of  carbon  dioxid 
and  the  changes  of  composition  which  are  incident  to  nutrition  will  be  most 
marked  in  those  tissues  characterized  by  the  greatest  degree  of  physiologic 
activity.  Muscle-tissue  exhibits  these  changes  to  a  greater  degree  than 
bone.  Tissues  with  intermediate  degrees  of  activity  should  exhibit  corre- 
sponding degrees  of  respiratory  change.  Experiment  confirms  this  view. 
Thus,  100  grams  each  of  muscle,  spleen,  and  broken  bone  from  a  recently 
living  animal  exposed  to  the  air  for  twenty-four  hours  absorbed  respectively 
50.8  c.c,  27.3  c.c,  and  17.2  c.c.  of  oxygen,  while  each  discharged  during  the 
same  period  56.8  c.c,  15.4  c.c,  and  8.1  c.c.  of  carbon  dioxid  respectively. 


AtjnospJzerir  Air. 

Ox-jjs  ynnt.Hg ,  or  20.8 S  per  ce/it , 

CO;^'0.:im.m.Hcf,  or  0.0 f-  per  cent, 

of  a/i  at//iospkere . 


Ox-Tens  I  Of?, 

38 mm  Jf^^, 

S per  cent. 


CO:t-Te?isioft 
4i8  7n.m.  He/', 
S,5  per  ce/it. 


'Vz-VmsiMt 

J.JOmm//^} 

l'/perce//,t. ' 
\C();f-Tensioji 


Alveolas 


Venam 


A/ier/al 


Blood. 


Shod. 


OxTejtsirm  0.007?,?nEff, 
6  ta  S  per  ce/it. 


Ox'Tensio/t. 

iOGyn,?}!.  N^, 

J4 per  cent. 


-CO3 -Tension 

38  777.  ?n,     M^, 

J  per  cent. 


Tissues. 

Fig.  197. — Diagram  showing  the  Relative  Tension  of  Oxygen  and  Carbon  Dioxid  in 
THE  Lungs,  in  the  Blood,  and  in  the  Tissues. 

In  another  series  of  experiments  by  a  different  observer  100  grams  of  muscle 
absorbed  in  three  hours  23  c.c.  of  oxygen,  and  100  grams  of  bone  5  c.c.  of 
oxygen.  Both  tissues  discharged  carbon  dioxid  in  amounts  proportional  to 
the  oxygen  absorbed.  The  same  respiratory  changes  may  be  more  satis- 
factorily dem_onstrated  by  passing  blood  through  the  tissues  of  isolated 
organs  and  the  tissues  of  recently  living  animals.  The  analysis  of  the  blood 
before  and  after  perfusion  shows  a  loss  of  oxygen  and  a  gain  in  carbon 
dioxid. 

Tension  of  the  Gases  in  the  Tissues. — As  the  presence  of  free  oxygen 
cannot  be  demonstrated,  its  tension  there  must  be  regarded  as  nil.     The 


420  TEXT-BOOK  OF  PHYSIOLOGY 

tension  of  the  carbon  dioxid  is  quite  high,  though  difficult  of  exact  deter- 
mination. It  has  been  estimated  at  from  45  to  68  mm,  Hg.,  or  from  6  to  9 
per  cent,  of  an  atmosphere. 

The  variations  of  tension  or  pressure  of  these  two  gases  in  the  lungs,  in 
different  parts  of  the  vascular  apparatus,  and  in  the  tissues,  and  their  rela- 
tions to  each  other,  are  shown  in  Fig.  197,  expressed  in  mm.  Hg.  and 
percentages  of  an  atmosphere. 

The  Mechanism  of  the  Gaseous  Exchange. — In  these  pressure  differ- 
ences sufficient  cause  is  found  for  the  exchange  of  the  gases.  The  oxygen 
pressure  in  the  alveoli  being  in  excess  of  that  in  the  blood,  the  gas  passes 
through  the  thin  alveolo-capillary  wall  into  the  plasma.  As  the  oxygen 
pressure  in  the  plasma  rises  and  approximates  that  in  the  alveoli,  a  portion  of 
the  oxygen  combines  with  the  hemoglobin  until  the  latter  is  almost  saturated. 
The  corpuscle  is  then  carried  through  the  arterial  system  surrounded  by 
oxygen  under  a  definite  pressure  which  is  sufficient  to  keep  the  absorbed 
oxygen  in  union  with  the  hemoglobin.  On  passing  into  the  systemic  capil- 
laries, the  blood  enters  a  region  in  which  the  oxygen  tension  in  the  surround- 
ing tissues  is  nil.  At  once  the  oxygen  dissolved  in  the  plasma  passes  through 
the  capillary  wall  into  the  surrounding  tissue-spaces.  The  pressure  removed 
from  the  corpuscle,  a  dissociation  of  the  oxygen  and  of  the  hemoglobin  takes 
place,  after  which  the  dissociated  oxygen  also  passes  through  the  capillary 
wall  into  the  surrounding  lymph  and  so  to  the  tissue  cells  where  it  is  stored 
and  utilized.  On  passing  into  the  venous  system  the  dissociation  of  the 
oxygen  and  the  hemoglobin  is  checked  by  the  rise  of  oxygen  pressure  in  the 
plasma.  On  reaching  the  lungs  the  oxygen  again  passes  into  the  blood 
until  the  former  condition  is  regained. 

The  sojourn  of  the  blood  in  the  capillaries  being  short,  the  oxyhemo- 
globin can  part  with  but  a  portion  of  its  oxygen,  sufficient,  however,  to  sat- 
isfy the  needs  of  the  tissues. 

The  carbon  dioxid  pressure  in  the  tissues  being  in  excess  of  that  in  the 
blood,  it  passes  through  the  capillary  wall  into  the  blood,  where  it  exists  in 
the  free  and  combined  states.  On  passing  into  the  pulmonic  capillaries  the 
blood  enters  a  region  in  which  the  carbon  dioxid  in  the  alveoli  is  less  than 
in  the  blood.  At  once  a  diffusion  and  dissociation  of  the  carbon  dioxid 
takes  place  through  the  alveolo-capillary  wall  until  equilibrium  is  established. 
This,  however,  is  of  very  short  duration,  for  the  carbon  dioxid  so  eliminated 
is  rapidly  removed  from  the  lungs  by  the  respiratory  movements. 

While  diffusion,  in  response  to  physical  and  chemic  conditions,  thus 
plays  a  large  part  in,  and  is  sufficient  to  account  for,  the  exchanges  of  gases, 
it  is  possible  that  the  alveolar  or  respiratory  epithelium  may  also  play  an 
essential  role.  It  is  beheved  by  some  investigators  that  it  is  active  in  both 
the  absorption  of  oxygen  and  the  excretion  of  carbon  dioxid.  This  view  has 
been  suggested  as  a  means  of  interpreting  the  results  of  the  experiments  of 
more  recent  investigators,  made  with  a  view  of  determining  the  tension  of  the 
blood  gases.  It  was  found  by  Bohr  that  the  tension  of  the  oxygen  in  arterial 
blood  was  often  as  high  as  loi  to  144  mm.  Hg.,  and  in  many  instances  higher 
than  the  tension  of  the  oxygen  in  the  trachea,  while  the  carbon  dioxid  tension 
in  the  trachea  was  higher  than  in  the  blood.  Haldane  and  Smith  by  a  dif- 
ferent method  found  an  oxygen  tension  in  the  arterial  blood  of  200  mm.  Hg. 
If  these  results  should  prove  to  be  correct,  though  they  are  at  present  subject 


RESPIRATION  421 

to  considerable  criticism  and  not  generally  accepted,  some  other  force  than 
diffusion  would  have  to  be  found  to  explain  the  facts.  It  would  then  remain 
to  be  determined  in  how  far  the  alveolar  epithelium  could  be  regarded  as 
an  active  agent  in  both  absorption  and  excretion  in  opposition  to  pressure. 

THE  TOTAL  RESPIRATORY  EXCHANGE 

The  total  quantities  of  oxygen  absorbed  and  carbon  dioxid  discharged 
by  a  human  being  in  twenty-four  hours  are  measures  of  the  intensity  of  the 
respiratory  process,  and  an  indication  of  the  extent  and  character  of  the 
chemic  changes  attending  all  life  phenomena.  Their  determination  and 
their  relation  to  each  other  are  matters  of  interest  and  importance.  The 
quantities  which  have  been  obtained  by  different  observers  are  the  outcome 
of  calculations  based  on  certain  groups  of  data  and  of  experiments  made  with 
special  forms  of  apparatus. 

Thus  from  the  total  air  breathed  daily,  estimated  from  the  amounts 
obtained  during  a  longer  or  shorter  period  by  experiments  with  spirometric 
apparatus,  and  from  the  percentage  loss  of  oxygen  and  gain  of  carbon  dioxid 
shown  by  an  analysis  of  the  respired  air,  it  can  be  calculated  at  least  ap- 
proximately what  the  total  amounts  of  oxygen  absorbed  and  carbon  dioxid 
exhaled  must  be.  If  it  be  assumed  that  the  minimum  daily  volume  of  air 
breathed  is  8500  hters  and  the  maximum  volume  12,752  liters,  and  the 
percentage  loss  of  oxygen  is  4.78,  then  the  total  volume  of  oxygen  absorbed 
is  406  liters  (580  grams)  or  609  liters  (870  grams).  By  the  same  method  the 
total  carbon  dioxid  exhaled  daily  is  found  to  be  either  372  liters  (735  grams) 
or  558  liters  (1103  grams).  The  direct  experiments  which  have  been  made 
with  specially  devised  forms  of  apparatus,  both  on  human  beings  and  animals, 
have  yielded  similar  results.  With  those  forms  which  are  adapted  for  both 
human  beings  and  animals — Scharling's,  Pettenkofer  and  Volt's — it  is  only 
possible,  however,  to  determine  theamountof  carbon  dioxid  and  water  exhaled 
and  from  them  to  calculate  the  amount  of  oxygen  absorbed.  This  is  done  by 
deducting  the  loss  in  weight  by  the  man  or  animal  during  the  experiment 
from  the  combined  weights  of  the  carbon  dioxid  and  water  discharged. 
The  difference  represents  the  oxygen  absorbed. 

The  Pettenkofer- Voit  apparatus  consists  essentially  of  a  chamber  large 
enough  to  admit  a  man  and  capable  of  being  made  air-tight  with  the 
exception  of  an  inlet  for  air  for  breathing  purposes.  The  respired  air  is 
drawn  through  a  tube  and  measured  by  a  large  meter  turned  by  a  water  or 
gas  motor.  By  means  of  a  side  tube  a  fractional  quantity  of  the  main 
column  of  air  is  diverted  to  an  absorption  apparatus  by  a  small  pump. 
This  air  first  passes  into  a  vessel  containing  HgSO^,  by  which  the  water  is 
collected;  then  into  long  tubes  containing  barium  hydroxid,  by  which  the 
carbon  dioxid  is  absorbed;  thence  into  a  small  meter,  by  which  its  amount  is 
registered.  From  the  amount  of  water  and  carbon  dioxid  thus  obtained 
the  amounts  of  both  in  the  total  air  breathed  are  calculated.  The  water  and 
carbon  dioxid  previously  present  in  the  air  are  simultaneously  determined  by 
a  corresponding  absorption  apparatus  and  deducted  from  the  amounts  ob- 
tained from  the  respired  air.  As  the  apparatus  is  traversed  constantly  by  a 
column  of  air  of  normal  composition  and  the  waste  products  removed  as  rapidly 
as  discharged,  the  experiment  can  be  continued  for  periods  varying  from 
six  to  twenty-four  hours  without  detriment  to  the  subject  of  the  experiment. 


422  TEXT-BOOK  OF  PHYSIOLOGY 

With  those  forms  adapted  only  for  animals — Regnault's  and  Reiset's,  or 
Jolyet  and  Regnard's — it  is  possible  to  determine  simultaneously  the  ab- 
sorption of  oxygen  and  the  discharge  of  carbon  dioxid.  As  the  apparatus 
employed  is  completely  closed,  the  carbon  dioxid  must  be  removed  as  soon 
as  discharged  and  the  oxygen  renewed  as  soon  as  absorbed.  The  former  is 
accomplished  by  the  aspiratory  action  of  moving  bulbs  containing  an  alkali, 
the  latter  by  a  steadily  acting  pressure  on  a  reservoir  of  oxygen.  This 
apparatus  consists  essentially  of  a  glass  bell-jar  in  which  the  animal  is 
placed.  This  is  brought  into  connection  by  tubes,  on  the  one  hand,  with 
the  oxygen  reservoir,  and,  on  the  other  hand,  with  the  aspiratory  bulbs, 
kept  in  motion  by  some  form  of  motor.  The  construction  of  each  of  these 
forms  of  apparatus  is  so  complex,  the  conduct  of  an  experiment  and  the 
final  determination  of  the  results  so  complicated,  that  a  detailed  description 
would  be  out  of  place  in  a  work  of  this  character. 

Of  the  results  obtained  by  these  and  other  methods  a  few  are  given  in 
the  following  table: 

Oxygen    Absorbed.  ,  Observer.  Carbon    Dioxid    Discharged. 

746  grams.  Vierordt.  "  876  grams. 

700  grams.  Pettenkofer  and  Voit.  800  grams. 

663  grams.  Speck.  770  grams. 

The  amounts  of  oxygen  absorbed  in  Pettenkofer  and  Voit's  experiments 
varied  from  594  to  1072  grams;  of  carbon  dioxid  exhaled,  from  686  to  1285 
grams. 

In  all  these  results  it  is  evident  on  examination  that  the  volume  of  oxygen 
absorbed  is  always  greater  than  the  volume  of  carbon  dioxid  exhaled,  or, 
what  amounts  to  the  same  thing,  the  weight  of  the  oxygen  absorbed  is  always 
greater  than  the  weight  of  the  oxygen  entering  into  the  formation  of  the 
carbon  dioxid  exhaled.  The  reason  for  this  difference  between  the  amounts 
of  oxygen  in  the  inspired  air  and  in  the  carbon  dioxid  exhaled  is  found  in 
the  fact,  that  on  a  mixed  diet — one  containing  fat — a  portion  of  the  oxygen 
is  utilized  in  the  oxidation  of  the  surplus  hydrogen  of  the  fat  with  the  forma- 
tion of  water.  Under  such  a  diet  the  respiratory  quotient  is  always  less 
than  unity,  usually  0.907.  On  a  purely  carbohydrate  diet — one  in  which 
there  is  no  surplus  hydrogen — all  the  oxygen  will  combine  wdth  carbon  and 
be  returned  as  carbon  dioxid,  and  hence  the  respiratory  quotient  will  be 
unity.  The  respiratory  quotient  therefore  indicates  the  extent  to  which  the 
oxygen  absorbed  is  utilized  in  oxidizing  carbon,  on  the  one  hand,  and 
hydrogen,  on  the  other. 

Since  the  total  oxygen  absorbed  and  carbon  dioxid  discharged  will  vary 
considerably  with  the  size  of  the  animal,  it  is  customary,  for  purposes  of 
comparison,  to  reduce  all  total  results  to  the  unit  of  body-weight  (one  kilo- 
gram) and  to  the  unit  of  time  (one  hour). 

Respiratory  Activity. — The  activity  or  the  intensity  of  the  respiratory 
process  may  be  measured  either  by  the  oxygen  absorbed  or  by  the  carbon 
dioxid  discharged.  But  as  the  carbon  dioxid  is  more  easily  estimated  than 
the  oxygen,  the  former  is  usually  taken  as  the  index  of  the  activity,  though 
there  are  reasons  for  believing  that  it  would  be  more  accurately  indicated  or 
represented  by  the  oxygen. 

Whatever  factor  may  be  accepted  as  the  measure,  it  is  certain  that  the 
respiratory  activity  varies  in  different  tissues  in  accordance  with  their  func- 


RESPIRATION  423 

tional  activities,  being  least  in  bones  and  greatest  in  muscles.  This  is  shown 
by  the  relative  amounts  of  oxygen  absorbed  and  carbon  dioxid  discharged 
by  equal  amounts  of  each  of  these  and  other  living  tissues  in  twenty-four 
hours,  as  given  in  the  following  table: 

QUANTITY  OF  O,  AND  CO^  ABSORBED  AND  EXHALED  DURING  TWENTY-FOUR 
HOURS,  IN  CUBIC  CENTIMETERS 

Oxygen  Carbon  Dioxid 

By  100  Grams  of:  Absorbed.  Exhaled. 

Muscle 50.8  c.c.  56.8  c.c. 

Brain 45.8  c.c.  42.8  c.c. 

Kidneys 37.0  c.c.  15.6  c.c. 

Spleen 27.3  c.c.  15.4  c.c. 

Testicles 18 .3  c.c.  27  . 5  c.c. 

Pounded  bones 17.2  c.c.  8.1  c.c. 

The  total  respiratory  change  therefore  of  the  body  as  a  whole  is  the  resultant 
of  the  respiratory  changes  of  its  individual  organs  and  tissues,  and  is  condi- 
tioned by  all  influences  which  retard  or  hasten  their  activity.  Among 
these  influences  the  more  important  are  the  following: 

Muscle  Activity. — As  the  muscles  constitute  a  large  part  of  the  body, 
about  40  per  cent.,  and  as  muscle-tissue  absorbs  and  discharges  relatively 
large  quantities  of  oxygen  and  carbon  dioxid,  it  is  readily  apparent  that  an 
increase  in  their  activity  would  be  followed  or  attended  by  an  increase  in  the 
respiratory  exchange.  In  passing  from  a  condition  of  body  repose  to  one  of 
marked  activity  there  ought  to  be  an  increase  in  the  amount  of  oxygen 
absorbed  and  CO2  discharged.  Pettenkofer  and  Voit  found  that  a  man  in 
repose  who  absorbed  daily  807.8  grams  of  oxygen  and  discharged  930 
grams  COj,  absorbed  duringwork  1006  grams  of  oxygen  and  discharged  1 137 
grams  of  COj.  Edward  Smith,  who  estimated  only  the  COj,  found  that  a 
man  in  repose  who  discharged  carbon  dioxid  at  the  rate  of  16 1.6  c.c.  per 
minute  increased  the  amount  while  walking  at  the  rate  of  two  and  three 
miles  an  hour  to  569  c.c.  and  851  c.c.  respectively.  Similar  results  have  been 
obtained  by  other  investigators. 

Digestive  Activity. — The  activity  of  the  alimenta,ry  canal,  involving 
contraction  of  its  muscle  coat  through  its  entire  length  as  well  as  secretion 
of  its  related  glands  called  forth  by  the  ingestion  of  food,  materially  influences 
the  absorption  of  oxygen  and  discharge  of  carbon  dioxid,  independent  of  the 
increase  due  to  the  oxidation  of  food  materials  after  absorption.  It  was 
found  that  in  a  fasting  man  a  dose  of  sodium  sulphate  increased  the  absorp- 
tion of  oxygen  as  much  as  17  per  cent,  and  the  discharge  of  COj  24  per  cent. 
(Lowy) .  It  is  difficult  to  determine  how  much  of  the  increase  after  a  meal 
is  therefore  due  to  food  oxidation  and  how  much  to  functional  activity 
of  the  canal  itself.  The  consumption  of  nitrogenized  meals,  however,  has 
a  greater  effect  than  non-nitrogenized  meals. 

Temperature. — A.  rise  in  temperature  of  the  surrounding  air  has  as 
an  eff'ect  diminution  in  the  amounts  of  oxygen  consumed  and  carbon 
dioxid  discharged.  A  fall  in  temperature  has  the  opposite  effect.  Thus 
a  cat  at  a  temperature  of  3.2°C.  consumed  during  a  period  of  six  hours 
21.39  grams  of  oxygen  and  discharged  22  grams  of  carbon  dioxid,  while  at 
a  temperature  of  29.6°C.  the  corresponding  amounts  for  the  same  period  of 
time  were  for  oxygen  13.9  grams  and  for  carbon  dioxid  13.12  grams.  Lavoi- 
sier and  Sequin,  having  reference  only  to  the  oxygen,  found  that  a  man 


424  TEXT-BOOK  OF  PHYSIOLOGY 

at  a  temperature  of  i5°C.  consumed  38.31  grams  of  oxygen,  while  at  a  tem- 
perature of  32,8°C.  the  corresponding  amount  was  but  35  grams.  Similar 
results  have  been  obtained  by  other  observers  with  different  animals.  The 
explanation  of  these  facts  is  to  be  found  in  the  increased  activity  of  all 
physiologic  mechanisms  coincident  with  a  fall,  and  in  the  decreased  activity, 
coincident  with  a  rise  in  temperature.  The  lower  temperatures  act  as  a 
stimulus  to  the  peripheral  terminations  of  the  nerve  system,  bringing  about 
reflexly  increased  activity  of  the  body  at  large.  The  muscles  especially  are 
not  only  reflexly  but  volitionally  excited  to  greater  activity.  This  leads 
naturally  to  an  increase  in  the  consumption  of  oxygen  and  in  the  production 
of  carbon  dioxid  and  in  the  evolution  of  heat. 

In  cold-blooded  animals  the  respiratory  exchange  is  influenced  in  a 
manner  the  reverse  of  that  observed  in  warm-blooded  animals.  With  a 
rise  of  external  temperature  and  a  corresponding  rise  of  body-temperature 
the  discharge  of  carbon  dioxid  steadily  increases.  Thus  a  frog  in  an  atmos- 
phere at  o°C.  with  a  body- temperature  of  i°C.  discharged  per  kilogram 
per  hour  4.31  c.c.  of  carbon  dioxid;  in  an  atmosphere  of  35°C.  with  a  body- 
temperature  of  34°C.  there  was  discharged  325  c.c.  per  kilo  per  hour. 
Intermediate  temperatures  were  attended  by  corresponding  increases  in 
the  amounts  of  CO2  discharged.  The  reason  for  this  difference  in  the  two 
classes  of  animals  is  probably  to  be  found  in  the  cold-blooded  animals,  in 
the  want,  of  a  self-adjusting  heat-regulating  mechanism. 

Age. — In  early  youth,  as  a  result  partly  of  the  more  pronounced  activity 
of  the  nutritive  energies  and  partly  of  a  cutaneous  surface  relatively  greater, 
as  compared  with  the  mass  of  the  body,  than  in  adult  life,  the  absorption  of 
oxygen  and  the  discharge  of  carbon  dioxid  are  greater  both  absolutely  and 
relatively.  Thus,  in  a  boy  of  nine  and  a  half  years  with  a  weight  of  22 
kilograms  it  was  found  that  in  twenty-four  hours  there  was  a  discharge  of 
carbon  dioxid  amounting  to  488  grams,  or  0.92  gram  per  kilo  per  hour,  and 
in  man  with  a  weight  of  65.5  kilograms  there  was  a  discharge  of  804,72 
grams,  or  0.51  gram  per  kilo  per  hour. 

THE  NERVE  MECHANISM  OF  RESPIRATION 

The  nerve  mechanism  by  which  the  respiratory  muscles  are  excited  to 
and  coordinated  in  activity  is  extremely  complex  and  involves  the  action  of 
both  afferent  and  efferent  nerves  and  their  related  nerve-centers  in  the 
central  nerve  system.  For  the  free  introduction  of  air  into  the  lungs  it  is 
essential  that  the  nasal  and  laryngeal  passages  and  the  cavity  of  the  thorax 
be  simultaneously  enlarged.  The  muscles  by  which  these  results  are  ac- 
complished have  already  been  mentioned  and  described.  Their  simultane- 
ous and  coordinate  contraction  implies  the  coordinate  activity  of  nerve- 
centers  and  their  related  motor  nerves;  thus  the  action  of  the  nasal  and 
laryngeal  muscles  (the  dilatator  naris  and  the  posterior  crico-arytenoid) 
involves  the  activity  of  the  facial  and  inferior  laryngeal  nerves  respectively, 
the  centers  of  origin  of  which  lie  in  the  gray  matter  beneath  the  floor  of 
the  fourth  ventricle;  the  diaphragm  and  intercostal  muscles  involve  re- 
spectively the  activity  of  the  phrenic  and  intercostal  nerves,  the  centers 
of  origin  of  which  lie  in  the  anterior  horn  of  the  gray  matter  of  the  spinal 
cord  at  a  level,  for  the  phrenic,  of  the  fourth,  fifth,  and  sixth  cervical  nerves, 


RESPIRATION  425 

and  for  the  intercostals  at  the  level  of  the  upper  thoracic  nerves.  Division 
of  any  one  of  these  nerves  is  followed  by  paralysis  of  its  related  muscle. 

Inspiratory  Center. — The  coordinate  contraction  of  the  inspiratory 
muscles  implies  a  practically  simultaneous  discharge  of  nerve  impulses  from 
each  of  the  foregoing  nerve-centers,  accurately  graduated  in  intensity  in 
accordance  with  inspiratory  needs.  This  has  been  supposed  to  necessitate 
the  existence  in  the  central  nerve  system  of  a  single  group  of  nerve-cells  from 
which  nerve  impulses  are  rhythmically  discharged  and  conducted  to  the 
previously  mentioned  nerve-centers  in  the  medulla  oblongata  and  spinal 
cord,  by  which  they  are  in  turn  excited  to  activity.  To  this  group  of  cells  the 
term  "inspiratory  center"  has  been  given. 

For  the  free  exit  of  air  from  the  lungs  it  is  essential  not  only  that  the  air- 
passages  be  open,  but  that  the  air  in  the  lungs  be  compressed  until  its  pressure 
rises  above  that  of  the  atmosphere.  This  is  accomplished  by  the  recoil  of  the 
elastic  tissue  of  the  lungs  and  thorax,  the  return  of  the  displaced  abdominal 
organs  aided  by  atmospheric  pressure,  and  the  contraction  of  the  expiratory 
muscles.  In  how  far  muscle  action  is  necessary  for  expiratory  purposes 
will  depend  on  the  resistance  offered  to  the  outflow  of  air  and  on  the  degree 
of  efl&ciency  of  the  elastic  forces. 

Expiratory  Center.— The  sim^ultaneous  and  coordinate  activity  of  the 
expiratory  muscles  in  impeded  expirations  also  involves  the  action  of  motor 
nerves  and  nerve-centers.  The  simultaneous  and  coordinate  discharge  of 
nerve  impulses,  also  graduated  in  intensity  for  expiratory  needs,  apparently 
implies  the  existence  in  the  central  nerve  system  of  a  single  center  from  which 
nerve  impulses  are  rhythmically  discharged  which  excite  and  coordinate 
the  lower  nerve-centers.  To  this  group  of  cells  the  term  "expiratory  center" 
has  been  given.  The  two  centers  taken  together  constitute  the  so-called 
"  respiratory  center. ' ' 

The  anatomic  existence,  however,  of  a  definite  group  of  cells  which 
initiates  the  respiratory  movements  has  not  as  yet  been  demonstrated. 
Nevertheless  there  is  in  the  dorsal  portion  of  the  medulla  oblongata,  at  the 
level  of  the  sensory  end-nucleus  of  the  vagus  nerve,  a  region  the  sudden  de- 
struction of  which  on  one  side  is  followed  by  a  cessation  of  respiratory  move- 
ments on  the  corresponding  side,  though  they  continue  on  the  opposite  side, 
a  fact  which  indicates  that  the  area,  though  acting  as  a  unit,  is  bilateral. 
The  bilateral  character  of  the  area  is  also  shown  by  the  continuance  of  the 
respiratory  movements  on  both  sides  after  longitudinal  division  of  the 
medulla.  Destruction  of  the  entire  region  is  followed  by  a  complete  cessation 
of  respiratory  activity  and  death  of  the  animal.  For  this  reason  the  term 
"noeud  vital"  (vital  point)  was  applied  to  it.  In  this  area  the  respiratory 
center  was  located.  It  has,  however,  been  shown  by  Gad  that  if  this  area 
be  gradually  destroyed  by  cauterization  the  respiratory  movements  do  not 
cease,  but  continue  until  the  cauterization  has  reached  a  point  far  forward 
in  the  formatio  reticularis,  in  which  the  respiratory  center  was  assumed 
to  lie. 

Though  its  existence  has  not  been  anatomically  determined  beyond 
question,  it  is  permissible  to  speak  of  the  central  mechanism  as  a  "center" 
located  in  the  medulla  oblongata. 

The  Cause  of  the  Rhythmic  Activity  of  the  Inspiratory  Center. — 
It  has  long  been  a  subject  of  discussion  as  to  whether  the  periodic  activity 


426  TEXT-BOOK  OF  PHYSIOLOGY 

of  the  inspiratory  center  is  automatic  or  autochthonic  (Gad)  in  character, 
expressive  of  the  idea  that  the  rhythmic  discharge  of  nerve  impulses  is  due 
to  some  stimulating  agent  generated  in  the  nerve-cells  of  the  center  itself,  the 
activity  of  which  is  conditioned  by  the  gaseous  condition  of  the  blood; 
or  whether  it  is  reflex  in  character,  that  is,  due  to  the  action  of  nerve  im- 
pulses transmitted  from  different  regions  of  the  body  through  afferent  nerves. 
The  solution  of  this  problem  has  apparently  been  settled  by  experiments 
the  object  of  which  was  the  division  of  all  afferent  nerve-paths  that  might 
have  central  connections  with  the  center.  The  results  of  experiments  of 
this  character  are  somewhat  as  follows:  When  the  vagus  nerves  are  divided 
the  respiratory  movements  at  once  diminish  in  number  per  minute  but  at 
the  same  time  increase  in  depth  and  amplitude.  The  number  of  respiratory 
movements  under  these  circumstances  varies  in  different  animals  from  four 
to  eight  per  minute,  a  rate  which  continues  practically  constant  so  long  as  the 
animal  lives,  which  may  be  a  period  varying  from  a  few  days  to  several 
weeks.  The  relative  duration  of  the  respiratory  phases  also  undergoes  a 
change,  inspiration  becoming  longer  than  expiration  and  at  the  same  time 
becoming  more  or  less  spasmodic  in  character. 

Inasmuch  as  it  is  a  familiar  observation  that  the  normal  rate  of  the  respira- 
tory movement  is  frequently  disturbed  by  nerve  impulses  transmitted  to  the 
center,  through  afferent  nerves  other  than  the  vagi,  as  well  as  from  higher 
centers  in  the  brain,  section  of  the  vagi  has  been  supplemented  by  a  trans- 
verse section  of  the  spinal  cord  at  the  level  of  the  first  dorsal  nerve,  by  section 
of  the  dorsal  roots  of  the  cervical  nerves  and  by  a  transverse  section  of  the 
region  of  the  brain  just  posterior  to  the  corpora  quadrigemina,  a  series  of  pro- 
cedures which  practically  isolates  the  center  from  all  transmitted  impulses. 
Nevertheless,  the  inspiratory  center  still  continues  to  discharge  nerve  im- 
pulses to  the  respiratory  muscles  at  a  rate  not  differing  much  from  that 
witnessed  after  section  of  the  vagi.  At  most  the  diminution  in  the  rate 
will  not  be  more  than  two  or  three  more  per  minute.  The  results  of  these 
experimental  procedures  would  seem  to  indicate  that  the  fundamental  rate 
of  discharge  of  nerve  impulses  is  approximately  from  four  to  six  per  minute. 
This  conclusion  has  been  strengthened  by  the  results  of  experiments  designed 
to  suspend  the  activity  for  some  minutes  by  the  withdrawal  of  the  blood 
by  temporarily  occluding  the  blood-vessels  passing  to  the  head.  With  the 
resuscitation  of  the  center,  after  the  release  of  the  blood-stream  and  at  a 
time  when  there  are  reasons  for  believing  that  the  afferent  paths  are  still 
incapable  of  conduction,  the  initial  rate  of  discharge  was  practically  constant, 
about  four  per  minute  in  the  cat.  The  same  result  was  observed  in  some 
instances  in  cats  when  in  addition  to  producing  anemia  the  vagi  as  well 
as  the  region  posterior  to  the  corpora  quadrigemina  were  divided  (Stewart). 

It  may  therefore  be  assumed  that  the  respiratory  center  possesses  an  in- 
dependent automatic  rhythm  which  is,  however,  much  slower  than  that 
characteristic  of  it  when  all  afferent  paths  leading  to  it  are  intact. 

Accepting  the  statement  that  the  fundamental  rhythm  of  the  inspiratory 
center  is  automatic — that  is,  due  to  a  stimulus  generated  within  itself — 
the  question  at  once  arises  as  to  the  nature  of  the  stimulating  agent.  By 
some  investigators  it  has  been  assumed  that  the  stimulus  is  connected  with 
the  content  or  pressure  of  oxygen,  by  others  with  the  content  or  pressure  of 
carbon  dioxid,  and  that  the  variations  in  the  respiratory  rhythm  are  depend- 


RESPIRATION  427 

ent  on  variations  in  the  pressure  of  one  or  the  other  of  these  two  gases. 
As  a  result  of  a  long  series  of  experiments  made  on  animals  and  human 
beings,  with  the  respiratory  nerve  mechanism  intact,  it  is  now  the  generally- 
accepted  opinion  that  the  more  efficient  cause  for  the  respiratory  rhythm 
is  an  increase  in  the  pressure  of  carbon  dioxid  in  the  blood  and  hence  in  the 
center  itself  rather  than  a  decrease  in  the  pressure  of  the  oxygen.  Whether 
the  pressure  of  the  carbon  dioxid  be  the  efficient  cause  or  not  of  the  funda- 
mental respiratory  rhythm,  there  is  abundant  evidence  that  the  activity  or  the 
irritability  of  the  center  is  modified  to  an  extraordinary  extent  by  variations 
in  the  pressure  of  the  carbon  dioxid  when  the  nerve  system  is  intact.  Proofs 
in  support  of  this  statement  will  be  given  in  a  subsequent  paragraph. 

Reflex  Stimulation  of  the  Inspiratory  Center. — Whether  the  inspira- 
tor}''  center  is  automatic  in  character  or  not,  it  may  be  influenced  directly 
by  nerve  impulses  descending  from  the  brain  in  consequence  of  volitional 
acts  or  emotional  states,  and  indirectly  by  nerve  impulses  brought  to  it 
from  the  general  periphery  through  various  afferent  nerves,  in  consequence  of 
agencies  acting  on  iheir  peripheral  terminations:  e.g.,  cold  applied  to  the  skin, 
irritating  gases  to  the  nasal  and  bronchial  mucous  membrane,  distention  and 
collapse  of  the  pulmonary  alveoli. 

The  Relation  of  the  Vagus  Nerves  to  the  Inspiratory  Center. — 
The  vagus  nerves,  of  all  the  afferent  nerves,  appear  to  be  the  most  influential 
in  maintaining  the  normal  rhythmic  discharge  of  nerve  impulses  from  the 
inspiratory  center,  as  shown  by  the  effects  that  follow  their  separation  from 
the  center.  (Fig.  198.)  .  Thus,  if  while  the  animal  is  breathing  regularly 
and  quietly  both  vagi  are  cut,  the  respiratory  movements  become  much  slower, 
falling  perhaps  to  one-third  their  original  number  per  minute.  At  the  same 
time  the  inspirations  become  deeper  and  somewhat  spasmodic  in  character. 
The  duration  of  the  inspiratory  movement  is  also  increased  beyond  that 
of  the  expiratory  movement.  If  now  the  central  end  of  one  of  the  divided 
vagi  be  stimulated  with  weak  induced  electric  currents,  the  respiratory 
movements  are  again  increased  in  frequency  and  their  depth  diminished 
until  the  normal  rate  is  restored.  With  the  cessation  of  the  stimulation  the 
former  condition  at  once  returns.  This  would  seem  to  indicate  that  the 
vagus  nerve  contains  nerve-fibers  which,  under  physiologic  conditions,  trans- 
mit nerve  impulses  which  inhibit  the  inspiratory  discharge  and  lead  to  an 
expiratory  movement  sooner  than  would  otherwise  be  the  case,  and  thus 
maintain  the  normal  rate  and  extent  of  the  inspiratory  discharge.  The 
stimulus  to  the  development  of  these  nerve  impulses  is  generally  believed 
to  be  the  distension  of  the  air  cells  at  the  beginning  of  the  inspiratory 
movement. 

If  however  the  central  end  of  the  divided  vagus  be  stimulated  with 
induced  electric  currents  of  moderate  intensity,  the  opposite  effect  is  pro- 
duced, viz.:  an  increase  in  the  extent  of  the  inspiratory  movement  and  a 
decrease  in  the  extent  of  the  expiratory  movement  until  the  inspiratory 
muscles  pass  into  the  tetanic  state  and  the  chest  walls  come  to  rest  in  the 
condition  of  a  forced  inspiration.  This  would  indicate  that  the  vagus  nerve 
also  contains  nerve- fibers  which,  with  the  proper  degree  of  stimulation  are 
capable  of  so  exciting  or  augmenting  the  activity  of  the  inspiratory  center, 
and  therefore  the  extent  of  the  inspiratory  movement,  as  to  lead  to  the  con- 
dition of  tetanus  of  the  inspiratory  muscles.     If,  on  the  other  hand,  the 


428 


TEXT-BOOK  OF  PHYSIOLOGY 


central  end  of  the  divided  superior  laryngeal  nerve  be  stimulated  with 
induced  electric  currents,  an  effect  the  opposite  of  the  foregoing  is  produced, 
viz.:  a  decrease  in  the  extent  of  the  inspiratory  and  an  increase  in  the  extent 
of  the  expiratory  movement  until  the  inspiratory  muscles  pass  into  the 
state  of  relaxation  and  the  chest  walls  come  to  rest  in  the  condition  of  com- 
plete expiration.  This  would  indicate  that  the  superior  laryngeal  nerve 
contains   nerve-fibers   which  are  capable,  when  sufficiently  stimulated  of 


y7?ca'.  ob. 


Fig.  198. — Diagram  Showing  the  Relation  of  the  Pulmonic  Fibers  of^the  Vagus  to 
THE  Inspiratory  Center  and  the  Connections  of  the  Latter  with  the  Phrenic  and 
Intercostal  Nerve  Centers  and  Their  Related  Muscles. — (G.  Bachman).  med.  ob.  Medulla 
oblongata.  5/».  c.  Spinal  cord.  />.  i/.  r.  Pulmonic  vagus  nerve,  excitator  and  inhibitor,  insp.c. 
Inspiratory  center,  phr.  c.  Phrenic  nerve-centers,  phr.  n.  Phrenic  nerve,  int.  n.  c.  Intercostal 
nerve-centers,    int.  c.  n.  Intercostal  nerves,   ext.  int.  c.  m.  External  intercostal  muscles. 

inhibiting  the  activity  of  the  inspiratory  center,  and  therefore  the  extent  of 
the  inspiratory  movement,  as  to  lead  to  the  condition  of  complete  relaxation 
of  the  inspiratory  muscles  with  a  consequent  expiratory  standstill. 

The  same  results,  viz.:  a  complete  relaxation  of  the  inspiratory  muscles 
leading  to  an  expiratory  standstill,  not  infrequently  follows  strong  stimulation 
of  the  central  ends  of  divided  vagi,  and  always  after  the  administration 
of  large  doses  of  chloral. 


RESPIRATION 


429 


The  results  of  these  experiments  would  seem  to  indicate  that  the  vagus 
nerve  contains  two  classes  of  nerve-fibers,  one  of  which,  when  stimulated 
with  weak  induced  electric  current,  inhibits  and  regulates  the  discharge 
of  nerve  energy  from  the  inspiratory  center,  and  thereby  the  extent  and 
frequency  of  the  inspiratory  movement;  the  other  of  which  when  stimulated, 
excites  or  augments  the  discharge  of  nerve  energy  from  the  inspiratory 
center  and  thereby  leads  to  an  increase  in  the  depth  or  amplitude  of  the 
inspiratory  movement.  According  as  the  one  or  the  other  of  these  two 
classes  of  fibers  are  excessively  stimulated,  will  the  inspiratory  center  be 
inhibited  or  augmented  in  its  activity  to  such  an  extent  that  the  chest  walls 

will  come  to  rest  in  the  first 
instance  in  the  state  of  expira- 
tory standstill,  in  the  second 
instance  in  the  state  of  inspira- 
tory standstill. 

The  stimulus  adequate  to 
the  excitation  of  the  pulmonic 
terminations  of  the  vagus 
nerve-fibers  in  the  physiologic 
condition  was  formerly  be- 
lieved to  be  the  chemic  action 
of  carbon  dioxid;  it  is  now 
believed  to  be  a  mechanic  action,  the  result  of  the  alternate  distention  and 
collapse  of  the  walls  of  the  pulmonic  alveoli.  Thus,  it  has  been  shown  by 
Head  that  if  the  lungs  are  actively  inflated  (positive  ventilation)  there  will 
be  produced  an  inhibition  of  the  inspiratory  and  an  augmentation  of  the 
expiratory  movement  until  the  inspiratory  muscles  are  completely  relaxed 
as  indicated  by  the  relaxation  of  the  diaphragm,  the  movements  of  which  are 
simultaneously  recorded  (Fig.  200),  a  result  similar  in  all  respects  to  that 


Diaphragm. 
Seconds. 


Fig.  199. — Positive  Ventilation  {Head).  Under 
the  influence  of  positive  ventilation,  the  inspiratory 
contractions  of  the  diaphragm  become  less  and  less 
till  they  disapoear  completely. 


Seconds. 

Fig.  200. — Negative  Ventilation.  {Head).  At  a  negative  ventilation  was  commenced. 
The  expiratory  relaxation  of  the  diaphragm  is  seen  to  become  more  and  more  incomplete,  until  it 
finally  enters  into  continued  contraction. 

produced  by  stimulation  of  the  superior  larnygeal  nerve.  On  the  other  hand, 
if  the  lungs  are  collapsed  by  the  artificial  withdrawal  of  air  (negative  ventila- 
tion) there  will  be  produced  an  augmentation  of  the  inspiratory  and  an 
inhibition  of  the  expiratory  movements  until  the  inspiratory  muscles  are  in 
a  condition  of  tetanic  contraction  as  indicated  by  the  contraction  of  the 
diaphragm  (Fig.  201)  and  by  the  state  of  the  thorax  which  is  that  charac- 
teristic of  extreme  inspiration,  a  result  similar  in  all  respects  to  that  produced 
by  moderate  stimulation  of  the  central  end  of  the  divided  vagus. 


430  TEXT-BOOK  OF  PHYSIOLOGY 

Theories  of  the  Mode  of  Action  of  the  Respiratory-nerve  Mechanism. 

— A  satisfactory  explanation  of  the  action  of  the  respiratory-nerve  mechanism 
is  very  difficult  to  present.  Theories  vary  in  accordance  with  the  estimate 
of  an  investigator  as  to  the  degree  of  automaticity  of  the  inspiratory  center, 
of  the  effects  of  vagus  stimulation  and  as  to  the  extent  to  which  the  expira- 
tory center  is  involved  with  the  activity  of  the  inspiratory  center  either 
simultaneously  or  successively. 

The  following  theories  have  each  found  adherents. 

1.  If  it  is  assumed  that  the  inspiratory  center  is  automatic  and  in  a  state  of 
continuous  excitation  the  result  of  the  action  of  carbon  dioxid  in  the  blood 
circulating  around  it,  then  it  is  only  necessary  to  assume  the  existence,  in 
the  trunk  of  the  vagus  of  one  set  of  nerve-fibers,  viz.:  inhibitor  fibers, 
the  central  terminations  of  which  arborize  around  the  inspiratory  center, 
and  the  function  of  which  is  to  check  or  inhibit  the  discharge  of  the  inspiratory 
center  and  thus  permit  of  an  expiratory  movement.  The  inhibitor  fibers 
are  supposed  to  be  stimulated  peripherally  by  the  expansion  of  the  lungs. 
With  the  recoil  of  the  lungs  the  inhibitor  effect  gradually  dies  away,  while  the 
inherent  excitation  of  the  inspiratory  center  again  returns,  to  be  followed 
by  another  discharge  of  nerve  impulses  and  a  new  inspiratory  movement, 
which  will  in  turn  be  again  inhibited  as  the  inhibitor  fibers  are  stimulated 
by  the  expanding  lung.  This  explanation  is  in  accordance  with  the  results 
which  follow  stimulation  of  the  superior  laryngeal  nerve  or  the  trunk  of 
the  vagus  with  induced  electric  currents  of  slight  intensity. 

2.  If  it  is  assumed,  on  the  contrary,  that  the  inspiratory  center  is  not  in  a 
state  of  constant  excitation  leading  to  a  frequent  periodic  discharge  of  nerve 
impulses,  but  requires  the  arrival  of  a  stimulus  to  call  forth  its  normal 
activity,  then  this  theory  does  not  suffice,  inasmuch  as  it  leaves  out  of  con- 
sideration the  presence  of  nerve-fibers  in  the  vagus  which  increase  or  aug- 
ment the  activity  of  the  inspiratory  center;  and  that  such  fibers  are  present  is 
apparently  indicated  by  the  effects  of  stimulation  of  the  central  end  of  the 
vagus  nerve  with  moderately  strong  induced  electric  currents,  and  from  the 
experiments  of  Hering  and  Breuer,  and  later  of  Head.  These  observers 
assume,  therefore,  that  in  addition  to  the  inhibitor  fibers  there  are  also  present  in 
the  vagus  excitator  fibers,  the  central  terminations  of  which  are  in  relation 
with  the  inspiratory  center  also  (Fig.  198);  and  just  as  the  inhibitor  fibers 
are  stimulated  by  the  expansion  of  the  lungs,  so  the  excitator  fibers  are  stimu- 
lated in  turn  by  the  recoil  of  the  lungs.  The  nerve  impulses  developed  by 
the  recoil  of  the  walls  of  the  alveoli  ascend  the  vagus  nerve  to  the  inspiratory 
center,  excite  it  to  activity,  and  thus  call  forth  a  new  inspiration  sooner 
than  it  would  otherwise  take  place.  The  nerve  impulses  developed  by 
the  expansion  of  the  walls  of  the  alveoli  in  turn  ascend  the  vagus  nerve  to  the 
inspiratory  center  and  inhibit  its  activity  and  thus  lead  to  an  expiratory 
movement,  sooner  than  it  would  otherwise  take  place.  According  to  this 
view  the  respiratory  mechanism  is  self-regulative  and  maintained  by  the 
alternate  expansion  and  recoil  of  the  lungs.  =r-| 

3.  Another  explanation  which  is  satisfactory  in  many  respects  has  been 
presented  by  Meltzer.  This  investigator  asserts  also  the  existence  in  the 
trunk  of  the  vagus  the  two  classes  of  nerve-fibers,  the  inhibitor  and  the 
excitator;  but  that  for  some  reason  they  do  not  respond  to  stimulation  at 
the  same  time  as  shown  by  the  effects  which  follow;  the  inhibitor  fibers 


RESPIRATION  431 

respond  first  and  the  excitator  fibers  somewhat  later.  Therefore  when 
they  are  stimulated  simultaneously  the  primary  effect  is  an  inhibition  of 
the  inspiratory  center  followed  by  an  expiratory  movement.  The  secondary 
effect  is  a  stimulation  of  the  inspiratory  center  followed  by  a  new  inspiratory 
movement.  In  this  view  expansion  of  the  lungs  stimulates  both  the  inhibitor 
and  the  excitator  fibers,  but  during  the  expansion  and  for  a  short  time  after, 
the  effect  of  the  inhibitor  stimulation,  viz.:  cessation  of  inspiration  and  the 
advent  of  expiration,  alone  manifests  itself.  With  the  cessation  of  expira- 
tion, the  inliibitor  stimulation  dies  away  and  the  late  effect  or  the  long  after- 
effect of  the  excitator  stimulation,  viz.:  a  new  inspiration,  manifests  itself. 
This  investigator  assumes  the  surface  of  the  lung  to  be  the  peripheral  organ 
of  the  respiratory  reflexes. 

When  it  is  assumed  that  both  inspiratory  and  expiratory  centers  cooper- 
ate in  a  respiratory  movement,  as  they  do  in  labored  respiration  either 
simultaneously  or  successively,  the  difficulties  of  the  problem  are  manifestly 
much  greater.  In  this  case  it  may  be  supposed  that  afferent  impulses,  de- 
veloped during  the  expansion  of  the  lung,  inhibit  the  inspiratory  while  aug- 
m^enting  the  expiratory  center,  and  that  impulses  developed  during  the  re- 
coil of  the  lungs  inhibit  the  expiratory  while  stimulating  the  inspiratory 
center. 

The  Effect  of  a  Change  in  the  Pressure  of  the  Blood  Gases  on  the 
Activity  of  the  Inspiratory  Center. — It  has  long  been  known  that  the  in- 
spirator)^ center  is  very  sensitive  to  a  change  in  the  composition  of  the  blood  in 
so  far  as  its  gaseous  constituents  are  concerned.  So  long  as  the  composition 
remains  normal  the  center  retains  its  normal  irritability  and  rhythm.  As 
stated  in  a  previous  paragraph  it  has  been  a  subject  of  discussion  as  to  whether 
the  center  is  more  responsive  to  an  increase  in  the  pressure  of  the  carbon 
dioxid  or  to  a  decrease  in  the  pressure  of  the  oxygen.  As  the  outcome  of 
a  long  series  of  experiments  it  is  now  the  generally  accepted  opinion  that  an 
increase  in  the  percentage  and  pressure  of  the  carbon  dioxid  in  the  blood 
and  hence  in  the  center  itself  is  more  efficient  in  raising  the  irritability  of  the 
center  than  a  decrease  in  the  percentage  and  pressure  of  the  oxygen.  Thus 
if  an  animal  is  caused  to  inhale  air  containing  but  2  per  cent,  of  CO2  more 
than  normal  the  respiratory  movements  will  be  increased  in  frequency  and 
depth,  while  a  corresponding  diminution  in  the  percentage  of  oxygen  will 
be  without  effect. 

It  has  been  shown  by  Haldane  and  Priestley  that  when  an  individual  was 
breathing  normal  air  and  the  rate  of  the  respiratory  movement,  14  per  minute, 
the  average  depth  was  637  c.c.  and  the  total  ventilation  was  8.918  liters  per 
minute.  On  raising  the  percentage  of  the  CO,  in  the  inspired  air  from 
0.04  per  cent,  to  0.79  per  cent,  the  average  depth  increased  to  739  c.c.  and 
the  total  ventilation  to  10.346  Uters  per  minute,  the  rate  remaining  the  same. 
When  the  percentage  of  the  CO 2  was  raised  to  2  per  cent,  the  average  depth 
increased  to  864  c.c,  the  rate  to  15,  and  the  total  ventilation  to  12.960  liters 
per  minute;  and  when  the  CO 2  in  the  inspired  air  was  raised  to  6  per  cent, 
the  average  depth  was  increased  to  2104  c.c,  the  rate  to  27  per  minute,  and 
the  total  ventilation  to  56.808  liters.  The  results  of  these  experiments 
indicate  that  an  increase  in  the  percentage  of  the  CO 2  in  the  inspired  air 
leads  to  an  increase  in  the  percentage  and  pressure  of  the  CO,  in  the  ar- 
terial blood  and  hence  in  the  inspiratory  center,  as  a  result  of  which  the 


432  TEXT-BOOK  OF  PHYSIOLOGY 

center  becomes  more  irritable  and  discharges  its  energy  more  frequently 
and  to  a  greater  degree  as  shown  by  the  increase  in  the  rate  and  the  depth 
of  the  inspiratory  movement. 

The  same  observers  have  also  shown  that  when  an  individual  is  caused 
to  inhale  air  the  percentage  of  the  oxygen  of  which  had  been  reduced  from 
20  to  13  and  therefore  to  about  8  per  cent,  in  the  alveolar  air  instead  of  about 
15  per  cent,  no  particular  change  in  either  the  frequency  or  the  depth  of 
the  inspiratory  movements  was  noticed,  but  when  the  percentage  of  the 
oxygen  was  lowered  below  this  amount  the  inspiratory  center  became  more 
irritable  as  shown  by  an  increase  in  the  rate  and  depth  of  the  inspiratory 
movement.  As  a  rule  the  oxygen  percentage  in  the  alveolar  air  must  be 
reduced  fully  one-half  and  thereby  the  percentage  and  pressure  of  the  oxygen 
in  the  arterial  blood  fully  one-third  before  the  respiratory  center  is  stimulated 
to  increased  activity.  A  reason  assigned  for  this  result  is  the  presence  in 
the  blood  of  some  non-oxidized  metabolic  product,  probably  lactic  acid,  that 
is  acting  as  the  stimulus.  All  recent  experimental  work  confirms  the  view 
that  the  specific  stimulus  to  the  inspiratory  center  is  the  normal  pressure  of 
the  CO 2  in  the  blood  and  so  responsive  is  it  to  this  agent  that  an  increase 
in  even  0.2  per  cent,  in  the  alveolar  air  is  sufficient  to  almost  double  the 
respiratory  ventilation. 

The  Establishment  of  Respiration  after  Birth. — ^Previous  to  birth 
the  exchange  of  the  gases  which  constitutes  the  respiratory  activity  in  the 
mammalian  fetus,  takes  place  in  the  placenta.  The  venous  blood  is  carried 
by  the  umbilical  artery  to  this  organ  in  which  the  blood  of  the  fetus  comes 
into  close  relation  with  the  blood  of  the  mother,  the  two  fluids  being  separated 
only  by  an  extremely  thin  partition.  The  venous  blood  while  passing 
through  the  placenta  yields  up  its  carbon  dioxid  to  and  receives  oxygen 
from  the  maternal  blood;  after  this  exchange  of  gases  the  now-arterialized 
blood  is  returned  to  the  fetus  by  the  umbilical  veins. 

Immediately  after  birth  and  the  detachment  of  the  placenta,  this  method 
of  gaseous  exchange  is  abolished  and  if  the  life  of  the  child  is  to  be  main- 
tained the  respiratory  movement  must  be  established.  The  cause  of  the 
first  inspiration,  therefore,  must  be  associated  with  an  increase  in  the  per- 
centage of  carbon  dioxid  or  a  decrease  in  the  percentage  of  oxygen  in  the 
blood.  In  accordance  with  statements  in  foregoing  paragraphs  the  former 
condition  is  more  likely  to  be  the  efficient  cause.  The  rapid  accumulation 
of  carbon  dioxid  with  its  increasing  pressure  in  the  inspiratory  center  so 
raises  its  irritability  as  to  lead  to  a  discharge  of  nerve  impulses  which  are 
conducted  to  the  inspiratory  muscles  and  cause  their  contraction.  With 
the  first  inspiration  thus  established  the  nerve  mechanism  described,  pages 
424,  426  comes  into  play. 

Inasmuch  as  cold  water  applied  to  the  skin  of  the  adult  profoundly  ex- 
cites at  times  the  inspiratory  center  it  has  been  assumed  that  an  additional 
factor  leading  to  an  excitation  of  the  inspiratory  center  is  the  rapid  cooling  of 
the  surface  of  the  child  by  the  evaporation  of  the  amniotic  fluid  from  the 
surface  of  the  skin.  The  nerve  impulses  thus  developed  are  transmitted 
through  cutaneous  nerves  to  the  inspiratory  center.  This  assumption  is 
somewhat  strengthened  by  the  fact  that  in  delayed  inspiration  the  stimula- 
tion of  the  skin  by  the  application  of  cold  water  frequently  leads  to  a  sudden 
inspiratory  movement. 


RESPIRATION  433 

MODIFICATIONS  OF  THE  RESPIRATORY  RHYTHM 

The  rhythmic  character  of  the  respiratory  movements  is  materially 
modified  by  a  change  in  the  quantitative  and  qualitative  composition  of  the 
air  and  blood  as  well  as  by  changes  of  a  pathologic  nature  of  the  respiratory 
apparatus  itself. 

Eupnea. — So  long  as  the  air  retains  its  normal  composition  and  the 
respiratory  mechanism  its  structural  integrity,  so  long  do  the  respiratory 
movements  exhibit  a  normal  rhythm  and  frequency.  To  the  condition  of 
easy  tranquil  breathing  the  term  eiipiea  is  given.  In  this  condition  the 
percentages  of  oxygen  and  carbon  dioxid  in  the  blood  are  such  as  to  favor  at 
least  the  rhythmic  discharge  of  nerve  impulses  from  the  inspiratory  center 
to  the  inspiratory  muscles,  of  sufficient  energy  and  frequency  for  the  main- 
tenance -of  normal  respiration. 

Hyperpnea. — The  normal  rate  of  the  respiratory  movements  is  increased 
by  a  rise  in  body-temperature,  by  active  exercise,  and  by  emotional  states. 
Whatever  the  cause,  the  increase  in  rate  and  probably  in  depth  is  termed 
hyperpnea. 

Febrile  states  characterized  by  a  rise  in  the  temperature  of  the  blood 
increase  considerably  the  respiratory  activity.  This  is  due  in  all  probability 
to  a  warming  of  the  inspiratory  center,  in  consequence  of  which  its  ir- 
ritability is  heightened;  for  surrounding  the  carotid  arteries  with  warm  tubes 
and  heating  the  blood  on  its  way  to  the  medulla  has  the  same  effect.  It  is 
also  possible,  however,  that  the  high  temperature  of  febrile  conditions  may 
interfere  with  the  absorbing  power  of  hemoglobin,  and  thus  by  diminish- 
ing the  quantity  of  oxygen  absorbed  lead  to  more  frequent  respirations. 
To  the  hyperpnea  induced  by  heat  the  term  titer  mo- poly  p^iea  is  frequently 
given. 

Muscle  activity,  especially  if  it  is  violent  and  indulged  in  by  those  unac- 
customed to  exercise,  is  generally  followed  by  increased  rate  and  depth  of 
breathing,  and  not  infrequently  it  is  attended  with  such  extreme  difficulty 
that  the  condition  approximates  that  of  dyspnea.  This  condition  is  attrib- 
uted to  the  production  and  discharge  into  the  blood  of  metabolic  products 
which  act  as  stimuli  to  the  respiratory  center  and  thus  increase  its  activity. 
Of  these  metabolic  products  CO 2  is  undoubtedly  one  of  the  most  efficient, 
as  stated  in  foregoing  paragraphs.  Emotional  states  temporarily  increase 
respiratory  activity.     With  their  disappearance  the  normal  condition  returns. 

Apnea. — Apnea  may  be  defined  as  a  temporary  cessation  of  the  respira- 
tory movements.  It  may  be  developed  by  rapid  and  deep  inspirations  due 
to  volitional  efforts,  by  rapid  mechanic  inflation  of  the  lungs,  and  by  stimu- 
lation of  various  afferent  nerves.  If  one  volitionally  breathes  rapidly  and 
deeply  for  a  period  varying  from  two  to  ten  minutes,  it  will  be  found  on 
cessation  of  the  effort  that  a  condition  of  apnea  is  established  which  may 
last  for  from  thirty  seconds  to  several  minutes.  One  experimenter  succeeded 
after  forcible  inspiration  for  two  and  a  half  minutes,  in  establishing  in 
himself  an  apnea  that  lasted  for  several  minutes  before  there  was  the  slightest 
desire  to  breathe.  Before  the  cessation  of  the  apnea,  the  face  became  pale 
and  corpse-like,  indicative  of  a  marked  condition  of  anoxhemia.  If  the 
lungs  of  an  animal  be  rapidly  inflated  through  a  cannula  inserted  in  the 
trachea,  a  similar  condition  is  developed.  Whether  the  apnea  be  estab- 
28 


434  TEXT-BOOK  OF  PHYSIOLOGY 

lished  by  volitional  efforts  or  by  mechanic  inflation,  the  respiratory  move- 
ments gradually  return.  At  first  they  are  feeble  but  soon  increase  in 
amplitude  and  frequency  until  the  normal  is  reached.  At  one  time  the 
apnea  that  results  from  rapid  ventilation  of  the  lungs,  whether  volitional  or 
mechanical,  was  attributed,  on  the  assumption  that  a  deficiency  of  oxygen 
in  the  arterial  blood  is  the  physiologic  stimulus  to  the  activity  of  the  inspira- 
tory center,  to  an  excess  of  oxygen  in  the  blood,  the  result  of  the  forced 
ventilation,  complete  saturation  of  the  plasma  and  the  hemoglobin,  in 
consequence  of  which  the  inspiratory  center  remained  inactive.  The 
apneic  state  is  at  present  attributed,  on  the  assumption  that  carbon  dioxid 
in  the  arterial  blood  is  the  physiologic  stimulus  to  the  inspiratory  center,  to  a 
diminution  in  the  percentage  of  the  carbon  dioxid  in  the  alveolar  air  (to  4  per 
cent,  or  less),  in  the  blood,  and  therefore  in  the  center,  the  result  of  the  forced 
ventilation.  The  increased  ventilation  eliminates  the  carbon  dioxid  to  such 
an  extent  that  the  percentage  and  pressure  in  the  blood  is  insufficient  to 
arouse  the  center  to  activity.  To  the  condition  of  the  blood  that  results 
from  this  rapid  ventilation,  viz.:  a  diminished  percentage  of  CO2,  the  term 
acapnia  has  been  given.  An  apnea  which  is  thus  developed  is  termed  apnea 
chemica  or  apnea  vera.  As  previously  stated,  stimulation  of  certain  afferent 
nerves,  especially  the  vagus,  will  induce  a  similar  cessation  of  the  respiratory 
movements.  Thus  if  the  central  end  of  the  divided  vagus  be  stimulated 
with  induced  electric  currents  of  marked  intensity,  the  thorax  will  come  to 
rest  in  the  state  characteristic  of  deep  expiration  from  inhibition  of  the  in- 
spiratory center.  Inasmuch  as  stimulation  of  the  vagus  causes  an  apnea 
resembling  that  caused  by  rapid  inflation  of  the  lungs,  it  has  been  suggested 
that  in  the  development  of  apnea  the  inspiratory  center  is  inhibited  in  its 
activity  simultaneously  with  the  elimination  of  the  CO2,  from  the  mechanic 
stimulation  of  the  pulmonic  terminations  of  the  vagus.  An  apnea  caused 
by  stimulation  of  the  vagus  is  termed  apnea  vagi  or  apnea  inhibitoria. 

In  the  apnea  that  results  from  voluntary  or  mechanic  inflation  of  the 
lungs  it  is  difficult  to  state  in  how  far  the  condition  is  due  to  a  diminution 
in  the  pressure  of  the  COj  and  in  how  far  to  a  stimulation  of  the  vagus. 
But  inasmuch  as  apnea  can  be  established,  though  not  of  such  long  duration, 
after  division  of  the  vagus  nerves,  the  probabilities  are  that  the  diminished 
percentage  of  the  CO 2  is  the  main  cause. 

Dyspnea. — Excessive  and  laborious  respiratory  movements  constitute 
a  condition  known  as  dyspnea.  Movements  of  this  character  indicate  that 
the  blood  contains  a  greater  percentage  of  COj  than  normal  or  a  diminished 
percentage  of  oxygen.  In  either  case  the  irritability  of  the  inspiratory  center 
is  abnormally  heightened.  Of  the  two  conditions,  the  former  is  by  far  the 
more  common.  While  it  is  true  that  a  deficiency  of  oxygen  in  the  arterial 
blood  gives  rise  to  an  increase  in  the  rate  and  depth  of  the  inspiratory  move- 
ments, this  does  not  arise  until  the  deficiency  of  the  oxygen  falls  to  about 
one-third  of  the  normal.  On  the  other  hand,  an  increase  of  even  0.2  per 
cent,  of  CO2  in  the  alveolar  air  will  almost  double  the  inspiratory  activity. 

These  conditions  of  the  blood  may  be  caused:  (i)  By  all  those  pathologic 
conditions  of  the  respiratory  apparatus  which  limit  the  free  entrance  of  oxygen 
into  and  the  free  exit  of  carbon  dioxid  from  the  blood;  (2)  by  those  alterations 
in  the  composition  of  the  air  and  subsequently  in  the  blood  which  arise  when 
the  individual  is  confined  in  a  space  of  moderate  size  with  imperfect  ventila- 


RESPIRATION  435 

tion.  A  deficiency  in  the  amount  or  the  quality  of  the  hemoglobin  is  usually 
attended  with  more  or  less  dyspnea. 

Asphyxia. — If  the  state  of  the  blood  observed  in  dyspnea  be  exaggerated 
— that  is,  if  the  increase  in  the  percentage  of  carbon  dioxid  become  more 
marked — the  respiratory  movements  become  more  laborious.  A  con- 
tinuance of  this  changed  composition  of  the  blood  eventuates  in  death. 
Before  this  occurs  the  individual  exhibits  a  succession  of  phenomena,  to  the 
totality  of  which  the  term  asphyxia  is  given. 

Asphyxia  may  be  caused:  (i)  By  a  sudden  interference  with  the  entrance 
of  oxygen  into  and  the  exit  of  carbon  dioxid  from  the  blood,  as  in  drowning, 
occlusion  of  the  trachea  from  any  cause,  double  pneumothorax,  etc.  (2) 
By  confinement  in  a  small  space  the  air  of  which  speedily  undergoes  a  loss 
of  oxygen  and  an  accumulation  of  carbon  dioxid.  In  the  first  instance 
death  may  occur  in  a  few  minutes;  in  the  second  instance  it  may  be  postponed 
several  hours  or  more,  the  time  varying  with  the  size  of  the  space. 

The  succession  of  phenomena  presented  by  an  individual  in  the  asphyxi- 
ated condition  is  as  follows:  Increased  rate  and  depth  of  the  respiratory 
movements,  passing  rapidly  from  hyperpnea  to  dyspnea,  with  an  active  con- 
traction of  all  the  muscles  concerned  in  respiration,  ordinary  and  extraor- 
dinary ;  a  blue,  cyanosed  condition  of  the  face  from  the  rapid  accumulation 
of  carbon  dioxid  and  disappearance  of  the  oxygen  of  the  blood;  a  diminution 
in  the  depth  of  inspiration  and  an  increase  in  the  force  and  extent  of  ex- 
piration, followed  by  general  convulsions;  collapse,  characterized  by  un- 
consciousness, loss  of  the  reflexes,  relaxation  of  the  muscles,  a  weak  action  of 
the  heart,  a  disappearance  of  the  pulse,  and  death.  As  shown  by  observation 
of  the  circulatory  apparatus  in  artificially  induced  asphyxia,  there  is  primarily 
an  increase  in  the  activity  of  the  heart,  soon  followed  by  retardation;  a  rise 
of  blood-pressure  in  the  early  stages  and  a  fall  to  zero  after  collapse  has  set 
in.  The  retardation  and  final  cessation  of  the  heart,  as  well  as  the  rise  of 
the  blood-pressure,  are  to  be  attributed  to  stimulation  of  the  cardio-inhibi- 
tory  and  vaso-motor  centers  from  the  accumulation  of  the  carbon  dioxid. 
With  the  exhaustion  of  the  nerve-centers,  there  is  a  general  relaxation  of  the 
skeletal  muscles,  the  cardiac  muscle,  a  fall  of  the  blood-pressure,  and  dilata- 
tion of  the  pupils. 

The  Cheyne-Stokes  Respiration. — A  modification  of  the  respiratory 
movements  characterized  by  periods  of  rest  alternating  with  periods  of 
activity  was  described  in  1818  and  in  1854  by  the  two  writers  whose  names  it 
bears.  The  periods  of  rest  vary  in  duration  from  twenty  to  thirty  seconds; 
the  periods  of  activity  from  thirty  to  sixty  seconds  and  may  include  from 
twenty  to  thirty  respiratory  movements. 

Each  period  of  rest  of  the  respiratory  mechanism  is  closed  by  the  appear- 
ance of  a  slight  shallow  respiratory  movement,  which  is  immediately  fol- 
lowed by  a  second,  slightly  deeper,  and  this  in  turn  by  a  third,  a  fourth,  a 
fifth,  and  so  on,  each  becoming  deeper  than  the  preceding  until  a  certain 
maximum  is  reached,  after  which,  each  succeeding  movement  gradually 
diminishes  in  depth  until  finally  the  movement  becomes  imperceptible  and  a 
new  period  of  rest  supervenes.  A  graphic  representation  of  the  Cheyne- 
Stokes  type  of  respiration  is  shown  in  Fig.  201.  This  type  of  respiration  is 
frequently  an  accompaniment  of  certain  pathologic  conditions,  e.g.,  uremic 
states,  cerebral  hemorrhage,  heart  diseases,  arteriosclerosis,  etc.,  though  no 


436  TEXT-BOOK  OF  PHYSIOLOGY 

satisfactory  explanation  of  it  has  yet  been  presented.  A  similar  though  far 
less  marked  periodicity  in  the  respiratory  movements  is  frequently  observed 
during  sleep,  especially  in  children.  A  periodicity  can  also  be  developed 
by  dividing  transversely  the  medulla  oblongata  just  above  the  calamus 
scriptorius,  which  either  injures  the  respiratory  center  or  removes  from  it 
some  cerebral  influence. 


Fig.  20I. — Tracing  Showing  the  Cheyne-Stokes  Form  of  Respiration. — {Hill.) 

THE  EFFECT  OF  THE  RESPIRATORY  MOVEMENTS  ON  THE  FLOW  OF 

BLOOD  THROUGH  THE  INTRA-THORACIC  VESSELS,  AND  ON 

THE  ARTERIAL  PRESSURE 

I.  On  the  Intra-thoracic  Vessels. — The  forces  which  cause  the  air 
to  flow  into  and  out  of  the  lungs  will  at  the  same  time  and  in  a  similar  way 
cause  the  blood  of  the  extra-thoracic  veins  to  flow  into,  through,  and 
out  of  the  intra-thoracic  vessels.  From  the  tendency  of  the  pulmonic 
elastic  tissue  to  recoil,  the  blood-vessels  in  the  thorax  at  the  end  of  an  expira- 
tion sustain  a  positive  pressure,  the  intra-thoracic  pressure  (see  page  305), 
about  six  millimeters  of  mercury  less  than  that  in  the  lungs,  or,  in  other  words, 
a  pressure  negative  to  that  of  the  atmosphere  by  six  millimeters.  As  a  result 
the  blood  in  the  systemic  vessels  under  atmospheric  pressure  will  flow 
steadily  toward  the  intra-thoracic  veins,  the  venae  cavae,  and  the  right 
side  of  the  heart,  i.e.,  from  a  point  of  high  to  a  point  of  low  pressure.  During 
inspiration  there  is  a  decrease  in  the  intra-thoracic  pressure,  the  decrease 
being  proportional  to  the  extent  of  the  inspiration.  With  this  decrease 
of  pressure,  the  intra-thoracic  veins  expand  and  their  internal  pressure  falls. 
As  the  systemic  or  extra-thoracic  veins  are  subjected  to  atmospheric  pressure, 
the  blood  in  these  vessels  is  forced,  by  reason  of  the  difference  of  pressure 
between  these  two  regions,  to  flow  more  rapidly  and  freely  into  the  intra- 
thoracic veins  and  right  side  of  the  heart.  The  right  heart  being  more 
generously  filled  with  blood  will  discharge  a  larger  volume  with  each  con- 
traction into  the  pulmonic  artery. 

Coincident  with  these  effects  a  similar  effect  is  produced  in  the  arterioles 
and  capillaries  of  the  pulmonic  alveoli.  Situated  between  the  two  elastic 
layers  of  the  alveolar  wall,  embedded  in  a  meshwork  of  connective  tissue, 
the  pressure  to  which  they  are  subjected  at  the  end  of  an  expiration  will 
also  be  a  few  mfllimeters  less  than  the  intra-pulmonic  pressure;  and  at  the 
end  of  an  inspiration  it  will  be  considerably  less.  With  the  inspiration  there- 
fore there  will  occur  a  dilatation  of  these  vessels,  and  hence  a  larger  flow  of 
blood  through  them  and  into  the  pulmonic  veins.     The  left  heart,  being 


RESPIRATION  437 

in  consequence  more  generously  filled  with  blood,  will  discharge  a  larger 
volume  into  the  aorta  at  each  contraction.  During  expiration  the  flow  of 
blood  through  the  intra-thoracic  vessels  will  be  diminished  for  the  reverse 
reasons. 

2.  On  the  Arterial  Pressure. — An  examination  of  a  tracing  of  the  arterial 
pressure  will  show  that  it  is  characterized  by  small  undulations  due  to  the 
cardiac  beat  and  large  undulations  due  to  the  respiratory  movements, 
inspiration  being  accompanied  by  a  rise,  expiration  by  a  fall  of  pressure. 
These  results  are  readily  accounted  for  by  the  difference  in  the  volume  of 
blood  discharged  by  the  left  heart  into  the  aorta  during  the  time  of  the  two 
movements.  If  a  tracing  of  the  respiratory  movements  and  of  the  blood- 
pressure  be  taken  simultaneously,  it  will  be  found  that  the  rise  of  pressure 
does  not  exactly  correspond  with  inspiration,  nor  the  fall  of  pressure  with 
expiration;  for  a  certain  period  after  the  beginning  of  an  inspiration  the 
pressure  continues  to  fall,  and  for  a  certain  period  after  the  beginning  of  an 
expiration  the  pressure  continues  to  rise.  During  the  remainder  of  the 
period,  however,  inspiration  is  attended  by  a  rise,  expiration  by  a  fall  of 
pressure.  The  explanation  of  these  results  lies  in  the  fact  that  at  the  begin- 
ning of  the  inspiration,  when  the  vessels  dilate,  the  blood-flow  momentarily 
slows;  the  left  heart  continuing  to  discharge  small  volumes  into  the  aorta, 
the  pressure  continues  to  fall.  So  soon  as  the  left  heart  begins  to  be  better 
filled,  the  pressure  at  once  begins  to  rise.  At  the  end  of  an  expiration  the 
flow  of  blood  into  the  left  heart  continues  and  the  pressure  rises,  but  with 
the  return  of  the  intra-thoracic  pressure  the  vessels  diminish  in  caliber,  the 
volume  of  blood  transmitted  by  them  becomes  less,  the  output  of  the  left  heart 
declines,  and  the  pressure  falls. 

3.  On  the  Flow  of  Lymph. — The  fall  of  the  intrathoracic  pressure 
during  an  inspiratory  movement  has  an  accelerating  effect  on  the  flow  of 
lymph  from  the  abdominal  portion  of  the  thoracic  duct  into  the  thoracic 
portion.  The  rise  of  the  pressure  during  an  expiratory  movement  causes  a 
more  abundant  discharge  of  lymph  from  the  end  of  the  duct,  into  the  venous 
blood.     The  advantages  of  this  mechanism  have  been  alluded  to  on  page  225. 


CHAPTER  XVI 
ANIMAL  HEAT 

The  animal  body  possesses  a  temperature  that  is  perceptible  to  the  sense 
of  touch  and  determinable  by  a  thermometer.  This  temperature  is  the 
result  of  the  liberation  of  heat  which  attends  the  chemic  changes  taking 
place  in  the  tissues  and  organs  of  the  living  body  and  which  underlie  all 
manifestations  of  life.  In  consequence  of  this  each  animal  acquires  a 
certain  body-temperature. 

In  man,  as  well  as  in  other  mammals  and  in  birds,  the  chemic  changes 
are  extremely  active  and  the  evolution  of  heat  very  great.  Through  some 
special  heat-regulating  mechanism,  by  which  heat-production  and  heat- 
dissipation  are  kept  in  equilibrium,  these  animals  have  acquired  and  main- 
tain within  limits  a  constant  temperature  which  is  independent  of  and  gen- 
erally above  that  of  the  surrounding  atmosphere.  As  the  temperature  of 
these  animals  is  high  and  perceptible  to  the  sense  of  touch,  they  were  origi- 
nally designated  "warm-blooded"  animals.  As  this  temperature  is  con- 
stant notwithstanding  the  great  variations  in  external  temperature  during 
the  summer  and  winter  seasons,  they  are  more  appropriately  termed  con- 
stant-temperatured  or  homoio-thermous  animals.  The  intensity  of  the  body- 
temperature  determined  by  the  insertion  of  a  thermometer  in  the  rectum 
varies  in  different  classes  of  mammals  from  37.2°C.  to  40°C.  The 
causes  of  this  variation  are  doubtless  connected  with  peculiarities  of  organi- 
zation. In  birds  the  rectal  temperature  is  usually  higher,  varying  from 
40.9°C.  in  the  pigeon  to  44°C.  in  the  titmouse  and  the  swift. 

In  reptiles,  amphibians,  and  fish,  chemic  changes  as  a  rule  are  not  very 
active  and  heat-production  relatively  slight.  As  they  are  devoid  of  a  suffi- 
ciently active  heat-regulating  mechanism,  the  temperature  of  the  body  is 
largely  dependent  on  that  of  the  medium  in  which  they  live,  though  it  is 
always  one  or  more  degrees  above  it.  In  winter  the  body-temperature  of 
frogs,  for  example,  may  decline  as  low  as  8.9°C.,  the  temperature  of  the 
surrounding  medium  being  6.7°C.  When  subjected  to  temperatures  below 
zero,  the  temperature  of  the  body  may  fall  below  the  freezing-point  also, 
when  the  lymph  and  fluids  of  the  body  become  ice.  Though  apparently 
dead,  the  gradual  elevation  of  the  temperature  restores  their  vitality.  In 
summer-time,  on  the  contrary,  the  body-temperature  may  attain  to  38°C. 
Similar  variations  have  been  observed  in  other  animals.  As  the  temperature 
of  these  animals  is  low  and  perceptibly  below  that  of  our  own  bodies,  they 
were  originally  termed  "cold-blooded"  animals;  as  their  temperature  is 
inconstant,  varying  with  the  temperature  of  the  surrounding  medium,  they 
are  more  appropriately  termed  "  variable- temperatured"  or  poikilo-ther- 
mous  animals. 

THE  TEMPERATURE  OF  THE  HUMAN  BODY 

The  determination  of  the  temperature  of  the  human  body  under  the 
changing  conditions  of  life  is  a  matter  of  the  greatest  physiologic  and 

438 


ANIMAL  HEAT 


439 


clinical  interest.  The  temperature  of  the  superficial  portions  of  the  body 
may  be  obtained  by  the  introduction  of  a  thermometer  into  the  mouth, 
the  rectum,  the  vagina,  or  the  axilla.  As  a  result  of  many  observa- 
tions it  has  been  found  that  the  temperature  of  the  rectum  is,  on 
the  average,  37.2°C.;  that  the  mouth,  36.8°C.;  that  of  the  axilla,  36.9°C. 
Owing  to  radiation  and  conduction,  the  surface  temperature  is  lower 
than  that  of  either  the  mouth  or  rectum,  and  varies  to  a  slight  extent 
in  different  regions  of  the  body:  e.g.,  at  a  room-temperature  of  20°C. 
the  skin  of  the  pectoral  region  has  a  temperature  of  34-7°;  that  of  the 
cheek,  34.4°;  that  of  the  calf,  33.6°;  that  of  the  tip  of  the  ear,  only 
28.8°,  etc. 

In  the  interior  of  the  body,  especially  in  organs  in  which  oxidation  takes 
place  rapidly,  and  which  at  the  same  time  are  protected  by  their  anatomic 
surroundings  from  rapid  radiation,  the  temperature  is  higher  than  that 
observed  in  the  rectum.  From  an  investigation  of  the  temperature  of  the 
blood  as  it  emerges  from  the  liver,  the  muscles,  the  brain,  alimentary  canal, 
etc.,  it  is  evident  that  these  organs  have  a  higher  temperature  than  the 
rectum. 


Fig.  202.— Scheme  to  show  the  Topogr.a.phic  Distribution  of  the  Temperature  in 
THE  Large  Blood-vessels.  The  degree  of  temperature  in  these  vessels  is  expressed  at  each 
point  by  its  height  above  a  conventional  abscissa  which  is  here  the  aorta,  in  which  the  tem- 
perature is  practically  uniform. — (After  Bernard.) 

As  the  chemic  changes  underlying  physiologic  activity  vary  in  intensity 
and  extent  in  different  regions  of  the  body,  there  would  be  marked  varia- 
tions in  their  temperature  were  it  not  that  the  blood,  having  a  large 
capacity  for  heat-absorption,  distributes  the  heat  almost  uniformly  to  all 
portions  of  the  body,  so  that  at  a  short  distance  beneath  the  surface  the 
temperature  varies  but  a  few  degrees. 

In  the  dog  the  temperature  of  the  blood  in  the  aorta  and  in  its  principal 
branches  is  approxim.ately  38.6^0.  In  passing  through  the  systemic  cap- 
illaries the  temperature  falls  from  radiation  and  conduction  to  surface 
temperature,  to  again  rise  as  the  venous  blood  approaches  the  deeper  regions 
of  the  body  (Fig.  202).  In  the  neighborhood  of  the  renal  veins  and  in  the 
superior  vena  cava,  the  temperature  is  again  that  of  the  aorta.  In  the 
portal  vein  the  temperature  rises  to  39°C.;  in  the  hepatic  vein,  to  8g.y°C. 
In  the  right  ventricle,  owing  to  the  admixture  of  blood  from  different 
localities  having  different  temperatures,  the  temperature  falls  to  38. 8°C  In 
passing  through  the  pulmonic  capillaries  the  temperature  of  the  blood  again 
falls,  so  that  in  the  left  ventricle  it  will  register  38.6°C.  There  is  thus 
usually  a  difference  between  the  two  sides  of  the  heart  of  about  o.2°C. 


440  TEXT-BOOK  OF  PHYSIOLOGY 

Variations  in  the  Mean  Temperature. — The  mean  temperature  of 
the  human  body  for  twenty-four  hours,  which  for  the  mouth  and  the  rectum 
may  be  accepted  at  36.8°C.  and  37.2°C.  respectively,  is  subject  to  variations 
from  a  variety  of  circumstances,  such  as  age,  periods  of  the  day,  food, 
exercise,  etc. 

Age. — At  birth  the  temperature  of  the  infant  is  slighdy  higher  than 
that  of  the  mother,  registering  in  the  rectum  about  37.5°C.  In  a  few  hours 
it  rapidly  declines  to  about  36.5°,  to  be  followed  in  the  course  of  twenty-four 
hours  by  a  rise  to  the  normal  or  slightly  beyond.  During  childhood  the 
temperature  gradually  approximates  that  of  the  adult.  In  old  age  the  tem- 
perature rises,  as  a  rule,  and  attains  a  maximum  at  eighty  years  of  37.4°C. 

Periods  oj  the  Day. — The  observations  of  Jiirgensen  show  that  there  is 
a  diurnal  variation  in  the  mean  temperature  of  from  o.5°C.  to  i.5°C.,  the 
maximum  occurring  late  in  the  afternoon,  from  5  to  7  o'clock,  the  minimum 
early  in  the  morning,  from  4  to  7  o'clock.  This  diurnal  variation  in 
the  mean  temperature  is  related  to  corresponding  variations  in  many  other 
physiologic  processess,  and  its  causes  are  to  be  found  in  the  ordinary  habits 
of  life  as  regards  the  time  of  meals,  periods  of  exercise,  sleep,  etc. 

Food  and  Drink. — The  ingestion  of  a  hearty  meal  increases  the  tempera- 
ture but  slightly — not  more  than  o.5°C.  Insufficiency  of  food  lowers 
the  temperature;  total  withdrawal  of  food,  as  in  starvation,  is  followed  by 
a  steady  though  slight  decline,  until  just  preceding  the  death  of  the  animal, 
when  it  falls  abruptly  to  from  6°  to  8°C.  Cold  drinks  lower,  hot  drinks 
raise  the  temperature.  Food  and  drinks,  however,  only  temporarily  change 
the  mean  temperature,  and  after  a  short  period  equilibrium  is  restored 
through  the  activity  of  the  heat-regulating  mechanism.  Alcoholic  drinks 
lower  the  temperature  about  o.5°C.  In  large  toxic  doses  in  persons  un- 
accustomed to  their  use  the  temperature  may  be  lowered  several  degrees. 
This  is  attributed  not  to  a  diminution  in  heat-production,  but  rather  to  an 
increase  in  heat-dissipation  (Reichert)  from  increased  action  of  the  heart, 
dilatation  of  the  blood-vessels  of  the  skin,  and  increased  activity  of  the 
sweat-glands. 

Exercise. — The  temperature  may  be  raised  by  active  muscular  exercise 
from  1°  to  i.5°C.  as  a  result  of  increased  activity  in  chemic  changes  in  the 
muscles  themselves.  A  rise  beyond  this  point  is  prevented  by  the  increased 
activity  of  the  circulatory  apparatus,  the  removal  of  the  heat  to  the  surface, 
and  its  rapid  radiation. 

External  Temperature. — The  external  temperature  influences  but  slightly 
the  mean  temperature  of  the  human  body.  In  the  tropic,  as  well  as  in  the 
arctic  regions,  notwithstanding  the  change  in  the  temperature  of  the  air, 
the  temperature  of  the  body  remains  almost  constant.  The  same  is  true  for 
the  seasonal  variations  in  the  temperature  of  the  temperate  regions. 

The  Residual  Heat  of  the  Body. — ^As  a  preliminary  to  a  consideration 
of  heat-production  and  heat-dissipation,  it  is  of  interest  to  determine  the 
actual  quantity  of  heat  expressed  in  Calories,  that  resides  in  the  body  at  all 
times.  This  can  be  approximately  determined  from  the  chemic  composi- 
tion and  the  temperature.  A  chemic  analysis  of  the  body  shows  that  it  con- 
sists of  water  0.6,  and  of  tissue  0.4.  If  the  weight  be  assumed  to  be  70 
kilograms  then  42  kilograms  consist  of  water,  and  as  the  temperature  is 
37°C.,  the  42  kilos  of  water  will  contain  42  X  37  or  1554  kilogram  calories; 


ANIMAL  HEAT  441 

the  remaining  28  kilograms  consist  of  tissues,  the  specific  heat  of  which  is 
but  0.8  that  of  water,  hence  the  28  kilograms  of  tissue  will  contain  28  X  0.8 
Calories,  the  equivalent  of  22.4  kilograms  of  water.  Since  the  temperature 
of  the  body  is  37°C.  the  additional  number  of  Calories  will  be  22.4  X^37 
or  828,  making  a  total  of  2382  Calories  an  amount  of  heat  absolutely  necessary 
to  maintain  the  body-temperature  at  the  physiological  level.  Notwith- 
standing the  constant  liberation  of  large  amounts  of  heat  each  day,  it  is 
dissipated  more  or  less  rapidly  in  accordance  with  variations  in  temperature, 
character  of  clothing  and  a  variety  of  other  conditions,  and  so  accurately 
is  this  done,  that  at  the  end  of  the  twenty-four  hours  the  body  possesses 
its  customary  quantity  of  heat  and  its  physiologic  temperature. 

HEAT-PRODUCTION.     THERMOGENESIS 

The  Source  of  Heat. — The  immediate  source  of  the  body-heat  is  to  be 
found  in  the  chemic  changes  which  take  place  in  all  the  tissues  and  organs 
of  the  body.  Each  contraction  of  a  muscle,  each  act  of  secretion,  each 
exhibition  of  nerve-force,  is  accompanied  by  the  evolution  of  heat.  The 
chemic  changes  are  for  the  most  part  of  the  nature  of  oxidations,  the  union 
of  oxygen  with  the  elements,  carbon  and  hydrogen,  of  the  food  principles 
either  before  or  after  they  have  become  constituents  of  the  tissues.  The 
ultimate  source  of  the  body-heat  is  the  latent  or  potential  energy  in  the  food 
principles,  which  was  absorbed  from  the  sun's  energy  and  stored  up  during 
the  growth  of  the  vegetable  world.  In  the  metabolism  of  the  animal  body 
the  food  principles  are  again  reduced  through  oxidation,  directly  or 
indirectly,  to  relatively  simple  bodies,  such  as  urea,  carbon  dioxid,  and 
water,  with  a  liberation  of  a  large  portion  of  their  contained  energy  which 
manifests  itself  as  heat  and  mechanic  motion. 

The  Total  Quantity. — The  total  quantity  of  heat  liberated  in  the  body 
daily  may  be  approximately  determined  in  at  least  two  ways:  (i)  By  deter- 
mining experimentally  the  heat  values  of  different  food  principles  by  direct 
oxidation;  (2)  by  collecting  and  measuring  with  a  suitable  apparatus,  a  cal- 
orimeter, the  heat  evolved  by  the  oxidation  of  the  food  within,  and  dissipated 
from,  the  body  daily. 

I.  Direct  Oxidation. — The  amount  of  heat  which  any  given  food  prin- 
ciple will  yield  can  be  determined  by  burning  a  definite  amount — e.g.,  i 
gram — to  carbon  dioxid  and  water  and  ascertaining  the  extent  to  which 
the  heat  thus  liberated  will  raise  the  temperature  of  a  given  amount  of  water, 
e.g.,  I  kilogram.  The  amount  of  heat  may  be  expressed  in  gram  or  kilo- 
gram degrees  or  calories;  a  gram  calorie  or  kilogram  calorie  being  the  amount 
of  heat  required  to  raise  the  temperature  of  a  gram  or  a  kilogram  (1000 
grams)  of  water  i°C.  The  apparatus  employed  for  this  purpose  is  termed 
a  calorimeter,  which  consists  essentially  of  a  closed  chamber,  in  which  the 
oxidation  takes  place,  surrounded  by  a  water-jacket.  The  rise  in  tempera- 
ture of  the  water  indicates  the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calorimeters 
and  different  food  principles  of  the  same  class  vary,  though  within  narrow 
limits:  e.g.,  i  gram  casein  yields  5.867  kilogram  calories;  i  gram  of  lean 
beef,  5.656;  I  gram  of  fat,  9.353,  9.423,  9.686  Calories;  i  gram  of  starch  or 
sugar,  4.116,    4.182,    4.479,   etc..  Calories.     These  results    are,  however. 


442  TEXT-BOOK  OF  PHYSIOLOGY 

physical  values,  and  indicate  the  quantity  of  heat  such  quantities  of  foods 
give  rise  to  when  completely  oxidized  to  carbonic  acid  and  water.  In  the 
animal  body  the  carbohydrates  and  the  fats,  with  the  exception  of  the  small 
portion  which  escapes  digestion,  are  reduced  to  carbon  dioxid  and  water, 
and  hence  practically  liberate  as  much  heat  as  they  do  when  oxidized  outside 
the  body.  The  proteins,  however,  are  only  reduced  to  the  stage  of  urea.  As 
this  compound  is  capable  of  further  reduction  in  the  calorimeter  to  carbon 
dioxid  and  water,  with  the  liberation  of  heat,  the  quantity  of  heat  it  contains 
must  therefore  be  deducted  from  the  physical  heat  value  of  the  protein. 
According  to  Rubner,  i  gram  of  urea  will  yield  2.523  kilogram  calories. 
As  about  one-third  of  a  gram  of  urea  results  from  the  oxidation  of  i  gram 
of  protein,  the  amount  of  heat  to  be  deducted  from  the  heat  value  of  the 
protein  is  ^  of  2.523,  or  0.841  Calories.  It  has  also  been  shown  by  the  same 
investigator  that  some  of  the  ingested  protein  is  found  in  the  feces,  the  heat 
value  of  which  must  also  be  determined  and  deducted.  This  having  been 
done,  the  physiologic  heat  value  of  protein  thus  becomes  4.124  Calories. 

The  following  estimates  give  approximately  the  number  of  kilogram 
calories  which  should  be  liberated  within  the  body  when  the  protein  is  burned 
to  the  stage  of  urea,  and  the  fat  and  carbohydrate  to  the  stage  of  carbon  dioxid 
and  water: 

I  gram  of  protein 4.124  Calories 

I  gram  of  fat 9-353  Calories 

I  gram  of  carbohydrate   4.116  Calories 

The  total  number  of  kilogram  calories  yielded  by  the  various  diet  scales 
can  be  readily  determined  by  multiplying  the  quantities  of  the  food  prin- 
ciples consumed  by  the  foregoing  factors. 

2.  Calorimetric  Measurements. — It  has  been  determined  from  a  long 
series  of  experiments  that  the  animal  body  dissipates  a  variable  amount  of 
heat  from  day  to  day,  an  amount  that  can  be  collected  and  measured  by 
placing  the  animal  in  a  suitable  apparatus — a  calorimeter.  A  calorimeter 
is  therefore  an  apparatus  designed  for  the  direct  determination  of  the  quan- 
tity of  heat  dissipated  from  the  body  in  any  given  time.  The  amount 
obtained  by  this  method  expressed  in  Calories  is  taken  as  a  measure  of  the 
heat  liberated  by  the  oxidation  of  the  food,  providing  of  course,  the  tempera- 
ture of  the  animal  remains  unchanged.  The  substance  employed  for  col- 
lecting and  measuring  the  heat  is  usually  water.  The  calorimeters  in 
general  use  consist  essentially  of  two  metallic  boxes  placed  one  within 
the  other,  though  separated  by  a  space  sufficiently  large  to  hold  a  defi- 
nite amount  of  water  (Fig.  203).  The  animal  is  placed  in  the  inner  box, 
which  is  also  provided  with  tubes  for  the  entrance  of  fresh  and  the  exit 
of  expired  or  vitiated  air.  The  heat  radiated  is  absorbed  by  the  water  and 
its  temperature  raised.  To  prevent  loss  by  radiation  and  to  render  it  in- 
dependent of  changes  in  the  surrounding  temperature  the  calorimeter  is 
surrounded  by  a  poorly  conducting  material,  such  as  wool.  The  tempera- 
ture of  the  animal  is  taken  at  the  beginning  and  the  end  of  the  experiment. 
If  the  temperature  of  the  animal  remains  the  same  at  the  end  of  the  experi- 
ment, then  the  heat  absorbed  by  the  water  represents  the  amount  produced 
by  the  animal.  If,  on  the  contrary,  the  temperature  of  the  animal  rises 
or  falls,  the  number  of  calories  so  retained  or  lost  must  be  added  to  or  sub- 
tracted from  the  amount  absorbed  by  the  calorimeter.     In  the  determination 


ANIMAL  HEAT 


443 


of  the  absolute  amount  of  heat  retained  or  lost  by  the  animal  above  or 
below  the  initial  temperature,  as  well  as  that  absorbed  by  the  materials  of 
the  apparatus  in  these  various  instances,  the  water  equivalent  of  the  tissues  of 
the  animal  and  the  materials  of  the  calorimeter  must  be  obtained,  and  then 
added  to  or  subtracted  from,  as  the  case  may  be,  the  amount  of  water  in  the 
calorimeter,  and  the  amount  thus  obtained  multiplied  by  its  rise  in  temperature. 

When  means  are  taken  to  collect  for  purposes  of  analysis,  the  urine  and 
the  feces  and  also  the  expired  air,  the  apparatus  is  designated  a  respira- 
tion calorimeter.  The  nitrogen  in  the  urine  and  feces  and  the  carbon  dioxid 
in  the  expired  air  permit  of  the  determination  of  the  amounts  of  protein 
and  fat  metabolized.  The  ventilation  of  the  chamber  is  accomplished  by 
some  form  of  aspirating 
apparatus. 

In  properly  conducted 
experiments  in  which  the 
sources  of  error  are  reduced 
to  a  minimum  there  is  a 
very  close  correspondence 
between  the  total  physio- 
logic heat  value  of  the  food 
and  the  amount  collected 
by  the  calorimeter.  Thus, 
in  an  experiment  detailed 
by  Rubner,  a  dog  was 
given  during  twelve  days 
228.06  grams  of  protein  and 
340.4  grams  of  fat,  the 
physical  heat  value  of  which 
was  estimated  at  4429  Cal- 
ories. The  urine  and  feces 
during  this  period  were  col- 
lected and  their  heat  value 
determined,  which  amounted  to  305  Calories.  The  heat  which  theoretic- 
ally therefore,  should  have  been  produced  was  4124  Calories.  During  the 
experiment  the  calorimeter  actually  absorbed  3958  Calories,  a  difference 
between  the  theoretic  and  experimental  results  of  166  Calories;  thus  of  the 
total  energy  liberated  about  96  per  cent,  appeared  as  heat. 

Calorimetric  experiments  on  human  beings  corresponding  in  general 
to  those  made  on  dogs  and  other  animals,  have  been  made  possible  in 
recent  years  by  the  employment  of  the  ingenious  and  finely  constructed 
respiration  calorimeters  devised  by  Atwater,  Benedict  and  their  co-workers. 
With  these  forms  of  apparatus  the  same  close  correspondence  between  the 
amount  of  heat  actually  absorbed  by  the  calorimeter  and  the  amount  which 
theoretically  would  be  liberated  by  the  oxidation  of  the  food,  has  been  found 
to  hold  true.  Inasmuch  as  the  calorimeter  of  Benedict  differs  in  some  re- 
spects from  those  hitherto  employed,  a  brief  description  of  the  principle  on 
which  it  is  constructed  may  not  be  inappropriate.  The  calorimeter  chambers, 
though  sufficiently  large  to  accommodate  a  human  being,  vary  in  size  and 
general  arrangement  in  accordance  with  the  character  of  the  experiment. 
The  construction  of  each  however  is  essentially  the  same. 


Fig.  203. — Water  Calorimeter  of  Dulong.  D 
and  D'.  Tubes  for  the  entrance  and  exit  of  air.  T  and  T'. 
Thermometers  for  ascertaining  the  temperature  of  the 
water.  S.  A  mechanic  contrivance  for  stirring  the  water 
for  the  purpose  of  distributing  the  absorbed  heat  uni- 
formly. To  prevent  the  escape  of  heat  with  the  expired 
air,  the  tube  D'  is  wound  many  times  in  the  water-space 
beneath  the  animal  cage. 


444 


TEXT-BOOK  OF  PHYSIOLOGY 


These  calorimeters  consist  of  two  parts,  one  for  the  collection  of  the 
heat  liberated  by  the  subject  of  the  experiment,  and  the  other  for  the  col- 
lection of  the  carbon  dioxid  and  water  exhaled  and  for  the  measurement 
of  the  oxygen  consumed.  Means  are  also  provided  for  the  collection  of  the 
urine  and  feces  when  necessary. 

The  first  part  of  the  calorimeter  consists,  Fig.  204,  of  a  chamber  of  sufi&cient 
size  with  metallic  walls,  copper  and  zinc  so  arranged  as  to  prevent  either 
absorption  or  radiation  of  heat,  in  other  words  that  they  will  be  heat- 
proof. At  one  side  there  is  a  door  of  sufficient  size  to  admit  the  subject 
as  well  as  a  window  to  admit  light.  The  heat  dissipated  by  the  subject  of 
the  investigation  is  taken  up  by  a  current  of  water  of  even  but  low  tem- 
perature and  flowing  constantly  under  a  steadily  acting  pressure  through  a 
copper  pipe  of  6  mm.  internal  and  10  mm.  external  diameter.  The  tem- 
perature of  the  water  is  determined  before  it  enters  and  after  it  leaves  the 


H^SO^         Soda  Lime       HgSO^ 
Fig.  204. — Diagram    to    show  the  Principle  of  the  Atwater-Benedict  Calorimeter. 

chamber.  The  amount  of  water  passing  through  the  tube  in  a  given  time, 
is  collected  and  weighed.  The  difference  in  temperature,  multiplied  by 
the  weight  of  the  water  gives  the  amount  of  heat  dissipated  by  the  individual. 
To  facihtate  the  absorption  of  the  heat  a  large  number  of  copper  discs  5 
centimeters  in  diameter  are  soldered  to  the  water  pipe  at  a  distance  of  5 
millimeters  from  one  another.  As  the  absorption  pipe  is  some  5  meters 
in  length,  the  area  presented  for  absorption  approximates  about  4.7  square 
meters.  The  heat  thus  absorbed  is  soon  conducted  to  the  water  followed  by  a 
rise  in  its  temperature.  Inasmuch  as  many  experiments  extend  over  a 
period  of  several  days  or  more,  special  arrangements  have  been  devised  for 
the  introduction  of  food  which  do  not  interfere  with  the  accurate  working 
of  the  calorimeter.  The  door  of  the  food  aperture  is  provided  with  glass 
for  the  transmission  of  light  into  the  chamber.  In  some  calorimeters  a 
bed  is  provided,  in  others  a  chair  as  well  as  devices  which  enable  the  subject 
to  perform  a  measurable  quantity  of  mechanical  work. 


ANIMAL  HEAT 


445 


The  second  part  of  the  calorimeter  consists  of  a  closed  system  of  tubes 
and  absorption  vessels  through  which  the  air  is  kept  moving  under  the  action 
of  a  blower  and  thus  kept  in  a  respirable  condition.  As  the  air  leaves  the 
chamber  it  passes  through  two  absorption  vessels  by  which  the  water  and 
carbon  dioxid  are  successively  absorbed  and  collected  by  sulphuric  acid  and 
soda  lime  respectively.  The  air  then  passes  through  an  additional  sul- 
phuric-acid vessel  which  absorbs  any  water  carried  from  the  soda-lime  vessel 
by  the  air  and  so  back  into  the  chamber.  By  weighing  these  absorbing 
vessels  before  and  after  the  experiment  the  amount  of  water  and  carbon 
dioxid  are  readily  determined.  The  oxygen  of  the  air  of  the  chamber  that 
is  utilized  for  respiration  purposes  is  restored  by  the  admission  of  oxygen 
from  a  cylinder  in  necessary  amounts  by  special  automatic  devices.  The 
amount  of  oxygen  entering  the  chamber  is  determined  by  weighing  the 
cylinder  before  and  after  the  experiment.  The  loss  in  weight  shows  the 
oxygen  consumed.  Of  course,  the  air  of  the  chamber  must  be  analyzed 
to  correct  certain  errors  that  may  possibly  arise. 

With  the  calorimeters  described  in  the  foregoing  paragraphs  it  is  pos- 
sible not  only  to  determine  directly,  but  also  indirectly,  from  the  amounts 
of  protein,  fat,  and  carbohydrate  metabolized  (calculated  from  the  C,H,0, 
and  N  eliminated  and  O  absorbed)  the  heat  produced  and  dissipated  each 
day  under  a-  great  variety  of  conditions  e.g.,  when  the  subject  is  fasting  or 
living  on  the  customary  diet;  when  resting  or  doing  a  fair  days  work;  when 
in  health  or  in  disease,  etc. 

I.  During  Fasting. — ^In  an  experiment  extending  over  a  fasting  period 
of  seven  days'  duration,  recorded  at  length  by  Benedict,  the  heat  was 
determined  directly,  and  ^Iso  indirectly,  from  the  materials  metabolized. 
Some  of  the  results  of  this  experiment  are  shown  in  the  following  tables: 

METABOLISM  OF  S.  A.  B.  DURING  A  SEVEN-DAY  FAST 


Day 


Calories 

Grams 

•  Q. 

Urine 

Directly 
Deter- 
mined 

Calcu- 
lated 
from 

Metab. 

Per 
Kg. 

Per  Sq. 
M. 

Pro- 
tein 

Fat 

R 
Glyco- 
gen 

Ratio 

N:S. 

Ratio 
NiPzOi 

1765 

1796 

29.7 

941 

73-4 

126.4 

64.9 

78 

19.6 

8.55 

176S 

1790 

29.9 

946 

74-7 

147 

5 

23.1 

75 

18.6 

5 

55 

1797 

1785 

30.8 

969 

78.1 

153 

0 

5-4 

74 

17.38 

6 

34 

1     177s 

1734 

30.8 

966 

69.8 

144 

7 

25.2 

75 

16. II 

4 

83 

1649 

1636 

29.0 

985 

65.2 

144 

7 

8.2 

74 

16.26 

5 

23 

1553 

1547 

27-5 

856 

64.4 

129 

8 

21.7 

75 

16.27 

5 

19 

1568 

1546 

28.0 

869 

60.8 

132.5 

18.7 

74 

16.28 

4.87 

Many  similar  experiments  have  been  made  by  Benedict  and  by  others  with 
different  forms  of  apparatus.  As  an  average  result  it  may  be  stated  that 
in  the  fasting  condition  and  doing  light  work  there  is  an  average  heat-dissipa- 
tion of  31  or  32  calories  for  each  kilogram  of  body-weight,  thus  making  a 
total  for  a  man  weighing  70  kilograms  of  2170  or  2240  Calories. 

2.  On  the  Customary  Diet. — ^\Vhen  an  individual  is  supplied  with  the  food 
materials  composing  a  customary  diet,  and  is  therefore  in  the  physiological 
condition,  the  amount  of  heat  produced  and  liberated   is  necessarily  in- 


446  TEXT-BOOK  OF  PHYSIOLOGY 

creased.  Under  these  circumstances  there  will  be  an  increase  of  from  lo  to 
14  per  cent,  thus  raising  the  heat-production  and  dissipation  per  kilogram  of 
body-weight  from  approximately  32  Calories  to  35.5  or  36.5  or  a  total  of 
2485  to  2555  Calories.  The  heat  liberated  by  the  metabolism  during  inan- 
ition is  not  sufficient  to  maintain  the  body  in  heat  equilibrium  and  there- 
fore, an  increase  in  the  food  supply  of  from  10  to  14  per  cent,  beyond  that 
metabolized  is  necessitated.  A  reason  assigned  for  this  necessity  is  as 
follows:  During  digestion  and  in  the  earlier  stages  of  assimilation,  the  foods 
undergo  a  reduction  and  cleavage,  in  consequence  of  which  a  portion  of  their 
contained  energy  is  liberated  as  free  heat  which  however  cannot  be  utilized  by 
tissue  cells  in  the  performance  of  their  various  activities.  This  heat  is 
probably  directly  dissipated  in  an  environment  of  33°C.  The  energy  that 
cells  require  for  the  manifestations  of  their  physiologic  activities  must 
apparently  be  derived  only  from  the  metabolism  of  food  materials  within 
themselves.  Therefore,  food  materials  in  amount  just  sufficient  to  replace 
the  amount  metabolized  in  inanition,  would  furnish  an  amount  of  heat 
which  would  be  insufficient  to  maintain  the  body  in  heat  equilibrium. 
Though  this  holds  true  for  a  man  weighing  70  kilograms  it  is  not  strictly 
true  for  men  who  weigh  less  or  have  a  different  skin  area.  It  has  been 
pointed  out  by  Rubner  that  the  number  of  Calories  dissipated  per  kilogram 
of  body-weight  varies  with  the  weight  and  the  skin  area :  that  the  metabolism 
is  proportional  to  the  surface  area  or,  in  other  words,  whatever  the  weight, 
there  will  be  for  a  given  surface  area,  a  certain  heat  loss  leading  in  turn  to  a 
given  metabolism  of  food  materials.  In  the  following  table  the  weights  of 
different  men,  their  skin  areas,  total  heat-production  as  well  as  Calories  per 
kilogram  while  doing  light  work  are  presented.      (Rubner.) 

Weight  Area  in  Calories  of  Calories 

in  Kg.  Sq.  M.  Metabolism.  per  Kg. 

80 2.283  2864  35.8 

70 2.088  2631  37.7 

60 1.885  2368  39.5 

50 1.670  2102  42.0 

40 1-438  1810  45-2 

In  the  foregoing  table  it  will  be  observed  that  in  the  man  weighing  80 
kilograms,  the  ratio  of  skin  area  to  kilogram  is  0.0285,  while  in  the  man 
weighing  40  kilograms  the  ratio  is  0.0359.  The  heat  liberated  by  this  slight 
increase  in  skin  area  amounts  to  9.4  calories  per  kilogram  weight. 

3.  During  Work. — ^The  foregoing  estimates  as  to  the  amounts  of  heat 
produced  have  reference  mainly  to  the  body  in  repose.  When  the  body 
passes  into  a  state  of  muscle  activity,  there  is  at  once  a  notable  increase 
in  heat-production  in  consequence  of  the  increase  in  the  activity  of  the 
chemic  changes  which  underlie  body  activity,  as  shown  by  the  increase  in 
the  consumption  of  oxygen  and  the  production  of  carbon  dioxid.  Not 
all  of  the  potential  energy  set  free,  however,  appears  as  heat;  for,  if  the 
muscles  are  engaged  in  doing  work  a  part  of  the  energy,  which  would  other- 
wise manifest  itself  as  heat  is  converted  into  mechanic  motion.  From  the 
work  done  during  a  period  of  eight  hours  it  has  been  estimated  that  about 
500  calories  are  so  transformed  or  utilized.  Him  calculated  from  an  aver- 
age of  five  experiments  that  a  man  weighing  67  kilos  in  repose  produced 
154.4  calories  per  hour  and  absorbed  30.7  grams  of  oxygen  per  hour;  but 


ANIM.\L  HEAT  447 

when  engaged  in  active  muscle  movements  produced  271.2  calories  and 
absorbed  119.84  grams  of  oxygen  per  hour.  The  increase  in  heat-produc- 
tion per  hour  during  activity  was  thus  almost  doubled,  though  the  sum  total 
produced  daily  in  which  there  was  a  working  period  of  eight  or  ten  hours 
was  only  about  one-third  more  than  during  a  day  of  repose.  During  sleep 
there  is  a  greatly  diminished  heat-production,  not  more  than  40  calories  per 
hour  being  produced.  The  preceding  data  may  be  tabulated  as  follows 
(Hirn): 

Day  of  Rest.  Day  of  Work. 

Heat  units  (Calories)  pro-    "I  Rest  16  hrs.     Sleep  8  hrs.     Rest  8  hrs.     Work  8  hrs.     Sleep  8  hrs. 
duced /  2470.4  320  1235.2  2169.6  320 


2790.4  3724-8 

Increased  muscle  activity  is  associated  with  increased  metabolism  and 
a  more  rapid  dissipation  of  heat,  which  must  be  compensated  for  by  a  cor- 
responding heat-production  if  heat  equilibrium  is  to  be  maintained.  This 
necessitates  a  larger  amount  of  food  ingested.  From  Calorimetric  inves- 
tigations, as  well  as  from  a  calculation  based  on  the  amounts  of  the  excreted 
products,  various  diet  scales  have  been  arranged  by  different  investigators, 
which  it  is  believed  will  furnish  the  extra  amount  of  energy.  A  few  are 
here  appended. 

DIETARIES  FOR  A  MAN  OF  70  KILOS,  DURING  HARD  WORK 

Voit.              Rubner.           Playfair.  Hultgren. 

Protein 145                   165                   155  134  grams 

Fat 100                    70                    70  79  grams 

Carbohydrate 500                  565                   567  522  grams 

3574                 3362                 3619  3436  Calories 

The  Mechanism  of  Heat -production. — Heat  is  liberated  to  a  greater 
or  less  degree  in  all  tissues  in  the  cells  of  which  food  materials  are  under- 
going disruption  and  oxidation  or  metabolism.  The  tissues  in  which  metabo- 
hsm  and  therefore  heat  liberation  takes  place  most  energetically,  e.g.,  muscles 
and  glands  are  more  especially  to  be  regarded  as  thermogenic  tissues. 

Heat-production  varies  in  intensity  and  amount,  in  accordance  with  a 
number  of  conditions,  but  principally  with  variations  in  physiologic  activity, 
the  quantity  and  quality  of  the  food,  and  changes  in  the  external  temperature. 

It  will  be  recalled  that  all  muscles  possess  tonicity  by  which  is  meant  a 
slight  degree  of  contraction,  the  result  of  the  continuous  arrival  of  nerve 
impulses  through  eft'erent  nerves  discharged  from  motor  nerve-cells  in  the 
spinal  cord  this  discharge  being  maintained  largely  by  nerve  impulses  com- 
ing through  afferent  nerves  from  the  muscles  themselves,  the  joints,  tendons, 
and  skin.  As  a  result  of  this  slight  but  constant  stimulation  of  the  spinal 
cord,  the  metabolic  changes  in  muscle  material  are  maintained  at  a  certain 
level,  with  a  corresponding  liberation  of  heat.  The  chief  result  of  the  tonicity 
would  thus  be  the  production  of  heat.  Any  physiologic  condition  that 
leads  to  a  greater  discharge  of  nerve  impulses  from  the  spinal  cord  and 
hence  increased  muscle  activity,  must  be  attended  by  increased  heat-produc- 
tion. Therefore  work  and  exercise  of  all  kinds  which  involve  a  more  rapid 
contraction  of  the  skeletal  muscles  is  attended  with  increased  heat-produc- 
tion. By  reason  of  their  mass  and  more  or  less  continuous  activity  the  muscles 
are  justly  regarded  as  the  chief  thermogenic  organs. 


448  TEXT-BOOK  OF  PHYSIOLOGY 

The  consumption  of  foods  that  have  a  higher  potential  heat  value  also 
contribute  to  the  amount  of  heat  produced.  Foods  have  different,  physio- 
logic heat  values.  If  the  food  consumed  contains  much  potential  energy  and 
quantity  consumed  be  larger  than  the  average  daily  requirements,  there  will 
be  an  increase  in  heat-production.     (See  page  442.) 

The  chief  external  factor  that  increases  metabolism  in  these  and  other 
organs  and  tissues  is  a  low  external  temperature.  A  fall  of  the  external 
temperature,  such  as  is  experienced  in  the  fall  and  winter  seasons,  causes, 
through  cutaneous  afferent  nerve  stimulation,  a  stimulation  of  the  spinal 
motor  centers,  and  to  a  larger  discharge  of  nerve  impulses  to  muscles,  glands, 
and  other  tissues.  The  increased  metabolism  thus  developed  leads  to  the 
consumption  of  increased  amount  of  food  and  oxygen.  As  a  result  there  is 
an  increased  heat-production.  If  at  the  same  time  volitional  activities  of 
muscles  be  evoked,  as  is  not  infrequently  the  case,  there  will  be  a  still  further 
increase  in  metabolism  and  heat  liberation.  When  all  these  conditions, 
increased  muscle  activity,  increased  amount  of  food  with  high  potential 
energy,  and  a  low  external  temperature  coexist,  heat-production  attains  its 
maximum,  amounting  to  as  much  as  4726  Calories  daily  (Hultgren). 

HEAT-DISSIPATION.    THERMOLYSIS 

From  the  preceding  statements  it  is  evident  that  the  body  is  continually 
liberating  heat  in  amounts  daily,  far  in  excess  of  that  necessary  for  the  main- 
tenance of  the  body-temperature.  Should  this  heat  be  retained,  the  tem- 
perature of  the  body  would  be  raised  at  the  end  of  twenty-four  hours,  an 
additional  18°  or  20°C. — a  temperature  far  in  excess  of  that  compatible 
with  the  maintenance  of  physiologic  processes.  That  the  body  may  be 
kept  at  the  mean  temperature  of  37°C.  it  is  essential  that  the  heat  liberated 
be  dissipated  as  fast  as  it  is  produced,  or  to  state  the  problem  in  another 
way,  the  heat  dissipated  by  the  body  must  be  replaced  by  an  equal  amount 
liberated,  if  equilibrium  of  temperature  is  to  be  maintained.  The  dis- 
sipation of  the  heat  is  accomplished  in  several  ways:  (i)  In  warming  the 
food  and  drink  to  the  temperature  of  the  body.  (2)  In  warming  the  inspired 
air  to  the  same  temperature.  (3)  In  the  evaporation  of  water  from  the 
lungs.  (4)  In  evaporating  water  from  the  skin.  (5)  In  radiation  and  con- 
duction from  the  skin.  The  quantities  of  heat  lost  to  the  body  by  these 
different  processes  it  is  difficult  for  obvious  reasons  to  accurately  determine, 
and  the  estimates  usually  given  must  be  regarded  only  as  approximative. 

The  number  of  Calories  which  are  theoretically  liberated  by  the  various 
diet  scales  can  be  readily  determined  by  multiplying  the  quantities  of  food 
principles  consumed  by  the  usual  factors  (see  page  442).  Thus  the  total 
number  of  Calories  furnished  by  the  various  diet  scales  would  be  as  follows: 
Voit's,  3387;  Vierordt's,  2695;  Ranke's,  2335;  Moleschott's,  2984;  Atwater's, 
3331;  Hultgren's,  3436.  As  the  body- weight  may  not  increase  and  as  the 
temperature  in  physiological  conditions  does  not  rise,  the  assumption  is 
that  the  food  is  oxidized  in  the  body  to  urea,  carbon  dioxid  and  water  with 
the  liberation  of  the  foregoing  amounts  of  heat  and  their  equally  rapid 
dissipation. 

Assuming  2500  Calories  to  be  an  average  of  heat  liberated  during  a  day 
of  repose,  the  losses,  in  the  ways  stated  in  the  foregoing  paragraph,  may  be 
tabulated  as  follows: 


ANIMAL  HEAT  449 

1.  In  Warming  Food  and  Drink. — The  average  temperature  of  food  and 

drink  is  about  i2°C.;  the  amount  of  both  together  is  about  3  kilograms; 
the  specific  heat  of  food  about  0.8  that  of  water.  The  absorption 
of  body-heat  therefore  by  the  food  amounts  approximately  to  3X0.8 
X25°C.  =  60  Calories  =  2.8  per  cent.  With  the  removal  of  the  end- 
products  of  the  foods  and  drink  from  the  body  an  equal  amount  of 
heat  is  carried  out. 

2.  In  Warming  the  Inspired  Air. — The    average   temperature  of   the   air 

is  i2°C.;  the  amount  of  inspired  air,  about  15  kilograms;  the  specific 
heat  of  air,  0.26.  The  absorption  of  body-heat  by  the  air  until  it  at- 
tains the  temperature  of  the  body  will  therefore  amount  to  15  X0.26  X  25° 
=  97.5  C8iories  =  3.8  per  cent.  The  expired  air  removes  from  the 
body  a  corresponding  amount. 

3.  In  the  Evaporation  of  Water  from  the  Lungs. — The  quantity  of  water 

evaporated  from  the  lungs  may  be  estimated  at  400  grams;  as  each 
gram  requires  for  its  evaporation  0.582  Calorie,  the  quantity  of  heat 
lost  by  this  channel  would  be  400X0.582  =  232.8  Calories=9.4  per 
cent. 

4.  In  the  Evaporation  of  Water  from  the  Skin. — The  quantity  of  water  evapor- 

ated from  the  skin  may  be  estimated  at  660  grams,  causing  a  loss  of 
heat  by  this  channel  of  660X0.582  =  384.1  Calories  =  15.3  per  cent. 

5.  In  Radiation  and  Conduction  from  the  Skin. — The  amount   of  heat  lost 

by  this  process  can  be  indirectly  determined  only  by  subtracting  the 
total   amount   lost  by   the   above-mentioned   channels  from  the  total 
amount  produced.     Thus,  2500—7774.4=1725.6  Calories  =  69  per  cent, 
would  represent  the  average  amount  lost  by  radiation  and  conduction. 
The  Mechanism  of  Heat-dissipation. — ^As  stated  in  foregoing  para- 
graphs heat-dissipation  is  accomplished  mainly  by  radiation  and  conduction 
from  the  skin,  70  per  cent.,  and  by  evaporation  of  water  from  the  skin  and 
lungs,  25  per  cent.     The  heat  dissipated  in  warming  food,  drink  and  air 
inspired,  or  what  amounts  to  the  same  thing  in  raising  the  excretions,  urinary, 
fecal  and    respiratory  to  the  body-temperature,  may  be   here  neglected. 
The  relative  amounts  dissipated  by  these  two  routes,  radiation  and  evapora- 
tion of  water,  will  depend  largely  on  the  external  temperature  in  so  far  as 
it  is  not  interfered  with  by  clothing  and  artificial  temperatures. 

The  mechanism  by  which  the  dissipation  is  accomplished  consists  of 
the  cutaneous  and  respiratory  blood-vessels  and  the  sweat-glands,  together 
with  the  heart  and  respiratory  apparatus,  which  collectively  therefore  may 
be  regarded  as  thermolytic  organs,  all  of  which  are  made  to  cooperate  by 
the  intermediation  of  the  nerve  system,  especially  the  vaso-motor  and 
secretory  portions  of  it.  With  a  given  external  temperature  such  as  charac- 
terizes the  spring  months,  there  is  a  certain  ratio  between  the  percentage  of 
heat  lost  by  radiation  and  by  water  evaporation.  As  the  temperature  rises 
as  it  does  during  the  summer  months,  the  cutaneous  vessels  dilate  as  a 
result  of  a  reflex  inhibition  of  the  vaso-motor  center  due  to  the  stimulating 
action  of  the  heat  on  the  cutaneous  nerve  endings,  which  brings  to  the  surface 
a  larger  volume  of  blood,  a  condition  favorable  to  increased  radiation.  This, 
however,  is  to  some  extent  prevented  by  reason  of  a  diminution  of  the  differ- 
ence in  temperature  between  that  of  the  atmosphere  and  that  of  the  body. 
The  sweat-glands  at  the  same  time  are  stimulated  to  increased  activ- 
29 


45° 


TEXT-BOOK  OF  PHYSIOLOGY 


ity,  and  in  consequence  of  the  additional  volumes  of  blood  brought  to 
the  skin  a  larger  amount  of  sweat  is  secreted,  which  speedily  undergoes 
evaporation.  As  each  gram  of  water  for  its  evaporation  requires  0.582 
of  a  calorie,  it  is  evident  that  increased  secretion  of  sweat  favors  heat-dissipa- 
tion. The  nerve-centers  influencing  the  activity  of  the  sweat-glands  may  be 
stimulated  not  only  reflexly,  but  directly  by  an  excess  of  heat  in  the  blood. 
If,  however,  the  atmosphere  itself  possesses  a  high  percentage  of  moisture, 
evaporation  from  the  body  is  much  diminished  and  the  value  of  sweating  as 
a  means  of  lowering  the  body-temperature  is  much  impaired.  Evaporation 
is  hastened  by  air  in  motion.  Hastened  respiratory  movements  and  the 
dilatation  of  blood-vessels  of  the  respiratory  surface  also  increase  the  evapora- 
tion of  water  from  the  lungs  and  thus  occasion  a  greater  loss  of  heat. 

When  the  external  temperature  falls  as,  it  does  in  the  autumn  and  winter 
months,  there  is  a  decrease  in  the  physiologic  activity  of  the  skin  from  a 
contraction  of  the  blood-vessels,  a  diminution  of  the  blood-supply,  and  a 
cessation  in  the  secretion  of  sweat  due  to  increased  activity  of  the  general 
vaso-motor  center.  The  blood,  being  prevented  from  coming  to  the  surface, 
is  retained  in  the  deeper  portion  of  the  body,  and  in  consequence  the  con- 
ditions for  radiation  are  diminished.  Nevertheless  by  reason  of  the  in- 
crease in  the  difference  of  temperature  between  that  of  the  air  and  that  of  the 
body,  radiation  takes  place  to  a  greater  extent  than  during  the  summer 
months.  These  variations  in  the  cutaneous  circulation  in  response  to  varia- 
tions in  the  external  temperature  are  brought  about  by  the  vaso-motor 
nerve  mechanism;  and  as  they  take  place  with  extreme  promptness  heat- 
dissipation  and  heat-production  are  quickly  adjusted  and  the  mean  tem- 
perature maintained. 

Radiation  from  the  skin  is  modified  to  some  extent  by  clothing.  An 
excess  of  clothing  diminishes,  a  diminution  of  clothing  increases  radiation. 
The  quality  of  clothing  is  also  an  important  factor.  Wool  is  a  poor 
conductor  of  heat  but  a  good  absorber  and  retainer  of  rnoisture,  and  hence 
is  adapted  for  cold  weather.  Linen  and  cotton  possess  the  opposite  quali- 
ties, and  hence  are  adapted  for  warm  weather.  Radiation  from  the  skin  is 
somewhat  interfered  with  by  subcutaneous  fat,  the  extent  of  the  interference 
being  dependent  on  its  amount. 

Inasmuch  as  the  mean  temperature  of  the  body  remains  practically 
constant,  notwithstanding  seasonal  variations,  it  is  apparent  that  heat-dis- 
sipation must  be  exactly  balanced  by  heat-production.  Should  there 
be  any  want  of  correspondence  between  the  two  processes,  there  would 
arise  either  an  increase  or  a  decrease  in  the  mean  temperature.  As  both 
heat-production  and  heat-dissipation  are  variable  factors,  dependent  on  a 
variety  of  internal  and  external  conditions,  their  adjustment  is  accomplished 
by  a  complex  self-regulating  mechanism  involving  muscle,  vascular,  and 
secretor  elements,  coordinated  by  the  nerve  system. 

The  Influence  of  the  Nerve  System. — Even  though  it  is  admitted  that 
the  heat  liberated  is  the  result  of  the  metabolizing  power  of  muscles 
and  glands,  and  other  tissues  as  well,  and  even  though  it  is  further  admitted 
that  the  heat  liberated  is  increased  when  the  tissues  (muscles)  are  excited  to 
action  under  the  stimulating  influence  of  the  efferent  nerve  system,  never- 
theless the  question  has  been  raised  whether  in  addition,  there  are  not  special 
nerves  and  nerve-centers,  the  specific  function  of  which  is  to  stimulate 


ANIMAL  HEAT  451 

njetabolism  at  all  times  and  thus  provide  for  a  continuous  production  of  heat, 
the  amount  of  which  from  day  to  day,  varying  with  the  extent  of  the  losses. 
In  this  case  the  heat  liberated  by  muscles  at  the  time  of  their  contraction 
would  be  incidental  and  not  to  be  regarded  as  a  necessary  part  of  the  fun- 
damental heat-producing  mechanism. 

This  view  is  apparently  supported  by  the  increase  in  heat-production 
that  follows  injuries  or  lesions  of  certain  portions  of  the  nerve  system.  Thus 
injury  to  the  caudate  nucleus  or  to  the  tuber  cinereum,  in  an  animal  is  promptly 
followed  by  a  rise  in  the  body-temperature  of  from  four  to  about  seven  de- 
grees; while  lesions  of  the  cortex  of  the  animal  in  the  neighborhood  of  the 
cruciate  sulcus  and  at  the  junction  of  the  supra-sylvian  and  post-sylvian 
fissures  is  promptly  followed  by  a  fall  in  the  body-temperature.  These  and 
many  other  facts  have  led  to  the  formation  of  a  theory  as  to  the  manner  in 
which  heat-production  is  excited  and  regulated  by  the  nerve  system  in  ac- 
cordance with  variations  in  external  temperature.  In  this  theory  it  is 
assumed  that,  throughout  the  anterior  horns  of  the  gray  matter  of  the  spinal 
cord  there  are  nerve-cells  that  give  origin  to  nerve-fibers  that  pass  out  in  the 
ventral  roots  of  the  spinal  nerves,  to  be  distributed  in  company  with  them 
to  muscles  at  least,  and  excite  heat-production  even  though  the  muscles  are 
not  in  active  contraction.  These  centers  and  nerves  are  termed,  respectively, 
thermogenic  centers  and  thermogenic  nerves.  The  thermogenic  centers  it  is 
further  assumed,  though  continuously  active,  are  not  influenced  directly  by 
nerve  impulses  transmitted  to  them  from  the  skin  in  consequence  of  changes 
in  the  external  temperature  nor  by  changes  in  the  temperature  of  the  blood, 
but  are  influenced  by  nerve  impulses  descending  the  nerve  system  from 
higher  levels. 

Experimental  investigations  have  apparently  demonstrated  that  an  injury 
or  stimulation  of  the  caudate  nucleus  or  of  the  tuber  cinereum  is  very 
promptly  followed  by  an  increase  in  heat-production,  which  leads  to  the 
inference  that  there  are  in  these  regions  groups  of  nerve-cells  which  when 
stimulated  increase  heat-production.  To  this  group  of  cells  the  term  thermo- 
accelerator  has  been  given. 

Experimental  investigations  have  also  apparently  demonstrated  the  fact 
that  injuries  or  stimulation  of  the  cerebral  cortex  in  the  neighborhood  of  the 
cruciate  sulcus  and  at  the  junction  of  the  supra-sylvian  and  post-sylvian 
fissures  (dog)  is  promptly  followed  by  a  decrease  in  heat-production  which 
leads  to  the  inference  that  there  are  in  these  regins  groups  of  nerve-cells  stimu- 
lation of  which  decreases  heat-production.  To  this  group  of  cells  the  term 
thermo-inhihitor  has  been  given.  Both  these  centers  are  in  direct  connec- 
tion by  nerve-fibers  with  the  thermogenic  centers  in  the  spinal  cord  and 
through  which  they  alternately  accelerate  or  inhibit  their  activity. 

The  thermo-accelerator  and  thermo- inhibitor  centers  it  is  assumed  are 
both  connected  with  the  cutaneous  surface  through  the  intermediation  of 
afferent  nerves  and,  therefore,  in  a  position  to  be  influenced  by  changes  in 
the  external  temperature.  Evidence  is  also  at  hand  that  they  may  be  in- 
fluenced by  changes  in  the  temperature  of  the  blood  passing  to  and  around 
them.  A  fall  of  the  external  temperature  develops  nerve  impulses  which 
transmitted  to  the  thermo-accelerator  center  excite  it  to  action  and  thus 
increase  heat-production;  a  rise  in  external  temperature,  on  the  other  hand, 
has  an  opposite  eft'ect  by  stimulation  of  the  inhibitor  center. 


452  TEXT-BOOK  OF  PHYSIOLOGY 

The  regulation  and  maintenance  of  the  mean  temperature  or  thermotaxis 
is,  therefore,  the  result  of  an  adjustment  between  heat-production  and  heat- 
dissipation,  both  of  which  are  variable  factors,  and  is  accomplished  by  a 
complex,  self-regulatmg  mechanism  consisting  of  muscle,  vascular  and  secretor 
elements  coordinated  by  the  nerve  system.  Given  this  mechanism  and  the 
variations  in  the  external  temperature  the  metabolizing  power  of  the  living 
material  is  caused  to  vary  in  one  direction  or  another  as  the  external  tem- 
perature rises  and  falls. 


CHAPTER  XVII 

EXCRETION 

Excretion  may  be  defined  as  the  process  by  which  the  end-products  of 
tissue  metabohsm  are  removed  from  the  body,  the  nature  of  the  process,  how- 
ever, differing  in  no  essential  particulars  from  that  underlying  the  process 
of  secretion  as  stated  in  preceding  chapters.  The  histologic  structures 
involved  and  the  forces  at  work  being  of  the  same  general  character,  it  is 
impossible  to  draw  any  sharp  line  of  distinction  between  them.  As  a  general 
fact  it  may  be  stated,  that  in  their  composition  all  the  characteristic  ingredi- 
ents of  the  excretions  are  incapable  either  of  entering  into  the  formation  of 
tissue  or  of  undergoing  oxidation  for  the  purpose  of  heat-production.  As 
the  retention  of  these  end-products  in  the  body  would  exert  a  deleterious 
influence  on  normal  metabolism,  their  prompt  removal  becomes  essential  to 
the  maintenance  of  physiologic  activity.  The  principal  excretions  of  the 
body — urine,  perspiration,  and  bile — are,  with  the  exception  of  those  given 
off  in  the  lungs,  complex  fluids  in  which  are  to  be  found  in  varying  propor- 
tions the  chief  end-products  of  metabolism. 

THE  URINE 

The  urine  is  a  fluid  formed  by  the  activities  of  the  kidneys  and  character- 
ized by  well-marked  physical  properties  and  a  complex  chemical  composi- 
tion. After  its  formation  in  the  kidneys  it  passes  through  the  ureters  into 
the  bladder,  where  it  is  temporarily  retained  before  being  discharged  from 
the  body. 

Physical  Properties. — Normal  urine  has  a  pale  yellow  or  amber  color, 
an  aromatic  odor,  an  acid  reaction,  and  a  specific  gravity  of  1.020.  As  a 
rule,  it  is  perfectly  transparent,  though  its  transparency  may  be  diminished 
from  the  presence  of  mucus,  calcium  and  magnesium  phosphates,  and  mixed 
urates. 

The  color,  which  varies  within  physiologic  limits  from  a  pale  yellow  to  a 
reddish-brown,  is  due  to  the  presence  of  the  coloring-matters,  urochrome, 
'Urohilm,  and  uroerythrin,  all  of  which  are  derivatives  of  the  bile  pigments  ab- 
sorbed from  the  liver  or  the  alimentary  canal. 

The  odor  of  the  urine  is  characteristic  and  due  to  the  presence  of  volatile 
organic  compounds. 

The  reaction  of  the  urine  is  generally  acid  to  litmus.  The  reaction  has 
been  for  a  long  time  attributed  to  the  presence  of  sodium  dihydrogen  phos- 
phate, though  it  is  more  probably  due  to  the  presence  of  various  acid  radicals. 
The  intensity  of  the  acidity  will  depend  on  the  extent  to  which  any  particular 
acid  is  dissociated  with  the  liberation  of  the  hydrogen  ions  as  it  is  the  latter 
factor  that  imparts  acidity  to  the  fluid.  The  degree  of  acidity,  however, 
varies  at  different  periods  of  the  day.  Urine  passed  in  the  morning  is  strongly 
acid,  while  that  passed  during  and  after  digestion,  especially  if  the  food  be 

453 


454  TEXT-BOOK  OF  PHYSIOLOGY 

largely  vegetable  in  character  and  rich  in  alkaline  salts,  is  either  neutral  or 
alkaline  in  reaction.  The  diminished  acidity  after  meals  is  attributed  to  the 
formation  of  hydrochloric  acid  by  the  gastric  glands  and  the  conseqiient 
liberation  of  bases  which  are  excreted  in  the  urine.  The  phosphoric  acid 
which  enters  into  combination  with  sodium  and  potassium  bases  is  a  product 
of  tissue  metabolism. 

The  specific  gravity  is  about  1.020,  though  it  varies  from  1.015  to  1.025. 
It  will  diminish,  other  things  being  equal,  with  increased  consumption  of 
water  and  diminished  activity  of  the  skin;  it  will  be  increased  of  course  by 
the  opposite  conditions. 

The  quantity  of  urine  excreted  in  twenty-four  hours  varies  from  1200  to 
1700  c.c.  Amounts  both  above  and  below  these  are  frequently  passed  from 
a  variety  of  causes. 

Chemic  Composition. — The  chemic  composition  of  the  urine  is  very 
complex  and  is  determined  partly  by  the  metabolism  of  the  constituents  of 
the  tissues  and  partly  by  the  quantity  and  the  quality  of  the  food  consumed 
and  metabolized.  Hence  the  composition  will  vary  from  day  to  day  in 
accordance  with  the  character  of  the  food.  An  average  composition  is 
presented  in  the  following  table: 

THE  CHEMIC  COMPOSITION  OF  URINE 

Water 1500.00  c.c. 

Total  solids 72  .00  grams. 

Urea 33  -18  grams. 

Uric  acid  (urates) o  •  55  grams. 

Hippuric  acid  (hippurates) 0.40  grams. 

Kreatinin,    xanthin,    hypoxanthin,    guanin,    ammonium ) 

salts,  pigment,  etc j- 11 .21  grams. 

Inorganic  salts;  sodium  and  potassium  sulphates,  phos- 
phates, and  chlorids;  magnesium  and  calcium  phos- 
phates         >■  2  7  .  00  grams. 

Organic  salts:  lactates,  acetates,  formates  in  small 
amounts 

Sugar a     trace 

Gases,  nitrogen,  and  carbonic  acid. 

The  Total  Solids. — It  is  frequently  a  matter  of  clinic  interest  to  deter- 
mine the  total  amount  of  the  solid  constituents  excreted  in  twenty-four  hours. 
This  may  be  attained  approximately  by  multiplying  the  last  two  figures  of 
the  specific  gravity  by  the  coefficient  2.33  of  Haeser  or  Christison.  The 
coefficient  of  Jones,  2.6,  is  believed  by  some  observers  to  give  more  accurate 
results  for  conditions  existing  in  this  country.  The  result  expresses  the  total 
solids  in  1000  parts:  e.g.,  urine  with  a  specific  gravity  of  1.020  would  contain 
20  X  2.33,  or  46.60  grams  of  solid  matter  per  1000  c.c.  If  the  amount 
passed  in  twenty-four  hours  be  1500  c.c,  the  total  solids  would  amount  to 
69.9  grams  daily. 

The  Water  of  the  Urine. — The  amount  of  urinary  water  and  its  ratio 
to  the  solid  constituents  will  vary  with  the  amount  consumed  and  the  activity 
of  the  skin  and  lungs.  In  summer  the  foods,  liquid  and  solid,  remaining  the 
same,  the  quantity  of  water  in  the  urine  is  diminished  in  consequence  of 
increased  activity  of  skin  and  lungs  and  the  ratio  of  water  to  solids  decreased. 
In  winter  the  reverse  conditions  obtain.  The  food  remaining  the  same,  the 
consumption  of  large  quantities  of  water  hastens  at  least  the  removal  of  end- 
products  from  the  tissues  and  thus  increases  the  urinary  solids. 


EXCRETION  455 

Urea. — Urea  is  the  most  abundant  of  the  organic  constituents  of  the 
urine  and  is  present  to  the  extent  of  from  2  to  3  per  cent.  It  is  a  colorless 
neutral  substance,  crystallizing  under  varying  conditions  in  long  silky  needles 
or  in  rhombic  prisms.  It  is  soluble  in  water  and  alcohol.  It  is  composed  of 
CONjH^.  When  subjected  to  prolonged  boiling,  it  combines  with  water, 
giving  rise  to  ammonium  carbonate.  The  presence  of  Micrococcus  urea 
in  urine  will  also  convert  the  urea,  by  combining  it  with  two  molecules  of 
water,  into  ammonium  carbonate,  C0N2H^  +  2H20  =  (NHJ2C03. 

The  amount  of  urea  excreted  each  day  varies  from  30  to  40  grams,  the 
average  being  about  34  grams  and  therefore  represents  an  amount  of 
protein  metabolized  equivalent  to  from  90  to  120  grams  or  an  average 
of  about  100  grams.  The  remaining  nitrogen-holding  compounds  in  the 
urine  represent  as  shown  by  their  nitrogen  content  a  protein  metabolism 
of  about  12  grams.  As  to  how  much  of  the  urea  or  of  the  total  nitrogen  is 
derived  from  the  metabolism  of  tissue  protein  and  how  much  from  the 
metabolism  of  the  food  protein  that  is  not  elaborated  into  tissue  protein, 
is  difficult  to  state.  It  has  been  observed  however  in  human  beings  in  the 
fasting  condition  that  for  a  period  of  10  days,  there  is  a  daily  excretion 
of  about  21  grams  of  urea  equivalent  to  about  63  grams  of  protein  metabo- 
lized. If  it  be  accepted  that  approximately  63  grams  of  tissue  protein  are 
metabolized  each  day  then  of  the  34  grams  of  urea  excreted,  13  grams  must 
come  from  about  40  grams  of  metabolized  food  protein.  That  the  urea  that 
comes  from  the  tissue  protein  is  a  rather  constant  factor  and  that  the  urea 
that  comes  from  the  food  protein  is  a  variable  factor  is  shown  by  the  fact  that 
the  amount  of  urea  excreted  rises  and  falls  proportionately  to  the  protein  con- 
sumed. As  to  the  particular  tissues  that  are  undergoing  protein  metabolism 
there  is  much  obscurity.  Contrary  to  what  might  be  expected  there  is  ap- 
parently but  little  protein  metabolism  in  muscle  tissue  for  there  is  no  parallel 
ism  between  urea  production  and  muscle  work.  Even  after  severe  labor 
extending  over  a  period  of  some  hours  there  is  no  noticeable  increase  in  the 
urea  excreted. 

Seat  of  Formation  and  Antecedents  of  Urea. — It  has  been  stated  in.  a 
foregoing  paragraph  that  the  excretory  organs  are  engaged  in  the  process  of 
eliminating  from  the  blood,  rather  than  in  elaborating,  the  end-products  of 
metabolism.  Therefore  the  supposition  is  that  the  kidneys  are  not  the  seat 
of  urea  formation  but  only  the  means  by  which  it  is  eliminated  from  the 
blood.  This  supposition  is  rendered  highly  probable  from  the  following 
facts:  the  blood  of  the  renal  artery  contains  from  one-third  to  one-half 
more  urea  than  the  blood  of  the  renal  vein;  ligation  of  the  renal  arteries 
or  removal  of  the  kidneys  leads  to  an  accumulation  of  urea  in  the  blood  to  an 
extent  four  times  the  normal  amount  in  24  hours;  perfusion  of  the  excised 
kidney,  which  still  retains  its  physiologic  activity,  with  blood  containing 
known  antecedents  of  urea  is  unattended  with  urea  formation.  These  and 
other  facts  of  a  similar  character  confirm  the  view  that  the  kidney  does  not 
manufacture  but  simply  excretes  urea  brought  to  it  by  the  blood.  Since 
urea  is  always  present  in  the  blood  to  an  extent  of  from  0.04  per  cent,  to 
0.06  per  cent.,  i.e.^  from  4  to  6  grams  per  10,000  grams,  and  that  it  is  being  ex- 
creted at  the  rate  of  about  1.5  grams  per  hour,  it  is  evident  that  it  is  being  as 
constantly  formed  in  some  one  or  more  organs,  and  discharged  into  the 
blood.  • 


4S6  TEXT-BOOK  OF  PHYSIOLOGY 

The  experimental  evidence  now  at  hand  indicates  the  Hver  as  the  chief 
organ  engaged  in  this  process.  The  following  facts  support  this  view,  viz, : 
destructive  diseases  of  the  liver,  e.g.,  acute  yellow  atrophy,  interstitial  hepa- 
titis, and  suppuration,  largely  diminish  the  production  of  urea  but  increase  the 
amount  of  the  ammonium  salts  in  the  urine;  the  establishment  of  an  Eck 
fistula  (the  union  of  the  portal  vein  with  the  ascending  vena  cava  whereby 
the  liver  is  almost  entirely  excluded  from  receiving  compounds  absorbed 
from  the  intestine)  is  followed  by  a  decrease  in  the  production  of  urea  and 
an  increase  in  the  ammonium  content  of  the  urine;  the  perfusion  of  the 
liver  of  a  recently  killed  animal  with  a  given  amount  of  blood  containing 
ammonium  salts  will  be  followed  after  the  lapse  of  several  hours  by  an  amount 
of  urea  in  the  blood  two  or  three  times  the  normal  quantity.  These  and 
other  facts  indicate  that  the  chief  seat  of  urea  formation  is  to  be  found  in 
the  liver  cells. 

The  antecedents  of  urea,  out  of  which  the  hepatic  cells  construct  urea 
have,  for  chemic  reasons  as  well  as  from  the  foregoing  experimental  results, 
been  shown  to  be  the  salts  of  ammonia  the  carbonate,  carbamate,  and 
lactate.  The  increase  in  the  ammonia  of  the  urine  simultaneously  with 
the  decrease  in  the  urea  renders  it  extremely  probable  that  these  salts  are 
antecedents  of  urea  and  that  the  transformation  takes  place  in  the  liver 
cells.  The  chemic  change  that  takes  place  is  simply  the  abstraction  of  two 
molecules  of  water  as  shown  in  the  following  formula: 

(NHJ2C03-2H20  =  C0N2H,. 

The  source  of  the  ammonia  is  probably  in  part  the  intestine,  as  this 
compound  is  one  of  the  products  of  the  hydrolysis  and  cleavage  of  the  proteins 
during  digestion.  That  this  is  the  case  is  apparent  from  the  fact  that  the 
blood  of  the  portal  vein  always  contains  more  ammonia  that  the  blood  of 
any  other  region  of  the  vascular  apparatus.  The  advantage  to  the  body 
that  results  from  the  conversion  of  ammonia  to  urea  is  that  it  prevents  an 
ammonia  intoxication  with  its  attendant  evils  that  would  otherwise  arise. 

It  will  be  remembered  that  the  amino-acids,  as  tyrosin,  leucin,  glutamic, 
and  aspartic  acids,  diamino-acids  and  bases,  as  lysin,  arginin,  histidin  which 
are  also  products  of  the  hydrolysis  of  proteins  during  digestion  are  capable 
of  being  absorbed  as  such  by  the  epithelial  cells  of  the  villi  and  mucous  mem- 
brane. After  absorption  they  are  transmitted  by  the  blood  to  the  tissues  of 
the  body  generally  by  which  they  are  directly  utilized  in  the  formation  of 
tissue  protein  or  perhaps  stored  for  future  use,  for  there  is  evidence  for  the 
belief  that  tissues  have  a  capacity  for  the  storage  of  amino-acids,  which  in 
the  case  of  muscles  amounts  to  from  70  to  80  milligrams  per  100  grams. 
After  saturation  of  the  tissues,  so  to  speak,  with  amino-acids,  there  yet  re- 
mains a  certain  percentage  in  the  blood  which  undergoes  a  cleavage  into  an 
NH2  portion  and  an  organic  portion;  the  former  is  then  converted  to  am- 
monia and  subsequently  to  urea  by  the  liver  cells,  the  latter,  the  organic 
portion,  contributes  to  the  production  of  fat  or  sugar,  which  in  due  time  is 
oxidized  and  thus  contributes  to  the  store  of  body-heat. 

From  the  foregoing  facts  it  is  evident  that  given  the  presence  of  ammonia 
salts  in  the  blood  of  the  portal  vein  the  appearance  of  urea  in  the  urine  is 
readily  accounted  for.  There  is  also  evidence  for  the  belief  that  in  the 
muscles  and  perhaps  »ther  tissues  as  well,  amino-acids,  those  resulting  from 


EXCRETION  457 

the  metabolism  of  tissue  protein,  as  well  as  those  stored,  undergo  a  similar 
chemic  change  with  the  formation  of  urea.  This  being  the  case,  the  muscles 
must  be  regarded  as  a  seat  of  urea  formation. 

Uric  Acid. — Uric  acid  is  one  of  the  constant  ingredients  of  the  urine. 
It  is  a  crystalline  nitrogen-holding  body  closely  resembling  urea,  its  formula 
being  CsH^N^Og.  The  total  quantity  excreted  daily  varies  from  0.2  to  i 
gram.  It  is  doubtful  if  uric  acid  exists  in  a  free  state  in  the  urine,  the  indi- 
cations being  that  it  is  combined  with  sodium  and  potassium  in  the  form  of  a 
quadriurate.  The  urates  when  in  excess  are  frequently  deposited  from  the 
urine  as  a  brick-red  sediment,  the  color  being  due  to  their  combination  with 
the  coloring-matter  uroerythrin.  When  pure,  uric  acid  crystallizes  in  the 
rhombic  form,  though  it  assumes  a  variety  of  forms.  Uric  acid  was  long 
regarded  as  a  product  of  general  protein  metabolism.  This  view  has  been 
abandoned.  At  present  it  is  believed  that  it  is  a  cleavage  product  of  nuclein,  a 
constituent  of  all  cell  nuclei.  In  the  metabolism  of  nuclein  a  protein  and 
nucleic  acid  are  formed,  from  the  latter  of  which  uric  acid  is  derived.  Nu- 
cleic acid  when  decomposed  yields  bases,  such  as  guanin,  adenin,  etc.,  which 
under  the  action  of  the  enzymes  guanase  'dndadenase  are  combined  with  water, 
deaminized,  and  converted  into  xanthin  and  hypoxanthin.  Because  of  the 
fact  that  these  bodies  can  also  be  obtained  from  a  synthesized  body  termed 
purin  they  are  known  collectively  as  the  purin  bases.  Though  there  is  a  close 
relationship  between  uric  acid  and  the  purin  bases,  it  has  been  impossible 
experimentally  to  derive  one  from  the  other.  When  hypoxanthin,  however, 
is  given  internally  it  is  oxidized  and  converted  into  uric  acid.  It  is  ex- 
tremely probable,  therefore,  that  uric  acid  is  an  oxidation  product  of  one  or 
more  of  the  purin  bases.    - 

It  is  probable,  however,  that  not  all  of  the  uric  acid  eliminated  is  derived 
from  the  nuclein  of  tissue  cells  and  their  decomposition  products,  the 
purin  bases.  Some  of  it  is  undoubtedly  derived  from  the  nucleins  contained 
in  foods.  The  uric  acid  eliminated  is  therefore  partly  endogenous  and 
partly  exogenous  in  origin. 

There  is  some  evidence  that  not  all  the  uric  acid  produced  in  the  body  is 
excreted  as  such,  but,  that  a  portion,  perhaps  one-half,  is  changed  to  urea. 

Adenin,  Guanin,  Xanthin,  Hypoxanthin. — These  compounds  are  also 
found  in  urine  in  small  but  variable  amounts.  They  are  nitrogenized  com- 
pounds derived  mainly  from  the  metabolism  of  the  nuclein  bodies. 

Kreatinin. — This  is  a  crystalline  nitrogenous  compound  closely  resem- 
bling kreatin,  one  of  the  constituents  of  muscle-tissue.  The  amount  excreted 
daily  is  about  i  gram.  The  origin  of  kreatinin  is  not  very  clear.  It  is 
probable,  however,  that  if  kreatin  is  capable  of  transformation  into  kreatinin 
a  certain  portion  is  derived  from  the  kreatin  contained  in  the  meat  con- 
sumed as  food.  But  as  kreatinin  is  steadily  excreted  though  in  less  amounts 
on  a  diet  from  which  meat  is  excluded  it  is  certain  that  this  portion  at  least 
must  have  some  other  source  containing  nitrogen,  and  the  inference  is  that 
it  is  one  of  the  end-products  of  the  protein  metabolism  that  is  taking 
places  in  tissues  generally  and  more  particularly  in  muscle-tissue. 

Hippuric  Acid. — ^This  acid  in  combination  with  sodium  and  potassium 
is  very  generally  present  in  urine,  though  in  small  amounts.  It  is  more 
abundant  in  the  urine  of  the  herbivora  than  the  carnivora.  In  man  the 
amount  excreted  daily  is  about  0.7  gram,  though  the  amount  may  be  raised 


458  TEXT-BOOK  OF  PHYSIOLOGY 

by  a  diet  of  asparagus,  plums,  cranberries,  etc.,  and  by  the  administration  of 
benzoic  and  cinnamic  acids.  There  is  evidence  that  hippuric  acid  is  formed 
in  the  kidney  from  benzoic  acid,  its  precursors,  or  related  bodies.  Various 
compounds  of  this  class  are  found  in  vegetable  foods,  a  fact  which  may  ac- 
count for  the  increase  in  the  excretion  of  hippuric  acid  on  a  vegetable  diet. 

Indol,  Skatol,  Phenol,  CresoL— These  compounds,  products  of  the 
putrefactive  changes  in  the  derivatives  of  protein  are  present  in  variable 
amounts,  associated  with  potassium  sulphate  (see  page  205).  These  com- 
pounds are  known  as  the  ethereal  sulphates.  The  extent  to  which  they  are 
present  is  taken  as  a  measure  of  the  extent  of  intestinal  putrefaction;  their 
presence  can  be  determined  by  various  tests.  Of  these  compounds  the  one 
generally  tested  for  is  potassium  indoxyl  sulphate  or  indican.  If  hydro- 
chloric acid  and  a  small  quantity  of  sodium  hypochlorite  be  added  to  sus- 
pected urine,  together  with  a  few  cubic  centimeters  of  chloroform,  the  indican 
if  present  will  be  separated  by  the  acid  into  indoxyl  and  potassium  sulphate. 
The  former  compound  will  then  be  oxidized  by  the  oxygen  set  free  from  the 
sodium  hypochlorite  and  form  indigo  blue.  The  chloroform  will  absorb  the 
indigo  blue  as  fast  as  formed  and  when  the  reaction  is  completed  will  accumu- 
late at  the  bottom  of  the  test-tube.  The  depth  of  the  color  is  indicative  of 
the  quantity  present  and  of  the  extent  of  the  intestinal  putrefaction. 

Inorganic  Salts. — Sodium  and  potassium  phosphates,  known  as  the 
alkaline  phosphates,  are  found  in  both  blood  and  urine.  The  total  quantity 
excreted  daily  is  about  4  grams.  Calcium  and  magnesium  phosphates, 
known  as  the  earthy  phosphates,  are  present  to  the  extent  of  i  gram.  Though 
insoluble  in  water,  they  are  held  in  solution  in  the  urine  by  its  acid  constitu- 
ents. If  the  urine  be  rendered  alkaline,  they  are  at  once  precipitated. 
Sodium  and  potassium  sulphates  are  also  present  to  the  extent  of  about  2 
grams.  The  phosphoric  and  sulphuric  acids  which  are  combined  with  these 
bases  enter  the  body  for  the  most  part  in  the  foods,  though  there  is  evidence 
that  they  also  arise  by  oxidation  in  consequence  of  the  metabolism  of  proteins 
which  contain  phosphorus  and  sulphur.  Sodium  chlorid  is  the  most 
abundant  of  the  inorganic  salts.  It  is  derived  mainly  from  the  food.  The 
amount  excreted  is  about  15  grams  in  twenty-four  hours. 

THE  KIDNEYS 

The  kidneys  are  the  organs  engaged  in  the  excretion  of  the  urinary 
constituents  from  the  blood.  Each  kidney  resembles  a  bean  in  shape,  is 
from  10  to  12  centimeters  in  length,  2  centimeters  in  breath,  and  weighs  from 
144  to  170  grams.  These  organs  are  situated  in  the  lumbar  region,  one  on 
each  side  of  the  vertebral  column  behind  the  peritoneum,  and  extend  from 
the  eleventh  rib  to  the  crest  of  the  ilium.  The  anterior  surface  is  convex, 
the  posterior  surface  concave.  The  latter  presents  a  deep  notch — thehilum. 
The  kidney  is  surrounded  by  a  thin  smooth  membrane  composed  of  white 
fibrous  and  yellow  elastic  tissue;  though  it  is  attached  to  the  surface  of  the 
kidney  by  minute  processes  of  connective  tissue,  it  can  very  readily  be  torn 
away.     The  substance  of  the  kidney  is  dense  but  friable. 

Upon  making  a  longitudinal  section  of  the  kidney  it  will  be  observed  that 
the  hilum  extends  into  the  interior  of  the  organ  and  expands  to  form  a 
cavity  known  as  the  sinus,  in  which  are  found  the  blood-vessels,  nerves,  and 


EXCRETION 


459 


duct  (Fig.  205).  This  cavity  is  mainly  occupied  by  the  upper  part  of  the 
renal  duct,  the  ureter,  the  interior  of  which  is  termed  the  pelvis.  The  ureter 
divides  into  several  portions  which  terminate  in  small  caps  or  calyces  which 
receive  the  apices  of  the  pyramids.  The  parenchyma  of  the  kidney  consists 
of  two  portions,  viz. : 

1.  An  internal  or  medullary  portion,  consisting  of  a  series  of  pyramids  or 

cones,  some  twelve  or  fifteen  in  number,  which  present  a  distinctly 
striated  appearance. 

2.  An  external  or  cortical  por- 

tion,   half    an    inch    in 
thickness  and  distinctly 
friable  in  character. 
A  histologic   analysis  of 
the   kidney  shows  that  it  is 
composed   of   a    connective- 
tissue  framework  supporting 
secreting  tubules,  blood-ves- 
sels, lymphatics  and  nerves, 
all  of  which  are  directly  con- 
nected  with  the  removal  of 
the  urinary  constituents  from 
the  blood. 

The  Tubules  of  the  Kid- 
ney.— ^From  its  structure  it 
is  apparent  that  the  kidney 
is  a  compound  tubular  gland, 
the  orifices  of  exit  being  di- 
rected toward  the  pelvis.  If 
the  apex  of  each  pyramid  be 
examined  with  a  lens,  it  will 
a  number  of  small 
which  may  be  re- 
as  the  beginnings 
uriniferous   tubules. 


present 

orifices 

garded 

of    the    urmnerous    lUOUies.  ^ig.  205.  — Longitudinal   Section   through  the 

From   this  point  the  tubules    Kidney,  the  Pelvis  of  the  Kidney,  and  a  Number 

pass  outward  in  a  Straight  but    ^^    ^^-^^    Calyces.     A.    Branch  of   the   renal   arteiy, 

somewhat  diverging  manner 
toward  the  cortex,  giving  off 
at  acute  angles  a  number  of 
branches  (Fig.  206).  From 
the  apex  to  the  base  of  the 
pyramids  they  are  known  as 
the  tubules  of  Bellini.  In  the  cortical  portion  of  the  kidney  the  tubule  be- 
comes enlarged  and  twisted,  and,  after  pursuing  an  extremely  convoluted 
course,  turns  backward  into  the  medullary  portion  for  some  distance,  form- 
ing the  ascending  limb  of  Henle's  loop;  it  then  turns  upon  itself,  forming 
the  descending  limb  of  the  loop,  reenters  the  cortex,  again  expands  and  be- 
comes convoluted,  and  finally  terminates  in  an  ovoid  invaginated  enlarge- 
ment known  as  Miiller's  or  Bowman's  capsule,  which  receives  a  small  tuft 
of  blood-vessels — the  glomerulus.     Each  tubule  consists  of  a  basement  mem- 


Renal  Calyces.  A. 
U.  Ureter.  C.  Renal  calyx,  i.  Cortex,  i'.  Medullary 
rays.  i".  Labyrinth,  or  cortex  proper.  2.  Medulla. 
2'.  Papillary  portion  of  medulla,  or  medulla  proper. 
2".  Border  layer  of  the  medulla.  3,  3.  Transverse  sec- 
tion through  the  axes  of  the  tubules  of  the  border  layer. 
4.  Fat  of  the  renal  sinus.  5,  5.  Arterial  branches. 
*.  Transversely  coursing  medulla  rays. — (Tyson,  after 
Henlc.) 


46o 


TEXT-BOOK  OF  PHYSIOLOGY 


brane  lined  throughout  its  entire  extent  by  epithelial  cells.  The  epithelium 
as  well  as  the  tubule  varies  in  shape  and  size  in  different  parts  of  its  course. 
In  the  capsule  the  epithelium  is  flattened,  lining  not  only  the  inner  surface 


Connecting 
piece. 

Intercalated 
Piece. 


Convoluted 
tubule. 


Renal  corpuscle 


Intercalated 
piece. 


Thick 


.Thin 
division  of  thL 
loop  of  Henle, 


Collecting    -   - 
tubule. 


Papillary  duct. 


Lobule. 


Lobule. 


Tunica  albuginea. 


— —  Stellate  vein. 


Arciform  artery 


Arcitorm  vein. 


— Interlobar  artery. 
Interlobar  vein. 


Fig.  206. — Scheme  of  the  Course  of  the  Uriniferous  Tubules  and  the  Renal 

Vessels.     {Slohr.) 

of  the  capsule  but  reflected  over  the  blood-vessels  as  well.  This  is  known 
as  the  glomerular  epithelium.  In  the  convoluted  portions  of  the  tubules  the 
epithelium  is  cuboid,  granular,  and  somewhat  striated;  in  Henle's  loop  it  is 
more  or  less  flattened. 


EXCRETION 


461 


The  Blood-vessels  of  the  Kidney. — The  renal  artery  enters  the  kidney 
at  the  hilum  behind  the  ureter;  it  soon  divides  into  several  large  branches 
which  penetrate  the  substance  of  the  kidney  between  the  pyramids  and  pass 
outward  into  the  cortex.  At  the  base  of  the  pyramids  branches  of  the 
arteries  form  an  anastomosing  plexus.  From  this  plexus  vessels  are  given 
off,  some  of  which  follow  the  straight  tubules  toward  the  apex  of  the  pyramids, 
vasa  recta,  while  others  enter  the  cortex  and  pass  to  its  surface  (Fig.  218). 
In  the  course  of  the  latter,  small  branches  are  given  off,  each  of  which  soon 
divides  and  subdivides  to  form  a  ball  of  capillary  vessels  known  as  the  glomer- 
ulus. These  capillaries,  however,  do  not  anastomose,  but  soon  reunite  to 
form  an  efferent  vessel  the  caliber  of  which  is  less  than  that  of  the  afferent 
artery.  In  consequence  of  this,  there  is  a  greater  resistance  to  the  outflow 
of  blood  than  to  the  inflow,  and  therefore  a  higher  blood-pressure  in  the 
glomerulus  than  in  capillaries  generally.  The  relation  of  the  glomerulus  to 
the  tubule  is  important  from  a  physiologic  point  of  view.  As  stated  above, 
the  glomerulus  is  received  into  and  surrounded  by  the  terminal  expansion 
or  capsule  of  the  tubule.  This  capsule,  formed  by  an  invagination  of  the 
terminal  portion  of  the  tubule,  consists  of  two  walls,  an  outer  one  consisting 
of  an  extremely  thin  basement  membrane,  covered  by  flattened  epithelial 
cells,  and  an  inner  one  consisting  apparently  only  of  flattened  epithelium 
which  is  reflected  over  and  closely  invests  the  glomerular  blood-vessels 
(Fig.  207).  The  blood  is  thus  separated  from  the  interior  of  the  capsule 
by  the  epithelial  wall  of  the  capillary  and  the  epithelium  of  the  reflected  wall 
of  the  capsule.  During  the  periods 
of  secretor  activity  the  blood-vessels 
of  the  glomerulus  are  filled  with  blood 
to  such  an  extent  that  the.  sac  cavity 
is  almost  obliterated.  After  its  exit 
from  the  capsule  the  efferent  vessel 
of  the  glomerulus  soon  again  divides 
and  subdivides  to  form  an  elaborate 
capillary  plexus  which  surrounds  and 
closely  invests  the  convoluted  tubules. 
From  this  plexus  as  well  as  from  the 
plexus  which  surrounds  the  straight 
tubules  veins  arise  which  pass  toward 
and  empty  into  the  interlobular  veins 
as ;  well  as  the  veins  at  the  base 
of  the  pyramids.  The  renal  veins 
formed  by  the  union  of  these  latter 
veins  emerges  from  the  kidney  at  the 
hilum  and  finally  empties  into  the 
vena  cava  inferior. 

The  Nerves  of  the  Kidney. — 
The  nerves  distributed  to,  and  associated  wdth  the  functional  activities  of 
the  kidney  consist  of  both  pre-  and  post-ganglionic  fibers.  The  latter  have 
their  origin  in  the  cells  of  small  ganglia  situated  close  to  the  semilunar 
ganglion.  From  their  origin  they  pass  through  the  renal  plexus  and  follow 
the  course  of  the  blood-vessels  to  their  termination.  The  former,  the  pre- 
ganglionic nerves,  have  their  origin  in  the  lower  portion  of  the  thoracic  region 


Fig.  207. — Scheme  of  the  Renal  or  Mal- 
PiGHiAX  Corpuscle,  i.  Interlobular  artery. 
2.  Afferent  vessel.  3.  Efferent  vessel.  4.  Outer 
wall.  5.  Inner  wall.  6.  Glomerulus.  7.  Neck 
of  tubule. — iStohr.) 


462  TEXT-BOOK  OF  PHYSIOLOGY 

of  the  spinal  cord  and  pass  forward  in  the  small  splanchnic  nerves.  Experi- 
ment has  shown  that  these  nerve-fibers  have  both  vaso-constrictor  and 
vaso-dilatator  functions.  The  presence  of  specific  secretor  fibers  for  the 
epithelium  has  not  been  satisfactorily  demonstrated. 

The  Renal  Duct. — ^The  renal  duct,  the  ureter,  is  a  musculo-membranous 
tube  about  5  mm.  in  diameter  when  distended,  30  cm.  in  length,  and  extends 
from  the  hilum  to  the  base  of  the  bladder  into  which  it  empties.  The  upper 
extremity  is  expanded  and  within  the  renal  sinus  becomes  irregularly 
branched,  giving  rise  to  a  number  of  short  tubes,  called  calyces,  each  of 
which  embraces  the  apex  of  a  Malpighian  pyramid.  The  interior  of  the 
expanded  portion  of  the  ureter  is  known  as  the  pelvis.  The  wall  of  the 
ureter  consists  of  a  mucous  membrane,  a  muscle  coat,  and  an  external 
fibrous  investment. 

MECHANISM  OF  URINE  SECRETION 

The  physiologic  mechanism  concerned  in  the  secretion  of  the  urine,  re- 
duced to  its  simplest  terms,  consists  of  (i)  the  afferent  vessel,  the  glomeru- 
lus, the  efferent  vessel,  the  capillaries  and  the  veins;  (2)  the  glomerular  cap- 
sule and  the  epithelium  of  the  tubule  throughout  its  entire  extent.     Fig.  208. 

The  secretion  of  urine  by  this  mechanism  is  a  complex  process  and 
susceptible  of  several  interpretations.  It  was  originally  inferred  by  Bowman, 
an  inference  based  on  the  histologic  structure  of  the  blood-vessels  and  tubules 
and  their  relation  one  to  the  other,  that  there  is  here  present  (i)  an  ap- 
paratus for  filtration,  the  capsule  with  its  enclosed  glomerulus,  and  (2)  an 
apparatus  for  secretion,  the  tubule  with  its  epithelium,  and  therefore  the 
elimination  of  the  urinary  constituents  from  the  blood  is  accomplished  by  the 
two  processes  of  filtration  and  secretion;  that  the  water  and  highly  diffusible 
inorganic  salts  simply  pass  by  filtration,  under  pressure,  through  the  walls  of 
the  glomerular  capillaries,  while  the  organic  constituents  are  removed  by  the 
epithelium  lining  the  tubules;  that  the  water,  descending  the  tubules  from  the 
capsules,  plays  the  part  of  a  solvent  for  the  crystallizable  compounds  secreted 
by  the  epithelium,  in  consequence  of  which  they  are  the  more  readily  carried 
by  the  moving  fluid  into  the  pelvis  of  the  kidney. 

Ludwig,  influenced  largely  by  the  facts  of  blood-pressure,  advanced  the 
view  a  few  years  later  that  the  factors  concerned  in  the  secretion  of  urine 
are  purely  physical;  that  in  consequence  of  the  high  pressure  in  the  vessels  of 
the  glomeruli,  due  to  the  high  pressure  in  the  renal  artery  on  the  one  hand, 
and  to  the  resistance  offered  by  the  smaller  efferent  vessel  on  the  other  hand, 
all  the  urinary  constituents  are  filtered  off  in  a  state  of  extreme  dilution. 
In  order  to  account  for  the  higher  percentage  of  the  organic  constituents  in 
the  urine  than  m.  the  blood,  it  was  assumed  that  as  the  dilute  urine  passed 
through  the  tubules  the  water  and  possibly  other  substances  as  well,  that  were 
necessary  for  the  nutrition  of  the  body,  were  partly  reabsorbed,  passing  by 
diffusion  into  the  lymph  and  blood  until  the  urine  acquired  its  normal 
characteristics  and  degree  of  concentration.  In  support  of  this  view,  a  large 
number  of  facts  relating  to  the  influence  of  an  increase  and  a  decrease  of 
pressure  in  the  blood-vessels  of  the  glomeruli,  the  velocity  of  the  blood-stream, 
etc.,  in  determining  the  rate  of  urinary  flow  were  adduced,  all  of  which 
apparently  indicated  that  the  former  stood  to  the  latter  in  the  relation  of 


EXCRETION 


463 


cause  and  effect,  and  that  the  formation  of  urine  was  accomplished  entirely 
by  purely  physical  forces. 

The  progress  of  physiologic  investigation,  however,  has  thrown  some 
doubt  on  the  validity  of  this  physical  interpretation. 

Heidenhain  brought  forward  a  series  of  facts  which  supported  the  view, 
that  all  the  constituents  of  the  urine  are  eliminated  from  the  blood  by  a  pro- 
cess of  secretion  on  the  part  of  the  epithelial  cells  both  of  the  glomerulus  and 
the  tubules;  that  these  cells  have  the  power  of  selection  in  removing  from  the 
blood  normal  diffusible  constituents  when  in  excess,  and  abnormal  con- 
stituents, i.e.,  urea,  uric  acid,  etc.,  when  present  in  any  amount,  recognizing 
at  the  same  time  that  the  secretor  activity  of  these  cells  is  modified  in  one 
direction  or  another  by  the  blood-pressure  and  the  variations  which  it  under- 


IV  m 


Fig.  208. — The  Mechanism  for  the  Secretion  of  the  Urine.  I  A,  interlobular  artery; 
av,  afferent  vessel;  ev,  efferent  vessel;  c,  capillary  vessels;  v,  vein;  IV,  interlobular  vein;  ct, 
contorted  tubule. 

goes  from  moment  to  moment.  In  other  words  that  the  secretion  of  urine  is 
a  physiologic  or  vital  process,  rather  than  a  physical  or  mechanical  process. 
As  evidence  that  the  cells  of  the  tubules  possess  a  selective  power,  he  pre- 
sented the  following  experiment:  The  spinal  cord  of  an  animal  is  divided  in 
the  neck  for  the  purpose  of  lowering  the  blood-pressure  in  the  kidney  below 
the  pressure  at  which  the  urine  is  secreted.  Five  to  twenty  c.c.  of  a  satur- 
ated solution  of  indigo-carmine  are  injected  into  the  blood-vessels;  after 
intervals  varying  from  ten  minutes  to  one  hour  the  animal  is  killed,  the 
blood-vessles  washed  out  with  alcohol  for  the  purpose  of  precipitating  the 
indigo-carmine  in  situ.  Section  of  the  kidney  shows  a  uniform  blue  stain 
of  the  cortex  alone  (Fig.  209).  Microscopic  examination  reveals  the  fact 
that  the  blue  stain  is  due  to  the  deposition  of  the  pigment  in  the  lumen  and 
in  the  lumen  border  of  the  cells  of  the  convoluted  tubules  (Fig.  210)  and 


464 


TEXT-BOOK  OF  PHYSIOLOGY 


the  ascending  limb  of  Henle's  loop;  while  the  epithelium  of  Bowman's  cap- 
sule as  well  as  the  glomerular  epithelium  present  no  evidence  of  pigmentation. 
The  physiologic  action  of  the  cells  of  the  convoluted  tubules  in  elimination 
of  indigo-carmine,  is  supposed  to  indicate  their  action  in  the  elimination  of 
urea  and  other  nitrogen-holding  compounds.  The  absence  of  the  pigment 
from  the  glomerular  epithelium  lends  support  to  the  view,  that  its  function 
is  the  elimination  of  water  and  highly  diffusible  inorganic  salts. 

Another  experiment  which  apparently  shows  the  action  of  the  epithelium 
is  the  following:  A  solution  of  uric  acid  in  piperazin  is  injected  into  the 
blood  of  a  rabbit.  At  the  end  of  from  twenty  to  sixty  minutes  the  animal  is 
killed.  The  kidney  is  sectioned  and  examined  microscopically,  whereupon 
it  is  found  that  crystals  of  uric  acid  are  present  in  the  epithehal  cells  of  the  con- 
torted tubules,  especially  toward  their  inner  border,  while  in  the  medullary 
tubules  the  crystals  are  confined  to  the  lumen.  These  facts  lead  to  the 
inference  that  under  these  conditions  at  least,  the  uric  acid  passes  from  the 


/'  .:'.7**Jj**  '  * ''""•'"•.••'it  ^ 


3^     .# 


Fig.  209. — Kidney  of  a  Rab- 
bit. Cortex  alone  stained  with 
the  indigo-carmine  at  the  end  of 
one  hour. — {Heidenhain.) 


Fig.  210. — Microscopic  Appearance  of 
THE  Lumen  of  the  Convoluted  Tubules 
Containing  the  Indigo-carmine. — {Hei- 
denhain.) 


blood  by  way  of  the  epithelium  into  the  interior  of  the  tubule  to  be  finally 
discharged  into  the  pelvis  of  the  kidney. 

Nussbaum  attempted  to  establish  the  secretory  power  of  the  epithelium 
in  another  way.  In  the  frog  the  kidney  receives  blood  from  two  sources:  the 
glomeruli  receive  their  blood  from  the  renal  artery,  the  tubules  from  the 
capillaries  formed  by  the  anastomosis  of  branches  of  the  efferent  vessel 
of  the  glomerulus  and  the  branches  of  the  renal  portal  vein.  Nussbaum 
believed  that  by  ligating  the  renal  artery  all  glomerular  activity  could  be 
abolished  and  the  part  played  by  the  epithelium  could  be  determined.  After 
so  doing  the  flow  of  urine  was  at  once  checked;  the  injection  of  urea  at 
once  reestablished  it.  This  fact  was  taken  as  a  proof  that  the  tubular 
epithelium  not  only  excreted  urea,  but  water  and  perhaps  other  constituents 
as  well.  It  was  also  found  that  sugar,  peptones,  carmine,  etc.,  which  are 
always  eliminated  from  the  blood  under  normal  conditions,  are  not  removed 
after  ligation  of  the  renal  artery.  It  was  concluded  from  these  experiments 
that  the  secreting  structures  of  the  kidney  consist  of  two  distinct  systems, 
the  glomerular  and  the  tubular;  the  former  secreting  water,  salts,  sugar, 


EXCRETION  465 

peptone,  etc.,  the  latter  urea,  uric  acid,  etc.  These  and  similar  facts  in- 
dicate that  the  renal  epithelium  possesses  a  secretor  function.  Heidenhain 
and  those  who  agree  with  him  assert  that  even  the  water  and  inorganic  salts 
which  pass  through  the  glomerular  epithelium  do  so  in  consequence  of  cell 
selection  and  cell  activity;  that  the  entire  process  is  one  of  secretion,  though 
conditioned  by  blood-pressure,  blood  velocity,  etc. 

The  Present  View  as  to  the  Mode  of  Formation  of  the  Urine. — 

The  still  further  progress  of  physiologic  investigation  has  apparently  re- 
established the  view,  subject  to  some  modifications,  that  the  glomerulus  is 
after  all  but  a  passive  apparatus  permitting  the  filtration  of  water  and  diffus- 
ible salts,  as  Bowman  and  Ludwig  suggested,  and  has  confirmed  the  view  of 
Heidenhain  that  the  epithelium  is  a  secretory  apparatus,  and  in  addition 
has  established  the  further  fact  that  the  epithelium  is  an  absorptive  apparatus 
as  well.  Without  presenting  the  details  of  a  long  series  of  experiments  it 
suffices  to  say  that  the  results  of  these  investigations  make  it  reasonably 
certain  that  the  glomerulus  permits  of  the  passage  of  a  filtrate,  not  of  the 
character  and  composition  asserted  by  Ludwig,  but  having  the  character- 
istics and  composition  of  the  blood-plasma,  «.e.,  having  the  chemic  composi- 
tion and  the  same  degree  of  concentration  as  this  fluid,  less  its  protein  con- 
tent; that  as  this  fluid  passes  through  the  tubules  it  receives  the  organic  con- 
stituents which  are  secreted  by  the  epithelium  and  in  addition  a  certain 
volume  of  water  as  well;  also  that  it  loses  water  and  inorganic  salts  which 
are  returned  to  the  blood  not  by  diffusion  as  Ludwig  believed  but  by  an 
act  of  absorption  on  the  part  of  the  epithelial  cells,  when  these  substances 
are  needed  for  nutritive  purposes. 

The  probability  that  the  renal  epithelium  possesses  absorptive  functions 
is  indicated  by  a  greater  or  less  constriction  of  the  tubules  in  those  animals 
in  which  the  retention  of  water  at  least,  is  desirable.  Thus  in  fish,  where 
the  supply  of  water  is  abundant  and  no  need  for  its  retention  in  the  body 
exists,  the  tubules  are  short,  wide  and  free  from  any  narrowing  which  would 
retard  the  downward  flow  of  the  fluid;  in  frogs  which  live  partly  on  land  and 
which  lose  water  readily  by  evaporation  from  the  skin,  the  retention  of  water 
is  necessary,  hence  in  these  animals  the  tubules  are  long  and  much  con- 
stricted; in  tortoises  where  there  is  no  evaporation  from  the  skin,  the  tubes 
are  short,  wide  and  devoid  of  constrictions.  In  mammals  the  constricted 
portion,  the  loop  of  Henle,  is  long  and  narrow,  and  very  probably  serves  to 
retard  the  flow  of  the  liquid  thus  permitting  of  the  absorption  of  water  and 
other  constituents. 

Summarizing  the  foregoing  it  may  be  stated  that  the  formation  of  urine 
is  accomplished  by  the  cooperation  of  the  glomerulus  and  the  epithelium  of 
the  renal  tubules. 

The  glomerulus  permits  the  passage  of  a  filtrate  similar  if  not  identical 
with  the  blood  plasma — minus  the  protein. 

The  epithelium,  especially  that  lining  the  contorted  portion  of  the  tubule, 
(i)  secretes  water,  various  salts  previously  in  combination  with  protein,  e.g., 
phosphates,  urea,  uric  acid,  various  other  nitrogen-holding  compounds  and 
organic  matters;  (2)  absorbs  such  constituents  of  the  urinary  fluid  as  water 
and  inorganic  salts  which  may  be  necessary  to  the  nutrition  of  the  body.  The 
final  result  of  the  entire  process  is  the  maintenance  of  the  normal  composition 
of  the  blood  and  body  fluids. 
30 


466 


TEXT-BOOK  OF  PHYSIOLOGY 


The  Influence  of  Blood-pressure, — Whether  the  elimination  of  the 
urinary  constituents  is  entirely  secretor  (physiologic)  in  character  or  not  there 
can  be  no  doubt  that  the  whole  process  is  largely  determined  by  the  pressure 
and  velocity  of  the  blood  in  the  glomerular  capillaries,  or,  to  state  it  more  ac- 
curately, on  the  difference  of  pressure  between  the  blood  in  the  capillaries  and 
the  urine  in  the  capsules.  As  a  rule,  this  latter  pressure  is  at  a  minimum.  If 
the  urine  should  accumulate  in  the  ureter  and  tubules  either  from  ligation  or 
mechanical  obstruction  until  its  pressure  approximated  that  of  the  blood,  the 
secretion  should  be  diminished  if  not  abolished.  It  is  difficult  to  determine 
the  average  pressure  or  velocity  of  the  blood  in  the  glomerular  capillaries, 
though  they  both  must  be  greater  than  in  capillaries  in  other  parts  of  the 
body,  from  the  fact  that  the  efferent  vessel  is  narrower  than  the  afferent, 
and  therefore  offers  great  resistance  to  the  outflow  of  blood,  a  condition  most 
favorable  to  the  production  of  a  high  pressure  in  the  glomerulus. 

Glomerular  Pressure. — The  pressure  of  the  blood  in  the  glomeruli  is 
the  resultant  of  the  pressure  in  the  renal  artery  and  the  resistance  to  the 
outflow  of  blood  through  the  efferent  vessel  and  the  capillaries  beyond. 

The   pressure   of  blood  in   the   renal  artery  may  be  augmented  and 
the  velocity  of  the  blood  stream  increased: 
I.  By  an  increase  in  blood-pressure  generally. 

2.  By  an  increase  in  the  blood-pressure 
of  the  renal  artery  alone. 

The  first  condition  may  be  caused 
by  an  increase  in  either  the  force  or 
frequency  of  the  heart's  action  or  by 
a  contraction  of  the  arterioles  of  the 
vascular  areas  in  any  or  all  parts  of  the 
body,  excepting,  of  course,  the  renal  vas- 
cular area.  Should  this  condition  arise, 
the  blood  would  be  forced  into  the  renal 
artery  in  larger  volumes  and  in  conse- 
quence its  pressure  would  be  increased. 
The  second  condition  is  brought  about  by 
a  dilatation  of  the  renal  artery  alone  and 
possibly  by  a  simultaneous  contraction 
of  the  efferent  vessels  of  the  glomeruli. 
The  pressure  of  the  blood  in  the 
renal  artery  and  therefore  in  the  glomer- 
uli may  be  diminished  and  the  velocity 
decreased : 

1.  By  a  decrease  in  the  blood-pressure 
generally. 

2.  By  a  decrease  in  the  blood-pressure 
of  the  renal  artery  alone. 

The  first  condition  may  be  caused  by  a  decrease  in  either  the  force  or 
frequency  of  the  heart's  action  or  by  a  dilatation  of  the  arterioles  of  large 
vascular  areas  in  any  or  all  parts  of  the  body.  Should  this  condition  arise, 
the  volume  of  blood  delivered  to  the  kidney  in  the  unit  of  time  would  be 
diminished  and  hence  its  pressure  would  fall.  The  dilatation  of  the  cutane- 
ous vessels  in  summer,  the  result  of  the  high   temperature  leads  to  a  di- 


FiG.  211. — To  Illustrate  the  Effect 
OF  Active  Changes  in  the  Vasa  Affer- 

ENTIA   AND    EffERENTIA   ON  THE   PRESSURE 

IN  THE  Glomerular  Capillaries.  A.  Re- 
nal arteries.  G.  Glomerular  capillaries. 
C.  Tubular  capillaries.  V.  Vein.  The  short 
thick  lines  represent  the  vasa  afferentia  and 
eiTerentia.  The  continuous  heavy  line  repre- 
sents the  mean  average  pressure.  If  the  vas 
afferens  dilates  and  the  vas  efferens  contracts 
separately  or  conjointly,  the  pressure  will  rise, 
as  indicated  by  the  upper  dotted  line.  If  the 
vas  afferens  contracts  and  the  vas  efferens 
dilates  separately  or  conjointly,  the  pressure 
will  fall,  as  indicated  by  the  lower  dotted 
line. — (After  Moral.) 


EXCRETION 


467 


minished  blood-supply  to  the  kidney  and  a  diminution  in  the  amount  of  urine 
secreted.  The  second  condition  is  brought  about  by  contraction  of  the 
renal  artery  alone  and  possibly  by  a  simultaneous  dilatation  of  the  efferent 
vessels  of  the  glomeruli.  Moreover  the  pressure  in  the  vessels  of  the  glomer- 
uli may  be  varied  according  to  the  degree  of  contraction  or  relaxation  of  the 
muscle  coat  of  the  aft'erent  and  eft'erent  vessels.  See  Fig.  211  and  the  ac- 
companying explanation. 

Coincident  with  the  rise  and  fall  of  pressure  in  the  glomerular  capillaries 
there  is  a  rise  and  fall  in  the  rate  of  urinary  flow.  Thus  it  has  been  found  that 
an  increase  in  the  aortic  pressure  from  127  to  142  mm.  of  mercury,  from 
ligation  of  the  carotid,  femoral,  and  vertebral  arteries,  increased  the  rate  of 
urinary  flow  from  8.7  grams  in  thirty  minutes  to  21.2  grams.  On  the 
contrary,  a  decrease  in  aortic  pressure  below  40  mm.  of  mercury  caused  by 
division  of  the  spinal  cord  is  followed  by  a  total  abolition  of  the  urinary  flow. 
These  facts  serve  to  indicate  the  dependence  of  the  secretion  on  blood- 
pressure. 

The  period  of  functional  activity  of  the  kidney  is  accompanied  by  an 
increase  in  the  volume  of  blood  flowing  through  it  as  is  evident  from  an  in- 


FlG. 


212. ^Scheme  of  a  Renal  Oncometer  or  Plethysmograph.     K,  kidney;  RT,  recei\-ing 
tambour  or  capsule;  PB,  pressure  bottle;  PR,  recording  piston. 


spection  of  the  organ.  At  this  time  it  is  enlarged,  swollen,  and  red  in  color. 
The  blood  in  the  renal  vein  is  bright  red  in  color  and  contains  more  oxygen 
and  less  carbon  dioxid  than  venous  blood  generally.  During  the  interv^als 
of  activity  the  kidney  is  supplied  with  a  less  amount  of  blood  and  hence  it 
diminishes  in  size,  becomes  pale  in  color  and  the  blood  of  the  renal  vein 
becomes  dark  and  venous  in  character.  These  variations  in  the  volume  of 
the  kidney  have  also  been  experimentally  determined  and  registered  by 
means  of  the  oncometer  and  oncograph  devised  by  Roy. 

The  Renal  Oncometer. — The  oncometer  consists  of  a  metallic  capsule, 
(Fig.  212)  composed  of  halves  which  open  and  close  by  means  of  a  hinge. 
The  capsule  encloses  two  thin  membranous  and  distensible  sacs.  These  are 
connected  with  a  piston  recorder  by  means  of  a  tube.  The  kidney,  with- 
drawn from  the  body,  is  placed  within  the  oncometer.  Through  an  openmg 
in  the  side  pass  the  artery,  vein,  and  ureter.  A  thin  light  oil  is  then  poured 
through  a  side  tube  from  a  pressure  bottle  until  the  membranous  sacs 
are   completely   filled  and  surround  the  kidney  on  all    sides.     When  the 


468 


TEXT-BOOK  OF  PHYSIOLOGY 


tube  from  the  pressure  bottle  is  closed,  the  conditions  are  such  that  all  varia- 
tions in  the  volume  of  the  kidney  are  taken  up  and  reproduced  by  the  lever 
attached  to  the  piston  recorder.  A  curve  of  the  variations  in  the  valume 
of  the  kidney  is  shown  in  Fig.  213.  An  examination  of  this  curve  shows  that 
the  volume-changes  exhibit  not  only  the  respiratory  but  also  the  cardiac 
undulations. 

The  Influence  of  the  Nerve  System.— The  influence  of  the  nerve  system 
in  regulating  the  blood-supply  to  the  kidney  is  evident  from  the  results  of 
experimentation.  If  the  nerves  which  accompany  the  renal  artery  into  the 
kidney  are  divided,  the  artery  at  once  dilates,  the  kidney  enlarges,  and  a 
copious  flow  of  urine  takes  place.  If  the  peripheral  ends  of  these  nerves  be 
stimulated  with  induced  electric  currents  the  artery  contracts,  the  kidney 
diminishes  in  size,  and  the  flow  of  urine  ceases.  In  addition  to  these  vaso- 
constrictor nerves,  there  is  evidence  that  the  kidney  also  receives  vaso-dilata- 
tor  nerves.  The  vaso-constrictor  nerves  emerge  from  the  spinal  cord  and  are 
found  in  the  anterior  roots  of  the  eleventh,  twelfth,  and  thirteenth  dorsal 
nerves,  in  the  dog.  Direct  and  reflex  stimulation  of  the  centers  of  origin 
of  these  nerves  gives  rise  to  contraction  or  to  a  dilatation  of  the  artery,  a 


I       15  Seconds  I  I 


Fig.  213.— Curve  of  the  variations  in  the  volume  of  the  kidney. 


diminution  or  a  swelling  of  the  kidney,  and  a  decrease  or  an  increase  in 
secretion,  independent  of  any  variation  in  general  blood-pressure  according 
to  the  nature  of  the  cause  acting. 

The  route  of  the  vaso-constrictor  nerves  is,  in  the  dog  at  least,  through 
the  lesser  splanchnics,  the  terminal  branches  of  which  arborize  around  the  cells 
of  the  renal  ganglia ;  from  these  ganglia  new  fibers  arise  which  pass  through 
the  renal  plexus  into  the  kidney  to  be  distributed  to  the  muscle  coat  of  the 
renal  artery  branches.  Section  of  these  nerves  is  followed  by  a  dilatation 
of  the  renal  vessels  and  an  increase  in  the  flow  of  urine.  Stimulation  of  the 
peripheral  ends  is  followed  by  a  constriction  of  the  vessels  and  a  cessation 
of  the  flow  of  urine. 

The  vaso-motor  center  for  the  blood-vessels  of  the  kidney  is  in  all  proba- 
bility situated  in  the  medulla  oblongata  in  close  proximity  to  the  general 
vaso-motor  centers,  though  subordinate  centers  are  doubtless  present  in  the 
spinal  cord.  It  was  found  by  Bernard  that  puncture  of  the  medulla  was 
occasionally  followed  by  a  profuse  secretion  of  urine  without  the  presence 
of  sugar.  The  route  of  the  vaso-motor  impulses  which  influence  the  renal 
blood-supply  is  down  the  cord  to  local  vaso-motor  centers,  thence  through  the 
splanchnics  to  the  renal  ganglia,  thence  through  the  renal  plexus  to  the 
blood-vessels. 


EXCRETION  469 

The  Influence  of  Variations  in  the  Composition  of  the  Blood. — ^As  it 

is  the  function  of  the  kidneys  to  excrete  water,  inorganic  saUs,  and  various 
end-products  of  metabolism  from  the  blood  and  thus  maintain  a  general 
average  composition,  it  is  highly  probable  that  as  soon  as  they  accumulate 
beyond  a  certain  percentage  they  themselves  act  as  stimuli  to  renal  activity, 
either  by  acting  directly  on  the  renal  epithelium  or  by  increasing  the  glomeru- 
lar pressure.  There  is  evidence  at  least  that  urea  acts  in  the  former  manner 
and  that  an  excess  of  water  in  the  blood,  from  copious  drinking  or  from  a  sud- 
den checking  of  the  skin  from  a  fall  of  temperature,  acts  in  the  latter  manner. 
The  introduction  into  the  blood  of  inorganic  salts,  such  as  potassium  nitrate, 
sodium  acetate,  etc.,  will  in  a  short  time  lead  to  increased  activity  of  the 
kidneys,  as  shown  by  an  increase  in  the  quantity  of  urine  excreted.  The 
manner  in  which  these  agents  and  other  members  of  their  class,  the  so-called 
saline  diuretics,  increase  renal  activity  is  yet  a  subject  of  discussion.  On  the 
one  hand,  it  is  stated  that  they  promote  an  absorption  of  water  from  the 
tissues  to  such  an  extent  that  a  condition  of  hydremic  plethora  is  produced, 
which  in  itself  increases  not  only  the  general  blood-pressure  but  the  local 
renal  pressure  as  well,  and  that  it  is  this  factor  which  is  the  cause  of  the 
increased  liow  of  urine.  On  the  other  hand,  it  is  asserted  that  though  the 
salts  increase  the  local  pressure  and  the  volume  of  the  kidney,  they  never- 
theless act  specifically  on  the  renal  epithelium,  and  therefore  may  be  regarded 
as  secreto-motor  agents.  An  increase  in  the  percentage  of  sugar  or  urea  in 
the  blood  has  a  similar  influence  on  the  kidney. 

The  Storage  and  Discharge  of  Urine. — Urination. — The  urinary  con- 
stituents, as  soon  as  they  are  eliminated  from  the  blood,  pass  into  and  through 
the  uriniferous  tubules  and  by  them  are  discharged  into  the  pelvis  of  the 
kidney.  They  then  enter  the  ureter  by  which  they  are  conducted  to  the 
bladder.  The  immediate  cause  of  this  movement  is  undoubtedly  a  difference 
of  pressure  between  the  terminal  portions  of  the  tubules  and  the  terminal 
portion  of  the  ureter,  aided  by  the  peristaltic  contraction  of  the  muscle  wall 
of  the  ureter. 

The  Bladder. — ^The  bladder  is  a  reservoir  for  the  temporary  reception  of 
the  urine  prior  to  its  expulsion  from  the  body.  When  distended  it  is  ovoid 
in  shape  and  is  capable  of  holding  from  600  to  800  cu.  cm.  The  bladder  is 
composed  of  four  coats:  viz.,  serous,  muscle,  areolar,  and  mucous.  The 
muscle  coat  consists  of  external  longitudinal  and  internal  circular  and  oblique 
layers  of  fibers  of  the  non-striated  variety  which  collectively  encircle  the  en- 
tire organ.  As  these  fibers  by  their  contraction  expel  the  urine  from  the 
bladder,  they  are  known  collectively  as  the  detrusor  urina  muscle.  At  the 
exit  of  the  bladder  the  circular  libers  are  somewhat  increased  in  number, 
giving  rise  to  the  appearance  of  a  distinct  muscle  band  which  has  been  termed 
the  sphincter  vesica  muscle.  The  presence  of  this  muscle  has,  however,  been 
denied  and  the  retention  of  the  urine  has  been  attributed  to  mechanic  condi- 
tions at  the  neck  of  the  bladder.  The  urethra  just  beyond  the  bladder  is  pro- 
vided with  a  distinct  circular  muscle  composed  of  striated  fibers,  the  sphincter 
nrethrcB  muscle.  At  the  close  of  an  act  of  urination  or  micturition  the  bladder 
is  small,  contracted,  and  its  cavity  is  almost  obliterated,  but  as  urine  is  con- 
tinually descending  the  ureter  and  entering  the  bladder  at  its  base,  the 
detrusor  muscle  gradually  relaxes  or  becomes  sufficiently  inhibited  from 
moment  to  moment  to  receive  it.     The  escape  of  urine  into  the  urethra  is 


470  TEXT-BOOK  OF  PHYSIOLOGY 

prevented  either  by  mechanic  conditions  or  by  the  contraction  of  the  sphincter 
muscle  at  the  vesic  orifice.  When  the  accumulating  urine  reaches  a  certain 
volume,  it  gives  rise  to  an  intra-vesic  pressure.  When  this  pressure  rises  to 
about  80  cm.  of  water  the  detrusor  urina  acquires  a  certain  degree  of  tension 
or  tonus.  This  is  followed  by  rhythmic  contractions  of  the  detrusor  urinse 
which  increase  in  extent  and  vigor  as  the  urine  continues  to  accumulate  until 
finally  a  general  contraction  develops,  the  force  of  which  overcomes  the 
constricting  influences  at  the  bladder  orifice  and  the  fluid  is  discharged. 
This  action  of  the  detrusor  muscle  is  generally  reinforced  by  the  contraction 
of  the  abdominal  muscles.  The  latter  portions  of  the  urine  are  ejected 
through  the  urethra  by  the  rhythmic  action  of  the  accelerator  urinae  muscles. 

The  Nerve  Mechanism  of  Urination. — ^The  expulsion  of  urine  is  pri- 
marily a  reflex  act  though  subject  to  a  variable  amount  of  volitional  control. 
The  reflex  character  of  the  act  is  especially  noticeable  in  young  children  in 
whom,  by  reason  of  the  imperfect  development  of  the  brain,  there  is  a  lack 
of  volitional  control.  During  the  intervals  of  urination  the  orifice  of  the 
bladder  is  closed  by  the  tonic  contraction  of  the  sphincter  vesicae  and  sphinc- 
ter urethrae  muscles,  thus  preventing  the  immediate  exit  of  urine  after  its 
descent  into  the  bladder.  The  tonic  contraction  of  both  muscles  is  main- 
tained by  the  activity  of  nerve-centers  in  the  lumbar  region  of  the  spinal 
cord.  The  detrusor  muscle  is  at  the  same  time  in  a  more  or  less  relaxed 
condition,  the  result  of  an  inhibition  of  its  governing  center  in  the  spinal  cord. 

When  the  accumulating  urine  reaches  a  certain  volume  it  causes,  as 
previously  stated,  an  intra-vesic  pressure,  an  increased  tonus  of  the  detrusor 
muscle,  followed  by  slight  rhythmic  contractions  of  its  fibers. 

When  the  desire  to  urinate  is  experienced  impressions  are  being  made  on 
the  afferent  nerve  endings  in  the  mucous  membrane  of  the  bladder.  The 
nerve  impulses  thus  developed  are  transmitted  to  the  urination  center  in  the 
spinal  cord  and  to  the  cerberum  and  influence  in  one  direction  or  another 
their  activities.  In  a  young  child  the  arrival  of  the  transmitted  impulses  in 
the  spinal  cord  is  immediately  followed  by  an  inhibition  of  the  sphincter 
centers  and  a  stimulation  of  the  detrusor  center,  as  a  result  of  which  the 
sphincter  muscles  relax  and  the  detrusor  muscle  contracts,  thus  expelling 
the  urine.  In  the  adult,  if  the  act  of  urination  is  to  be  permitted  the  same 
mechanism  is  brought  into  action.  In  its  expulsive  efforts  the  detrusor  mus- 
cle is  assisted  by  the  contraction  of  the  abdominal  muscles  and  possibly  the 
diaphragm  in  response  to  volitional  efforts.  After  the  discharge  of  urine 
there  is  a  return  to  the  former  condition, namely,  a  contraction  of  the  sphincter 
muscles  and  a  gradually  inhibition  of  the  detrusor  muscle.  If  the  act  of 
urination  is  to  be  suppressed  vohtional  impulses  descend  the  cord  and  cause 
an  increased  contraction  of  the  sphincter  urethrae  muscle,  whereby  the  action 
of  the  reflex  mechanism  is  for  a  while  opposed. 

The  nerve  mechanism  therefore  involves  both  efferent  and  afferent  nerves 
as  well  as  nerve-centers  in  the  lumbo-sacral  region  of  the  spinal  cord. 

Efferent  Nerves. — ^The  efferent  nerve-fibers  for  the  sphincter  urethrae 
muscle  have  their  origin  in  the  spinal  cord  from  which  they  pass  by  way  of 
the  third  and  fourth  sacral  nerves,  the  pelvic  nerve  and  the  inferior  hemor- 
rhoidal nerve  directly  to  the  muscle. 

The  efferent  nerve-fibers,  for  the  detrusor  muscle,  including  the  specialized 
portion,  the  internal  sphincter,  have  their  origin  in  nerve-cells  in  thelumbo- 


EXCRETION  471 

sacral  region  of  the  spinal  cord  and  pass  to  their  destination  by  two  paths. 
The  fibers  in  the  first  path  leave  the  spinal  cord  by  way  of  the  second  to  the 
fifth  lumbar  nerves,  then  pass  into  and  through  the  sympathetic  chain, 
through  the  inferior  splanchnics  to  the  inferior  mesenteric  ganglion  around 
the  cells  of  which  their  terminal  branches  arborize;  from  the  cells  of  this 
ganglion  new*  fibers  emerge  which  pass  through  the  hypogastric  nerves  to  the 
muscles.  The  fibers  of  the  second  path  leave  the  spinal  cord  by  way  of  the 
second  to  the  fourth  sacral  nerves,  then  pass  into  the  pelvic  or  erigens  nerve 
to  small  ganglia  (pelvic  ganglia)  along  the  sides  of  the  bladder  around  the 
cells  of  which  their  terminal  branches  arborize;  from  the  cells  of  these  ganglia 
new  nerve-fibers  emerge  which  pass  directly  to  the  muscles.  In  both  paths 
the  nerves  coming  from  the  cord  are  pre-ganglionic,  those  coming  from  the 
ganglia,  post-ganglionic. 

The  central  mechanism  that  excites  and  coordinates  the  activities  of  the 
vesical  muscles  is  situated  in  the  lumbo-sacral  region  of  the  spinal  cord  and 
is  designated  the  vesical  or  urination  center. 

Afferent  Nerves — ^The  afferent  nerve-fibers  that  excite  the  central  mech- 
anism to  activity,  are  contained  in  both  the  nerve-paths  described  in  fore- 
going paragraphs  and  enter  the  spinal  cord  in  the  dorsal  roots  of  the  lumbar 
and  spinal  nerves. 

Though  the  origin,  course  and  distribution  of  the  nerves  composing  this 
mechanism  are  fairly  well  known,  their  mode  of  action  is  somewhat  obscure 
and  the  results  of  experimentation  not  always  in  accord.  According  to  v. 
Zeissl  stimulation  of  the  peripheral  ends  of  the  divided  hypogastric  nerves 
causes  mainly  a  contraction  of  the  sphincter  muscles  and  a  relaxation  of  the 
detrusor  muscle,  while  a  stimulation  of  the  peripheral  ends  of  the  divided 
sacral  nerves  causes  a  \dgorous  contraction  of  the  detrusor  muscle  and  a  re- 
laxation of  the  sphincter  muscles.  The  lumbar  centers  would  therefore 
cause  a  reception  and  a  retention  of  the  urine,  and  the  sacral  centers  would 
cause  its  expulsion. 

PERSPIRATION.    SEBUM 

The  perspiration  or  sweat,  the  chief  secretion  of  the  skin,  is  a  clear  colorless 
fluid,  slightly  acid  in  reaction  and  saline  to  the  taste.  Its  specific  gravity 
varies  from  1.003  ^^  1.006.  Unless  collected  from  the  soles  of  the  feet  and 
the  palms  of  the  hand,  it  is  apt  to  be  mixed  with  epithelial  cells  and  sebum. 
The  total  quantity  of  perspiration  secreted  daily  has  been  variously  estimated 
at  from  700  to  1000  grams;  the  exact  amount,  however,  is  difficult  of  determi- 
nation, for  the  reason  that  the  rate  of  secretion  varies  greatly  with  variations  in 
temperature,  food,  drink,  season  of  the  year,  etc. 

Chemic  analysis  of  the  sweat  shows  that  it  contains  but  from  0.5  to  2.5 
per  cent,  of  solid  constituents,  the  variation  in  the  percentage  depending  on 
the  quantity  of  water  secreted.  The  solids  consist  of  traces  of  urea,  neutral 
fats,  lactic  and  sudoric  acids  in  combination  with  alkaline  bases,  and  inorganic 
salts  (Fovel).  Other  observ-ers,  however,  have  not  been  able  to  detect  the 
presence  of  either  lactic  or  sudoric  acid.  Urea  is  a  constant  ingredient, 
though  its  percentage  is  extremely  small,  possibly  not  more  than  o.i  per 
cent.  The  amount,  however,  may  be  very  much  increased  in  uremic 
conditions,  the  result  of  acute  or  chronic  disease  of  the  kidneys.  The  inor- 
ganic   constituents    consist  mainly  of  sodium  chlorid   and  alkaline  and 


472  TEXT-BOOK  OF  PHYSIOLOGY 

earthy  phosphates.  Carbonic  acid  is  also  present  in  the  free  state  as  well 
as  in  combination  with  alkaline  bases. 

The  very  small  quantity  of  the  solid  constituents  in  the  sweat,  taken  in 
connection  with  the  fact  that  it  is  excreted  most  abundantly  when  the  external 
temperature  is  high,  indicates  that  it  is  not  so  important  as  an  excrementi- 
tious  fluid  as  it  is  as  a  means  for  the  regulation  of  the  temperature  of 
the  body. 

The  sweat  is  a  product  of  the  secretory  activity  of  specialized  glands, 
the  sweat-glands,  embedded  in  the  skin,  to  the  histologic  structures  of 
which  they  bear  a  special  relation. 

THE  SKIN 

The  skin  is  a  complexly  organized  structure  investing  the  entire  external 
surface  of  the  body.  Its  total  area  varies  from  1.17  to  1.35  square  meters  in 
man  and  from  i.i  to  1.17  square  meters  in  woman.  It  varies  in  thickness 
in  different  localities  of  the  body  from  |  to  y^(j-  of  an  inch.  The  skin 
consists  of  two  principal  layers:  viz.,  a  deep  layer,  the  derma  or  corium, 
and  a  superficial  layer,  the  epidermis. 

The  derma  or  corium  may  be  subdivided  into  a  reticulated  and  a  pap- 
illary layer.  The  reticulated  layer  consists  of  white  fibrous  and  yellow 
elastic  tissue,  non-striated  muscle-fibers,  woven  together  in  every  direction 
and  forming  an  areolar  network,  in  the  meshes  of  which  are  deposited 
masses  of  fat  and  a  structureless  amorphous  matter;  the  papillary  layer  con- 
sists mainly  of  club-shaped  elevations  or  projections  of  the  amorphous 
matter  constituting  the  papillae.  The  reticulated  layer  serves  to  connect 
the  skin  with  the  underlying  structures  and  to  afford  support  for  the  blood- 
vessels, nerves,  and  lymphatics  which  are  distributed  to  the  papillae  (Fig.  225) . 

The  epidermis  is  an  extra-vascular  structure  consisting  entirely  of  epi- 
thelial cells.  It  may  also  be  subdivided  into  two  layers — the  Malpighian  or 
pigmentary  layer,  and  the  corneous  or  horny  layer.  The  former  is  closely  ap- 
plied to  the  papillary  layer  of  the  true  skin  and  is  composed  of  large  nucle- 
ated cells,  the  lowest  layer  of  which,  the  "prickle  cells,"  contains  the  pig- 
ment granules  which  give  to  the  skin  its  varying  hues  in  different  individu- 
als and  in  different  races  of  men;  the  corneous  layer  is  composed  of  flattened 
cells  which  from  their  exposure  to  the  atmosphere,  etc.,  are  hard  and  horny 
in  texture. 

The  Sweat-glands. — These  glands  are  tubular  in  shape,  the  inner 
extremity  of  each  being  coiled  upon  itself  a  number  of  times,  forming  a 
little  ball  situated  in  the  derma  or  the  subcutaneous  connective  tissue.  From 
this  coil  the  duct  passes  up  in  a  straight  direction  to  the  epidermis,  where 
it  makes  a  few  spiral  turns,  after  which  it  opens  obliquely  on  the  surface. 
The  gland  consists  of  a  basement  membrane  lined  with  epithelial  cells. 
It  is  supplied  abundantly  with  blood-vessels  and  nerves.  The  sweat- 
glands  are  extremely  numerous  all  over  the  cutaneous  surface,  though 
they  are  more  thickly  disposed  in  some  situations  than  others.  They 
probably  average  400  to  the  square  centimeter;  the  total  number  has  been 
estimated  at  from  2,000,000  to  2,500,000. 

The  Influence  of  the  Nerve  System  on  the  Production  of  Sweat. — 
The  secretion  of  sweat,  though  a  product  of  the  activity  of  epithelial  cells 
and  dependent  on  a  variety  of  conditions,  is  regulated  to  a  large  extent  by 


EXCRETION  473 

the  nerve  system.  Here,  as  in  other  secreting  glands,  the  fluid  is  derived 
from  materials  in  the  lymph-spaces,  furnished  by  the  blood.  Generally 
the  two  conditions,  increased  blood-flow  and  increased  glandular  action, 
coexist.  At  times,  however,  a  profuse  clammy  perspiration  is  secreted  with 
diminished  blood-flow.  Two  sets  of  nerves  are  evidently  concerned  in 
this  process:  viz.,  vaso-motor  nerves,  which  regulate  the  blood-supply,  and 
secretor  ner\'es,  which  stimulate  the  gland-cells  to  activity. 

The  Sweat  Nerves. — The  sweat  nerves  which  excite  the  activities  of  the 
epithelium  of  the  sweat-glands  have  their  origin  in  nerve-cells  located  in  the 
anterior  and  lateral  gray  matter  of  the  spinal  cord.  The  sweat  nerves,  like 
the  vaso-motor  nerves,  do  not  pass  directly  to  the  gland-cells,  but  indirectly 
by  way  of  the  ganglia  of  the  sympathetic  chain.  In  these  ganglia  the  sweat 
nerves  terminate,  their  end-branches  aborizing  around  the  nerve-cells.  From 
the  cells  of  these  ganglia  new  nerve-fibers  arise  which  then  pass  without 
interruption  to  their  final  destination.  The  former  are  termed  pre-gangli- 
onic,  the  latter  post-ganglionic.  From  their  origin  and  distribution  it  is 
apparent  that  the  sweat  nerves  are  constituent  portions  of  the  autonomic 
nerve  system. 

From  experiments  made  on  animals  and  from  observation  of  clinic  con- 
ditions in  human  beings,  it  may  be  stated  in  a  general  way,  that  the  pre- 
ganglionic fibers  which  emerge  from  the  spinal  cord  in  the  ventral  roots  of 
spinal  nerves  between  the  second  thoracic  and  third  lumbar  nerves  may  be 
divided  into  four  groups,  viz.:  (i)  Those  distributed  to  the  sweat-glands  of 
the  skin  of  the  trunk  of  the  body;  (2)  those  for  the  sweat-glands  of  the  fore 
limbs;  (3)  those  for  the  sweat-glands  of  the  head,  neck  and  face;  (4)  those 
for  the  sweat-glands  of  the  hind  limbs.  These  nerves  pursue  the  same  route 
as  the  vaso-motor  nerves  with  which  they  are  associated  in  function  (see 
page  377). 

The  exact  course  for  the  sweat  nerves  has  been  experimentally  deter- 
mined only  for  the  cat  and  dog.  In  these  animals,  however,  sweat-glands 
are  found  only  in  the  balls  of  the  feet.  According  to  Langley's  observations 
the  sweat  nerves  for  the  forefeet  leave  the  spinal  cord  in  the  thoracic 
nerves  from  the  fourth  to  the  tenth  inclusive.  After  passing  into  the  sym- 
pathetic chain  they  ascend  to  the  stellate  ganglion,  around  the  cells  of  which 
their  end-branches  arborize.  From  this  ganglion  non-medullated  fibers  pass 
in  the  gray  rami  communicantes  to  the  nerves  composing  the  brachial  plexus 
and  then  to  the  feet.  The  sweat  nerves  for  the  hind  feet  leave  the  cord 
mainly  in  the  first  and  second  lumbar  and  terminate  in  sympathetic  ganglia, 
from  which  the  post-ganglionic  nerves  pass  into  the  nerve-trunks  included 
between  the  sixth  lumbar  and  the  second  sacral  nerves,  which  enter  into 
the  formation  of  the  sacral  plexus  and  through  which  they  pass  to  the  feet. 

The  existence  of  a  dominating  sweat  center  in  the  medulla  oblongata  is 
probable,  though  its  location  has  never  been  definitely  determined. 

That  the  sweat-giands  are  stimulated  to  activity  by  nerve  impulses  is 
shown  by  the  fact  that  stimulation  of  the  peripheral  ends  of  the  divided 
cervical  sympathetic,  of  the  brachial  plexus,  or  of  the  sciatic  nerve  is  followed 
in  a  few  seconds  by  a  profuse  secretion.  Though  under  physiologic  con- 
ditions there  is  a  simultaneous  dilatation  of  the  blood-vessels  and  an  increased 
supply  of  blood,  this  is  merely  a  condition  and  not  a  cause  of  the  secretion; 
for  the  secretion  can  be  excited  and  the  flow  maintained  for  a  period  of  from 


474 


TEXT-BOOK  OF  PHYSIOLOGY 


ten  to  fifteen  minutes  after  ligation  of  the  blood-vessels  of  the  limb  or  even 
after  its  amputation,  when  the  corresponding  nerve  is  stimulated. 

As  the  sweat-glands  are  always  in  a  state  of  more  or  less  activity  it  is 
assumed  that  the  sweat  centers  possess  a  certain  degree  of  tonicity,  though 
the  cause  for  such  tonicity  has  not  been  made  apparent.  It  is  quite  possible 
that  the  presence  of  carbon  dioxid  in  the  blood,  as  well  as  the  temperature, 
may  be  important  factors,  since  an  increase  in  the  venosity  of  the  blood  or 
a  rise  in  temperature  is  usually  followed  by  an  increase  in  the  amount  of 
sweat  excreted.  The  centers  may  also  be  excited  to  increased  activity  by 
both  central  and  peripheral  causes,  e.g.,  psychic  states  of  an  emotional 
character  and  a  rise  in  the  external  temperature.  Pilocarpin  injected  into 
the  blood  causes  a  profuse  secretion  even  when  the  nerves  have  been  divided. 
Its  action  is  supposed  to  be  exerted  on  the  terminal  branches  of  the  nerv'es 

and  possibly  on  the  cells  themselves.  As  in 
the  case  of  the  sahvary  glands  atropin  sus- 
pends the  activity  of  the  terminal  branches  of 
secretor  nerves. 

Hairs. — Hairs  are  found  in  almost  all 
portions  of  the  body,  and  can  be  divided 
into — 

1.  Long,  soft  hairs,  on  the  head, 

2.  Short,  stiff  hairs,  along  the  edges  of  the 
eyelids  and  nostrils. 

3.  Soft,  downy  hairs  on  the  general  cuta- 
neous surface. 

They  consist  of  a  root  and  a  shaft.  The 
shaft  is  oval  in  shape  and  about  60  micro- 
millimeters  in  diameter;  it  consists  of  fibrous 
tissue,  covered  externally  by  a  layer  of  im- 
bricated cells,  and  internally  by  cells  contain- 
ing granular  and  pigment  material. 

The  root  of  the  hair  is  embedded  in  the 
hair-follicle,  formed  by  a  tubular  depression 
of  the  skin,  extending  nearly  through  to  the 
subcutaneous  tissue;  its  walls  are  formed  by  the  layers  of  the  corium,  covered 
by  epidermic  cells.  At  the  bottom  of  the  follicle  there  is  a  papillar}'  pro- 
jection of  amorphous  matter,  corresponding  to  a  papilla  of  the  true  skin, 
containing  blood-vessels  and  nerves,  upon  which  the  hair-root  rests. 
The  investments  of  the  hair-roots  are  formed  of  epithelial  cells,  consti- 
tuting the  internal  and  external  root-sheaths. 

The  lower  portion  of  the  hair-follicle  is  connected  with  the  upper  surface 
of  the  derma  by  bundles  of  non-striated  muscle-fibers  which  are  termed 
arrectores  pilortim  muscles.  Their  inclination  and  insertion  are  such  that 
their  contraction  is  followed  by  erection  of  the  hair-follicle  and  hair-shaft. 
These  muscles  are  excited  to  action  by  nerves  termed  pilo-motor  nerves. 

THE  SEBUM 

The  sebum  or  sebaceous  matter  is  a  peculiar  oily  material  produced 
by  specialized  glands  in  the  skin.  It  consists  of  water,  epithelium,  pro- 
teids,  fat,  cholesterin,  and  inorganic  salts. 


Fig.  214. — Large  Sebaceous 
Gland,  i.  Hair  in  its  follicle.  2,  3, 
4,  5.  Lobules  of  the  gland.  6.  Ex- 
creton-  duct  traversed  by  the  hair. 
— {Sai)pey.) 


EXCRETION  475 

The  sebaceous  glands  are  simple  and  compound  racemose  glands 
opening  by  a  common  excretory  duct  on  the  surface  of  the  epidermis  or 
into  the  shaft  of  a  hair-follicle  (Fig.  2 14) .  These  glands  are  extremely  numer- 
ous and  found  in  all  portions  of  the  body,  with  the  exception  of  the  palms 
of  the  hands  and  soles  of  the  feet,  and  most  abundantly  in  the  face.  They 
are  formed  by  a  delicate  structureless  membrane  lined  by  polyhedral 
epithelium. 

The  sebum  is  not  produced  by  an  act  of  true  secretion,  but  is  formed  by 
a  proliferation  and  degeneration  of  the  gland  epithelium.  When  first 
poured  on  the  surface,  the  sebum  is  oily  and  semiliquid  in  character,  but 
soon  hardens  and  acquires  a  cheese-like  consistence.  It  serves  to  lubri- 
cate the  hair  and  skin  and  prevent  them  from  becoming  dry  and  harsh. 

The  surface  of  the  fetus  is  generally  covered  with  a.  thick  layer  of  seba- 
ceous matter,  the  vernix  caseosa,  which  possibly  keeps  the  skin  in  a  normal 
condition  by  protecting  it  from  the  effects  of  the  long-continued  action  of 
the  amniotic  fluid  in  which  the  fetus  is  suspended. 


CHAPTER  XVIII 

EXTERNAL  SECRETIONS 

Secretion  is  a  term  applied  to  a  process  by  which  complex  fluids  are 
formed  from  the  constituents  of  the  lymph  which  are  separated  from  the 
blood-stream  by  the  activities  of  the  endothelial  cells  of  the  capillary  wall, 
as  the  blood  flows  through  the  capillary  blood-vessels.  In  this  process  the 
endothelial  cell  is  aided  by  the  physical  forces,  diffusion,  osmosis,  and  filtra- 
tion.    These  separated  materials  may  be  utilized  in  several  ways: 

1.  For  the  repair  of  the  tissues,  for  growth,  for  the  liberation  of  energy. 

2.  For  the  elaboration  or  production  by  specialized  organs  of  a  variety  of 

complex  fluids  and  specific  materials  of  widely  different  application. 
The  fluids  and  specific  materials  thus  formed  are  utilized  for  the  most 
part  to  meet  some  special  need  of  the  body.     All  such  fluids  and  mate- 
rials are  termed  secretions,  and  the  organs  by  which  they  are  formed  are 
termed  secretor  organs.     Secretions  whether  simple  or  complex  may  in 
a  general  way  be  divided  into  two  groups,  viz.:  external  and  internal. 
External  Secretions. — ^An  external  secretion  may  be  defined  as  a  more 
or  less  complex  fluid  formed  by  the  secretor  activities  of  epithelial  cells  of 
glands,  which  is  discharged  through  well-defined  ducts  on  the  surfaces  of 
the  body,  the  skin  or  mucous  membrane.     The  glands  by  which  they  are 
formed  or  secreted  are  known  as  glands  of  external  secretion. 

Internal  Secretions. — Internal  secretions  may  be  defined  as  more  or 
less  complex  materials  or  agents  formed  by  the  activities  of  epithelial  cells  of 
organs,  and  which  are  discharged  into,  and  distributed  by  the  blood  to  organs 
and  tissues  near  and  remote,  the  activities  of  which  they  influence  in  varying 
ways  and  degrees.  The  glands  by  which  they  are  formed  or  secreted  are 
known  as  glands  of  internal  secretion. 

Organs  of  External  Secretions. — ^All  organs  belonging  to  this  group 
consist  primarily  of  a  thin  delicate  homogeneous  membrane,  one  side  of 
which  is  covered  with  a  layer  of  epithelial  cells  and  the  other  side  of  which 
is  closely  invested  by  a  network  of  capillary  blood-vessels,  lymph-vessels, 
and  nerves.  Though  the  epithelial  cells  have  a  general  histologic  resem- 
blance one  to  another,  their  physiologic  function  varies  in  different  situations, 
in  accordance  probably  with  their  ultimate  chemic  structure,  a  fact  which 
determines  the  difference  in  the  character  of  the  secretions. 

These  organs  may  consist  of  a  single  layer  of  cells  or  a  group  of  cells,  and 
may  be  subdivided  into — 

1.  Secreting  membranes. 

2.  Secreting  glands. 

The  secreting  membranes  are  the  mucous  membranes  lining  the 
gastro-intestinal,  the  pulmonary,  and  the  genito-urinary  tracts,  and  the 
serous  membranes  lining  closed  cavities,  such  as  the  pleural,  pericardial, 
peritoneal,  and  synovial  membranes. 

The  mucous  membranes  are  soft  and  velvety  in  character  and  are  com- 

476 


EXTERNAL  SECRETIONS  477 

posed  of  a  condensed  connective  tissue  forming  a  basement  membrane 
beneath  which  is  a  layer  of  blood-vessels  and  muscle-fibers,  and  on  which 
is  a  layer  of  epithelium,  the  histologic  as  well  as  physiologic  characters  of 
which  vary  in  different  situations.  The  mucus  secreted  by  the  various 
epithelial  forms  will  very  naturally  possess  a  somewhat  different  composition, 
according  to  the  locality  in  which  it  is  formed.  In  a  general  way  it  may 
be  said  that  mucus  is  a  pale,  semi-transparent,  alkaline  fluid,  containing 
leukocytes  and  epithelial  cells.  It  is  composed  chemically  of  water,  mineral 
salts  and  an  albuminoid  body,  mucin,  to  the  presence  of  which  it  owes  its 
viscidity.  Much  of  the  mucus  is  secreted  by  the  goblet  cells  on  the 
surface  of  the  mucous  membranes.  The  principal  varieties  of  mucus  are 
the  nasal,  bronchial,  vaginal,  urinary,  and  gastro-intestinal. 

The  serous  membranes  are  composed  of  thin  membrane  formed  by  a 
condensation  of  connective  tissue  and  covered  by  a  single  layer  of  large, 
flat,  nucleated  cells  with  irregular  margins.  These  membranes  enclose 
v/hat  are  practically  large  lymph-sacs  or  spaces,  and  the  fluid  they  contain 
resembles  lymph  in  all  respects  and  is  practically  identical  with  it.  It  serves 
to  diminish  friction  when  the  viscera  they  enclose  move  over  one  another. 
The  most  important  of  the  serous  membranes  are  the  pleural,  pericardial, 
and  peritoneal. 

The  synovial  membranes  in  and  around  joints  resemble  serous  membranes. 
The  cells  covering  them,  however,  secrete  a  clear,  colorless  fluid  resembling 
lymph,  but  differing  from  it  in  containing  a  m.ucin-like  substance,  a  nucleo- 
albumin,  which  imparts  to  it  considerable  viscidity.  This  synovial  fluid 
serves  to  diminish  friction  between  the  opposing  surfaces  of  the  bones  as 
they  glide  over  one  another  during  movement. 

Other  secretions,  such  as  the  aqueous  and  vitreous  humors  of  the  eye, 
the  fluid  of  the  internal  ear,  the  cerebrospinal  fluid,  etc.,  will  be  considered 
in  connection  with  the  organs  with  which  they  are  associated. 

The  secreting  glands  are  formed  of  the  same  histologic  elements  as 
the  secreting  membranes.  They  are  formed  by  an  involution  of  the  mucous 
membrane  or  skin,  the  epithelium  of  which  is  variously  modified  structurally 
and  functionally  in  the  various  situation?  in  which  they  are  formed.  Like 
the  membranes  themselves,  the  glands  are  invested  by  capillary  blood- 
vessels and  supplied  with  lymph-vessels  and  nerves,  of  which  the  latter 
are  in  direct  connection  with  the  blood-vessels  and  epithelial  cells.  The 
interior  of  each  gland  is  in  communication  with  the  free  surface  by  one  or 
more  passageways  known  as  ducts. 

These  glands  may  be  classified  according  as  the  involution  is  cylindrical 
or  dilated  as — 

1.  Tubular.  The  tubular  glands  may  be  simple — e.g.,  sweat-glands, 
intestinal  glands,  fundus  glands  of  the  stomach;  or  compound — e.g.,  kidney, 
testicle,  salivary,  and  lachrymal  glands. 

2.  Alveolar.  The  alveolar  glands  may  also  be  simple — e.g.,  the  seba- 
ceous glands,  the  ovarian  follicles,  meibomian  glands;  or  compound,  as 
the  mammary  glands  and  salivary  glands. 

For  the  production  of  a  secretion  it  is  necessary  that  the  plasma  of  the 
blood,  the  common  material,  be  delivered  to  the  lymph-spaces  with  which 
the  epithelial  cells  are  in  close  relation.  The  processes  involved  in  the  pass- 
age of  the  plasma  across  the  capillary  wall  have  already  been  considered 


478  TEXT-BOOK  OF  PHYSIOLOGY 

in  connection  with  the  production  of  lymph.  They  include  the  physical 
processes,  diffusion,  osmosis,  and  filtration  combined  with  a  secretor  activity 
of  the  cells  of  the  capillary  wall.  The  question  as  to  which  of  these  pro- 
cesses is  the  more  active  is  yet  a  subject  of  investigation. 

As  the  chemic  composition  and  the  chemic  features  of  the  organic  con- 
stituents of  all  secretions  have  been  demonstrated  to  be  the  outcome  of 
metabolic  processes  going  on  within  the  epithelial  cells,  it  must  be  assumed 
at  least  that  these  differences  are  correlated  with  differences  in  the  histo- 
logic features  and  chemic  composition  of  the  epithelium.  The  discharge  of 
the  secretion  is,  as  a  rule,  intermittent;  that  is,  there  are  periods  of  inactivity 
or  rest.  In  rest  more  especially  the  epithelial  cells,  after  the  assimilation 
of  lymph,  accumulate  within  themselves  such  characteristic  products  as 
globules  of  mucin,  granules  which  apparently  are  the  antecedents  of  the  diges- 
tive enzymes,  granules  of  glycogen,  globules  of  fat,  sugar,  and  proteins, 
as  in  the  case  of  the  mammary  gland.  In  how  far  all  these  compounds  are 
the  result  of  secretor  activity  or  of  a  cell  degeneration  and  disintegration  it  is 
impossible  to  state  in  the  light  of  present  knowledge.  During  the  period 
of  gland  rest  the  blood-supply  to  the  gland  is  merely  sufficient  for  nutritive 
purposes.  When  the  occasion  arises  for  gland  activity,  the  blood-vessels, 
under  the  influence  of  the  vaso-motor  nerves,  dilate  and  deliver  to  the  gland 
an  amount  of  blood  far  beyond  that  required  for  nutritive  purposes.  As  a 
result  the  gland  becomes  red  and  vascular  and  the  blood  emerging  by  the 
veins  frequently  retains  its  customary  arterial  color.  The  increased  blood- 
supply  favors  a  rapid  transudation  of  water  and  salts  into  the  lymph-spaces 
from  which  they  are  speedily  absorbed  and  transmitted  by  the  epithelial 
cells  into  the  interior  of  the  gland  lumen.  Coincident  with  the  passage 
of  water  through  the  cell,  the  organic  constituents  are  extruded  from  the  end 
of  the  cell  bordering  the  lumen  to  become  dissolved,  or  in  the  case  of  fat  to 
be  suspended,  in  the  water.  The  secretion  thus  formed  accumulates  and 
with  the  rise  of  pressure  which  inevitably  follows  it  at  once  passes  into  the 
ducts  to  be  discharged  on  the  surface  of  the  mucous  membrane  or  skin,  as  the 
case  may  be. 

The  Influence  of  the  Nerve  System. — ^The  activity  of  every  gland  is  con- 
trolled by  nerve-centers  situated  in  the  central  nerve  system.  These  centers 
may  be  excited  to  activity  either  by  impressions  made  on  the  peripheral 
terminations  of  afferent  nerves  or  by  emotional  states,  or,  possibly,  by  changes 
in  the  composition  of  the  blood  itself.  As  a  rule,  all  normal  secretion  is  a 
reflex  act  involving  the  usual  mechanism:  viz.,  a  receptive  surface  (skin, 
mucous  membrane,  or  sense-organ),  an  afferent  nerve,  an  emissive  cell  from 
which  emerges  an  efferent  nerve  to  be  distributed  to  a  responsive  organ,  the 
gland  epithelium,  though  the  secretion  may  in  some  instances  be  initiated 
by  a  psychic  state. 

For  the  production  of  the  secretion  by  the  epithelial  cell  it  is  believed 
by  some  experimenters  that  two  physiologically  distinct,  efferent  nerve- 
fibers  are  involved — one  stimulating  the  production  of  the  organic  constituents 
{trophic  nerves),  the  other  stimulating  the  secretion  of  water  and  inorganic 
salts  {secretor  nerves).  The  evidence  for  the  influence  of  the  nerve  system 
on  secretion  and  the  mode  of  connection  of  the  nerve-fibers  with  the  gland- 
cells  have  been  alluded  to  (page  156)  and  will  again  be  in  subsequent  chapters. 

The  structure  of  the  glands  of  external  secretion,  the  composition  and 


EXTERNAL  SECRETIONS 


479 


physiologic  actions  of  their  secretions  have  in  large  part  been  considered 
in  the  foregoing  chapter  on  Digestion.  There  remains,  however,  to  be  con- 
sidered the  mammary  glands,  the  liver  and  the  sebaceous  glands. 

MAMMARY  GLANDS 

The  mammary  glands,  which  secrete  the  milk,  are  two  more  or  less 
hemispheric  organs  situated  in  the  human  female  on  the  anterior  surface  of 

the  thorax.  Though  rudimentary  in 
childhood,  they  gradually  increase  in 
size  as  puberty  approaches.  The  gland 
presents  at  its  convexity  a  small  conical 
eminence  termed  the  mammilla  or  nipple, 
surrounded  by  a  circular  area  of  pig- 
mented skin,  the  areola.  The  gland 
proper  is  covered  by  a  layer  of  adipose 


fkp... 


Fig.  215. — Mammary  Glajvd.  i. 
Lactiferous  ducts.  2.  Lobuli  of  the 
mammary  gland. 


Fig.  216  — Acini  of  the  Mammary 
Gland  of  a  Sheep  During  Lactation. 
a.  Membrana  propria.  b.  Secretory 
epitbelium. 


tissue  anteriorly  and  is  attached  posteriorly  to  the  pectoral  muscles  by  a  net- 
work of  fibrous  tissue. 

During  utero-gestation  the  mammary  glands  become  larger,  firmer, 
and  more  lobulated;  the  areola  darkens  and  the  blood-vessels,  especially 
the  veins,  become  more  prominent.  At  the  period  of  lactation  the 
gland  is  the  seat  of  active  histologic  and  physiologic  changes  correlated 
with  the  production  of  milk.  At  the  close  of  lactation  these  activities  cease, 
the  glands  diminish  in  size,  undergo  involution,  and  gradually  return  to  their 
former  non-secreting  condition. 

Structure  of  the  Mammary  Gland. — Each  mammary  gland  consists 
of  an  aggregation  of  some  15  or  20  irregular  pyramidal  lobes,  each  of  which 
is  surrounded  by  a  framework  of  fibrous  tissue.  This  tissue  affords  support 
for  blood-vessels,  lymph-vessels,  and  nerves.  Each  lobe  is  provided  with  a 
single  excretory  duct,  the  lactiferous  duct,  which  as  it  approaches  the  areola 
expands  into  a  fusiform,  ampulla  or  reser\^oir.  At  the  base  of  the  nipple 
the  ampullae  contract  to  form  some  15  or  20  narrow  ducts,  which,  ascending 
the  nipple,  open  by  constricted  orifices  0.5  mm.  in  diameter  on  its  apex 
(Fig.  215). 

On  tracing  the  lactiferous  duct  into  a  lobe,  it  is  found  to  divide  and 
subdivide  into  a  number  of  branches,  which  pass  into  smaller  masses — the 
lobules.  The  lobule  in  turn  is  composed  of  a  large  number  of  tubular  acini 
or  alveoli,  the  final  terminations  of  the  lobular  ducts.     Each  acinus  consists 


48o 


TEXT-BOOK  OF  PHYSIOLOGY 


of  a  basement  membrane  lined  by  a  single  layer  of  low  cuboidal  epithelial 
cells  (Fig.  216).  Externally  the  acinus  is  surrounded  by  blood-vessels, 
nerves,  and  lymphatics. 

MILK 

Physical  Properties. — Milk  as  obtained  during  active  lactation  is  an 
opaque  bluish-white  fluid,  almost  inodorous,  with  a  sweet  taste,  an  alkaline 
reaction,  and  a  specific  gravity  of  from  1.025  to  1.040.  Examined  micro- 
scopically, it  is  seen  to  consist  of  a  clear  fluid,  the  milk  plasma,  holding  in 
suspension  an  enormous  number  of  small,  highly  refractive  oil-globules, 
which  measure  on  the  average  about  y  o^i^o  of  an  inch  in  diameter.  It  has 
been  asserted  by  some  observers  that  each  globule  is  surrounded  by  a  thin 
protein  envelope  which  enables  it  to  maintain  the  discrete  form.  This,  how- 
ever, is  at  present  disbelieved. 

The  quantity  of  milk  secreted  daily  by  the  human  female  averages  about 
1200  c.c. 

Chemic  Composition. — ^The  chemic  analysis  of  milk  shows  that  it  con- 
sists of  all  the  different  classes  of  nutritive  principles,  which  are  necessary 
to  the  growth  and  development  of  the  body.  The  only  exception  appears 
to  be  an  insufficient  amount  of  iron  (3  or  4  milligrams  per  1000  grams  of 
milk)  for  the  formation  of  the  coloring  matter  of  the  blood,  the  hemoglobin. 
This  is  provided,  however,  by  the  liver  in  which  the  iron  accumulates  during 
intra-uterine  life.  According  to  Bunge  the  liver  of  a  rabbit  newly  born  con- 
tains as  much  as  18.2  milligrams  per  100  grams  of  body-weight,  while  at 
the  end  of  twenty-four  days  it  contains  only  3.2  milligrams  per  100  grams  of 
body-weight.  The  composition,  however,  varies  not  only  in  different  classes 
of  mammals  but  in  the  same  animal  at  different  times  and  under  different 
physiologic  conditions.  Average  compositions  of  milk  of  certain  animals  are 
shown  in  the  following  table: 

THE  COMPOSITION  OF  MILK 


Constituents 


Human 


Cow 


Goat 


Mare 


Ass 


Water , 

Caseinogen     \ 
Lactalbumin  / 

Fat 

Lactose 

Inorganic  Salts 


87.80 
1-50 

3-5° 
7  .00 
0.20 


87  .00 

3.20 

3.80 
5.00 
0.50 


86.91 
3  69 


90.00 
1.80 
0.30 

1-30 
5-50 
0.30 


90.00 

2  .10 

1.30 
6.30 
0.30 


Caseinogen  is  the  chief  protein  constituent  of  milk.  Associated  with  it, 
however,  are  two  other  proteins,  lactalbumin  and  lactoglobulin,  both  of 
which  are  present  in  but  small  quantity.  When  milk  is  treated  with  acetic 
acid,  sodium  chlorid,  or  magnesium  sulphate  to  saturation,  the  caseinogen 
is  precipitated  as  such,  and  after  the  removal  of  the  fat  with  which  it  is  entan- 
gled may  be  collected  by  appropriate  chemic  methods.  On  the  addition  of 
rennet,  an  alcoholic  extract  of  the  mucous  membrane  of  the  calf's  stomach, 
which  contains  the  enzyme  rennin  or  pexin,  the  caseinogen  undergoes  a 
conversion  into  an  insoluble  protein,  casein  or  tyrein.  To  this  process  the 
term  coagulation  has  been  given.  The  presence  of  calcium  phosphate 
appears  to  be  essential  to  this  process,  inasmuch  as  it  does  not  take  place  if 


EXTERNAL  SECRETIONS 


481 


the  milk  be  completely  decalcified  by  the  addition  of  potassium  oxalate. 
After  coagulation,  the  more  or  less  solid  mass  of  milk  separates  into  a  liquid 
portion,  the  serum,  and  a  solid  portion,  the  coagulum.  The  former,  gener- 
ally termed  whey,  consists  of  water,  salts,  lactalbumin,  sugar;  the  latter,  the 
curd,  consists  of  the  casein  and  entangled  fat.  Boiling  the  milk  retards  and 
even  prevents  the  coagulation  by  rennet,  owing  to  the  precipitation  of  the 
calcium  phosphate.  When  milk  is  taken  into  the  stomach,  it  is  probable 
that  the  rennin  coagulates  the  caseinogen  in  a  manner  similar  to,  if  not  iden- 
tical with,  this  process,  which  appears  to  be  essential  to  the  normal  digestion 
of  the  milk. 

Fat  is  present  in  the  condition  of  a  very  fine  emulsion  and  is  more  or  less 
solid  at  ordinary  temperatures.  It  is  a  compound  of  olein,  palmitin,  and 
stearin  with  small  quantities  of  butyrin  and  caproin.  The  melting-point  of 
butter  varies  between  31°  and  34°C.  When  milk  is  allowed  to  stand  for 
some  time,  the  fat-globules  rise  to  the  surface  and  form  a  thick  layer  known 
as  cream..  Churning  the  milk  or  cream  causes  the  fat-globules  to  run  to- 
gether and  form  a  coherent  mass  termed  butter. 

Lactose  is  the  particular  form  of  sugar  characteristic  of  milk.  It  belongs 
to  the  saccharose  group  and  has  the  following  composition:  C^Ji^^O^^. 
Though  incapable  of  undergoing  fermentation  by  the  action  of  the  yeast  plant 
it  is  readily  reduced  by  the  Bacillus  acidi  lactici  to  lactic  acid  and  carbon 
dioxid,  the  former  of  which  imparts  to  milk  an  acid  reaction  and  a  sour 
taste.  With  the  accumulation  of  the  lactic  acid  the  caseinogen  is  precipi- 
tated as  a  more  or  less  consistent  mass. 

Inorganic  salts  are  always  present  and  are  chiefly  those  of  potassium, 
sodium,  calcium,  and  magnesium  phosphates  and  chlorids.  The  following 
table  of  Bunge  gives  the  quantitative  amounts  of  these  constituents  in  both 
human  and  cow's  milk: 


In  1000  Parts              ^Pj^^" 
Slum 

Sodium 

Calcium  ^fZ^ 
smm 

Iron 
Oxid 

Phos- 
phoric 
Acid 

Chlorin 

Human  milk 0.78 

0.2?             0    -3  2             0   nf\            n   c\riif\       \      n   ai 

0.43 
1.69 

Cow's  milk 1.76 

I. II 

1.59          0.21 

0.0030 

1.97 

Iron  is  also  present  in  small  amounts  possibly  from  3  to  5  milligrams  per 
1000  c.c.     Citric  acid  to  the  extent  of  0.05  per  cent,  is  also  present. 

Mechanism  of  Milk  Secretion. — During  the  time  of  lactation  the 
mammary  gland  exhibits  periods  of  secretory  activity  which  alternate  with 
periods  of  repose.  Coincidently  with  these  periods  certain  histologic  changes 
take  place  in  the  secreting  epithelium.  At  the  close  of  a  period  of  active 
secretion  and  after  the  discharge  of  the  milk  each  acinus  presents  the  follow- 
ing features:  The  epithelial  cells  are  short,  cubical,  nucleated,  and  border  a 
relatively  wide  lumen,  in  which  is  found  a  variable  quantity  of  milk.  After 
the  gland  has  rested  for  some  time  active  metabolism  again  begins.  The 
cells  grow  and  elongate;  the  nucleus  divides  into  two  or  three  new  nuclei; 
constriction  takes  place  and  the  inner  portion  is  detached  and  discharged 
into  the  lumen  of  the  acinus.  During  the  time  these  changes  are  taking 
place  oil-globules  make  their  appearance  in  the  cell  protoplasm,  some  of 
31 


482 


TEXT-BOOK  OF  PHYSIOLOGY 


Fig.  217. — Section  OF  the 
Mammary  Gland  of  a  Cat 
IN  THE  Early  Stages  of  Lac- 
tation. A.  Cavity  of  alveoli 
filled  with  granules  and  glob- 
ules of  fat.  I,  2,  3.  Epithe- 
lium in  various  stages  of 
milk-formation. — {Yeo.) 


which  are  discharged  separately  into  the  lumen,  while  others  remain  for  a 
time  associated  with  the  detached  portion  of  the  cell  (Fig.  217).  From 
these  histologic  changes  it  is  inferred  that  the  caseinogen  and  fat  are  products 
of  the  metabolism  of  the  cell  protoplasm  and  not  derived  directly  from 
the  lymph  from  the  blood.  The  lactose  apparently  has  a  similar  origin,  as 
appears  from  the  fact  that  it  is  not  found  either  in  the  blood  or  any  other 
tissue,  and  that  it  is  formed  independently  of  carbohydrate  food.  The  water, 
and  especially  the  inorganic  salts,  are  the  result  of  secretor  activity  rather 

than  of  diffusion  and  filtration.  This  is  ren- 
dered probable  from  the  fact  that  the  propor- 
tions of  the  inorganic  salts  of  milk  are  more 
closely  allied  to  those  of  the  tissues  of  the  new- 
born child  than  to  blood.  With  the  passage  of 
the  water  and  salts  into  the  lumen  of  the  acinus 
the  proteids  undergo  disintegration  and  solution 
and  the  liquid  assumes  the  characteristics  of 
milk. 

The  discharge  of  milk  is  occasioned  by  the 
suction  efforts  on  the  part  of  the  child,  aided  by 
atmospheric  pressure  and  the  contractions  of 
the  non-striated  muscle-fibers  of  the  lactiferous 
ducts. 

The  Developmental  Changes  and  the  Se- 
cretion of  Milk. — Previous  to  menstruation  the 
glands  are  quite  rudimentary,  but  with  the  estab- 
lishing of  this  function  they  increase  in  size  as  a  result  of  the  growth  of  con- 
nective tissue  and  the  deposition  of  fat.  The  glandular  tissue  is,  however, 
more  or  less  inactive,  but  with  impregnation  it  too  undergoes  a  rapid  devel- 
opment. The  alveoli  increase  in  number  and  size  and  toward  the  end  of 
the  period  of  pregnancy  begin  to  secrete  the  forerunner  of  milk,  the  fluid 
colostrum.  From  the  foregoing  it  is  apparent  that  there  is  a  close  relation- 
ship between  the  activities  of  the  ovaries  and  the  uterus  and  the  activities 
of  the  mammary  glands  though  the  mechanism  by  which  the  two  groups  of 
activities  are  mediated  is  not  very  clear.  Starling  and  Lane-Claypon  have 
thrown  some  light  on  the  mechanism  by  the  demonstration  that  the  injection 
of  water  extracts  of  developing  feti  into  the  body  of  a  non-pregnant  mam- 
mal— a  rabbit — for  a  period  of  from  15  to  17  days  was  followed  by  the  devel- 
opment of  the  mammary  glands,  similar  if  not  identical  with  the  develop- 
ment that  occurs  during  an  actual  pregnancy.  From  this  fact  the  inference 
is  drawn  that  during  intra-uterine  life,  the  fetus  as  it  develops  secretes  an 
agent  which,  entering  the  maternal  blood,  is  carried  to  the  mammary  glands 
and  by  it  stimulated  to  growth.  The  agent  thus  produced  by  the  fetus 
would  belong  to  the  class  of  agents  known  as  hormones.  Since  those  kata- 
bolic  changes  in  the  cells  of  the  glands  which  eventuate  in  the  formation  of 
milk  do  not  arise  during  the  intra-uterine  life  of  the  fetus,  but  which  soon 
arise  after  birth,  the  further  assumption  is  made  that  this  hormone  has  an 
inhibitor  influence  on  the  gland-cells  which  prevents  the  formation  of  milk; 
but  with  its  withdrawal  after  birth  secretion  at  once  begins. 

Influence    of    the    Nerve    System. — Judging    from    analogy,    it    is 
probable  that  the   secretion  of  milk  is  regulated  by  impulses   emanating 


EXTERNAL  SECRETIONS  483 

from  the  nerve  system,  though  the  exact  nerve-channels  for  the  trans- 
mission of  such  impulses  have  not  been  determined  experimentally. 
The  results  of  experiments  made  on  the  nerve  distribution  to  the 
mammary  gland  of  the  goat  are  unsatisfactory  and  contradictory  and 
do  not  shed  much  light  on  the  subject.  Even  after  division  of  all  nerves 
to  this  gland  in  this  animal,  secretion  continues  in  an  apparently  normal 
manner.  Nevertheless,  it  is  well  known  that  emotional  states  on  the  part  of 
the  mother  modify  the  quantity  as  well  as  quality  of  milk,  indicating  a  con- 
nection between  the  gland-cells  and  the  central  organs  of  the  nerve  system. 
Nerve  terminals  have  been  discovered  in  and  around  the  epithelial  cells — a 
fact  which  supports  this  view. 

Colostrum. — Within  a  day  or  two  after  parturition  the  alveoli  become 
filled  with  a  fluid  which  in  some  respects  resembles  milk  and  which  has  been 
termed  colostrum.  This  is  a  watery  fluid  containing  disintegrated  epithelial 
cells  and  fat-globules,  as  well  as  a  colostrum  corpuscles,  which  are  probably 
leukocytes  containing  line  fat-globules.  Colostrum  is  distinguished  from 
milk  in  being  richer  in  sugar  and  inorganic  salts.  It  also  differs  from  milk 
in  undergoing  coagulation  by  heat  which  is  supposed  to  be  due  to  the  pres- 
ence of  a  globulin.  Its  coagulation  point  is  about  72°C.  It  is  said  to 
possess  constituents  which  act  as  a  laxative  to  the  young  child. 

THE   LIVER 

The  liver  is  a  large  gland  situated  in  the  upper  and  right  side  of  the 
abdominal  cavity,  where  it  is  held  in  position  largely  by  ligaments  formed  by 
reduplications  of  the  peritoneal  investment.  In  the  adult  it  weighs,  freed 
of  blood,  from  1300  to  1700  grams.  The  liver  is  connected  with  the  duo- 
denal portion  of  the  intestine  by  the  hepatic  duct.  It  receives  blood  both 
from  the  hepatic  artery  and  from  the  portal  vein,  and  in  this  respect  differs 
from  all  other  glands  in  the  body.  The  epithelial  structures  of  the  liver  are 
inclosed  by  a  firm  fibrous  membrane,  known  as  Glisson's  capsule.  At  the 
transverse  fissure  it  invests  and  follows  the  blood-vessels,  which  there  enter, 
in  all  their  ramifications  through  the  gland. 

Structure  of  the  Liver. — The  liver  is  composed  of  an  enormous  num- 
ber of  small  masses,  rounded,  ovoid,  or  polygonal  in  shape,  called  lobules, 
measuring  about  one  millimeter  in  diameter  and  separated  from  one  another 
by  a  narrow  space  in  which  are  to  be  found  blood-vessels,  lymphatics,  and 
hepatic  ducts,  supported  by  connective  tissue.  In  the  pig  this  space  and  its 
contained  elements  is  quite  distinct,  sharply  marking  out  the  border  of 
the  lobule  (Fig.  218).  This  is  not  so  apparent  in  man.  Each  lobule'Iis 
made  up  of  irregular  or  polygonal-shaped  cells  measuring  about  30  to  40 
micromillimeters  in  diameter.  These  cells  are  arranged  in  a  radial  manner 
from  the  center  to  the  circumference  of  the  lobule  (Fig,  219).  Each  cell 
possesses  one  and  at  times  two  nuclei.  There  is  no  evidence  for  the  exist- 
ence of  a  distinct  cell-wall.  The  cell  protoplasm  frequently  contains  glob- 
ules of  fat,  granules  of  a  protein  nature,  granules  of  glycogen,  pigment  ma- 
terial, etc.  The  appearance  presented  by  the  cell  will  vary  considerably,  ac- 
cording to  the  time  it  is  observed.  Thus  there  may  be  a  complete  absence  of 
these  constituents,  when  the  cell  may  present  a  series  of  vacuoles  separated 
by  bands  of  protoplasm.     The  cells  are  the  secreting  structures  of  the  liver, 


484 


TEXT-BOOK  OF  PHYSIOLOGY 


and  hence  are  in  close  relation  to  capillary  blood-vessels,  lymph  spaces, 
nerves,  and  irregular  channels  or  passageways.  The  latter  running  between 
the  epithelial  cells  may  be  compared  to  the  lumen  of  other  secreting  glands. 
Blood-vessels  and  Their  Distribution. — The  blood-vessels  which  are 
in  relation  with  the  liver  are: 

1.  The  portal  vein. 

2.  The  hepatic  artery. 

3.  The  hepatic  vein. 

The  portal  vein  and  the  hepatic  artery  enter  the  liver  at  the  transverse 
fissure.  After  penetrating  its  substance  they  divide  and  subdivide  into 
smaller  and  smaller  branches,  which  ulimately  occupy  the  space  between  the 
lobules,  completely  surrounding  and  lim- 
iting them.  From  their  situation  they  are 
termed  interlobular  veins  and  arteries. 

The  interlobular  veins  give  off  small  cap- 
illary vessels  which  penetrate  the  lobule  at 
all  points  of  its  surface.     These  capillaries, 


^  Trabeculae  of 
hepatic  cells. 


Antral  vein. 


Fig.  218. — Section  of  Liver  of  Pig,  showing 
/ERY  Di.\GRAMMATiCALLY  THE  LOBULES.  a.  Interlobu- 
lar connective  tissue,  h,  c.  Branches  of  portal  vein 
and  of  hepatic  artery,  d.  Bile-ducts,  e.  Intralobular 
vein. — {Pier  sol.) 


Interlobular  vein.    Hepatic  duct. 


Fig.  aiQ.^ScHEME  of  a  Hepatic 
Lobule,  represented  in  Transverse 
Section  below  and,  by  Partial  Lev- 
eling, IN  Longitudinal  Section 
Above.  In  the  left  half  the  blood- 
vessels are  drawn;  in  the  right  half 
onlv  the  cords  of  hepatic  cells.  X  20. 
■—(Stohr.) 


though  frequently  anastomosing,  form  a  radial  meshwork  which  converges 
toward  the  center  of  the  lobule.  In  the  meshes  of  this  plexus  are  found, 
arranged  in  a  corresponding  radial  manner,  the  liver  cells.  The  interlobular 
arteries  are  distributed  to  the  walls  of  the  portal  vein,  to  the  connective 
tissue,  and  finally  terminate  in  the  portal  vein  capillaries.  The  intralobu- 
lar capillaries  thus  receive  and  transmit  blood  which  is  an  admixture  of  both 
arterial  and  venous  blood.  In  the  center  of  each  lobule  there  is  a  large  vein, 
formed  by  the  union  of  the  intralobular  capillaries,  known  as  the  intralobular 
vein,  which  collects  all  the  blood  of  the  lobule  and  transmits  it  through  the 
lobule  to  an  underlying  or  sublobular  vein  (Fig.  220).  These  latter  vessels, 
uniting  and  reuniting,  ultimately  form  the  hepatic  vein,  which  empties  the 
blood  into  the  inferior  vena  cava. 

Bile  Capillaries  and  Hepatic  Ducts. — The  bile  capillaries  are  narrow 
channels  which  penetrate  the  lobule  in  all  directions  and  are  generally  found 
running  along  the  sides  of  the  cells.     These  channels,  which  are  devoid  of 


EXTERNAL  SECRETIONS  485 

walls,  receive  from  the  cells  some  of  the  products  of  their  secretor  activity, 
and  hence  are  comparable  to  the  lumen  of  the  alveoli  of  other  secreting 
glands.  At  the  periphery  of  the  lobules  the  bile  capillaries  communicate 
with  larger  channels  which  are  the  beginnings  of  the  hepatic  or  bile-ducts 
lying  in  the  interlobular  spaces.  The  interlobular  bile-ducts  possess  a  dis- 
tinct wall  lined  by  flattened  epithelium.  There  is,  however,  no  distinct  line 
of  demarcation  between  the  cells  of  the  interlobular  ducts  and  the  secreting 
cells  of  the  liver  proper,  as  the  two  blend  insensibly,  the  one  into  the 
other.  As  the  hepatic  ducts  increase  in  size  they  gradually  acquire  the 
structure  characteristic  of  the  main  hepatic  duct:  viz.,  a  mucous,  a  muscle, 
and  a  fibrous  coat. 


-^Z; 


Fig.  220. — TR.4NSVEB5E  Section  of  a  Single  Hepatic  Lobule,  i.  Intralobular  vein,  cut 
across.  2,2,2,2.  Afferent  branches  of  the  intralobular  vein.  3,3,3,3,3,3,3,3,3-  Interlobular 
branches  of  the  portal  vein,  w"ith  its  caJDillary  branches,  forming  the  lobular  plexus,  extending  to 
the  radicles  of  the  intralobular  vein. — (Sappey.) 

The  Nerve  Supply. — Experimental  investigations  have  demonstrated 
that  the  liver  is  supplied  with  nerves  derived  from  the  central  nerve  system. 
The  route  of  these  nerves  is  probably  by  way  of  the  vagi  and  of  the  splanch- 
nics  as  far  as  the  semilunar  ganglion,  around  the  cells  of  which  the  terminal 
fibers  arborize.  From  the  cells  of  this  ganglion  post-ganglionic  fibers  arise 
which  pass  by  way  of  the  hepatic  plexus  along  the  course  of  the  hepatic 
artery  and  portal  vein.  Many  of  the  nerves  which  enter  the  liver  are  vaso- 
motor in  function;  as  to  W'hether  others  are  secretor  in  character  is  yet  a  sub- 
ject of  investigation.  It  has  been  asserted  that  nerve  filaments  have  been 
demonstrated  running  between  the  cells  and  even  penetrating  their  substance. 
This  fact  would  indicate  that  the  metabolic  processes  of  the  liver  are  under 
the  control  of  the  central  nerve  system. 

Functions  of  the  Liver. — The  anatomic  and  histologic  peculiarities  of 
the  liver  would  indicate  that  it  has  a  variety  of  relations  to  the  general  pro- 
cesses of  the  body.  Experimental  investigation  has  brought  some  of  these 
relations  to  light.  Though  its  physiologic  actions  are  not  yet  wholly  under- 
stood, it  may  be  said  that  it  is  engaged  in: 

1.  The  elaboration  and  excretion  of  bile. 

2.  The  production  of  starch  (glycogen)  and  sugar  (glacose). 


486  TEXT-BOOK  OF  PHYSIOLOGY 

3.  The  formation  of  urea. 

4.  The  conjugation  of  products  of  protein  putrefaction. 

The  Elaboration  of  Bile. — The  physical  properties  and  chemical  com- 
position of  the  bile  have  already  been  considered  (page  195).  The  character- 
istic salts  of  the  bile,  sodium  glycocholate  and  taurocholate,  do  not  pre-exist 
in  the  blood,  and  therefore  must  be  formed  by  the  liver  cells  out  of  materials 
derived  from  the  blood  of  the  intralobular  capillaries.  The  antecedents  of 
the  bile  salts,  glycocoll  and  taurin,  are  crystallizable  nitrogenized  compounds, 
and  known  chemically  as  amido-acetic  and  amido-ethylsulphonic  acids. 
Their  chemic  composition  indicates  that  they  are  derivatives  of  the  proteins, 
though  the  intermediate  stages  in  their  production  are  unknown.  The 
origin  of  the  cholalic  acid  with  which  they  are  combined  is  equally  obscure. 
The  bile  salts  as  they  are  found  in  the  bile  are  produced  however  in  the  liver 
cells  by  metabolic  activity. 

The  primary  coloring-matter  of  the  bile,  bilirubin,  has  been  shown  to 
be  a  derivative  of  hematin,  a  product  of  the  disintegration  of  hemoglobin. 
It  is  supposed  that  the  liver  cells  bring  about  this  change  by  combining  water 
with  hematin,  with  the  abstraction  of  iron.  The  product  thus  formed  is  bili- 
rubin, which  is  excreted,  while  the  iron  is  for  the  most  part  retained  in  the 
liver  cells. 

Cholesterin  is  a  waste  product  derived  largely  from  the  nerve-tissue. 
It  is  brought  to  the  liver  and  simply  excreted  by  the  cells.  The  remaining 
constituents  of  the  bile,  water  and  inorganic  salts,  are  secreted  here  in  the 
same  way  as  in  all  other  glands. 

When  once  formed,  the  liver  cells  discharge  these  various  compounds 
into  the  channels  by  which  they  are  surrounded;  they  then  pass  into  the  open 
mouths  of  the  bile-ducts  at  the  periphery  of  the  lobules.  Under  the  increas- 
ing pressure  which  arises  from  the  secretion  and  accumulation  of  bile,  this 
fluid  flows  from  the  smaller  into  the  larger  bile-ducts,  and  finally  is  emptied 
either  directly  into  the  intestine  or  into  the  gall-bladder,  where  it  is  stored 
until  required  for  digestive  purposes.  The  secretion  of  bile,  as  observed 
by  means  of  a  biliary  fistula,  is  continuous  and  not  intermittent,  though  the 
rate  of  flow  is  subject  to  considerable  variation. 

The  liver  cells,  as  far  as  the  secretion  of  bile  is  concerned,  appear  to  be 
independent  of  the  nerve  system.  Their  activity,  however,  is  stimulated  by 
the  increased  blood-supply  which  arises  during  digestion  in  consequence  of 
the  dilatation  of  the  intestinal  vessels,  since  it  is  at  this  period  that  the  rate 
of  discharge  is  the  greatest.  The  same  results  have  been  shown  by  experi- 
ment. Thus,  division  of  the  splanchnic  nerves  is  followed  by  an  increased 
discharge  of  bile,  apparently  due  to  the  dilatation  of  the  portal  vessels;  stimu- 
lation of  their  peripheral  ends  is  followed  by  a  decreased  discharge  of  bile 
in  consequence  of  the  contraction  of  the  portal  vessels.  The  bile  salts  appear 
to  be  the  most  efficient  stimulants  to  the  activity  of  the  liver  cells,  for  their 
administration  and  absorption  is  followed  by  an  increase  not  only  in  the 
amount  of  water,  but  of  the  inorganic  salts  and  other  solid  constituents  as 
well. 

The  flow  of  bile  from  the  bile  capillaries  to  the  main  hepatic  duct,  though 
primarily  dependent  on  differences  of  pressure,  is  aided  by  the  contraction 
of  the  muscular  walls  of  the  bile-ducts  and  the  inspiratory  movements  of  the 
diaphragm.     Any  obstacle  to  the  discharge  of  bile  leads  to  its  accumulation, 


EXTERNAL  SECRETIONS  487 

a  rise  of  pressure  beyond  that  of  the  capillary  blood-vessels,  and  a  reabsorp- 
tion  by  the  lymph-vessels  of  the  bile  constituents.  After  their  discharge  into 
the  blood  from  the  thoracic  duct  these  constituents  are  deposited  in  part 
in  various  tissues,  giving  rise  to  the  phenomena  of  jaundice,  and  in  part  are 
eliminated  in  the  urine. 

The  Production  of  Starch  (Glycogen)  and  Sugar  (Glycose  or  Glucose). 
— In  1857  Bernard  discovered  the  fact  that  the  liver  normally  during  life 
produces  a  substance,  analogous  in  its  chemic  composition  to  starch  and 
known  as  liver  starch  or  animal  starch.  This  substance  can  be  obtained  by 
the  following  method:  Small  pieces  of  the  liver  of  an  animal  recently  killed, 
preferably  after  a  meal  rich  in  carbohydrates,  are  placed  in  acidulated  boiling 
water  for  a  few  minutes;  then  rubbed  up  in  a  mortar  with  sand,  again  boiled, 
after  which  the  proteins  are  removed  by  filtration.  The  filtrate  thus  obtained 
is  opalescent  and  resembles  a  solution  of  starch.  The  starch  may  be 
precipitated  from  this  solution  with  alcohol.  It  may  subsequently  be  ob- 
tained free  by  drying,  when  it  presents  itself  as  a  white  amorphous  powder, 
soluble  in  hot  or  cold  water.  Chemic  analysis  shows  that  it  consists  of 
CgHioOj,  or  a  multiple  of  it. 

When  either  the  original  solution  obtained  by  boiling  or  a  solution  of 
this  amorphous  powder  is  treated  with  iodin,  it  strikes  a  port-wine  color. 
When  digested  with  saliva,  pancreatic  juice,  or  boiled  with  dilute  acids,  the 
solution  becomes  clear,  and  testing  with  Fehling's  solution  reveals  the  pres- 
ence of  sugar. 

For  the  reason  that  this  starch  is  capable  of  being  transformed  into  or  of 
generating  glycose  or  glucose  it  received  the  name  of  glycogen;  and  inasmuch 
as  the  liver  continually  produces  glycogen  it  may  be  said  to  have  a  starch- 
forming  or  an  amylogenetic  or  a  glycogenic  function. 

If  the  liver  be  allowed  to  remain  in  the  body  of  an  animal  for  a  period 
of  twenty-four  hours  before  the  decoction  is  made  as  above  described,  it  will 
be  found  that  the  solution  contains  only  a  small  amount  of  starch  but  a 
relatively  large  amount  of  sugar.  The  inference  drawn  is  that  after  death 
the  starch  is  transformed  by  some  agent,  possibly  a  ferment,  into  sugar 
(glucose).  From  this  fact  as  well  as  from  the  results  of  different  lines  of 
investigation,  it  is  the  generally  received  opinion  that  the  same  change  is 
constantly  taking  place  in  the  living  condition  and  therefore  the  liver  is  said 
to  have  a  sugar-forming  or  a  glycogenetic  function.  The  liver  cells  are  thus 
characterized  by  two  processes  amylogenesis  a,ndglyco genesis.  To  the  trans- 
formation of  glycogen  into  sugar  the  term  glycogenolysis  has  recently  been 
applied. 

The  presence  of  glycogen  in  the  liver  cells  can  be  shown  microscopically 
in  the  form  of  discrete  hyaline  and  refractive  masses,  which  show  a  blue  or 
violet  color  with  iodin.  As  they  are  soluble  in  water  they  can  be  readily 
dissolved  out  fromx  the  cells,  leaving  small  vacuoles  separated  from  one 
another  by  strands  of  cell  substance.  The  amount  of  glycogen  in  a  well- 
fed  animal  varies  from  1.5  to  4  per  cent,  of  the  total  weight  of  the  liver.  By 
experimental  methods  it  has  been  shown  that  the  production  of  glycogen  is 
dependent  very  largely  on  the  consumption  of  carbohydrates,  the  greater 
the  amount  of  sugar  and  starch  in  the  food,  the  greater  being  the  production 
of  glycogen.  Nevertheless  it  is  also  certain  that  glycogen  can  be  derived 
from  proteins;  for  if  the  carbohydrates  are  excluded  from  the  food  and  the 


488  TEXT-BOOK  OF  PHYSIOLOGY 

animal  fed  on  a  pure  protein  diet  plus  fat,  glycogen  will  continue  to  be 
formed  in  the  liver  though  in  far  less  amounts.  1^1 

The  facts  connected  with  the  formation  of  glycogen,  as  well  as  witli  its 
transformation  as  at  present  generally  accepted,  may  be  stated  as  follows: 
The  dextrose  or  glucose  into  which  the  carbohydrates  are  mainly  converted  by 
the  action  of  the  digestive  fluids  is  absorbed  into  the  blood  of  the  portal  vein 
and  carried  direct  to  the  liver,  where  a  certain  portion  of  it  diffuses  across 
the  capillary  walls  into  the  surrounding  lymph-spaces;  by  the  action  of  the 
cells  or  by  a  special  enzyme  it  is  then  dehydrated,  and  temporarily  deposited 
under  the  form  of  the  non-difi"usible  body  glycogen.  At  a  subsequent  period 
and  in  proportion  to  the  needs  of  the  system  the  liver  cells,  through  the  agency 
of  a  ferment,  transform  the  glycogen  into  glucose  or  dextrose  by  the  addition 
of  a  molecule  of  water,  after  which  it  is  returned  to  the  blood,  by  which  it 
is  transported  to  the  systemic  capillaries,  where  it  disappears  again,  diffusing 
across  the  walls  of  the  capillaries  into  the  surrounding  lymph-spaces  to  play 
a  part  in  the  general  nutritive  process.  Though  the  final  disposition  of  the 
sugar  is  uncertain  it  is  highly  probable  that  after  its  delivery  to  the  muscles, 
for  example,  it  may  be  directly  oxidized  or  temporarily  stored  as  glycogen 
or  possibly  be  used  in  the  formation  of  living  material.  Ultimately,  how- 
ever, through  oxidation  (glycolysis)  it  yields  heat  and  contributes  to  the 
production  of  muscle  energy.  Should  there  be  a  failure  on  the  part  of  the 
liver  cells  to  store  up  its  usual  percentage  of  the  absorbed  sugar,  lo  to  20 
per  cent.,  by  reason  of  impaired  nutrition,  disturbance  of  the  portal  circu- 
lation, or  a  larger  excess  of  sugar  in  the  blood  of  the  portal  vein,  it  would 
pass  through  the  liver  into  the  blood  of  the  general  circulation  and  increase 
the  percentage  amount  of  sugar  above  the  normal  (o.i  to  0.2  per  cent.) 
establishing  the  condition  of  hyperglycemia.  This  would  soon  be  followed 
by  its  elimination  from  the  blood  by  the  kidneys  and  its  appearance  in 
the  urine,  giving  rise  to  a  glycosuria. 

In  opposition  to  this  view.  Dr.  Pavy,  after  years  of  accurate  experimentation, 
states  that  the  blood  on  the  cardiac  side  of  the  liver  never  under  normal  circum- 
stances contains  a  larger  percentage  of  sugar  than  is  to  be  found  in  any  part  of 
the  circulation,  except  in  the  portal  vein.  He  states  that  glycogen  is  never  re- 
converted into  sugar,  and  denies  that  the  liver  produces  sugar,  to  be  discharged 
into  the  blood;  the  function  of  the  liver  is  merely  to  arrest  the  passage  of  sugar, 
and  so  to  shield  the  general  circulation  from  an  excess;  the  sugar  which  arises 
in  the  liver  after  death  is  a  post-mortem  product  and  not  an  illustration  of  what 
takes  place  during  life.  Dr.  Pavy,  having  apparently  demonstrated  the  glucosid 
constitution  of  protein  material  in  general,  accounts  for  the  presence  of  glycogen 
in  muscles  and  other  tissues  on  the  assumption  that  during  the  cleavage  of  the 
protein  molecule  the  carbohydrate  element  is  set  free  and  temporarily  stored  as 
glycogen.  He  thus  accounts  for  the  production  of  sugar  in  the  body,  even  in  the 
absence  of  all  sugar  and  starch  from  the  food.  Pavy  believes  that  the  glycogen 
produced  in  the  liver  is  utilized  in  the  formation  of  fat  and  the  synthesis  of  complex 
proteins  necessary  to  the  construction  of  the  tissues. 

The  Formation  of  Urea. — It  is  now  generally  believed  that  the  liver 
is  the  most  active  of  all  the  organs  which  may  be  engaged  in  the  production 
of  urea.  This  belief  is  based  on  numerous  physiologic  and  pathologic 
data.  The  compounds  out  of  which  the  hepatic  cells  construct  urea  have 
been  for  chemic  reasons  asserted  to  be  the  ammonium  salts,  e.g.,  the  car- 


EXTERN-'VL  SECRETIONS  489 

bonate,  carbamate,  and  lactate,  which  are  constantly  present  in  the  blood. 
These  salts,  which  result  from  protein  metabolism,  may  be  absorbed  from 
the  tissues  or  from  the  intestines,  carried  to  the  liver,  and  there  synthesized  to 
urea.  This  supposition  is  supported  by  an  experiment  as  follows:  The 
liver  of  an  animal  recently  living  is  removed  from  the  body  and  its  vessels 
perfused  continuously  with  blood  (the  urea  content  of  which  is  known) 
containing  the  ammonium  salts.  An  analysis  of  this  blood  shows,  after  a 
time,  a  diminution  of  these  salts,  and  a  large  increase  in  the  amount  of  the 
urea.  After  the  establishment  of  an  Eck  fistula  (the  union  of  the  portal 
vein  with  the  ascending  vena  cava  whereby  the  liver  is  largely  excluded  from 
acting  on  products  absorbed  from  the  intestines)  there  is  a  marked  diminu- 
tion in  the  production  of  urea  while  the  ammonia  content  of  the  urine 
largely  increases.  One  large  source  for  the  ammonium  which  is  trans- 
formed into  urea  by  the  liver  cells,  is  that  split  off  from  the  protein  molecule 
during  digestion  by  the  action  of  the  gastric  and  pancreatic  juices.  The 
ammonia  is  then  absorbed  and  combined  with  carbon  dioxid  with  formation 
of  ammonium  carbonate.  When  this  compound  is  transported  to  the  liver 
by  the  portal  blood,  the  cells  convert  it  into  urea  in  a  manner  shown  in  the 
following  formula: 

(NH  J  2CO3  -  2H2O  =  CON2H,. 

Destructive  diseases  of  the  liver — e.g.,  acute  yellow  atrophy,  suppuration, 
cirrhosis — largely  diminish  the  production  of  urea,  but  increase  the  quanti- 
ties of  the  ammonium  salts  in  the  urine.  The  same  is  true  when  the  liver 
cells  are  destroyed  during  acute  phosphorus  poisoning. 

The  Conjugation  of  Products  of  Protein  Putrefaction. — One  of  the 
important  functions  of  the  liver  is  the  conversion  of  toxic  compounds,  the 
products  of  the  putrefaction  of  proteins,  into  non-toxic  compounds.  These 
compounds  are  formed  in  the  intestine,  are  absorbed  and  carried  by  the  blood 
of  the  portal  vein  to  the  liver.  In  their  passage  through  the  capillaries  of 
the  liver  they  are  conjugated  for  the  most  part  with  potassium  sulphate  by 
the  action  of  the  liver  cells  and  thus  deprived  of  their  toxicity.  Among  the 
substances  thus  conjugated  are  indol,  skatol,  phenol,  and  cresol.  After 
absorption  indol  and  skatol  are  oxidized  to  indoxyl  and  skatoxyl  and  then 
combined  with  potassium  sulphate  giving  rise  to  potassium  indoxyl  sul- 
phate and  potassium  skatoxyl  sulphate.  Phenol  and  cresol  are  apparently 
directly  combined  with  potassium  sulphate.  All  of  these  compounds  then 
pass  into  the  blood  of  the  general  circulation  and  finally  are  eliminated  by 
the  kidneys.  Potassium  indoxyl  sulphate  or  indican  is  the  source  of  the 
indigo-forming  substance  found  in  the  urine.  Other  compounds  are  like- 
wise reduced  in  toxicity  by  the  liver  cells  though  the  methods  by  which  this 
is  accomplished  vary  with  the  nature  of  the  compound.  The  liver  thus 
presents  a  chemic  defense  against  the  entrance  of  more  or  less  toxic  agents 
into  the  blood  of  the  general  circulation. 


CHAPTER  XIX 
INTERNAL  SECRETION 

An  internal  secretion  may  be  defined  as  a  more  or  less  complex  material 
or  agent,  produced  by  the  secretor  activities  of  epithelial  cells  of  organs  and 
tissues,  and  which  are  discharged  into  the  blood  and  distributed  to  organs 
more  or  less  remote,  the  activities  of  which  they  influence  in  varying  ways 
and  degrees.  Some  increase,  some  inhibit  physiologic  processes  while  others 
stimulate  growth  and  in  different  ways  modify  metabolism.  The  internal 
secretion  in  many,  if  not  all  instances  belongs  to  a  class  of  agents  known  as 
hormones,  agents  which  according  to  Starling,  are  of  known  or  unknown 
composition,  characterized  by  a  relatively  simple  chemic  or  molecular  com- 
position, an  easy  diffusibility  across  the  walls  of  the  capillary  blood-vessels, 
a  ready  susceptibility  to  oxidation  and  a  rapid  elimination,  as  a  result  of 
which,  their  action  does  not  continue  indefinitely.^ 

The  internal  secretions  and  the  organs  by  which  they  are  produced, 
constitute  a  secondary  coordinating  mechanism,  which  supplement  the  pri- 
mary coordinating  nerve  mechanisms  of  the  body;  the  cooperation  of  both, 
however,  being  necessary  for  the  maintenance  of  the  activities  of  individual 
organs  as  well  as  of  the  activities  of  the  body  as  a  whole. 

Though  the  term  internal  secretion  has  been  applied  to  various  sub- 
stances such  as  urea  and  carbon  dioxid,  which  arise  in  consequence  of  tissue 
metabolism  and  which,  after  being  discharged  into  the  blood,  influence  in 
many  ways  physiologic  processes,  yet  the  term  is  more  applicable  to  the 
secretions  of  organs  possessing  epithelial  cells  in  anatomic  relation  to  blood- 
vessels. The  glands  by  which  these  specific  materials  or  hormones  are 
secreted  are  known  as 

Glands  of  Internal  Secretion  or  Endocrinous  Glands. — ^The  glands 
consist  mainly  of  epithelial  cells  in  close  relation  to  the  walls  of  capillary 
blood-vessels  and  lymphatics,  and  in  some  instances,  if  not  all,  under  the 
control  of  the  central  nerve  system.  By  reason  of  the  absence  of  ducts  and 
their  relation  to  blood-vessels  they  have  also  been  termed  ductless  glands 
and  vascular  glands  and  inasmuch  as  the  secretion  is  discharged  internally 
(into  the  blood)  they  have  been  designated  endocrinous  glands. 

The  glands  which  fall  into  this  category  are  the  thyroid,  the  parathyroids, 
the  hypophysis  cerebri  or  pituitary,  the  adrenals,  the  pancreas,  the  ovaries, 
and  testicles. 

THE  THYROID 

The  thyroid  gland  or  body  consists  of  two  lobes  situated  on  the  lateral 
aspect  of  the  upper  part  of  the  trachea  (Fig.  221).     Each  lobe  is  pyriform 

^  By  reason  of  the  fact  that  some  of  the  active  principles  of  the  internal  secretions  instead 
of  exciting,  relax  or  inhibit  activity,  the  term  hormone  for  the  class  is  inapplicable  according  to 
Schafer,  and  he  therefore  suggests  for  the  latter  class,  the  term  'chalone'  from  xa^'i''',  to  relax 
or  make  slack.  Because  they  act  like  certain  drugs  he  also  proposes  the  term  'acoid'  from 
'dkos,  a  remedy,  to  denote  this  quaUty,  and  to  denote  that  they  arise  within  the  body,  the  prefix 
aut  can  be  employed.  We  thus  get  the  complete  expression  autocoid  substance  to  denote  any 
drug-like  principle  which  is  produced  in  or  can  be  separated  from  the  internally  secreting  organs 
and  we  may  divide  these  autocoid  substances  into  hormones  or  excitatory  agents,  and  chalones 
or  inhibitory  agents. 

490 


INTERNAL  SECRETION 


491 


in  shape,  the  base  being  directed  downward  and  on  a  level  with  the  fifth  or 
sixth  tracheal  ring.  The  lobe  is  about  50  mm.  in  length,  20  mm.  in  breadth, 
and  25  mm.  in  thickness.  As  a  rule,  the  lobes  are  united  by  a  narrow  band 
or  isthmus  of  the  same  tissue.  The  gland  is  reddish  in  color,  and  abundantly 
supplied  with  blood-vessels  and  lymphatics. 

Microscopic  examination  shows  that  the  thyroid  consists  of  an  enormous 
number  of  closed  sacs  or  vesicles,  variable  in  size,  the  largest  not  measuring 
more  than  o.i  mm.  in  diameter.  Each  sac  is  composed  of  a  thin  homogen- 
ous membrane  lined  by  cuboid  epithelium.  The  interior  of  the  sac  in  adult 
life  contains  a  transparent,  viscid  fluid  containing  albumin  and  termed 
"colloid"  substance.  Externally,  the  sacs  are  surrounded  by  a  ^'plexus  of 
capillary  blood-vessels  and  lymphat- 
ics. The  individual  sacs  are  united 
and  supported  by  connective  tissue, 
which  forms,  in  addition,  a  cover- 
ing for  the  entire  gland.  Non- 
medullated  nerve  fibers  derived  from 
the  middle  and  inferior  cervical  gan- 
glia pass  into  the  gland  and  are  dis- 
tributed to  the  walls  of  the  smaller 
blood-vessels  and  to  the  epithelium 
lining  the  vesicles  (Berkley). 

The  role  played  by  the  thyroid 
gland  and  its  secretion,  especially  in 
mammals  in  the  regulation  of  the 
functional  activities  of  various 
organs  and  tissues  of  the  body  has 
been  determined  from  the  examina- 
nion  of  the  effects  that  follow  its 
arrested  development  in  the  early 

years  of  childhood,  its  degeneration  in  adults,  its  surgical  removal  in  human 
beings  when  this  is  necessitated  by  pathologic  conditions,  its  extirpation  in 
animals,  and  the  ingestion  of  extracts  of  the  gland  or  the  gland  itself. 

The  Effects  of  Arrested  Development. — A  congenital  absence  of  the 
thyroid  or  an  arrested  development  in  early  childhood  is  followed  by  a  de- 
fective physical  and  mental  development  characterized  by  a  group  of  phe- 
nomena termed  cretinism.  The  most  characteristic  of  these  phenomena  are, 
a  small  and  irregular-shaped  body;  a  swollen  face;  puffy  eyelids;  a  broad  flat 
nose,  a  large  tongue  inclined  to  hang  out  of  the  mouth;  a  swollen  abdomen; 
short  thick  legs;  mental  dullness  and  stupidity;  and  even  more  or  less  idiocy. 
The  thyroid,  as  shown  on  post-mortem  examination  presents  an  atrophied 
appearance  and  consists  mainly  of  hard  fibrous  tissue. 

The  Effects  that  Follow  Degeneration  in  the  Adult.— The  degen- 
erative processes  that  occur  in  the  thyroid  in  the  adult  give  rise  to  a  group 
of  phenomena  the  most  striking  of  which  is  a  swollen  condition  of  the  skin, 
the  result  of  the  hyperplasia  of  the  subcutaneous  connective  tissue,  of  an 
embryonic  type  and  rich  in  mucinoid  material,  to  which  the  term  myxedema 
is  given.  Partly  in  consequence  of  this  change  in  the  skin,  the  face  becomes 
broader,  swollen  and  flattened,  giving  rise  to  a  loss  of  expression.  The  fea- 
tures in  some  instances  become  coarse  and  irregular.     With  the  progress  of  the 


Fig.  221. — View  of  Thyroid  Body.  i. 
Thyroid  isthmus.  2.  Median  portion  of  crico- 
thyroid membrane.  3.  Crico-thyroid  muscle. 
4.  Lateral  lobe  of  thyroid  body. — {After  Morris.} 


492  TEXT-BOOK  OF  PHYSIOLOGY 

disease  the  mind  becomes  dull  and  clouded,  the  memory  becomes  defective, 
dementia  supervenes  and  finally  the  condition  of  idiocy  may  be  established. 

The  Effects  of  Surgical  Removal  in  Man  and  Animals. — ^The  com- 
plete removal  of  the  thyroid  gland  by  surgical  procedures  necessitated  by 
grave  pathologic  conditions  has  been  followed  in  human  beings  by  a  serious 
morbid  condition  to  which  the  terms  cachexia  strumapriva  or  thyroprivia 
have  been  applied.  Within  a  relatively  short  time  the  patients  complained 
of  muscle  fatigue,  a  sense  of  heaviness  in  the  limbs  and  more  or  less  pain. 
With  the  progress  of  the  disordered  condition  there  developed  a  swelling  of 
the  face,  hands  and  feet  similar  to  that  observed  in  myxedema.  The  mental 
processes  became  sluggish,  the  speech  difficult  and  movements  slow.  A 
mental  condition,  approximating  idiocy  was  finally  developed  before  death 
supervened.  With  the  development  of  knowledge,  regarding  the  function 
of  the  thyroid  it  was  found  possible  to  remove  a  large  portion  of  the  diseased 
gland  without  the  development  of  unfortunate  results,  providing  a  small 
portion,  sufficiently  large,  however,  to  maintain  the  necessary  amount  of 
internal  secretion  was  left  behind. 

The  extirpation  of  the  thyroid  in  animals  is  usually  fatal  though  the 
effects  differ  according  to  the  animal  operated  on.  Herbivorous  animals 
as  a  rule  survive  the  removal  of  the  gland  far  better  than  carnivorous  animals, 
though  no  sufficient  reason  for  this  difference  has  been  presented.  Dogs 
usually  die  in  a  few  weeks,  death  being  preceded  by  tremors,  and  convu'- 
sions.  Monkeys,  according  to  Horsley's  experiments  and  observations,  gen- 
erally die  within  a  few  weeks.  Among  the  symptoms  which  developed 
within  a  few  days  after  the  removal  of  the  gland  may  be  mentioned  loss  of 
appetite,  fibrillar  contractions  of  muscles;  tremors  and  spasms;  mucinoid 
degeneration  of  the  skin,  giving  rise  to  puffiness  of  the  eyelids  and  face  and 
to  a  swollen  condition  of  the  abdomen;  hebetude  of  mind,  frequently  termin- 
ating in  idiocy;  fall  of  blood-pressure;  dyspnea;  albuminuria;  atrophy  of  the 
tissues,  followed  by  death  of  the  animal  in  the  course  of  from  five  to  eight 
weeks.  The  complexus  of  symptoms  observed  in  monkeys  was  divided  by 
Horsley  into  three  stages:  viz.,  the  neurotic,  the  mucinoid,  and  the  atrophic. 

It  is  evident  from  the  foregoing  facts,  that  the  presence  of  the  thyroid  is 
essential  to  the  normal  activity  of  the  tissues  generally.  As  to  the  manner 
in  which  it  exerts  its  favorable  influence,  there  is  some  difference  of  opinion. 
The  view  that  the  gland  removes  from  the  blood  certain  toxic  bodies,  render- 
ing them  innocuous  and  thus  preserving  the  body  from  a  species  of  auto- 
intoxication, is  gradually  yielding  to  the  more  probable  view  that  the  epi- 
thelium is  engaged  in  the  secretion  of  a  specific  material,  which  finds  its 
way  into  the  blood  or  lymph  and  in  some  unknown  way  influences  favorably 
tissue  metabolism.  This  view  of  the  function  of  the  thyroid  is  supported 
by  the  fact  that  successful  grafting  of  a  portion  of  the  thyroid  beneath  the 
skin  or  in  the  abdominal  cavity  will  prevent  the  usual  symptoms  which 
follow  thyroidectomy. 

The  Effects  of  Feeding  the  Thyroid  Gland. — On  the  supposition  that 
the  thyroid  gland  secreted  and  discharged  into  the  blood  an  agent  necessary 
to  the  maintenance  of  the  normal  metabolism,  it  was  thought  probable  that 
the  internal  administration  of  the  gland  or  its  extracts  might  provide  the 
body  with  the  necessary  amount  of  the  secretion.  Acting  on  the  suggestion, 
fresh  thyroids  of  various  animals,  principally  from  sheep,  have  been  admin- 


INTERNAL  SECRETION  493 

istered  to  myxedematous  patients  with  the  result  of  relieving  them  of  their 
unfortunate  symptoms  and  of  restoring  them  to  a  normal  physiologic  condi- 
tion. Equally,  if  not  more  surprising,  are  the  results  which  have  followed 
the  internal  administration  of  thyroids  to  cretins.  The  long-continued 
treatment  changed  the  entire  metabolism  resulting  in  an  increase  in  growth 
and  weight  and  an  awakening  of  the  child's  mentality. 

Hyperthyroidism. — If  the  gland  extracts  are  administered  in  sufficient 
quantity  and  for  a  sufficient  length  of  time  when  the  body  is  in  a  normal  con- 
dition the  internal  secretion  influences  very  markedly  the  general  metabolism 
as  shov>'n  by  an  increased  oxidation  of  fat  and  protein  and  a  decline  in  body 
weight.  If  the  dosage  be  large  toxic  symptoms  may  arise,  e.g.,  vertigo,  in- 
creased cardiac  action,  flushing,  tremors,  glycosuria,  and  in  monkeys,  exoph- 
thalmos and  a  widening  of  the  palpebral  fissure.  From  these  facts  the 
inference  has  been  draw^n  from  the  clinical  side  that  the  symptoms  com- 
prised under  the  term  exophthalmic  goiter,  viz.:  rapid  action  of  the  heart,  pul- 
sation of  the  large  arteries  at  the  base  of  the  neck,  protrusion  of  the  eyeballs 
and  fine  tremors  of  the  hands  are  due  to  an  enlargement  of  the  gland  and  a 
hypersecretion  of  the  thyroid  cells,  a  condition  spoken  of  as  hyperthyroidism. 
This  inference  has  apparently  been  confirmed  by  the  disappearance  of  the 
symptoms  after  the  removal  of  a  large  portion  of  the  gland,  care  being  taken 
to  leave  a  small  portion  sufficiently  large,  however,  to  produce  the  necessary 
amount  of  the  internal  secretion. 

The  Thyroid  Secretion. — ^The  chemic  features  of  the  material  secreted 
and  obtained  from  the  structures  of  the  thyroid  indicate  that  it  is  a  complex 
protein  containing  iodin,  which,  under  the  influence  of  various  reagents, 
undergoes  cleavage,  giving  rise  to  a  non-protein  residue,  which  carries  with 
it  the  iodin  and  phosphorus.  The  amount  of  iodin  in  the  thyroid  varies 
from.  0.33  to  I  milligram  for  each  gram  of  tissue.  To  this  compound  the 
term  iodo-thyrin  has  been  given.  The  administration  of  this  compound 
produces  effects  similar  to  those  which  follow  the  therapeutic  administration 
of  the  fresh  thyroid  itself,  viz. :  a  diminution  of  all  myxedematous  symptoms. 
In  normal  states  of  the  body,  iodo-thyrin  influences  very  actively  the  general 
metabolism.  It  gives  rise  to  a  decomposition  of  fats  and  proteins  and  to  a 
decline  in  body-weight.  In  large  doses  it  may  produce  toxic  symptoms,  e.g., 
increased  cardiac  action,  vertigo,  and  glycosuria. 

The  Functions  of  the  Thyroid  Gland. — ^The  functions  of  the  thyroid 
gland  which  have  been  drawn  from  the  results  that  have  followed  its  removal 
from  animals  by  surgical  procedures,  have  been  made  questionable,  since  the 
discovery  of  the  parathyroid  glands  and  a  study  of  the  phenomena  which 
follow  when  they  alone  are  removed.  From  their  situation  and  close  rela- 
tionship to  the  thyroid  gland  it  is  generally  accepted,  that  in  the  earlier  ex- 
periments, especially  those  made  on  cats  and  dogs,  and  some  other  carnivor- 
ous animals,  both  sets  of  glands  were  removed  and  hence  some  of  the  symp- 
toms which  developed  after  the  removal  of  the  thyroids  were  due  to  the  loss 
of  function  not  of  the  thyroid  but  of  the  parathyroids. 

This  is  especially  true  of  the  fibrillar  contractions,  tremors  and  spasms. 
These  it  is  now  more  generally  believed  arise  only  in  consequence  of  the 
simultaneous  removal  of  the  parathyroids. 

The  function  or  the  physiologic  action  of  the  thyroid  is  to  produce  an 
internal  secretion  which  after  its  entrance  into  the  blood  promotes  favorably 


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the  metabolism  of  the  neuro-muscular  systems  at  least.  The  myxedema  and 
the  failure  of  the  mental  powers  are  attributed  to  the  loss  or  degeneration  of 
the  gland  and  hence  its  internal  secretion,  and  cretinism  to  the  arrest  of  its 
development. 

THE  PARATHYROIDS 

The  parathyroids  are  small  bodies,  usually  four  in  number,  two  on  each 
side.  They  are  divided  into  superior  and  inferior.  The  superior  are  situ- 
ated internally  and  on  the  posterior  surface  in  close  relation  to,  and  frequently 
imbedded  in,  the  substance  of  the  thyroid;  the  inferior  are  situated  externally, 
sometimes  in  contact  with,  and  at  other  times  removed  a  variable  distance 
from  the  thyroid  (Fig.  222).  Microscopically  the  parathyroids  consist  of 
thick  cords  of  epithelial  cells  separated  by  septa  of  fine  connective  tissue 
and  surrounded  by  capillary  blood-vessels. 

Effects  of  Parathyroid  Removal. — The  surgical  removal  of  the  para- 
thyroids is  followed  in  the  course  of  from  two  to  five  days  by  the  death  of  the 
animal  preceded  in  most  instances  by  a  series  of  symptoms  which  are  em- 
braced under  the  general  term  "tetany."  These  symptoms  are  fibrillary 
contractions  of  muscles,  tremors,  spasmodic  contractures  and  paralyses  of 

groups  of  muscles  and  not  infrequently 
convulsive  seizures  and  coma.  During  the 
convulsion  there  is  an  acceleration  of  the 
heart-beat,  and  increase  in  the  respiratory 
movements  which  frequently  become  dysp- 
neic  in  character.  There  is  also  a  loss 
of  appetite,  nausea,  mucous  vomiting,  and 
diarrhea.  Death  may  occur  during  a 
convulsion  or  from  coma.  (Morat  and 
Doyon.)  The  administration  of  calcium 
causes  the  tetany  to  disappear  and  relieves 
the  animal  of  these  various  symptoms,  from 
which  it  is  inferred  that  the  parathyroids 
influence  or  regulate  calcium  metabolism. 

These  results  for  the  most  part  occur 
only  when  all  the  parathyroids  are  removed 
It  is  asserted  that  even  if  one  gland  is  re- 
tained the  animal  does  not  die.  The  above 
described  symptoms  may  manifest  them- 
selves, however,  but  they  are  slight  in 
degree.  Under  these  circumstances  there 
is  a  diminished  tolerance  for  sugar  as  shown 
by  the  appearance  of  sugar  in  the  urine 
Fig.  222.— The  position  of  the  para-  when  the  normal  amount  is  ingested. 
thyroid  glands.—iZiickerkandl.)  Vincent  and  Jolly  have  recently  pub 

lished  the  results  of  a  series  of  experiments 
which  seem  to  negative  to  some  extent  the  preceding  statements.  These 
experimenters  state  that  while  it  is  true,  that,  as  a  rule,  the  removal  of  both 
thyroids  and  parathyroids  in  the  carnivora  is  a  fatal  operation,  there  are 
nevertheless  many  exceptions;  and  in  the  mammalia  generally,  e.g.,  cats, 
dogs,  foxes,  guinea-pigs,  rats,  and  monkeys,  the  exception  becomes  the  rule 


INTERNAL  SECRETION  495 

as  more  than  51  per  cent,  of  animals  survived  the  operation  for  a  prolonged 
period  and  of  these  68  per  cent,  showed  no  specific  symptoms  of  any  kind. 
From  the  contradictory  observations  it  is  evident  that  the  subject  needs 
further  investigation. 

THE  PITUITARY 

The  pituitary  is  a  small  body  lodged  in  the  sella  turcica  of  the  sphenoid 
bone.  It  measures  14  mm.  from  side  to  side,  8  mm.  from  before  backward, 
and  6  mm.  from  above  down,  and  consists  of  an  anterior  lobe  somewhat 
pink  in  color,  and  a  posterior  lobe  yellowish-gray  in  color.  The  anterior 
lobe  is  much  the  larger  and  partly  embraces  the  posterior  lobe  (Fig.  223). 
The  anterior  lobe  is  developed  from  an  invagination  of  the  ectoderm  of  the 
buccal  cavity  and  consists  of  gland  tissue  surrounded  by  a  thin  envelope  of 
connective  tissue.  It  becomes  separated  from  the  mouth  by  the  fusion  of  the 
sphenoid  cartilages.  The  posterior  lobe  is  an  outgrowth  from  the  mid-brain 
and  is  connected  with  the  infundibulum  of  the  third  ventricle  by  a  short 
stalk.  In  the  early  stages  of  its  development  it  presents  a  central  cavity 
which  is,  however,  soon  obliterated  by  the  growth  of  special  tissues.  It 
persists  in  the  cat.  It  has  been  suggested  that  the  term 
hypophysis  cerebri  be  reserved  for  the  anterior  lobe  and 
the  term  infundibular  body  for  the  posterior  lobe.  This 
distinction  appears  to  be  desirable  inasmuch  as  in  their 
origin,  structure  and  functions  they  are  separate  and 
distinct  bodies. 

Histology  of  the  Pituitary  Body. — If  a  mesial 
sagittal  section  be  made'  through  the  pituitary  it  will 
present  an  appearance  which  in  a  general  way  is  the 
same  in  many  animals  though  the  details  vary  some-  section  "f' the  %^0- 
what  in  each  animal.  In  the  monkey  the  arrangement  itary  Bodyand  infun- 
of  the  anatomic  parts  (Fig.  224)  "is  similar  to  the  ar-  dibulum  with  Adjoin- 
^    ,^     -11  1        '  1  ^1     ^  ^1  ^     •        ING  Part  OF  Third  Ven- 

rangement  m  man.    It  will  be  observed  that  the  posterior  tricle.      a.    Anterioi 

lobe  is  solid  and  that  there  is  no  open  connection  with  lobe.  a'.  A  projection 
the  cavity  of  the  third  ventricle;  that  it  is  invested,  over  STheSSdibulumT 
a  large  part  of  its  surface,  by  a  thin  layer  of  epithelium.  Posterior  lobe  connected 
The  anterior  lobe,  which  lies  in  front  of  it  is  separated  by  a  stalk  with  the  in- 
by  a  cleft  which  is  the  remnant  of  the  cavity  of  the  ^^^^  of^optk  chiasm.— 
buccal  pouch.  Though  the  appearance  of  the  ante-  {Schwalhefrom  Quain.) 
rior  lobe,  and  the  epithelial  investment  of  the  posterior 
lobe  is  somewhat  different,  the  latter  is  but  a  differentiation  of  the  former,  a 
procedure  that  takes  place  in  fetal  life.  The  epithelial  investment  is 
usually  spoken  of  as  the  pars  intermedia,  and  regarded  histologically  and 
physiologically  as  a  part  of  the  posterior  lobe.  Superiorly  the  anterior  lobe 
and  the  pars  intermedia  are  united,  though  a  portion  of  the  latter  passes 
upward  and  embraces,  if  it  does  not  entirely  surround,  the  infundibular 
stalk;  inferiorly  and  posteriorly  the  two  bodies  also  unite.  The  posterior 
surface  of  the  posterior  lobe  is  free  from  epithelial  investment  in  the  mid-line. 
The  extent  to  which  the  epithelium  invests  the  posterior  lobe  varies  in  dif- 
ferent animals.     In  the  cat  and  dog  it  is  almost  complete. 

Microscopic  examination  of  the  anterior  lobe   shows  the  presence  of 
granular  epithelial  cells,  the  descendents  of  the  original  buccal  epithelium, 


496 


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arranged  in  columns  between  which  pass  large  thin-walled  blood-vessels. 
In  view  of  the  physiologic  importance  of  this  lobe  it  is  believed  that  the 
granules  of  the  cell  represent  an  internal  secretion,  which  passes  into  the 
blood-stream  and  is  thus  distributed  to  various  regions  of  the  body. 

The  pars  intermedia  consists  of  several  layers  of  finely  granular  epithelial 
cells  which  develop  a  colloid  material  that  subsequently  passes  into  the  pos- 
terior lobe  where  it  becomes  hyaline  in  character.  The  epithelial  investment 
is  separated  from  the  posterior  lobe  by  a  layer  of  blood-vessels  though 
columns  of  cells  penetrate  it. 

The  posterior  lobe  consists  of  neuroglia  cells  and  fibers.  True  nerve- 
cells  are  apparently  wanting.  Throughout  the  lobe  there  are  numerous  small 
hyaline  bodies  which  are  apparently  streaming  upward  to  the  ventricular 
cavity.  In  view  of  the  physiologic  importance  of  this  infundibular  body  or 
pars  nervosa,  these  hyaline  masses  are  believed  to  represent  an  internal  secre- 
tion which  passes  upward  through  loose  tissue  channels  toward  the  infundi- 
bulum  to  be  discharged  into  the  fluid  of  the  third  ventricle.  If  the  stalk  be 
divided  there  is  an  accumulation  of  these  bodies  in  the  posterior  lobe. 
Both  parts  of  the  pituitary  are  well  supplied  with  blood  though  from  different 
sources. 

Effects  of  Total  Removal. — The  effects  which  were  observed  by  the 
earlier  investigators  to  follow  total  removal  of  the  hypophysis  were  not 
always  in  accord  by  reason  of  the  difference  in  the  operative  methods 

pursued,  injuries  to  the  brain, 
infections,  imperfect  removals 
as  shown  by  post-mortem  ex- 
amination, etc.  Some  inves- 
tigators claimed  that  after  total 
removal  animals  lived  for  long 
periods  and  that  therefore  the 
gland  was  not  essential  to  life; 
others  claimed,  however,  its 
total  removal  was  followed  very 
shortly  by  death  preceded  by  a 
series  of  characteristic  symp- 
toms and  that  therefore  it  was 
absolutely  essential  to  life. 
The    introduction    of    a    new 

method  of  procedure  for  the  re- 

FiG.  224— Mesul  Saggital  Section  OF  THE  Pit-  -^^^„i     „f    .^^     hvnnnhv^i<;    bv 

uiTARY  Body  OF  THE  Monkey,     a,  Optic  chiasm;  6,  "lOval    Ot    tne     nypopn>SlS    Dy 

process  of  the  pars   intermedia;  c,  third    ventricle;  d,  PauIeSCO    and    itS    employment 

anterior  lobe  proper;/,  posterior  lobe  or  pars  nervosa;  by  Cushing  and  his  CO- workers 

f,  epithelium  investment  of  the  posterior  lobe;  ^,epithe-  1         1    rl   t  It        Vi'    V>  f 

Hum  of  the  pars  intermedia  passing  over  the  neighbor-  ^^^  '^^  '-^  reSUltS  wniCn  are  lOr 

ing  brain  mass;  e,  c\&it.— {After  Herring)  the  most  part  in  general  agree- 

ment. This  method  involves 
an  approach  to  the  gland  through  the  temporal  bone  instead  of  through  the 
buccal  cavity  as  was  formerly  the  case.  The  temporal  muscles  are  first 
dissected  away  from  the  skull  on  both  sides  and  reflected  downward. 
Large  openings  are  made  in  the  bone  and  dura  of  both  sides.  The  temporal 
lobe  on  one  side  is  lifted  up  with  a  spoon-shaped  spatula  sufficiently  large 
to  expose  the  hypophysis,  hanging  from  the  infundibulum.     Owing  to  the 


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INTERNAL  SECRETION  497 

opposite  opening  in  the  skull,  the  opposite  half  of  the  cerebrum  is  displaced 
and  protruded  so  that  injury  to  the  brain  from  compression  is  prevented. 
The  gland  can  then  be  picked  up  with  forceps  and  removed,  Paulesco 
reported  that  the  total  removal  of  the  gland  in  24  dogs  resulted  in  death  in 
24  hours.  In  seven  other  animals  the  fatal  result  was  postponed  for  variable 
periods.  One  animal  survived  for  five  months  and  another  for  a  year 
without  exhibiting  any  very  characteristic  symptoms.  As  a  post-mortem 
examination  showed  that  the  gland  was  only  partially  removed  it  was 
assumed  that  the  remaining  portion  had  been  sufficient  to  maintain  life. 
Removal  of  the  anterior  lobe  alone  was  followed  by  death  as  certainly  as  when 
the  entire  gland  was  removed.  Removal  of  the  posterior  lobe  alone  was  not 
followed  by  noticeable  eflfects.  From  these  facts  Paulesco  asserted  that  the 
hypophysis  is  an  organ  indispensable  to  life  as  its  removal  rapidly  eventuates  in 
death,  and  that  of  its  different  parts  the  anterior  lobe  is  the  more  important. 
Crowe,  Cushing  and  Homans  have  more  recently  reported  a  series  of  100 
operations  for  the  removal  of  the  hypophysis,  the  results  of  which  are  corrobo- 
rative in  many  respects  of  the  results  of  Paulesco.  It  was  found  that  the 
duration  of  lif-e  in  adult  dogs  was  from  two  to  three  days  and  in  young  dogs 
about  eleven  days.  In  a  few  cases  the  animals  survived  for  several  weeks 
but  a  post-mortem  examination  showed  that  small  viable  portions  of  the 
gland  had  escaped  removal.  Among  the  many  symptoms  that  followed  to- 
tal hypophysectomy  according  to  these  experimenters  the  more  striking  after 
24  hours  were  a  lowering  of  body-temperature,  unsteadiness  of  gait  and  stiff- 
ness of  movement,  a  fall  of  blood-pressure,  feeble  and  slow  respiration, 
muscle  twitchings,  lethargy,  coma,  and  death.  In  old  animals  there  was 
occasional  glycosuria;  in- young  animals  polyuria.  Total  removal  of  the 
anterior  lobe  alone  in  this  series  of  experiments  was  also  as  fatal  as  removal 
of  the  entire  gland. 

The  Effects  of  Partial  Removal  of  the  Anterior  Lobe. — When  only  a 
portion  of  the  anterior  lobe  was  removed  the  animal  survived  for  a  much 
longer  period  than  when  the  removal  was  complete.  The  duration  of  life 
apparently  depended  on  the  amount  and  the  cellular  activity  of  the  parts  left 
behind.  As  a  result  of  the  partial  removal  only  there  developed  a  series  of 
phenomena  to  which  the  term  cachexia  hypophyseopriva  has  been  given. 
These  phenomena  varied  somewhat  in  accordance  with  the  age  of  the 
animal.  Adult  animals  became  adipose  and  degenerated  sexually,  young 
animals  likewise  became  adipose  but  they  remained  undersized  and  failed 
to  develop  sexual  characteristics  and  hence  sexual  infantilism  persisted. 
The  organs  of  reproduction  in  both  sexes  remained  rudimentary.  The 
temperature  was  subnormal  and  nutritive  disorders  of  the  skin  developed. 
These  various  symptoms  were  attributed  at  the  time  to  the  partial  removal 
of  the  anterior  lobe  alone  and  hence  a  deficiency  of  secretion,  but  the  results 
of  a  series  of  experiments,  subsequently  published  by  Cushing,  led  to  the 
belief  that  some  of  these  phenomena  were  due  to  injury  or  impairment  of  the 
normal  function  of  the  posterior  lobe  at,  or  subsequent  to,  the  time  of  the 
operation.  Just  which  of  these  phenomena  were  to  be  attributed  to  a 
diminished  secretion  of  the  anterior  lobe  and  which  to  a  diminished  secretion 
of  the  posterior  lobe  was  left  for  future  experiments  to  determine.  However 
important  the  function  of  the  anteror  lobe  may  be  in  relation  to  the  phe- 
nomena just  mentioned,  it  is  apparent  that  it  must  have  some  additional  func- 
32 


498  TEXT-BOOK  OF  PHYSIOLOGY 

tion  of  even  greater  significance  or  else  its  removal  would  not  be  attended 
with  fatal  results. 

The  Effects  of  Pathologic  Conditions. — In  recent  years  the  idea  has 
gradually  developed  that  certain  pathologic  states  of  the  body  are  associated  in 
some  way  with  pathologic  states  of  the  pituitary  body.  Thus  the  condition 
of  giantism  which  begins  in  youth  and  the  condition  of  acromegaly  which 
appears  in  adult  life  are  believed  to  be  the  result  of  a  hypersecretion  of 
the  anterior  lobe,  which  in  turn  may  be  due  to  a  hyperplasia  of  the  gland 
elements  excited  by  a  variety  of  causes.  In  both  giantism  and  acromegaly 
there  is  an  increased  activity  in  the  nutritive  process  leading  to  an  over- 
growth of  osseous  tissue  and  the  overlying  structures.  In  the  former  con- 
dition the  overgrowth  is  general;  in  the  latter  it  is  confined  to  the  face  and 
the  extremities,  hands  and  feet.  To  this  phase  of  pituitary  activity  the  term 
hyperpituitarism  has  been  given. 

The  opposite  condition,  infantilism  and  adiposity,  have  also  been  shown 
to  be  associated  with  pathologic  changes  in  the  pituitary.  In  these  cases 
not  only  is  the  individual  of  small  size  but  the  genital  organs  are  undeveloped. 
In  addition  there  may  be  a  subnormal  temperature,  loss  of  hair,  etc.  These 
phenomena  may  be  due  to  a  diminished  or  defective  secretion  partly  of  the 
anterior  lobe  and  partly  of  the  posterior  lobe.  To  this  condition  the  term 
hypopituitarism  has  been  given. 

Goetsch  has  reported  recently  that  the  feeding  of  an  extract  of  the  anterior 
lobe  to  young  rats  "has  a  stimulating  effect  upon  the  growth  of  the  animal 
and  upon  its  sexual  development  and  activity."  Infantilism  and  a  want  of 
sexual  development  may  therefore  be  due  to  a  deficiency  of  the  anterior-lobe 
secretion  early  in  life. 

The  Effects  of  Removal  of  the  Posterior  Lobe. — Gushing  in  a  series 
of  experiments  (1911)  has  demonstrated  that  the  posterior  lobe  with  its  epi- 
thelial investment  exerts,  contrary  to  general  opinion,  a  profound  influence 
on  metabolism  and  more  especially  on  the  metabolism  of  the  carbohydrates, 
either  alone  or  in  conjunction  with  other  glands  having  internal  secretions. 
These  experiments  also  led  to  the  belief  that  some  of  the  phenomena  detailed 
in  the  foregoing  paragraph,  especially  the  deposition  of  fat,  the  subnormal 
temperature  and  perhaps  the  imperfect  development  of  the  sexual  organs  are 
due  rather  to  a  deficiency  or  absence  of  the  secretion  of  the  posterior  lobe 
than  to  a  deficiency  or  absence  of  the  secretion  of  the  anterior  lobe. 

It  has  apparently  been  demonstrated  by  Gushing  that  the  hyaline  bodies 
found  in  the  posterior  lobe  represent  an  internal  secretion;  that  they  are 
discharged  into  the  cavity  of  the  third  ventricle  where  they  undergo  solution 
in  the  cerebrospinal  fluid,  by  means  of  which  the  dissolved  material 
enters  the  blood-stream,  since  compression  of  the  infundibular  stalk  gives 
rise  to  an  accumulation  of  the  hyaline  material.  The  presence  in  the  cerebro- 
spinal fluid  of  an  agent  that  produces  the  same  physiologic  effects  when 
intravenously  injected,  as  injections  of  extracts  of  the  posterior  lobe  do,  has 
also  been  established. 

In  the  various  operative  procedures  incident  to  the  removal  of  the  entire 
hypophysis  or  of  the  anterior  lobe  a  transient  glycosuria  is  frequently  ob- 
served, a  phenomenon  attributed  to  the  discharge  under  the  circumstances  of 
an  excessively  large  amount  of  the  reserve  hyaline  substance  or  of  the 
posterior  lobe  secretion  into  the  cerebrospinal  fluid  in  the  third  ventricle. 


INTERNAL  SECRETION  499 

This  secretion  diminishes  temporarily  the  normal  toleration  or  assimilation 
of  sugar  and  in  some  unknown  way  leads  to  a  hyperglycemia  and  glycosuria.^ 
If  the  customary  amount  of  sugar  is  ingested  a  portion  of  it  will  be  eliminated 
in  the  urine. 

If  the  posterior  lobe  with  its  epithelial  investment  is  totally  removed 
or  if  the  infundibular  stalk  is  compressed  by  a  clip  so  as  to  prevent  the  dis- 
charge of  the  secretion  into  the  ventricle  the  animal  becomes  very  tolerant  of 
sugar  and  is  enabled  to  assimilate  larger  quantities  than  formerly  without 
the  development  of  alimentary  glycosuria.  As  a  probable  result  of  the  in- 
creased carbohydrate  assimilation,  a  condition  of  nutrition  is  established, 
characterized  by  a  general  deposition  of  fat  suggesting  a  conversion  of  the 
sugar  into  fat.  There  is  probably  at  the  same  time  an  imperfect  oxidation 
of  the  carbohydrates  as  indicated  by  the  lowered  temperature. 

That  the  condition  of  generalized  adiposity  is  probably  due  to  deficient 
posterior  lobe  secretion  is  shown  by  the  fact  that  the  increased  tolerance  for 
sugar  can  be  lowered  very  promptly  by  the  coincident  intravenous  or  sub- 
cutaneous injection  of  extracts  of  the  posterior  lobe. 

From  the  foregoing  facts  it  may  be  assumed  that  the  secretion  of  the 
posterior  lobe  in  some  unknown  way  influences  the  metabolism  of  sugar. 
From  the  facts  at  hand  it  may  be  assumed  that  a  hypersecretion  from  any 
cause  whatever,  leads  to  a  diminished  tolerance  for  or  assimilation  of  sugar, 
as  shown  by  the  hyperglycemia  and  glycosuria,  though  the  manner  in  which 
the  hyperglycemia  is  developed,  whether  by  a  more  rapid  conversion  of 
glycogen  to  sugar  or  by  an  inefficient  storage  of  sugar  as  glycogen  is  unknown. 
A  hyposecretion  from  any  cause  leads  to  an  increased  tolerance  for  or 
assimilation  of  sugar  which  eventually  contributes  to  the  formation  and 
deposition  of  fat.  In  the  complexus  of  symptoms  that  accompany  patho- 
logic changes  in  the  hypophysis  either  in  the  anterior  or  posterior  lobe  it  is 
difficult  to  indicate  those  which  are  to  be  attributed  to  increased  or  decreased 
secretion  of  either  the  anterior  or  posterior  lobe  by  reason  of  their  close 
juxtaposition  and  their  possible  simultaneous  involvement;  again  it  is  also 
uncertain  as  to  whether  the  secretions  produce  their  effects  alone  or  through 
the  cooperation  of  the  secretions  of  other  organs  having  more  or  less  influ- 
ence in  the  metabolism  of  the  carbohydrates. 

The  Effects  of  Injections  of  Extracts. — ^The  extracts  of  the  anterior 
lobe  when  intravenously  injected  appear  to  be  without  any  appreciable  effect 
on  any  of  the  physiologic  mechanisms.  Injections  of  the  extracts  of  the 
posterior  lobe,  however,  give  rise  very  promptly,  as  shown  by  Howell,  to  an 
increase  in  the  blood-pressure  which  appears  to  be  due  to  an  increased 
contraction  of  the  arteriole  muscle  rather  than  to  a  stimulation  of  the  vaso- 
motor centers,  as  the  contraction  takes  place  even  after  destruction  of  the 
spinal  cord  and  medulla  oblongata.  The  action  of  the  active  constituent 
of  the  extract  appears  to  be  very  general  as  there  is  a  simultaneous  diminution, 
as  shown  by  plethysmographic  investigations,  in  the  volume  of  various 
organs.  On  the  heart  the  extract  has  an  inhibitor  action  which  takes  place 
concomitantly  with  the  contraction  of  the  arterioles  and  the  rise  of  the 
pressure  as  shown  by  Howell.     This  is  attributed  to  a  direct  stimulation  of 

1  The  normal  tolerance  for  cane-sugar  in  the  case  of  the  dog,  when  it  is  given  by  the  mouth  is 
about  10  grams  per  kilo  of  body-weight  and  for  human  beings  about  2  grams  (glucose)  per  kilo 
of  body-weight  or  approximately  150  grams.  Any  increase  beyond  this  amount  appears  in  the 
urine  indicating  that  the  assimilation  limit  has  been  exceeded. 


500 


TEXT-BOOK  OF  PHYSIOLOGY 


the  cardio-inhibitor  center  as  the  retardation  is  partly  prevented  at  least 
when  the  vagus  is  divided  or  its  function  suspended  by  atropin.  Even  after 
this  has  been  done,  however,  a  slowing  of  the  heart  may  still  be  induced,  a 
fact  which  suggests  that  the  extract  acts  directly  on  the  heart-muscle  as  well. 
Schafer  and  his  co-workers  have  also  demonstrated  that  pituitary  extracts 
cause  dilatation  of  the  renal  vessels  and  stimulate  specifically  the  renal  cells 
to  activity,  thus  causing  a  marked  diuresis.  The  extract  also  stimulates  the 
non-striated  muscles  of  the  intestines,  bladder  and  uterus,  giving  rise  in  each  in- 
stance to  vigorous  contractions;  it  also  causes  a  marked  discharge  of  milk 
from  the  mammary  gland  during  lactation  due  to  the  contractioa  of  the  walls 
of  the  milk  ducts;  the  dilatator  muscle  of  the  iris  is  also  stimulated,  causing 
dilatation  of  the  pupil.  A  pharmaceutical  preparation  of  the  pituitary, 
"pituitrin",  in  a  similar  manner  raises  the  blood-pressure  for  a  considerable 
period,  stimulates  intestinal  peristalsis  and  excites  contractions  of  the  uterus 
during  and  after  labor. 

The  Functions  of  the  Pituitary  or  Hypophysis. — ^The  functions  of  the 
pituitary  body  are  related  to  the  activities  of  the  anterior  and  posterior  lobes. 
The  anterior  lobe,  through  its  internal  secretion  stimulates  the  growth  of  the 
skeleton  and  associated  tissues  as  apparently  shown  by  the  fact  that  an  excess 
of. secretion  in  early  life  leads  to  giantism  and  in  adult  life  to  acromegalia, 
while  a  deficiency  of  secretion  leads  to  defective  growth  and  the  establish- 
ment of  infantilism.  The  posterior  lobe  through  its  internal  secretion  assists 
in  the  regulation  of  carbohydrate  metabolism  as  shown  by  the  fact  that  an 
excess  of  secretion  lowers  the  assimilation  capacity  and  thus  develops  gly- 
cosuria, while  a  deficiency  of  the  secretion  raises  the  assimilation  capacity 
and  leads  to  the  production  and  accumulation  of  fat. 

ADRENAL  GLANDS 

The  adrenal  glands  or  suprarenal  capsules  are  two  flattened  bodies,  some- 
what crescentic  or  triangular  in  shape,  situated  each  upon  the  upper  extremity 
of  the  corresponding  kidney,  and  held  in  place  by  connective  tissue.  They 
measure  about  40  mm.  in  height,  30  mm.  in  breadth,  and  6  to  8  mm.  in 
thickness.  The  weight  of  each  is  about  4  gm.  Accessory  glands  are  some- 
times found  in  the  surrounding  connective  tissue  along  the  abdominal  sym- 
pathetic and  in  the  neighborhood  of  the  genital  organs. 

In  some  animals  such  as  the  dog,  cat  and  rabbit,  these  glands  have  no 
anatomic  connection  with  the  kidneys,  but  are  situated  at  varying  dis- 
tances from  them. 

Histology. — The  gland  is  covered  externally  by  a  fibrous  tissue  from 
which  septa  pass  into  the  more  central  portions  thus  forming  a  framework 
for  the  support  of  blood-vessels  and  cells. 

A  section  of  the  gland  shows  just  beneath  the  capsule  an  outer  portion 
termed  the  cortex  and  an  inner  portion  termed  the  medulla  (Fig.  225).  The 
cortex  consists  mainly  of  cuboid  cells  arranged  in  cylindric  columns.  The 
outer  layers  of  cells  are  arranged  in  irregular  masses  forming  what  has  been 
called  the  zona  glomerulosa.  The  medulla  consists  of  uniting  and  interlacing 
cords  of  polyhedral  cells,  the  cytoplasm  of  which  contains  granular  matter 
and  a  distinct  nucleus.  When  treated  with  chromic  acid  or  chromium  salts 
the  cytoplasm  stains  a  dull  brown  or  yellow  color.  For  this  reason  they  are 
termed  chromaffin  cells.     Similar  cells  are  found  in  sympathetic  ganglia. 


INTERNAL  SECRETION 


501 


The  gland  is  abundantly  supplied  with  blood-vessels  and  nerves.  The 
arteries  are  branches  of  the  aorta,  the  phrenic,  and  renal  arteries.  After 
penetrating  the  gland  they  divide  into  smaller  branches  and  capillaries  which 
ultimately  come  into  close  relation  with  the  cells  of  both  the  cortex  and 
medulla.  The  veins  emerge  from  the  gland  at  the  hilum  and  empty  on 
the  right  side  into  the  vena  cava  and  on  the  left  side  into  the  renal  vein. 
The  nerves  passing  to  the  gland  are  derived  for  the  most  part  from  the 
autonomic  system.  The  pre- ganglionic  fibers  pass  from  the  cord  by  way 
of  the  splanchnics  to  the  semilunar  ganglion.  The  post-ganglionic  pass 
from  the  semilunar  ganglia  through  its  branches  direct  to  the  gland.  Ac- 
cording to  Bergmann  nerves  come  from  the  phrenic  and  vagus  also. 

Embryologic  Development. — Embryologic 
investigations  have  shown  that  the  mature  adre- 
nal gland  consists  of  two  distinct  tissues  derived 
from  two  different  portions  of  the  embryo.  The 
cortex  is  derived  from  that  portion  of  the  meso- 
derm from  which  is  evolved  the  precursor  of  the 
kidney,  the  Wolffian  body;  the  medulla  is  de- 
rived from  the  embryonic  sympathetic  ganglia 
and  consists  primarily  of  nerve-cells,  which, 
however,  in  the  course  of  development  become 
profoundly  modified.  Comparative  anatomic 
studies  have  shown  the  relation  of  these  two 
components  of  the  adrenal  body.  In  the  elasmo- 
branch  fishes,  the  shark,  ray,  etc.,  the  cortex  is 
represented  early  by  an  internal  body  somewhat 
rod-shaped  and  elongated  and  situated  toward 
the  posterior  portion  of  the  kidney.  In  the 
bony  fishes  this  organ  becomes  paired.  The 
medulla  is  represented  by  a  series  of  paired 
bodies  extending  along  the  vertebral  column 
and  in  close  relation  to  the  sympathetic  ganglia 
and  contain  a  number  of  chromaffin  cells. 
These  two  elements  unite  to  form  the  com- 
pound adrenal.  In  the  mammals  the  larger  por-  supRSENfL^BoD™''  a^  fiZtS 
tion  of  this  chromaffin  material  fuses  and  be-  capsule;  b,  zona  glomerulosa;  c, 
comes  enveloped  on  each  side  by  the  internal  zona  fasciculata;(/,  zona  reticularis; 
11         .,1     ,1       p  ,•  r  ,1  •   ,•  1  1    e,  medullary  cords; /,  venous  cnan- 

body  With  the  formation  01  the  existing  adrenal  Qgi.  g^  ganglion-cells.    (Piersol). 

body.  The  unincorporated  portions  of  the  chro- 
maffin bodies  remain  as  masses  of  varying  size,  found  in  connection  with 
the  sympathetic  nerve  system.  The  most  important  of  these  bodies  is  the 
"abdominal  chromaffin  body"  extending  along  the  aorta  from  the  level  of 
the  adrenals  to  the  bifurcation.  It  is  readily  exposed  in  the  dog  by  stain- 
ing with  a  solution  of  bichromate  of  potassium.  Accessory  adrenal  bodies 
may  consist  of  either  cortical  or  chromaffin  material  alone  or  of  both. 

The  Effects  of  Disease  and  Removal  of  the  Adrenal  Glands. — ^There 
is  a  profound  disturbance  of  the  nutrition  first  described  by  Addison  and 
subsequently  termed  by  Trousseau,  Addison's  disease,  which  is  character- 
ized by  extreme  muscular  weakness  and  an  incapacity  for  sustained  muscle 
activity;  a  bronze-like  discoloration  of  the  skin  and  mucous  membranes,  dis- 


502  TEXT-BOOK  OF  PHYSIOLOGY 

turbance  of  the  digestive  functions,  indicated  by  indigestion,  vomiting  and 
diarrhea;  a  feeble  action  of  the  heart;  a  small  feeble  pulse;  a  low  blood-pres- 
sure; a  subnormal  temperature  and  a  feeble  respiration.  This  condition, 
which  is  thus  largely  characterized  by  a  loss  of  tone  in  the  skeletal  as  well  as 
the  vascular  musculature,  terminates  fatally  from  a  paralysis  of  the  respira- 
tory muscles.  Post-mortem  examination  reveals  in  all  cases  a  more  or  less 
extensive  disease  of  one  or  both  adrenals.  A  very  common  lesion  is  a  tuber- 
cular degeneration.  These  symptoms  were  attributed  by  Addison  to  a  loss 
of  function  of  the  glands. 

The  removal  of  these  bodies  from  various  animals  by  surgical  procedures 
is  invariably  and  in  a  short  time  followed  by  death,  preceded  by  some  of  the 
symptoms  characteristic  of  Addison's  disease.  Thus,  shortly  after  their  re- 
moval the  animal  becomes  tranquil  and  apathetic;  the  respiration  soon  be- 
comes feeble  and  difficult;  prostration  supervenes  and  the  animal  appears  as 
though  paralyzed,  but  the  irritability  of  the  skeletal  muscles  and  nerves  is 
normal;  the  heart  becomes  slow,  feeble  and  irregular;  the  blood-pressure  falls 
promptly  20  to  30  mm.  of  mercury,  after  which  it  steadily  falls  to  a  low  level; 
the  appetite  fails,  the  temperature  declines  and  death  occurs  in  from  twelve 
to  forty-eight  hours.  In  some  instances  a  pigmentation  of  the  skin  similar 
to  that  seen  in  Addison's  disease  has  been  observed.  From  the  fact  that 
animals  so  promptly  die  after  extirpation  of  these  bodies,  and  the  further  fact 
that  the  blood  of  some  animals  is  toxic  to  the  subjects  of  recent  extirpation, 
but  not  to  normal  animals,  the  conclusion  was  drawn  that  the  function  of  the 
adrenal  bodies  is  to  remove  from  the  blood  some  toxic  product  of  muscle 
metabolism.  Its  accumulation  after  extirpation  was  supposed  to  cause 
death  through  autointoxication.  This  view  is,  however,  not  generally 
accepted. 

The  Effects  of  the  Injection  of  Gland  Extracts. — On  the  supposition 
that  the  adrenals  might  secrete  and  pour  into  the  blood  a  specific  material 
that  favorably  influences  general  metabolism,  Schafer  and  Oliver  injected 
hypodermatically  glycerin  and  water  extracts  of  the  medulla  into  the  bodies 
of  various  animals  and  observed  at  once  an  increased  rate  of  the  heart- 
beats and  of  the  respiratory  movements.  The  effects  however  were  only 
transitory.  When  these  extracts  were  injected  into  the  veins  directly,  there 
followed  in  a  short  time  a  cessation  of  the  auricular  contraction  though  the 
ventricular  contraction  continued  vigorously  but  with  a  slower  rhythm. 
The  blood-pressure  at  the  same  time  was  markedly  increased.  If  the  vagi 
were  cut  previous  to  the  injection  or  if  the  inhibitor  influence  of  the  vagi 
was  removed  by  an  injection  of  atropin  the  reverse  effects  were  produced, 
viz.,  an  increase  in  the  rapidity  and  vigor  of  both  the  auricular  and  ventricular 
contraction  accompanied  by  a  still  more  marked  rise  of  blood-pressure.  This 
latter  effect  is  the  result  partly  of  the  increased  action  of  the  heart  but  very 
largely  the  result  of  a  vigorous  contraction  of  the  muscle-fibers  in  the  walls  of  the 
arterioles.  This  is  attributed  to  a  direct  stimulation  of  the  arterioles  and  not 
to  a  stimulation  of  the  vaso-constrictor  center.  The  contraction  of  the 
arterioles  is  quite  general  as  shown  by  plethysmographic  studies  of  the  limbs, 
the  spleen,  kidney,  etc.  The  arterioles  of  the  lungs  and  brain  do  not  con- 
tract under  its  influence  to  the  same  extent  as  do  the  arterioles  in  other 
regions  of  the  body,  possibly  for  the  reason  that  the  arteriole  muscles  in 
these  organs  are  not  so  abundantly  supplied  with  vaso-motor  nerves  as  they 


INTERNAL  SECRETION  503 

are  in  other  regions  of  the  body.  Applied  locally  to  the  mucous  membranes, 
adrenalin  extract  produces  contraction  of  the  blood-vessels  as  shown  by  the 
pallor  which  follows.  The  skeletal  muscles  are  affected  by  the  extract  very 
much  as  they  are  by  veratrin.  The  duration  of  a  single  contraction  is  very 
much  prolonged,  especially  in  the  phase  of  relaxation  or  of  decreasing  energy. 
In  the  foregoing  instances  the  extract  apparently  produces  its  effects  by 
an  augmentation  of  the  normal  tonus  of  the  arteriole  muscle.  The  effects 
follow  the  injection  of  an  extract  of  the  medulla  only.  An  extract  of  the 
cortex  appears  to  be  without  influence. 

The  injection  of  small  doses  of  the  active  principle  of  the  gland  into  the 
peritoneal  cavity  or  into  the  blood  is  also  followed  by  glycosuria  in  the 
course  of  an  hour  which  may  last  for  several  hours. 

The  Internal  Secretion. — It  is  apparent  from  the  results  of  these  experi- 
ments that  the  adrenal  bodies  are  engaged  in  elaborating  and  pouring  into 
the  blood  a  specific  material  which  on  the  one  hand  stimulates  to  increased 
activity  the  muscle-fibers  of  the  heart  and  arteries,  thus  assisting  in  main- 
taining the  normal  blood-pressure,  and  on  the  other  hand  maintaining  the 
tonicity  of  the  skeletal  muscles.  An  alkaloidal  substance  was  isolated  by 
Abel  from  extracts  of  this  gland,  to  which  the  term  epinephrin  was  given.  A 
crystallizable  substance  was  isolated  first  by  Takamine  and  later  by  Aldrich, 
to  which  the  term  adrenalin  was  given.  Both  substances  are  apparently 
equally  eflScacious  in  causing  contraction  of  the  blood-vessels  and  in  raising 
the  blood-pressure.  Epii^ephrin  or  adrenalin  represents  the  active  principle 
of  the  gland  and  is  regarded  as  a  product  of  the  secretor  activity  of  the 
cells  composing  the  medulla.  As  the  effects  following  the  intravenous  injec- 
tion of  adrenalin  are  of  sliort  duration,  the  supposition  is  that  it  is  speedily 
oxidized.     It  is  regarded  as  the  typical  hormone. 

The  action  of  adrenal  extract  however  is  not  limited  to  the  non-striated 
muscle-fibers  of  the  arterioles  but  extends  itself  to  the  non-striated  fibers 
found  in  the  the  walls  of  the  viscera,  e.g.,  stomach  and  intestines,  gall- 
bladder, urinary  bladder,  uterus,  etc.  The  administration  of  this  secretion 
is  followed  however,  in  these  regions,  by  an  inhibition  of  the  tonus  and 
subsequent  relaxation  of  the  visceral  walls,  with  the  exception  of  the  uterus. 
In  the  case  of  the  isolated  virgin  uterus  of  many  mammals,  adrenal  augments 
the  tone  and  the  vigor  of  the  spontaneous  contractions.  In  addition  to  the 
foregoing  phenomena,  dilatation  of  the  pupil,  increased  flow  of  saliva  and 
glycosuria  have  been  observed. 

From  an  examination  of  the  effects  that  follow  the  intravenous  injection 
of  adrenalin  or  adrenal  extracts,  it  is  apparent  that  they  closely  resemble  the 
effects  that  follow  stimulation  of  the  sympathetic  nerve  in  a  large  part  if  not 
in  all  parts,  of  its  distribution. 

It  has  been  a  subject  of  discussion  as  to  whether  adrenalin  acts  on  the 
muscle-fiber  directly  or  upon  the  endings  of  the  sympatheticnerves  with  which 
they  are  functionally  associated.  By  reason  of  the  fact  that  non-striated 
muscles  that  have  no  connections  with  the  sympathetic  nerve  system,  are 
not  influenced  by  adrenalin;  and  the  further  fact  that  non-striated 
muscles  that  have  been  deprived  of  their  nerve  connections  through  degenera- 
tive changes  following  division  of  the  nerves,  are  influenced  by  adrenalin, 
have  led  to  the  assumption  that  it  acts  neither  on  muscle  nor  nerve,  but  on 
some  material  which  intervenes  between  the  nerve  endings  and  the  muscle 


504  TEXT-BOOK  OF  PHYSIOLOGY 

but  which  is  intimately  related  to  the  muscle.  To  this  material  Langley 
has  applied  the  term  "receptive  substance."  In  all  instances  the  adrenalin 
stimulates  the  receptive  substances  as  a  result  of  which  the  normal  effect 
produced  by  the  sympathetic  nerve  impulse  is  intensified. 

The  Influence  of  the  Nerve  System. — The  secretory  activity  of  the 
adrenals  is  regulated  by  the  nerve  system.  Thus  Dreyer  found  that  the  blood 
of  the  adrenal  vein  after  stimulation  of  the  splanchnics  was  capable  of  causing 
to  a  much  greater  extent  the  usual  physiologic  effects  when  injected  into  an 
animal  than  blood  of  the  adrenal  vein  before  stimulation  and  this  independ- 
ent of  the  vascular  changes  that  were  simultaneously  provoked.  It  has  also 
been  shown  by  Ascher  that  a  high  blood-pressure  can  be  maintained  by 
prolonged  stimulation  of  the  splanchnics.  Cannon  has  reported  that  major 
emotional  disturbances  such  as  fright  lead  to  an  increase  in  the  secretion 
of  the  adrenals  as  shown  by  the  fact  that  the  blood  taken  from  the  vena  cava 
above  the  level  of  the  adrenal  veins  will  promptly  produce  an  inhibition 
of  a  contracting  intestinal  strip,  while  blood  taken  from  the  animal  previous 
to  the  fright,  had  no  such  effect.  After  ligation  of  the  veins  or  the  removal 
of  the  adrenals  there  was  a  failure  of  this  effect  upon  excitement. 

Emotional  excitement  in  cats  at  least  is  also  attended  with  hypergly- 
cemia and  glycosuria  which  is  probably  due  to  an  increase  of  the  adrenal 
secretion  in  the  blood  inasmuch  as  a  similar  effect  follows  the  injection  of  the 
extract  into  the  blood.  The  hyperglycemia  and  glycosuria  caused  either 
by  the  intravenous  injection  of  the  extract  or  by  an  increased  activity  of  the 
adrenals  following  emotional  excitement,  fear  or  rage,  is  difficult  of  explana- 
tion. It  may  be  the  result  of  a  direct  action  or  an  indirect  action  through 
secretor  nerves  on  the  liver  cells,  in  consequence  of  which  the  stored  glycogen 
is  rapidly  transformed  into  sugar  and  discharged  into  the  blood. 

An  advantage  that  would  accrue  to  the  animal  from  the  accumulation 
of  sugar  in  the  blood  under  these  circumstances,  would  be  a  quickly  avail- 
able source  of  energy-yielding  material,  for  the  continued  muscle  activity 
that  would  attend  either  flight  or  defense. 

The  Function  of  the  Adrenal  Gland. — ^The  function  of  the  adrenal 
gland,  at  least  of  the  medullary  portion,  is  to  furnish  an  internal  secretion 
which  serves  apparently  to  stimulate  the  receptive  substance  at  the  myo- 
neural junction  and  thus  cooperates  with  the  sympathetic  system  to  maintain 
that  degree  of  frequency  and  force  of  the  heart-beat  and  the  contraction  of 
the  arteriole  muscle  necessary  to  maintain  the  normal  blood-pressure;  to 
inhibit  as  occasion  requires,  the  tonus  of  muscle  walls  of  various  viscera; 
to  cause  a  mobilization  of  sugar  in  the  blood  when  this  is  necessary,  and 
to  increase  in  some  unexplained  way  the  tonus  and  activity  of  the  skeletal 
musculature. 

The  Chromaphil  Tissue. — The  chromaphil  tissue  lying  along  the  aorta 
and  in  other  situations  as  well — that  portion  of  the  original  embryonic 
chromaphil  tissue  that  was  not  incorporated  in  the  medulla  of  the  adrenal 
gland — has  been  shown  to  possess  physiologic  actions  similar  to,  if  not 
identical  with  extracts  of  the  adrenal  medulla  itself.  Thus  it  causes  not  only 
a  marked  rise  of  blood-pressure  but,  according  to  the  recent  investigations  of 
Fulk  and  Macleod,  in  many  mammals  it  inhibits  the  spontaneous  contrac- 
tions of  isolated  intestinal  muscle  and  augments  the  tone  and  contractions  of 
the  isolated  virgin  uterus. 


INTERNAL  SECRETION  505 

The  Pancreas. — ^The  pancreas  though  engaged  in  the  production  of  an 
external  secretion  is  yet,  by  reason  of  the  specialized  group  of  cells,  the 
islands  of  Langerhans,  to  be  regarded  as  an  organ  of  an  internal  secretion 
as  well.  These  islands  it  is  generally  believed  are  engaged  in  the  secretion 
of  an  agent  which  after  entering  the  blood  is  carried  to  the  muscles  where  it 
activates  or  assists  a  glycolytic  enzyme  in  promoting  the  oxidation  of  sugar; 
or  it  may  inhibit  normally  the  stimulating  action  of  adrenalin  on  the  liver 
cells  and  thus  prevent  an  excessive  output  of  sugar  and  the  development  of 
hyperglycemia  (see  page  519).  If  the  entire  pancreas  is  extirpated  and  the 
animal  survive  the  operation,  a  glycosuria  is  soon  established,  followed  by  a 
series  of  symptoms  resembling  those  observed  in  diabetes  mellitus  as  it  occurs 
in  man,  viz.:  thirst,  polyuria,  loss  of  energy,  decline  in  body-weight,  etc., 
followed  by  death  in  a  few  weeks.  Pathologic  processes  that  involve  a  large 
portion  of  the  pancreas  likewise  give  rise  to  a  similar  series  of  phenomena, 
as  ligation  of  the  pancreatic  duct,  a  procedure  that  leads  to  a  destruction  of 
all  portions  of  the  pancreas  except  the  islands  of  Langerhans  and  without 
developing  glycosuria  has  led  to  the  inference  that  these  islands  are  the 
agents  engaged  in  the  production  of  the  internal  secretion. 

The  Testicles  and  Ovaries. — The  testicles  and  ovaries  are  regarded  at 
the  present  time  as  glands  for  the  production  of  an  internal  secretion,  as  well 
as  for  the  production  of  the  characteristic  reproductive  elements.  The 
testicles  possess  in  the  epithelial  cells  lining  the  peripheral  portions  of  the 
seminiferous  tubules  an  apparatus  for  the  development  of  spermatozoa,  and 
in  the  cells  in  the  connective  tissue  between  the  tubules — the  so-called  inter- 
stitial cells — an  apparatus  for  the  production  of  the  internal  secretion  which 
is  discharged  into  the  blood,  and  favorably  influences  the  development  of  the 
secondary  sexual  characters,  as  well  as  the  body  as  a  whole.  This  general 
view  is  based  on  facts  such  as  the  following: 

The  removal  of  the  testicles  early  in  life  and  before  the  age  of  puberty 
leads  to  imperfect  development  of  the  vesiculae  seminales  and  the  prostate 
gland;  in  addition  to  these  defects,  there  is  a  failure  of  development  of  the 
various  and  distinctly  sexual  characters  peculiar  to  man  and  other  animals  as 
well.  Sexual  desire  is  wanting  and  the  body  frequently  remains  in  the  in- 
fantile state.  In  some  instances,  however,  there  is  an  increase  in  the  bones  of 
the  skeleton  and  hence  the  animal  increases  in  size.  It  has  been  suggested 
that  the  internal  secretion  controls  the  growth  of  the  bones  by  antagonizing 
the  action  of  some  other  agent,  as  the  secretion  of  the  anterior  lobe  of  the 
pituitary,  which  promotes  the  growth  of  the  skeleton.  That  these  results  are 
not  due  to  the  loss  of  the  structures  producing  the  sperm  elements,  is  shown 
by  the  fact  that  ligation  of  the  vas  deferens  only,  while  destroying  these 
structures,  does  not  interfere  with  development  of  the  sexual  characters. 
Transplantation  of  the  testicles,  in  cocks  and  in  certain  of  the  smaller  mam- 
mals that  have  been  castrated,  has  led  to  the  development  of  secondary 
sexual  characters  which  in  no  apparent  way  differed  from  those  of  control 
animals.  In  old  age  the  testicles  undergo  degenerative  changes  characterized 
by  a  decline  in  physical  vigor,  inactivity  and  a  cessation  of  sexual  activity. 
From  the  foregoing  facts  it  is  apparent  that  these  organs  are  engaged  in  the 
production  of  an  internal  secretion  which  influences  favorably  the  growth, 
development,  and  general  vigor  of  the  entire  body. 

Extracts  of  the  testicles  have  been  prepared  and  injected  into  the  body 


5o6  TEXT-BOOK  OF  PHYSIOLOGY 

in  advanced  years,  with  a  view  of  reestablishing  the  former  condition  of 
body  vigor.  Experiments  of  this  character  were  made  originally  by  Brown- 
Sequard  on  himself  and  reported  that  his  general  health,  his  muscular  power 
and  mental  activity  were  much  improved.  These  results  have  not  received 
general  confirmation. 

The  ovaries  are  also  regarded  as  glands  for  the  production  of  an  internal 
secretion,  as  well  as  for  the  production  of  characteristic  reproductive  ele- 
ments. The  ovary  possesses  in  the  epithelium  of  the  Graafian  follicles  an 
apparatus  for  the  development  of  the  ova,  and  in  the  cells  of  the  intervening 
stroma — the  so-called  interstitial  cells — an  apparatus  for  the  production  of 
an  internal  secretion  which  is  poured  into  the  blood  and  influences  not  only 
•  the  development  of  sexual  organs  but  influences  favorably  the  development 
of  the  secondary  sexual  characters  as  well  as  the  body  as  a  whole. 

The  removal  of  the  ovaries  of  human  beings  early  in  life  is  an  operation 
that  is  not  often  performed  and  hence  it  is  difficult  to  state  the  results  that 
might  arise.  Their  removal  in  certain  animals  leads  to  an  atrophy  of  the 
uterus,  and  in  addition,  to  a  failure  of  development  of  secondary  sexual 
characters.  Menstruation  does  not  occur  and  the  body  does  not  reach  matu- 
rity. The  removal  of  the  ovaries  in  adult  life  results  in  a  cessation  of  men- 
struation, and  the  appearance  of  a  variety  of  disorders  of  a  body  and  mental 
character.  Similar  phenomena  are  frequently  observed  at  the  menopause, 
when  the  ovaries  undergo  degenerative  changes.  The  administration  of 
extracts  of  the  ovaries — oophorin  tablets — is  claimed  to  relieve  some  of  the 
symptoms  following  the  removal  of  the  ovaries  or  occurring  during  the 
menopause.  The  transplantation  of  an  ovary  into  the  wall  of  the  uterus  or 
into  the  broad  ligament  after  ovariotomy  in  women  has,  even  after  the  lapse 
of  two  years,  reestablished  menstruation  and  awakened  sexual  desire.  From 
facts  such  as  the  foregoing  it  has  come  to  be  believed  that  ovaries  also  produce 
an  internal  secretion  which  has  a  marked  influence  on  the  body  and 
mental  states  of  women  and  mammals  generally. 

THE  SPLEEN 

The  spleen  is  a  soft  bluish-red  organ,  oval  in  shape,  from  twelve  to  fifteen 
centimeters  long  by  eight  broad  and  four  thick.  It  is  situated  in  the  left 
hypochondrium  between  the  stomach  and  the  diaphragm.  In  this  situation 
it  is  held  in  position  by  a  fold  of  the  peritoneum  which  passes  from  the  upper 
border  to  the  diaphragm. 

Structure. — A  section  of  the  spleen  shows  that  it  consists  of  connective 
tissue,  blood-vessels,  lymph-corpuscles,  and  lymphoid  tissue.  The  surface 
of  the  spleen  is  covered  by  a  capsule  composed  of  dense  fibrous  tissue, 
from  the  inner  surface  of  which  septa  or  trabeculse  pass  inward  toward  the 
center  of  the  organ.  In  their  course  they  give  off  a  series  of  processes  which 
unite  freely,  forming  a  spongy  connective-tissue  framework.  The  capsule 
and  the  main  trabeculae  in  some  animals  contain  numerous  non-striated 
muscle-fibers.  In  man  they  are  relatively  few  in  number.  The  blood-ves- 
sels which  enter  the  spleen  are  supported  by  the  connective-tissue  septa. 
As  they  pass  toward  the  center  of  the  organ  they  divide  very  rapidly  and 
soon  diminish  in  size.  In  their  course  small  branches  are  given  off,  which 
penetrate  the  inter-trabecular  tissue  and  become  encased  with  spheric  or 


INTERNAL  SECRETION  507 

cylindric  masses  of  adenoid  tissue  known  as  Malpighian  corpuscles.  These 
corpuscles  are  composed  largely  of  leukocytes.  In  some  animals  the  leuko- 
cytes, instead  of  being  arranged  in  masses,  are  distributed  along  the  walls 
of  the  artery  as  a  continuous  layer.  Within  the  corpuscles  the  arteries  pass 
into  capillaries;  whether  the  artery  passes  directly  to  the  splenic  pulp  or  indi- 
rectly by  way  of  the  corpuscles,  its  ultimate  branches  terminate  in  capillaries 
which  open  into  the  spaces  of  the  splenic  pulp.  From  these  spaces  a  net- 
work of  venules  gathers  the  blood  and  transmits  it  to  the  veins.  It  is  a  dis- 
puted question  as  to  whether  the  spaces  are  lined  by  epithelium,  thus  form- 
ing a  continuous  blood  channel,  or  whether  they  are  wanting  in  this  histologic 
element. 

The  Splenic  Pulp. — The  spaces  of  the  connective-tissue  framework 
are  filled  with  a  dark  red  semifluid  mass  known  as  the  splenic  pulp.  When 
microscopically  examined,  the  pulp  presents  a  fine  loose  network  of  adenoid 
tissue,  large  numbers  of  leukocytes  or  lymph-corpuscles,  red  corpuscles  in 
various  stages  of  disintegration,  and  pigment  granules.  Chemic  analysis 
reveals  the  presence  of  a  number  of  nitrogen-holding  bodies,  e.g.,  leucin, 
tyrosin,  xanthin,  uric  acid;  organic  acids,  e.g.,  acetic,  lactic,  succinic  acids; 
pigments  containing  iron,  and  inorganic  salts. 

The  Functions  of  the  Spleen. — Notwithstanding  all  the  experiments 
which  have  been  made  to  determine  the  functions  of  the  spleen,  it  cannot  be 
said  that  any  very  definite  results  have  been  obtained.  The  fact  that  the 
spleen  can  be  removed  from  the  body  of  an  animal  without  appreciably  inter- 
fering with  the  normal  metabolism  would  indicate  that  its  function  is  not  very 
important.  The  chief  changes  observ^ed  after  such  a  procedure  are  an  en- 
largement of  the  lymphatic  glands  and  an  increase  in  the  activity  of  the  red 
marrow  of  the  bones.  The  presence  of  large  numbers  of  leukocytes  in  the 
splenic  pulp  and  in  the  blood  of  the  splenic  vein  suggested  the  idea  that  the 
spleen  is  engaged  in  the  production  of  leukocytes,  and  to  this  extent  contrib- 
utes to  the  formation  of  blood.  The  presence  of  disintegrated  red  blood- 
corpuscles  has  suggested  the  view  that  the  spleen  exerts  a  destructive  action 
on  functionally  useless  red  corpuscles.  These  and  other  theories  as  to 
splenic  functions  have  been  offered  by  different  observers,  but  all  are  lack- 
ing positive  confirmation. 

Volume  Variations  of  the  Spleen. — It  was  shown  some  years  since  by 
Roy,  with  the  aid  of  the  plethysmograph,  that  the  spleen  undergoes  rhyth- 
mic variations  in  volume  from  moment  to  moment.  In  the  cat  and  in  the 
dog  the  diminution  in  the  volume  (the  systole)  and  the  increase  in  volume 
(the  diastole)  together  occupied  about  one  minute. 

This  fact  was  determined  by  withdrawing  the  spleen  through  an  opening 
in  the  abdominal  wall  and  enclosing  it  in  a  box  with  rigid  walls,  the  interior 
of  which  was  connected  with  a  piston  recording  apparatus.  The  system 
being  filled  with  oil,  each  variation  in  volume  was  attended  by  a  to-and-fro 
displacement  and  a  corresponding  movement  of  the  recording  lever.  The 
special  form  of  plethysmograph  used  for  this  purpose  is  known  as  the  on- 
cometer or  bulk  measurer,  and  the  recording  apparatus  as  the  oncograph. 

The  cause  of  these  variations  in  volume  Roy  attributed  to  a  rhythmic 
contractility  of  the  non-striated  muscle-fibers  in  the  capsule  and  trabeculae, 
and  not  to  changes  in  the  arterial  blood-pressure,  as  the  curve  of  the  pressure 
taken   simultaneously  remained   practically  uniform.     The   effect   of   the 


5o8  TEXT-BOOK  OF  PHYSIOLOGY 

rhythmic  contractions  of  the  splenic  muscle-tissue  is  to  force  the  blood 
through  the  organ,  a  condition  necessitated  perhaps  by  the  pressure  relations 
within,  though  what  function  is  thereby  fulfilled  is  not  apparent. 

It  was  subsequently  shown  by  Schafer  and  Moore  that  the  splenic 
volume  is  extremely  responsive  to  all  fluctuations  of  the  arterial  blood-pressure ; 
that  though  the  spleen  may  passively  expand  and  recoil  in  response  to  the 
rise  and  fall  of  the  blood-pressure,  nevertheless  the  reverse  conditions  may 
obtain:  viz.,  that  the  splenic  volume  may  diminish  as  the  pressure  rises,  if 
the  splenic  arterioles  contract  simultaneously  with  the  contraction  of  the 
arterioles  generally.  On  the  contrary,  the  splenic  volume  increase  is  coinci- 
dent with  a  dilatation  of  the  splenic  and  systemic  arterioles.  In  addition  to 
the  rhythmic  variations,  the  spleen  steadily  increases  in  volume  for  a  period 
of  five  hours  after  digestion,  and  then  gradually  returns  to  its  former 
condition. 

Influence  of  the  Nerve  System. — The  nerves  which  supply  the  vascu- 
lar and  visceral  muscles  in  the  spleen  are  derived  directly  from  the  semilunar 
ganglion  (post-ganglionic  fibers)  and  pass  to  it  in  company  with  the  splenic 
artery.  The  nerve-cells  from  which  they  arise  are  in  physiologic  relation 
with  nerve-fibers  (pre-ganglionic  fibers)  which  emerge  from  the  spinal  cord 
in  the  anterior  roots  of  the  third  thoracic  to  the  first  lumbar  nerves  inclusive, 
though  they  are  found  most  abundantly  in  the  sixth,  seventh,  and  eighth 
thoracic  nerves.     Their  center  of  origin  is  in  the  medulla  oblongata. 

Stimulation  of  the  nerves  in  any  part  of  their  course  gives  rise  to  a 
diminution  in  splenic  volume;  division  of  the  nerves  is  followed  by  an  increase 
in  the  volume.  In  asphyxia  the  spleen  is  small  and  contracted,  a  condition 
attributed  to  a  stimulation  of  the  centers  in  the  medulla  by  the  venosity  of 
the  blood. 

The  musculature  of  the  spleen  may  also  be  excited  to  contraction  by 
reflex  influences,  as  shown  by  the  fact  that  stimulation  of  the  central  end  of  a 
nerve  is  attended  by  a  diminution  of  volume. 

Inasmuch  as  the  excised  spleen  will  continue  to  exhibit  variations  in 
volume  when  perfused  with  blood,  it  would  appear  that  it  possesses  some 
contractile  mechanism  independent  to  some  extent  of  the  nerve  system. 


CHAPTER  XX 
METABOLISM 

It  will  be  recalled  that  each  cell  in  the  body  in  the  living  state  is  the  seat 
of  a  series  of  chemic  changes  which  vary  in  degree  from  moment  to  moment 
in  accordance  with  the  degree  of  functional  activity,  and  on  the  continuance 
of  which  all  life  phenomena  depend.  Some  of  these  chemic  changes  are 
related  to  or  connected  with  the  molecules  of  the  living  material,  while 
others  are  connected  with  the  food  material  supplied  to  them.  Of  the 
chemic  changes  occurring  within  the  molecules  some  are  destructive,  dis- 
similative  or  disintegrative  in  character,  whereby  the  molecule  is  in  part 
eventually  reduced  through  a  series  of  descending  chemic  stages  to  simpler 
compounds  which,  apparently  of  no  use  in  the  cell,  are  eliminated  from  it. 
It  is,  therefore,  said  that  the  living  material  undergoes  molecular  disinte- 
gration as  a  result  of  functional  activity.  To  these  changes,  the  term  katahol- 
ism  is  also  applied.  Other  of  these  changes  are  constructive,  assimilative  or 
integrative  in  character,  whereby  a  part  at  least  of  the  food  material  furnished 
by  the  blood-plasma  is  transformed  through  a  series  of  ascending  chemic 
stages  into  living  material,  and  whereby  it  is  repaired  and  its  former  physio- 
logic condition  restored.  It  is,  therefore,  said  that  the  living  material  under- 
goes molecular  integration  as  a  preparation  for  functional  activity.  To  these 
changes  the  term  anahoUspi  is  also  applied. 

Living  material  has  also  a  temperature  varying  in  degree  in  different 
species  of  animals  as  well  as  in  different  parts  of  the  same  animal.  Here  as 
elsewhere  the  temperature  is  due  to  heat  liberated  from  organic  compounds 
through  disruption  and  subsequent  oxidation  to  simpler  compounds. 
Though  some  of  the  heat  liberated  may  come  from  the  tissue  molecules, 
the  larger  part  by  far  comes  from  the  food  molecules — sugar,  fat,  and 
protein, — constituents  of  the  fluids  permeating  the  interstices  of  the  living 
material.  These  foods  carry  into  the  body  potential  energy,  ultimately 
derived  from  the  sun.  WTien  they  are  disrupted  and  oxidized  the  potential 
energy  is  transformed  into  kinetic  energy  which  manifests  itself  for  the  most 
part  as  heat.  To  the  sum  total  of  all  the  chemic  changes  occurring  in 
tissues  and  foods  the  term  metabolism  is  given. 

Though  the  foregoing  statements  are  applied  to  the  individual  cell  they 
are  equally  applicable  to  the  body  as  a  whole,  inasmuch  as  the  organs  and 
tissues  of  which  it  consists  are  composed  of  cells.  The  body  grows  in  size 
and  maintains  its  nutrition,  by  the  introduction  of  food  materials  which  are 
utilized  in  part,  for  the  repair  of  the  tissues  which  have  undergone  molecular 
disintegration  in  consequence  of  actiWty,  and  in  part  for  the  liberation  of 
energy.  As  a  result  of  the  disintegration  or  the  metabolism  of  tissue  and 
food  materials,  products  such  as  carbon  dioxid,  urea,  etc.,  are  formed  which, 
apparently  of  no  further  use,  are  discharged  from  the  body  by  eliminating 
organs  as  the  kidney,  lungs,  skin,  etc.  Assimilation  and  dissimilation  are 
constantly  taking  place.  If  the  food  assimilated  and  metabolized  exactly 
replaces  the  tissues  dissimilated  and  the  food  metabolized  the  body  will 
retain   a  condition  of  nutritive  equilibrium. 

509 


5IO 


TEXT-BOOK  OF  PHYSIOLOGY 


The  term  metabolism  may,  therefore,  be  api)lied  to  the  chemic  changes 
which  materials  undergo,  under  the  influence  of  living  cells.  It  comprises 
(i)  the  elaboration  of  substances  of  relatively  low  composition  into  sub- 
stances of  higher  and  more  complex  composition — anabolism;  and  (2) 
their  subsequent  reduction  to  substances  of  lower  and  less  complex  com- 
position, as  well  as  the  reduction  of  the  relatively  less  complex  food  materials 
— katabolism. 

For  many  reasons  it  is  desirable  to  know,  as  far  as  this  is  possible  the 
extent  and  character  of  the  tissue  changes,  as  well  as  the  successive  chemic 
changes  which  the  food  materials  undergo  in  their  transit  through  the 
body  from  day  to  day. 

In  order  to  obtain  this  information  it  is  necessary  to  place  the  subject  of 
the  experiment  under  conditions  which  permit  of  the  collection  of  the  ex- 
cretions for  purposes  of  analysis,  and  then  to  deduce  from  the  nitrogen  and 
carbon  they  contain,  the  amounts  of  the  tissue  materials  metabolized  in  the 
absence  of  food;  or  the  amounts  of  the  food  materials  metabolized  when  the 
ordinary  amounts  of  food  are  consumed.  Supplementary  facts  from  the 
field  of  physiologic  chemistry  throw  much  light  on  many  of  the  intermediate 
chemic  stages. 

The  methods  by  which  a  metabolism  experiment  is  conducted  have  been 
detailed  on  pages  1 18-120. 

In  various  sections  of  the  preceding  pages  some  of  the  facts  pertaining 
to  the  metabolism  of  the  body  have  been  presented  and  considered,  and 
it  will  suffice  in  this  connection  to  summarize  some  of  the  facts  already 
alluded  to  and  to  add  others  not  heretofore  mentioned. 

Before  considering  the  metabolism  of  the  individual  organic  food  ma- 
terials as  it  unfolds  itself  in  the  animal  body  under  physiologic  conditions, 
it  is  of  advantage  to  consider  the  metabolism  of  living  material  as  it  mani- 
fests itself  when  the  animal  or  the  human  being  is  deprived  of  food  for  a 
variable  length  of  time  or  during  the  starvation  process. 

THE  METABOLISM  OF  LIVING  MATERIAL  DURING  STARVATION 

When  an  animal  is  subjected  to  the  conditions  of  a  metabolism  experi- 
ment and  deprived  of  all  food  except  water  and  oxygen,  it  is  a  relatively  simple 
matter  to  collect  the  excretions,  to  analyze  them  for  nitrogen  and  carbon  and 
then  to  calculate  from  the  amounts  of  each  the  amounts  of  protein  and  fat 
metabolized  from  day  to  day.  The  carbohydrates  may  be  left  out  of  con- 
sideration in  a  prolonged  experiment.  The  following  experiment  is 
illustrative.  The  man  fasted  five  days  and  performed  light  work  in  the 
respiration  chamber. 

METABOLISM  OF  J.  A.  IN  STARVATION 


N  elimination 

C  elimination 

Day  of  fasting 

Urine 

Feces 

Total 

Urine 

Feces 

Respiration 

Total 

I 

12.04 

0.13 

12. 17 

8.0 

188.5 

197.6 

2 

12.72 

0.13 

12.84 

8.3 

179-4 

188.8 

3 

13.48 

0-I3 

13.61 

9.9 

172.2 

183.2 

4 

13-56 

0.13 

13.69 

10.3 

169.4 

180.8 

5 

11-34 

0.13 

11.47 

9-3 

165.8 

176.2 

METABOLISM 


;it 


From  the  foregoing  figures  the  following  table  of  the  general  metabolism 
is  made: 


Day  of  fasting 

Protein                   Fat 

Calories               Calories 
from  protein         from  fat 

Calories, 
total 

I 

2 

3 
4 
S 

76.1 
80.3 
85.1 
85.6 
71.7 

206.  I 
191 .6 
181. 2 
177.6 
181. 2 

303.5                1916.9 
320.5                1781.9 
339.4                1684.7 
341.4                1651.9 
286.1                1684.7 

2220.4 
2102.4 
2024.1 

1992.3 
1970.8 

— From  Lusk's  "Nutrition." 

From  the  foregoing  table  it  will  be  readily  observed  that  with  each  sus- 
ceeding  day  there  was  metabolized  as  the  result  of  activity  and  for  purposes 
of  heat-production  a  certain  quantity  of  protein  and  fat  which  on  the 
fifth  day  amounted  to  71.7  grams  and  181.2I  grams,  respectively,  yielding 
together  1970.8  Calories. 

In  prolonged  starvation  the  metabolism  of  protein  and  fat  continues 
though  in  diminishing  amounts  until  both  reach  the  minimum  just  previous 
to  death,  which  for  the  protein  is  about  18  or  20  grams.  The  minimum  for 
the  fat  at  this  period  depends  on  the  amount  of  fat  in  the  body  prior  to  the 
starvation  period. 

During  the  course  of  the  starvation  there  is  a  corresponding  decline  in 
the  body  weight.  Coincidently  certain  disorders  of  nutrition  arise.  (For 
further  particulars  regarding  starvation  metabolism,  the  loss  of  weight  of 
different  tissues  and  the  post-mortem  appearance  the  reader  is  referred  to 
pages  128  and  129.) 

THE  METABOLISM  OF  THE  BODY  ON  A  MIXED  DIET 

i\s  an  illustration  of  the  result  of  a  metabolism  experiment  on  a  mixed 
diet  the  following  experiment  of  Pettenkofer  and  Voit  may  be  cited.  The 
subject  was  a  man,  weighing  70  kilograms  and  at  rest. 

On  a  mixed  diet  the  materials  under  outcome  were  collected;  from 
their  amounts  it  was  calculated  that  the  body  had  received  the  amounts  of 
the  food  principles  under  income. 

COMPARISON  OF  THE  INCOME  AND  OUTCOME 


Income 

Outcome 

N  in               C  in 
grams            grams 

1                       1 

N  in              C  in 
grams           grams 

HiOin 
grams 

137  grams  albumin "1 

117  grams  fat I    in   c          21:    c 

352  grams  carbohydrate. .;  (      ^'^          0   :>-3 

Urine 

Feces  

17.4                12.7 

2.1                14.5 

248.6 

1279 

83 
828" 

2016  grams  water J 

195             275.8 

2190 

It  will  be  observed  that  the  body  was  in  nitrogenous  equilibrium ;  that 
it  stored  up  39.7  grams  of  carbon  equivalent  to  52  grams  of  fat,  and  that 
it  eliminated  174  more  grams  of  water  than  were  consumed. 

1  The  fat  is  calculated  from  the  carbon  remaining  after  deducting  the  carbon  derived  from 
the  protein  which  is  equal  to  3.28  grams  for  each  gram  of  nitrogen  eliminated. 


512  TEXT-BOOK  OF  PHYSIOLOGY 

THE  METABOLISM  OF  THE  PROTEINS 

It  will  be  recalled  that  the  protein  constituents  of  the  food  introduced 
into  the  alimentary  canal  are  reduced  by  the  action  of  the  enzymes  of  the 
gastric  and  pancreatic  juices  to  relatively  simple  compounds,  viz. :  ammonia, 
amino-  and  diamino-acids  which  are  for  the  most  part  at  once  absorbed  into 
the  blood.  A  portion  of  the  ammonia  and  a  portion  of  some  of  the  amino- 
acids  pass  into  the  large  intestine  where  they  give  rise  under  the  destructive 
action  of  bacteria  to  substances  of  a  simpler  character  such  as  indol,  skatol, 
and  phenol.  These  and  other  putrefactive  substances  are  in  part  discharged 
from  the  body  in  the  feces  and  in  part  absorbed  into  the  blood  and  ultimately 
eliminated  in  combination  with  potassium  sulphate  by  the  kidney  (see  pages 
458  and  488).  The  ammonia  absorbed  is  converted  during  its  passage 
through  the  liver  into  urea. 

The  absorbed  amino-acids,  after  passing  through  the  liver  and  gaining 
entrance  into  the  general  circulation,  are  distributed  to  the  tissues  where 
they  are  in  part  utilized  for  the  reconstruction  of  the  tissue  molecules  that 
have  undergone  metabolism  in  consequence  of  their  functional  activity,  and 
in  part  separated  by  the  action  of  tissue  enzymes  into  the  amine  element 
NHo,  and  a  carbonaceous  radical.  The  percentage  of  the  amino-acids  used 
for  the  former  purpose  varies  at  different  periods  of  life  and  under  varying 
circumstances.  In  youth  when  tissues  are  developing  and  growing  and  at 
the  same  time  are  undergoing  an  active  metabolism,  a  larger  percentage  is 
utilized  for  repair  and  growth  than  in  adult  life  when  the  body-growth  at 
least  has  ceased,  though  metabolism  continues  more  or  less  actively. 

The  amine  element,  NH2,  and  the  carbonaceous  radical  arising  from 
the  cleavage  of  the  amino-acids  not  utilized  for  tissue  growth  and  repair  are 
disposed  of  as  follows.  The  NH2  is  combined  with  hydrogen  and  then  with 
CO2  to  form  ammonium  carbonate  which,  perhaps  in  the  tissues,  certainly 
in  the  liver  is  transformed  into  urea;  the  carbonaceous  radical  is  oxidized 
to  CO2  and  water  with  the  liberation  of  heat.  The  heat-producing  power 
of  protein — 4  Calories  per  gram — is  almost  entirely  due  to  this  oxidation. 
Since  scarcely  more  than  half  the  protein  consumed  is  utilized  for  tissue 
repair  the  question  has  been  raised  as  to  whether  a  smaller  amount  than 
that  usually  consumed  viz.:  100  to  120  grams,  would  not  be  equally  satis- 
factory. For  statements  regarding  the  advantages  of  a  low-  and  high- 
protein  diet  see  pages  121  and  122. 

The  tissue  protein,  sooner  or  later,  also  undergoes  katabolism  in  conse- 
quence of  functional  activity,  and  the  natural  supposition  would  be  that  it 
too  is  reduced  to  its  constituent  amino-acids  which  are  then  disposed  of 
by  the  methods  already  alluded  to.  Nevertheless  there  are  reasons  for  think- 
ing that  this  is  not  wholly  the  case.  Folin  has  presented  a  series  of  facts 
that  lead  to  the  supposition  that  this  portion  of  the  protein  is  represented 
in  part  by  creatinin.  Thus  it  has  been  shown  that  while  the  urea  excreted 
rises  and  falls  with  the  protein  consumed,  the  amount  of  creatinin  excreted 
remains  more  or  less  stationary;  again,  in  individuals  of  pronounced  muscle 
development,  after  unusual  muscle  activity  and  in  diseases  involving  a 
destruction  of  muscle-tissue  there  is  an  increase  in  creatinin  elimination. 
In  the  opposite  conditions  the  amount  excreted  is  low.  For  these  and  other 
reasons  creatinin  is  regarded  as  a  specific  final  end-product  and  an  indicator 
of  the  metabolism  of  muscle  protein. 


METABOLISM  513 

The  Metabolism  of  the  Nudeo-proteins. — The  tissue  cells  of  animal  and 
many  vegetable  foods  contain  nuclei  into  the  composition  of  which  nucleo- 
proteins  enter.  In  the  metabolism  of  the  nucleo-proteins  it  is  assumed  on 
the  basis  of  their  reaction  with  hydrolyzing  agents  that  they  undergo  a 
cleavage  into  a  protein  and  nuclein.  The  nuclein  subsequently  gives  rise 
to  nucleic  acid  which,  under  the  influence  of  an  enzyme,  is  separated  into 
two  bases,  guanin  and  adenin;  these  bases  under  the  action  of  two  enzymes, 
guanase  and  adenase,  are  combined  with  water,  deaminized  and  transformed 
into  xanthin  and  hypoxanthin.  The  oxidation  of  these  latter  compounds 
gives  rise  to  uric  acid.     (See  page  13  and  457.) 

THE  METABOLISM  OF  THE  FAT 

It  will  be  recalled  that  during  the  digestive  process  the  neutral  fats  are 
gradually  reduced  by  the  action  of  the  enzyme  lipase  of  the  pancreatic  juice 
to  corresponding  fat  acids  and  glycerin;  that  subsequently  the  fat  acids  com- 
bine wdth  alkalis  to  form  soaps,  after  which  both  soaps  and  glycerin  are 
absorbed.  After  absorption  and  during  their  passage  through  the  epithelial 
cells  covering  the  \dlli,  the  soap  and  glycerin  are  synthesized  into  fat  globules 
and  deposited  in  the  cell  material.  Subsequently  the  fat  granules  enter 
the  lymph- vessels  of  the  mesentery  and  by  which  they  are  discharged  into 
the  thoracic  duct  and  finally  into  the  blood  at  the  junction  of  the  subclavian 
and  internal  jugular  veins.  After  a  meal  rich  in  fat,  the  amount  of  fat  in 
the  blood  may  be  so  great  as  to  impart  to  it  a  distinctly  pink  color.  In  the 
course  of  several  hours  the  fat  disappears.  As  to  the  change  it  undergoes 
before  it  passes  across  the  capillary  wall  there  is  much  obscurity.  It  is 
stated  by  Hanriot,  that  a  lipase  in  blood  again  hydrolizes  the  fat  to  fat  acids 
and  glycerin  and  in  consequence  of  their  solubiUty,  pass  out  into  the  lymph- 
spaces.  By  ultra-microscopic  methods  of  illumination  the  fat  may  be  ob- 
served as  ''blood  dust,"  in  vigorous  Brownian  movement  (Bayliss).  It  is 
possible  that  under  this  form  they  pass  into  the  lymph-spaces.  As  to  the 
final  disposition  of  the  fat  the  general  belief  is,  that  when  it  is  consumed 
in  normal  amounts  and  under  physiologic  conditions  it  promptly  undergoes 
oxidation  in  the  tissue  cells  yielding  carbon  dioxid  and  water  with  the  libera- 
tion of  heat. 

The  intermediate  stages  through  which  fat  passes  have  not  been  wholly 
determined.  Nevertheless  it  is  a  well-founded  supposition  that  the  first 
stage  is  a  cleavage  of  the  neutral  fats  to  fat  acids — -stearic,  palmitic  and 
oleic,  and  glycerin. 

These  fat  acids  are  the  higher  members  of  the  fat  acids  and  are  character- 
ized by  a  high  molecular  weight.  Before  they  can  undergo  complete  oxida- 
tion they  must  be  reduced  to  lower  acids  of  the  fat  acid  series,  acids  of  a  low 
molecular  weight. 

This  is  accomplished  presumably  by  successive  cleavages  and  oxidations. 
Thus  stearic  acid  which  consists  of  18  carbon  groups  is  successively  reduced 
by  oxidation  of  the  carbon  atom  in  the  beta  position  until  the  four  carbon 
stage  is  reached  when  the  resulting  acid  is  known  as  but5nric  acid,  e.g., 
CH3.CH0.CH2.COOH.  Upon  oxidation  of  the  beta  carbon  atom  there  is 
formed  /3-oxybutyric  acid,  viz.:  CH3.CHOH.CH2.COOH. 

The  succeeding  oxidation  results  in  the  removal  of  the  two  hydrogen 
atoms  attached  to  the  beta  carbon  atom  and  results  in  the  formation  of 
33 


514  TEXT-BOOK  OF  PHYSIOLOGY 

aceto-acetic  acid  or  diacetic  acid,  viz.:  CH3.CO.CH2.COOH,  which  by  the 
loss  of  a  molecule  of  CO2  yields  acetone,  viz.:  CH3.CO.CH3. 

Acetone  by  a  further  oxidation  is  converted  into  carbon-dioxid  and 
water.  Thus  by  these  successive  oxidations  the  stored-up  energy  of  the 
fat  is  liberated  as  heat. 

If  fat  is  consumed  in  too  large  a  quantity  the  last  stage  may  not  be 
who  lly  completed  and  some  of  the  acetone  produced  may  be  eliminated  in 
the  urine.  Analysis  of  the  urine  generally  shows  the  presence  of  acetone 
to  the  extent  of  0,01  to  0.03  grams  daily. 

In  diabetes  these  three  compounds — /S-oxybutyric  acid,  aceto-acetic  acid 
and  acetone — ^make  their  appearance  in  the  urine.  Since  all  oxidations  are 
preceded  by  a  cleavage  the  inference  is  that  in  this  disease  there  is  an 
absence  of  the  enzyme  necessary  to  effect  the  cleavage  of  the  oxybutyric 
acid  and  thus  permit  of  further  oxidation.  The  accumulation  of  this 
acid  in  the  body  establishes  the  condition  known  as  acidosis  and  in  turn  to 
coma  and  death. 

The  Origin  of  the  Body  Fat. — i.  From  Protein. — It  is  a  familiar 
observation  that  on  the  customary  diet  the  animal  body  frequently  accu- 
mulates a  large  amount  of  fat.  The  question,  therefore,  has  arisen  as  to  its 
origin,  and  many  experiments  have  been  made  to  determine  it.  If  an  animal 
is  placed  on  a  protein  diet,  fat  is  very  seldom  deposited.  If  the  protein  is 
consumed  in  normal  amounts,  and  the  fats  and  carbohydrates  in  excess  of 
the  normal  amounts,  there  is  generally  an  accumulation  and  storage  of  fat. 
It  was  for  a  long  time  taught  by  v.  Voit  that  the  carbonaceous  radical  set 
free  in  the  metabolism  of  protein  was  retained  in  the  body  and  deposited  as 
fat.  This  origin  of  fat  has  largely  been  disproven.  The  experimental 
work  of  subsequent  investigators  has  shown  that  v.  Voit's  results  were  due 
to  a  faulty  analysis  of  the  food  consumed ;  that  if  certain  errors  are  eliminated, 
all  the  carbon  arising  from  protein  metabolism  can  be  detected  in  the  usual 
excretions.     The  prevailing  opinion  is  that  fat  does  not  arise  from  protein. 

The  appearance  of  fat  granules  in  the  cells  of  tissues  undergoing  certain 
pathologic  changes  was  formerly  regarded  as  evidence  that  the  protein  con- 
stituents of  these  cells  in  their  metabolism  give  rise  to  fat.  This  view  is  also 
no  longer  entertained.  The  facts  observed  lead  to  the  inference  that  the 
fat  thus  appearing  is  either  transported  from  the  connective-tissue  cells  or  is 
deposited  directly  from  the  blood-stream ;  but  owing  to  the  destructive  changes 
in  the  cell  it  is  no  longer  able  to  metabolize  it.  The  fat  observed  in  muscle- 
and  gland  cells  is  to  be  regarded  as  an  evidence  of  deposition  rather  than  of 
degeneration. 

2.  From  Fat. — If  now  body  fat  is  not  a  derivative  of  protein,  the  only  two 
other  sources  are  the  fats  and  the  carbohydrates  of  the  food.  That  the  fat 
of  the  food  can  be  deposited  is  now  a  general  belief,  contrary  to  the  former 
belief,  that  the  fat  of  the  food  was  at  once  oxidized,  thus  sparing  from  oxida- 
tion the  fat  arising  from  the  protein.  This  belief  is  based  on  the  results  of 
experiments  which  however  are  not  strictly  physiologic  in  character.  Thus 
two  dogs  were  starved  for  a  definite  period.  One  was  then  given  a  large 
amount  of  linseed-oil  and  the  other  large  amounts  of  mutton  fat.  At  the 
end  of  several  weeks  both  had  accumulated  fat.  A  post-mortem  examina- 
tion of  the  fat  of  the  first  dog  showed  that  it  was  liquid  at  o°C.  while  the 
fat  of  the  second  dog  was  solid  at  50 °C.     In   another   experiment  a  dog 


METABOLISM  515 

was  allowed  to  starve  until  its  weight  had  been  reduced  40  per  cent.  It  was 
then  given  large  amounts  of  fat  with  small  amounts  of  meat.  At  the  end 
of  five  days,  the  animal  was  killed  and  found  to  contain  1353  grams  of  fat. 
Of  this  amount  only  131  could  have  come  from  the  protein.  The  inference 
therefore  is  that  the  food  fat  is  capable  of  being  deposited  as  such.  The 
presence  in  the  animal's  fat  of  foreign  fat  acids  such  as  erucic  acid  has 
been  detected  when  the  animal  has  been  fed  on  colza  or  rape-seed  oil. 

Though  these  facts  hold  true  it  is  highly  probable  that  under  physiologic 
conditions  all  the  varieties  of  fats  consumed  as  foods  are  digested  and  sub- 
sequently synthesized  by  the  epithelial  cells  of  the  villi  into  the  form  of  fat 
characteristic  of  the  animal,  and  if  not  immediately  needed  for  oxidation 
purposes  is  transported  to  the  connective  tissues  and  deposited.  Regardless 
of  the  nature  of  the  fat  in  the  food,  the  fat  of  the  animal  is  peculiar  to  it  and 
possesses  physical  and  chemic  properties  which  serve  in  large  measure  to 
distinguish  it. 

3.  From  Carbohydrates. — That  carbohydrates  are  capable  of  being 
transformed  into  fat  when  consumed  in  amounts  beyond  that  necessary 
for  heat-production  is  a  generally  well-recognized  fact,  though  the  successive 
steps  by  which  this  is  brought  about  have  never  been  disclosed.  It  has  not 
been  possible  to  effect  this  transformation  by  any  known  chemic  procedure. 
Animals  fed  on  a  diet  containing  the  customary  amounts  of  protein  and  fat 
but  containing  a  somewhat  larger  amount  of  carbohydrates  than  usual  soon 
begin  to  lay  on  fat.  The  many  experiments  on  the  fattening  of  animals  have 
placed  this  beyond  question.  This  article  of  food  must  be  looked  on  as  the 
chief  source  of  the  body  fat. 

THE  METABOLISM  OF  THE  CARBOHYDRATES 

It  was  stated  in  a  previous  chapter  that  all  the  starch  and  sugar  that  are 
consumed  daily  are  converted  by  the  action  of  the  salivary,  pancreatic  and 
intestinal  enzymes,  for  the  most  part  into  glucose ;  that  after  absorption  the 
glucose  is  transported  by  the  blood  of  the  portal  vein  to  the  liver;  that  while 
passing  through  the  liver  capillaries,  a  portion  passes  across  the  capillary  wall 
into  the  surrounding  lymph  and  into  the  liver  cells  in  which  it  is  dehydrated 
by  enzymic  action  and  converted  into  starch  (glycogen);  that  subsequently 
this  starch  is  again  hydrated  by  the  same  or  a  different  enzyme  and  con- 
verted into  sugar,  glucose  or  glycose,  after  which  it  passes  into  the  blood  to 
take  the  place  of  the  sugar  which  has  left  the  blood  and  which  has  been 
oxidized  in  the  tissues.  By  this  process,  glycogenolysis,  the  normal  percent- 
age of  the  sugar  in  the  blood  is  maintained.  The  metabolism  of  the  carbo- 
hydrates in  the  body  includes  a  brief  statement  of  the  formation  of  glycogen 
and  sugar  in  other  tissues  than  the  liver  and  more  especially  in  the  muscles. 

Muscle  Glycogen. — Glycogen  is  also  found  in  muscles  and  to  some 
extent  in  the  placenta,  and  embryonic  tissues  generally.  Chemic  analysis 
has  shown  that  muscles  contain  from  0.5  per  cent,  to  i  per  cent,  and  as 
these  organs  amount  to  about  40  per  cent.  (28  kgm.)  of  weight  of  the  body, 
70  kgm.,  they  generally  contain  from  140  to  280  grams  of  glycogen.  Inas- 
much as  chemic  analysis  has  failed  to  demonstrate  the  presence  of  glycogen 
in  the  blood,  the  inference  is  that  it  arises  in  the  muscle-cell  in  a  manner 
similar  to  that  observed  in  the  liver-cell,  viz.:  by  a  transformation,  through 


5i6  TEXT-BOOK  OF  PHYSIOLOGY 

de-hydration,  of  the  sugar  of  the  blood.  By  reason  of  this  fact  it  may  be  said 
that  the  muscle  also  possesses  a  starch-forming  or  a  glycogenic  or  an  amylo- 
genelic  function.  If  it  is  a  fact  that  of  the  sugar  absorbed  only  from  12  to 
20  per  cent,  is  temporarily  arrested  by  the  liver,  the  remainder  passing  on 
into  the  blood  of  the  general  circulation,  it  is  readily  conceivable  that  the 
storage  of  the  sugar  under  the  form  of  glycogen  by  the  muscle-cells  is  neces- 
sary not  only  for  the  activity  of  the  muscle  itself,  but  as  a  means  of  preventing 
an  abnormal  percentage  of  sugar  in  the  circulating  blood.  It  is  generally 
admitted  that  though  the  glycogen  is  the  source  of  the  energy  expended  by 
the  muscle,  it  cannot  be  disrupted  and  oxidized  as  such,  but  that  it  must 
first  be  transformed  into  sugar  (glucose) ;  and  for  this  purpose  the  assump- 
tion is  made  that  a  special  enzyme  is  present  and  active.  The  muscle  is 
therefore  said  to  possess  or  exhibit  a  sugar-forming  or  a  glycogenetic  func- 
tion. The  muscle-cells  are  thus  like  the  liver  cells  characterized  by  the  two 
processes  amylogenesis  and  glycogenesis.  During  the  periods  of  prolonged 
activity  of  the  muscles  the  percentage  of  glycogen  rapidly  diminishes,  a  fact 
that  leads  to  the  inference  that  it  is  the  source  in  large  part  of  the  energy 
expended  by  the  muscle.  During  the  period  of  rest  the  percentage  of 
glycogen  rapidly  increases  until  the  normal  is  regained. 

The  Influence  of  the  Nerve  System. — ^The  physiologic  mechanism  by 
which  glycogen  is  changed  to  glucose  (glycogenolysis)  in  amounts  just  suffi- 
cient to  supply  the  needs  of  the  tissues  without  perceptibly  increasing  the 
amount  of  sugar  in  the  blood  is  obscure  and  but  imperfectly  known.  That 
the  nerve  system  is  in  some  way  concerned  in  the  regulation  of  glycogenolysis 
is  apparent  from  observation  of  the  effects  of  injuries,  major  emotional  states 
as  well  as  of  the  results  of  experimental  procedures,  but  whether  the  regula- 
tion is  direct,  i.e.,  through  its  action  on  the  liver-cells,  or  indirect,  i.e.,  through 
its  action  on  the  glands  of  internal  secretion,  e.^.,  adrenals  and  hypophysis, 
is  a  subject  of  investigation  and  discussion.  It  was  discovered  by  Bernard 
that  puncture  of  the  floor  of  the  fourth  ventricle,  at  a  point  between  the 
acoustic  and  vagus  nerves,  near  the  middle  line,  is  followed  within  an  hour 
or  two  by  the  appearance  of  sugar  in  the  urine,  which  lasts  for  from  five  to 
six  hours  in  the  rabbit  and  from  two  to  three  or  even  seven  in  the  dog.  For 
this  reason  Bernard  gave  to  this  area  the  name  of  "diabetic  area." 

Coincident  with  the  appearance  of  sugar  in  the  urine  (glycosuria)  there 
is  an  increase  in  the  percentage  of  sugar  in  the  blood  (hyperglycemia).  The 
liver  at  the  same  time  contains  a  higher  percentage  of  sugar  than  normally. 
Apparently  the  initial  step  in  this  series  of  phenomena  is  an  increased  con- 
version of  glycogen  into  sugar  (glycogenolysis).  This  supposition  receives 
support  from  the  fact  that  the  degree  of  the  hyperglycemia,  and  the  subse- 
quent glycosuria,  will  depend  on  the  amount  of  glycogen  previously  in  the 
liver.  If  the  animal  has  been  well  fed  on  carbohydrates,  the  resulting  gly- 
cosuria will  be  pronounced;  if,  on  the  contrary,  it  has  been  allowed  to  fast 
for  several  days,  the  glycosuria  will  be  slight. 

Assuming  that  the  nerve-cells,  which  constitute  the  diabetic  area,  influ- 
ence the  conversion  of  glycogen  into  sugar,  the  question  arises  as  to  whether 
the  puncture  destroys  the  nerve-cells,  or  whether  it  stimulates  them  in  some 
way  to  increased  activity.  The  results  of  experiment  lead  to  the  latter  sup- 
position. Thus  stimulation  of  sensor  nerves  in  different  regions  of  the  body, 
injuries  to  the  central  nerve  system,  major  emotional  states,  etc.,  which  have 


METABOLISM  517 

the  same  effect  as  puncture  of  the  medulla  lead  to  the  inference  that  the 
"diabetic  center"  is  stimulated  by  the  puncture  and  hence  to  the  further 
inference  that  normally  it  is  stimulated  by  nerve  impulses  transmitted  to  it 
from  one  or  more  regions  of  the  body.  As  to  whether  the  puncture  of  the 
medulla  excites  the  activities  of  vaso-motor  or  of  secretor  cells  there  is  much 
obscurity.  Whatever  the  nature  of  the  impulses  thus  generated,  the  further 
question  arises,  as  to  the  pathway  by  which  the  nerve  impulses  pass,  whether 
by  way  of  the  vagi  or  by  way  of  the  spinal  cord  and  the  splanchnic  nerves 
and  the  hepatic  plexus  formed  in  part  by  the  post-ganglionic  fibers  from  the 
cells  of  the  semilunar  ganglion.  That  it  is  not,  by  way  of  the  vagi,  is  shown 
by  the  fact  that  division  of  these  nerves  does  not  interfere  with  the  develop- 
ment of  glycosuria,  nor  prevent  it,  after  it  is  once  established.  That  the 
pathway  is  down  the  spinal  cord  and  subsequently  through  the  splanchnics, 
is  indicated  by  the  fact  that  a  transverse  division  of  the  spinal  cord  above 
the  level  of  origin  of  these  nerves,  as  well  as  a  division  of  the  splanchnics, 
prevents  the  development  of  the  glycosuria  that  otherwise  would  follow  punc- 
ture of  the  medulla.  It  must  be  remembered,  however,  that  a  transverse 
division  of  the  spinal  cord  in  itself  frequently  gives  rise  to  a  temporary 
glycosuria.  Though  the  results  of  experimentation  are  not  in  all  respects 
in  agreement,  yet  there  is  evidence  that  the  pathway  is  continued  through  the 
splanchnic  nerve  particularly  on  the  left  side  as  far  as  the  semilunar  ganglion 
and  thence  through  the  hepatic  plexus. 

Though  stimulation  of  the  left  splanchnic  nerve  is  usually  followed  by 
glycosuria,  the  question  arises  whether  this  effect  is  due  to  direct  nerve  stimu- 
lation of  the  liver  cells,  thus  increasing  glycogenesis  or  glycogenolysis,  or 
whether  it  is  due  to  an  increased  amount  of  adrenal  secretion  in  the  blood, 
which  is  discharged  into  it  during  the  time  of  splanchnic  stimulation.  The 
following  facts  will  serve  to  elucidate  the  relationship  between  these  organs. 

It  is  well  known  that  the  subcutaneous  injection  of  one  milligram  of 
adrenalin  chlorid  per  1000  grams  of  body-weight,  will  give  rise  to  hyper- 
glycemia and  glycosuria.  Stimulation  of  the  left  splanchnic  nerve  just  where 
it  sends  branches  to  the  adrenal  gland,  gives  rise  to  an  increased  secretion 
and  its  discharge  into  the  blood  as  can  be  shown  in  many  ways.  From  this 
the  deduction  is  made  that  the  glycosuria  is  the  result  of  an  increase  in  the 
normal  percentage  of  adrenalin  in  the  blood  and  this  latter  fact,  is  due  to 
splanchnic  stimulation.  This  supposition  is  strengthened  by  the  further 
fact  that  after  removal  of  the  adrenal  glands,  neither  puncture  of  the  medulla 
nor  stimulation  of  the  splanchnics  gives  rise  to  glycosuria  except  in  rare 
instances. 

It  is  not  to  be  inferred,  however,  that  the  mere  presence  of  adrenalin  in 
the  blood  is  the  cause  of  the  increased  activity  of  the  liver  cells  and  hence  of 
the  hyperglycemia  and  glycosuria.  That  another  factor  is  present  is  evident 
from  the  following:  Stimulation  of  the  hepatic  plexus,  which  presumably  in- 
fluences the  production  of  sugar,  always  gives  rise  to  glycosuria  when  the 
adrenal  glands  are  intact  but  never  after  their  removal.  ■  Hence  the  assump- 
tion is  made  that  the  production  of  sugar  by  the  liver — glycogenolysis — is 
under  the  control  of  the  central  nerve  system  and  by  the  route  detailed  in 
foregoing  paragraphs,  but  only  when  a  sufficient  amount  of  adrenalin  is  pres- 
ent in  the  blood.  The  role  assigned  to  the  adrenalin  is  a  heightening  of  the 
irritability  of  the  terminal  branches  of  the  hepatic  plexus  whereby  arriving 


5i8  TEXT-BOOK  OF  PHYSIOLOGY 

nerve  impulses  become  more  efficient.  These  facts  do  not  decide  the  ques- 
tion, however,  as  to  the  character  of  the  nerves  involved,  that  is,  whether  they 
are  purely  vaso-motor  or  secretory  in  character.  The  evidence  for  the  exist- 
ence of  glycogenolytic  nerves  is  not  decisive. 

Glycosuria. — ^The  mechanism  by  which  the  sugar  is  stored  in  the  liver 
as  glycogen,  and  subsequently  transformed  into  glucose,  and  discharged  into 
the  blood  in  amounts  just  sufficient  to  meet  the  needs  of  the  tissues  without 
giving  rise  to  hyperglycemia  and  glycosuria,  as  well  as  the  mechanism  by 
which  it  is  oxidized  is  apparently  very  delicate  and  easily  disturbed  as  indi- 
cated by  the  ease  with  which  a  glycosuria  more  or  less  pronounced  and  of 
longer  or  shorter  duration  can  be  established.  The  causes  of  glycosuria  are 
many  and  their  mode  of  action  often  obscure. 

Alimentary  Glycosuria. — ^When  carbohydrate  food,  sugar  more  especially 
is  consumed  in  amounts  beyond  what  constitutes  the  assimilation  or  storage 
limit,  the  excess  soon  appears  in  the  urine.  This  limit  varies  somewhat  in 
different  animals.  The  tissues  collectively  of  a  normal  human  being  have 
an  assimilation  capacity  of  approximately  150  grams  of  glucose.  Should 
an  amount  of  sugar  be  consumed  much  beyond  this  at  one  time,  the  excess 
will  be  eliminated  in  the  urine.  Inasmuch  as  carbohydrate  material  con- 
sumed each  day  is  represented  mainly  by  starch,  and  as  digestion  and  ab- 
sorption proceed  slowly  and  metabolism  continually,  a  glycosuria  due  to  a 
large  amount  of  aliment  is  not  of  frequent  occurrence,  under  physiologic 
conditions.  The  assimilation  limit  may  be  lowered  or  raised  beyond  the 
normal  in  accordance  with  variations  in  the  activity  of  the  hypophysis. 
(See  chapter  on  Internal  Secretion.) 

Adrenal  Glycosuria. — As  stated  in  a  previous  paragraph  the  subcutaneous 
injection  of  adrenalin  chlorid  or  of  the  watery  extract  of  the  medulla  of  the 
gland  is  very  soon  followed  by  a  glycosuria  and  hyperglycemia  as  well.  The 
degree  of  the  glycosuria  will  depend  on  the  extent  to  which  glycogen  has 
been  stored  in  the  liver.  It  will  also  be  recalled  that  stimulation  of  the  left 
splanchnic  nerve  is  followed  by  glycosuria  by  reason  of  a  greater  discharge 
of  adrenalin  into  the  blood.  The  increased  transformation  of  glycogen  to 
sugar  has  been  attributed  to  an  exaltation  of  the  irritability  of  the  nerve  end- 
ings of  the  hepatic  plexus  or  to  a  direct  action  on  the  liver  cells  themselves. 
As  to  which  view  is  more  probable,  future  experiments  may  decide.  In 
either  case  the  adrenalin  excites  an  increased  transformation  of  glycogen  to 
sugar.  Any  factor  therefore  which  would  more  or  less  permantly  increase 
adrenal  activity  would  cause  a  more  or  less  permanent  glycosuria.  (See 
page  504.) 

Pancreatic  Glycosuria. — It  has  been  known  for  some  years  that  if  the 
pancreas  be  extirpated  and  the  animal  survive  the  operation,  glycosuria  is 
promptly  established,  followed  by  a  series  of  symptoms  which  gradually,  in- 
creasing in  severity,  and  lead  to  the  death  of  the  animal  in  from  two  to  four 
weeks.  In  addition  to  the  presence  of  sugar,  acetone,  aceto-acetic,  /3-oxybu- 
tyric  acid,  and  an  excess  of  urea  have  been  found  in  the  urine.  The 
quantity  of  sugar  excreted  and  the  gravity  of  the  attendant  symptoms  may 
be  much  diminished,  if  not  entirely  prevented  by  allowing  a  portion  of  the 
gland  to  remain  even  though  its  capacity  for  the  production  of  pancreatic 
juice  is  entirely  abolished.  Transplantation  of  various  portions  of  the  pan- 
creas into  the  subcutaneous  tissue,  in  the  walls  of  the  abdomen,  will  also 


METABOLISM  519 

prevent  the  glycosuria.  The  explanations  which  have  been  offered  as  to 
the  manner  in  which  the  pancreas  prevents  and  its  removal  gives  rise  to 
the  excretion  of  sugar  are  largely  hypothetical. 

It  has  been  claimed  by  many  investigators  that  the  pancreas  secretes  a 
specific  material  partaking  of  the  nature  of  a  hormone  which  is  discharged 
into  the  blood  and  by  it  distributed  to  various  tissues,  especially  the  muscles 
where  it  exerts  its  influence.  Since  the  discovery  of  the  islets  of  Langerhans 
in  the  body  of  the  pancreas  it  has  come  to  be  believed  that  the  cells  composing 
these  islets,  rather  than  the  cells  lining  the  pancreatic  acini  generate  the 
specific  material.  This  view  is  supported  by  the  fact  that  in  many  cases  of 
persistent  and  fatal  glycosuria  in  man,  there  is  in  the  large  majority  of  cases, 
a  concomitant  lesion  of  these  islets.  The  route  by  which  the  secreted  mate- 
rial reaches  the  blood  is  thought  to  be  through  the  lymph-vessels  and  thoracic 
duct,  for  ligation  of  this  latter  vessel  is  followed  glycosuria. 

As  to  the  manner  in  which  the  specific  material  prevents  and  its  removal 
gives  rise  to  glycosuria,  two  theories  at  least  are  supported  by  facts  and  are 
more  or  less  plausible,  e.g. : 

1.  That  it  promotes  the  oxidation  (glycolysis)  of  sugar. 

2.  That  it  prevents  a  too  rapid  production  of  sugar  on  the  part  of  the  liver  cells. 

It  is  assumed  according  to  the  first  view  that  there  is  present  in  the  mus- 
cles, an  enzyme,  the  action  of  which  is  the  disruption  of  the  sugar  molecule 
and  thereby  promotes  its  oxidation;  but  that  for  the  manifestation  of  its 
power  it  requires  the  activating  influence  of  the  pancreatic  hormone,  for  in 
its  absence  oxidation  of  sugar  does  not  take  place  and  hence  with  the  con- 
tinual production  of  sugar  by  the  liver  cells,  hyperglycemia  and  glycosuria 
result.  This  view  has  received  confirmation  in  recent  years  from  the  results 
of  the  experimental  work  of  Knowlton  and  Starling.  These  investigators 
were  able,  by  employing  a  heart-lung  preparation  of  a  normal  dog,  to  keep 
the  heart  beating  for  several  hours  in  a  practically  normal  manner  by  causing 
blood  of  the  sam.e  animal  to  circulate  through  its  cavities  and  the  coronary 
system  at  the  rate  of  about  300  c.c.  every  two  minutes.  To  this  blood  glucose 
was  added.  At  the  end  of  the  experiment  it  was  found  that  the  amount  of 
sugar  oxidized  v»'as,  on  the  average,  4  milligrams  per  gram  of  heart-muscle 
per  hour.  On  substituting  the  heart-lung  preparation  of  a  dog  that  had 
been  rendered  diabetic  by  the  removal  of  the  pancreas  it  was  found  that  the 
power  of  consuming  sugar  was  reduced  to  a  minimum  or  altogether  lost  by 
reason  presumably  of  the  absence  of  the  pancreatic  hormone.  They  then 
demonstrated  that  the  heart  of  the  diabetic  animal  acquired  a  sugar-consum- 
ing or  oxidizing  power  on  feeding  it  with  the  blood  of  a  normal  animal,  the 
consumption  being  in  one  animal  2.9,  5.8,  8.1  mflligrams  per  gram  of  heart- 
muscle  in  three  consecutive  hours,  and  in  another  experiment  there  was  an 
average  consumption  of  3.55  milligrams.  In  another  series  of  experiments 
it  was  demonstrated  that  the  diabetic  heart  very  promptly  developed  a  sugar- 
consuming  power  on  the  addition  to  the  blood  of  a  boiled  extract  of  the  pan- 
creas the  consumption  rising  from  almost  nothing  to  4  milligrams  in  the 
second  hour.  The  deduction  from  these  experiments  is  that  the  oxidation 
of  sugar  is  dependent  on  the  presence  of  a  specific  hormone  secreted  by  the 
pancreas.  If  this  same  fact  could  be  established  for  skeletal  muscle,  the 
view  that  the  pancreatic  hormone  promotes  and  its  absence  prevents  the 
oxidation  of  sugar  would  be  generally  acceptable. 


520  TEXT-BOOK  OF  PHYSIOLOGY 

It  is  assumed  by  the  supporters  of  the  second  view  that  the  production 
of  sugar  in  the  liver  cells  is  regulated  by  two  opposing  factors  or  hormones, 
that  secreted  by  the  adrenal  glands  and  that  secreted  by  the  pancreas,  the 
former  exciting,  the  latter  inhibiting  the  process.  In  the  absence  of  the 
pancreatic  hormone  the  adrenal  hormone  unduly  excites  the  production  of 
sugar.  The  following  facts  support  this  view:  After  extirpation  of  the  adre- 
nals or  ligation  of  the  adrenal  veins,  by  which  the  secretion  is  prevented  from 
entering  the  venous  blood,  the  removal  of  the  pancreas  is  not  followed  as 
usual  by  glycosuria.  The  simultaneous  injection  of  an  extract  of  pancreas 
and  an  injection  of  that  amount  of  adrenalin  ordinarily  necessary  to  produce 
glycosuria  prevents  its  development. 

Pharmacologic  Glycosuria. — It  is  well  known  that  various  pharmacologic 
agents  when  introduced  into  the  body  frequently  give  rise  to  glycosuria, 
though  the  manner  in  which  they  do  so  is  not  always  clear.  Thus,  the  ad- 
ministration of  ether,  curare,  the  uranium  salts,  phlorhizin,  etc.,  is  generally 
followed  by  the  elimination  of  sugar  in  varying  amounts.  Of  these  agents 
the  one  generally  used  for  experimental  purposes  is: 

Phlorhizin. — ^Phlorhizin  is  a  glucosid  obtained  from  the  root  bark  of  the 
cherry,  plum  and  apple  tree.  It  can  readily  be  separated  into  glucose  and 
phloretin  on  the  latter  of  which  its  action  depends.  If,  therefore,  either 
phlorhizin  or  phloretin  be  injected  subcutaneously  or  administered  by  the 
mouth,  sugar  very  promptly  will  appear  in  the  urine  in  amounts  varying, 
with  the  dosage,  from  5  to  15  per  cent.  This  glycosuria  is,  however,  tem- 
porary, but  may  be  made  more  or  less  continuous  by  repeated  injections  at 
least  three  or  four  times  daily.  Coincidently  there  is  a  diminution  in  the 
percentage  of  sugar  in  the  blood  (hypoglycemia),  the  opposite  condition  to 
that  observed  in  the  glycosurias  heretofore  mentioned.  Inasmuch  as  this 
deficiency  calls  for  a  larger  discharge  of  sugar  from  the  liver  this  organ  and 
others  as  well,  soon  become  free  from  glycogen  especially  if  the  animal  be 
deprived  of  carbohydrate  food.  Experimental  investigations  lead  to  the  con- 
clusion that  the  seat  of  action  of  the  phlorhizin  is  in  the  kidney  itself  (whether 
in  the  glomerular  or  in  the  renal  epithelium  is  uncertain),  in  consequence  of 
which  the  sugar  of  the  blood  is  permitted  to  pass  along  with  other  constitu- 
ents into  the  tubules  and  hence  into  the  urine.  As  to  the  change  which  the 
kidney  structures  undergo  under  the  action  of  phlorhizin  not  much  is  known. 
It  has  been  assumed  that  ordinarily  the  sugar  is  in  combination  with  colloid 
material  in  the  blood  and  by  reason  of  this  combination  cannot  pass  across 
the  wall  of  the  glomerular  blood-vessels.  Any  uncombined  sugar,  as  is  the 
case  in  hyperglycemia,  will  diffuse  readily  in  the  urine.  Under  the  influence 
of  phlorhizin  the  kidney  structure  is  presumed  to  acquire  the  power  of  break- 
ing up  this  combination  setting  the  sugar  free,  whereupon  it  at  once  diffuses 
into  the  urine.  When  phlorhizin  is  administered  to  an  animal  living  on  a 
meat  and  fat  diet  alone,  after  all  sugar  has  been  discharged  from  the  body, 
sugar  still  appears  in  the  urine  indicating  that  it  must  have  some  other  origin 
than  the  glycogen  of  the  liver  or  other  tissues.  The  ratio  of  the  dextrose  to 
the  nitrogen  in  the  urine — 2.8  to  i  (Minkowski)  or  3.65  to  i  (Lusk),  a  ratio 
which  corresponds  to  that  found  in  the  urine  of  diabetic  patients  living  on  a 
meat- fat  diet,  leads  to  the  conclusion  that  the  sugar  arises  from  the  metabo- 
lism of  protein  in  a  manner  already  alluded  to. 

Pituitary  Glycosuria. — It  was  discovered  by  Gushing  during  his  experi- 


METABOLISM  521 

mental  and  surgical  procedures  incidental  to  the  removal  of  the  pituitary 
in  whole  or  in  part,  that  glycosuria  frequently  develops  which,  however, 
gradually  passes  away.  Subsequently  it  was  shown  that  mechanical  or 
electrical  stimulation  of  the  intact  posterior  lobe  gave  rise  to  a  similar  glycosu- 
ria provided  of  course  there  was  a  sufficiently  large  amount  of  glycogen  in 
the  liver.  This  effect  has  been  attributed  to  an  increased  discharge  of  the 
internal  secretion  of  the  posterior  lobe  into  the  cerebro-spinal  fluid  and  finally 
into  the  blood,  which  in  some  way  lowers  the  assimilation  limit  of  the  ani- 
mal and  hence  leads  to  an  elimination  of  sugar  (see  chapter  on  Internal 
Secretion). 

Parathyroid  Glycosuria. — ^When  the  thyroids  and  at  least  three  of  the 
parathyroid  bodies  are  removed  in  animals,  there  is  a  disturbance  of  carbo- 
hydrate metabolism,  a  diminished  tolerance  for  sugar,  as  shown  by  the 
appearance  of  glycosuria.  A  similar  condition  is  established  when  three 
parathyroids  and  but  one  thyroid  are  removed.  It  would  thus  appear  that 
the  parathyroids  rather  than  the  thyroids  have  an  influence,  though  un- 
defined in  the  regulation  of  carbohydrate  metabolism. 

The  problem  of  the  storage  of  sugar  in  the  body,  its  release  and  subse- 
quent oxidation  in  accordance  with  metabolic  needs  is  of  great  interest  by 
reason  of  the  fact  that  it  is  intimately  related  with  that  grave  condition  of 
persistent  glycosuria  seen  in  man  and  to  which  has  been  given  the  term: 

Diabetes. — This  term  has  been  applied  to  a  syndrome  characterized  by 
persistent  hyperglycemia  and  glycosuria  accompanied  by  thirst,  wasting  pf 
the  tissues  and  imperfect  oxidation  of  the  fats.  The  sugar  discharged  into 
the  urine  varies  in  amounts  but  continues  even  in  the  absence  of  all  carbo- 
hydrate food.  The  condition  is  usually  permanent,  enduring  for  months  or 
years  but  eventually  terminating  in  the  death  of  the  individual. 

The  cause  of  the  disease  in  each  instance  is  not  always  clear  nor  the  man- 
ner in  which  the  storage,  release  or  oxidation  of  the  sugar  is  disturbed. 

A  characteristic  feature  of  the  disease  is  the  continuous  elimination  of 
sugar  even  in  the  absence  of  carbohydrate  food.  Under  these  circumstances 
there  is  evidence  that  the  sugar  is  derived  from  the  protein  consumed  or 
from  the  protein  of  the  tissues  or  from  both.  It  is  well  known  that  in  the 
metabolism  of  protein  it  undergoes  a  cleavage  into  an  NH2  element  and  an 
organic  acid  radicle.  It  is  further  known  that  the  NH2  element  is  combined 
successively  with  hydrogen  and  carbon  dioxid  to  form  ammonium  carbonate 
which  is  transformed  in  the  liver  into  urea;  that  the  organic  acid  radical  is 
converted  into  sugar.  It  has  also  been  determined  that  the  dextrose  yielded 
in  the  metabolism  of  protein  bears  to  the  nitrogen  yield,  the  ratio  of  3.65 
to  I,  indicating  that  for  every  3.65  grams  of  sugar  and  every  gram  of  nitrogen 
6.25  grams  of  protein  have  been  metabolized.  In  diabetes,  in  the  absence 
of  carbohydrate  from  the  diet,  this  ratio  of  dextrose  to  nitrogen  in  the  urine, 
expressed  by  the  symbol  D  :  N,  has  been  found  to  exist  from  which  the  de- 
duction is  made  that  the  origin  of  the  sugar  is  to  be  sought  for  in  the  metabo- 
lism of  the  proteins. 

The  necessity  for  heat-production  leads  to  a  larger  consumption  of  fat 
and  this  in  turn  impairs  or  overtaxes  the  capacity  of  the  body  tissues  for  fat 
oxidation  and  hence  there  arises  imperfectly  oxidized  cleavage  products  such 
as  j8-oxybutyric  acid,  aceto-acetic  acid  and  acetone  which  are  finally  elimi- 
nated in  the  urine  thus  establishing  the  condition  of  acidosis. 


522  TEXT-BOOK  OF  PHYSIOLOGY 

It  is,  therefore,  apparent  that  the  phenomena  of  diabetes  in  man  closely 
resemble  the  phenomena  that  arise  in  animals  when  the  pancreas  is  removed; 
hence  there  is  a  general  belief  that  destructive  disease  of  the  pancreas  is  the 
most  frequent  cause  of  diabetes. 

From  the  foregoing  facts  it  is  clear  that  a  glycosuria  more  or  less  pro- 
nounced may  be  due  to  the  following  causes: 

1.  An  imperfect  abstraction  of  sugar  from  the  portal  blood  and  its  storage 

as  glycogen  by  the  liver  cells. 
The  imperfect  abstraction  and  storage  of  sugar  may  be  due  to  an  impair- 
ment in  the  functional  activities  of  the  liver  cells  or  to  the  ingestion  and 
absorption  of  excessive  quantities  of  sugar  from  the  intestine.     In  the 
latter  instances  the  resulting  glycosuria  is  said  to  be  of  alimentary  origin. 

2.  A  too  rapid  conversion  of  glycogen  to  sugar  on  the  part  of  the  liver  cells 

(glycogenesis  or  glycogenolysis). 
The  increased  conversion  of  glycogen  to  glucose  may  be  due  to  impair- 
ment of  the  nerve-centers  regulating  the  normal  process  or  to  stimula- 
tion of  the  liver  cells  by  one  or  more  hormones  discharged  into  the  blood 
in  unusual  quantities  by  some  organ  of  internal  secretion,  e.g.,  adrenalin. 
See  page  504. 

3.  An  incomplete  oxidation  of  sugar  (glycolysis)  in  the  cells  of  muscle  and 

•perhaps  other  tissues  as  well,  in  consequence  of  which  it  accumulates 
in  the  blood  beyond  the  normal  amount,  thus  establishing  the  condition 
of  hyperglycemia. 
The  imperfect  oxidation  of  sugar  in  the  muscle-tissue  is  probably  the 
result  of  an  absence  of  the  necessary  glycolytic  enzymes.     The  more 
rapid  metabolism  of  the  protein  constituents  of  the  tissues  whereby 
their  glucose  radicals  are  liberated  in  large  amount  maybe  necessitated  by 
the  inability  to  oxidize  the  sugar  normally  brought  to  them  by  the  blood. 
The  final  disposition  of  sugar  in  the  body  is  an  oxidation  to  carbon 
dioxid  and  water.     The  chief  if  not  the  only  intermediate  stage  is  lactic 
acid.     It  appears  from  chemical  relations  that  the  molecule  of  sugar  is  trans- 
formed into  two  molecules  of  lactic  acid  and  these  in  turn  into  carbon  dioxid 
and  water.     In  its  final  oxidation  the  contained  energy  is  liberated  as  heat. 


CHAPTER  XXI 

THE  CENTRAL  AND  PERIPHERAL  ORGANS  OF  THE  NERVE 

SYSTEM 

The  nerve  system  has  been  resolved  by  histologic  investigation  into  single 
morphologic  units  termed  neurons.  Though  they  have  a  common  origin 
they  have  assumed  different  forms,  in  different  regions  of  the  system  in  the 
course  of  their  development.  Nevertheless  it  is  apparent  from  an  ex- 
amination of  their  structure  that  they  have  many  features  in  common. 
The  neurons  in  their  totality  constitute  the  neuron  or  nerve  tissue. 
Arranged  in  both  a  serial  and  a  collateral  manner  into  a  regular  and  con- 
nected whole,  they  form  the  neuron  or  nerve  system.  The  neurons,  more- 
over, are  grouped  into  more  or  less  complexly  organized  masses  termed  organs 
virhich  in  accordance  with  their  location  may  be  divided  into  (i)  central  organs 
and  (2)  peripheral  organs. 

The  Central  Organs. — ^The  central  organs  of  the  nerve  system  are 
the  encephalon  and  the  spinal  cord,  lodged  within  the  cavity  of  the  cranium 
and  the  cavity  of  the  spinal  or  vertebral  column  respectively.  The  general 
shape  of  these  two  portions  of  the  nerve  system  corresponds  with  that  of  the 
cavities  in  which  they  are  contained.  The  encephalon  is  broad  and  ovoid, 
the  spinal  cord  is  narrow  and  elongated. 

The  encephalon  is  subdivided  by  deep  fissures  into  four  distinct,  though 
closely  related  portions:  viz.,  (i)  the  cerebrum,  the  large  ovoid  mass,  occu- 
pying the  entire  upper  part  of  the  cranial  cavity;  (2)  the  cerebellum,  the 
wedge-shaped  portion  placed  beneath  the  posterior  part  of  the  cerebrum 
and  lodged  within  the  cerebellar  fossae  of  the  cranium;  (3)  the  isthmus  of 
the  encephalon,  the  more  or  less  pyramidal-shaped  portion  connecting  the 
cerebrum  and  cerebellum  with  each  other  and  both  with  (4)  the  medulla 
oblongata  (Fig.  226). 

The  spinal  cord  is  narrow  and  cylindric  in  shape.  It  occupies  the  spinal 
canal  as  far  as  the  second  or  third  lumbar  vertebra. 

The  central  organs  of  the  nerve  system  are  bilaterally  symmetric,  con- 
sisting of  distinct  halves  united  in  the  median  line.  The  cerebrum  is  sub- 
divided by  a  deep  fissure,  running  antero-posteriorly,  into  two  ovoid  masses 
termed  cerebral  hemispheres;  the  cerebellum  is  also  partially  subdivided  into 
hemispheres;  the  isthmus  likewise  presents  in  the  median  line  a  partial 
division  into  halves;  the  medulla  oblongata  and  spinal  cord  are  subdivided 
by  an  anterior  or  ventral  and  a  posterior  or  dorsal  fissure  into  halves,  a  right 
and  a  left. 

The  Peripheral  Organs. — ^The  peripheral  organs  of  the  nerve  system 
in  anatomic  and  physiologic  relation  with  the  central  organs  are  the  ence- 
phalic and  the  spinal  nerves. 

The  encephalic  nerves,  twelve  in  number  on  each  side  of  the  median  line, 
are  in  anatomic  relation  with  the  base  of  the  encephalon,  and  because  of 
the  fact  that  they  pass  through  foramina  in  the  walls  of  the  cranium  they 
are  usually  termed  cranial  nerves. 

523 


524 


TEXT-BOOK  OF  PHYSIOLOGY 


The  spinal  nerves,  thirty-one  in  number  on  each  side,  are  in  anatojnic 
relation  with  the  spinal  cord,  and  because  of  the  fact  that  they  pass  through 
foramina  in  the  walls  of  the  spinal  column  they  are  termed  spinal  nerves. 
As  both  cranial  and  spinal  nerves  are  ultimately  distributed  to  the  structures 
of  the  body — i.e.,  the  general  periphery — they  collectively  constitute  the 

peripheral  organs  of  the  nerve  system. 

The  spinal  nerves  consist  of  two  groups  of  nerve- 
fibers,  a  ventral  and  a  dorsal  group.  Though 
closely  intermingled  in  the  common  trunk  of  the 
spinal  nerve  they  are  distinctly  separated  near  the 
spinal  cord.  Owing  to  their  connection  with  the 
ventral  and  dorsal  surfaces  of  the  spinal  cord  they 
have  been  termed  respectively  the  ventral  and  dor- 
sal roots.  Peripherally  the  ventral  root  fibers  are 
distributed  to  skeletal  muscles,  glands,  walls  of 
blood-vessels  and  walls  of  various  viscera:  the  dor- 
sal root  fibers  are  distributed  to  skin,  mucous  mem- 
branes, muscles,  joints,  etc. 

The  central  organs  of  the  nerve  system  are  sup- 
ported and  protected  by  three  membranes  named, 
in  their  order  from  without  inward,  the  dura  mater, 
the  arachnoid,  and  the  pia  mater. 

The  Encephalo-spinal  Fluid. — The  general 
subarachnoid  space,  as  well  as  certain  cavities  within 
the  encephalon,  contain  a  clear  transparent  fluid, 
termed  the  encephalo-spinal  fluid.  This  fluid  has 
an  alkaline  reaction  and  a  specific  gravity  of  1.007 
or  1.008.  It  is  composed  of  water,  proteins  (pro- 
teoses and  serum  globulin),  and  pyrocatechin 
CgH^(0H)2,  capable  of  reducing  copper  salts, 
though  not  exhibiting  any  other  of  the  properties 
of  sugar.  In  many  respects  this  fluid  resembles 
lymph.  The  subarachnoid  space  and  the  general 
encephalic  cavities,  termed  ventricles,  communicate 
with  one  another  by  an  opening  in  the  pia  mater 
(the  foramen  of  Magendie)  as  it  passes  over  the 
lower  part  of  the  fourth  ventricle. 

The  Functions  of  the  Nerve  System. — It  will 
be  recalled  that  the  entire  nerve  system  can  be  re- 
solved into  simple  morphologic  units,  termed  neu- 
rones, each  of  which  possesses  certain  histologic  fea- 
tures and  physiologic  properties;  that  their  relation 
one  to  the  other  both  in  a  serial  and  collateral  man- 
ner gives  rise  to  the  general  architecture  of  the  nerve 
system.  Recalling  the  functions  of  neurons  in  their 
individual  and  collective  capacities  the  functions  of  the  nerve  system  may  be 
formulated. 

The  functions  of  the  nerve  system  are  twofold:  (i)  It  unites  and  asso- 
ciates the  organs  and  tissues  of  the  body  in  such  a  manner  that  they  are 
enabled  to  cooperate  for  the  accomplishment  of  a  definite  object.     (2)  It 


Fig.  226. — The  Central 
Organs  of  the  Nerve 
System,  f.  t.  o.  Frontal, 
temporal,  and  occipital 
lobes  of  the  cerebrum,  c. 
Cerebellum,  p.  Pons.  mo. 
Medulla  oblongata.  yns., 
ms.  The  upper  and  lower 
limits  of  the  spinal  cord. 
The  remaining  letters  in- 
dicate the  region  and  num- 
ber of  the  spinal  nerves. — ■ 
{Quain,  after  Bourgery.) 


THE  SPINAL  CORD  525 

serves  to  arouse  in  the  individual  a  consciousness  of  the  existence  of  an 
external  world,  by  virtue  of  the  impressions  which  it  makes  on  his  sense 
organs,  and  consequently  to  enable  him  to  adjust  himself  to  his  environment. 

By  virtue  of  the  anatomic  and  physiologic  association,  a  stimulus,  if  of 
sufl&cient  intensity,  applied  to  one  organ  or  tissue  will  call  forth  activity 
in  one  or  more  organs  near  or  remote  from  the  part  stimulated.  This 
coordination  of  action  is  accomplished  mainly  by  the  spinal  cord  and  the 
medulla  oblongata.  All  actions  which  take  place  in  response  to  a  periph- 
eral stimulus  and  independently  of  volition  are  termed  reflex  actions.  The 
reflex  activities  connected  with  digestion,  the  circulation  of  the  blood,  with 
respiration,  excretion,  etc.,  are  illustrations  of  the  coordinating  capabilities 
of  the  nerve-centers  located  in  these  portions  of  the  central  nerve  system. 

By  virtue  of  the  physiologic  activities  of  the  encephalon  and  more 
particularly  of  the  cerebrum  and  of  the  relation  existing  between  it  and  the 
sense  organs  of  the  body,  consciousness  of  the  existence  of  an  external 
world  and  the  individual's  relation  to  it  is  developed.  Experimental  and 
clinic  investigations  show  that  of  the  parts  of  the  encephalon,  the  cerebrum 
is  the  chief,  though  not  perhaps  the  sole  organ  of  the  mind  and  that  its 
functions  are  for  the  most  part  mental. 

The  function  of  a  part  at  least  of  the  peripheral  nerve  system  is  to  afford 
a  means  of  communication  between  the  central  organs  of  the  nerve  system 
and  the  remaining  structures  of  the  body. 

The  nerve-trunks  constituting  this  part  of  the  nerve  system  may  be 
divided  into  two  groups,  as  follows: 

1.  The  first  group  comprises  nerves  in  connection  with  the  special  sense- 

organs,  e.g.,  skin,  eye,  ear,  nose,  tongue,  as  well  as  nerves  in  connection 
with  the  general  or  organic  sense-organs,  e.g.,  mucous  membranes, 
viscera,  etc.,  which  are  connected  primarily  with  nerve-cells  in  the  spinal 
cord  and  medulla  oblongata,  and  secondarily  with  nerve-cells  in  local- 
ized areas  of  the  cerebral  cortex. 

2.  The  second  group  comprises  ner\'es  which  terminate  mainly  in  the 

muscle  apparatus  and  which  constitute  the  continuation  of  nerve  paths 
which  have  their  origin  in  nerve-cells  of  localized  areas  of  the  cerebral 
cortex. 

The  first  group  of  nerves,  the  afferent,  especially  those  connected  with 
the  special  sense-organs,  are  excited  to  activity  by  impressions  made  on  their 
peripheral  terminations  by  agencies  in  the  external  world.  The  nerve 
impulses  thus  generated  are  transmitted  in  part  only  as  far  as  the  spinal 
cord  and  m.edulla  oblongata  while  the  remainder  ascend  to  nerve-cells  in 
localized  areas  of  the  cerebral  cortex  where  they  evoke  sensations.  These 
sensations  by  their  grouping  and  combinations  become  the  primary  elements 
of  intelligence.  The  afferent  nerves  thus  become  a  means  of  communication 
between  the  physical  and  the  mental  worlds. 

The  second  group  of  nerves,  the  efferent,  are  excited  to  activity  by  those 
molecular  disturbances  in  their  related  nerve-cells  which  accompany  voli- 
tional efforts.  The  nerve  impulses  thus  developed  and  discharged  from 
localized  areas  in  the  cerebral  cortex  are  transmitted  by  way  of  the  me- 
dulla and  spinal  cord  to  the  muscles  of  the  face,  trunk  and  extremities 
which  are  in  consequence  excited  to  activity.  The  muscle  movements  thus 
become  physical  expressions  of  mental  states,  and  if  directed  in  a  definite 


526  TEXT-BOOK  OF  PHYSIOLOGY 

manner  to  the  overcoming  of  the  resistance  offered  by  the  external  world 
they  become  capable  of  modifying  it  in  accordance  with  the  mental  states. 
The  efferent  nerves  thus  become  a  means  of  communication  between  the 
mental  and  the  physical  worlds. 

The  central  nerve  system  is  thus  composed  of  a  number  of  separate 
though  closely  related  parts,  to  each  of  which  a  separate  function  has  been 
assigned.  In  the  study  of  the  structure  and  function  of  these  separate  parts 
it  will  be  found  convenient,  and  conducive  to  clearness,  after  a  brief  pre- 
sentation of  the  relation  of  the  spinal  nerves  to  the  spinal  cord,  to  consider 
them  in  the  order  of  their  complexity,  beginning  with  the  spinal  cord  and 
ending  with  the  cerebrum. 

The  Relation  of  the  Spinal  Nerves  to  the  Spinal  Cord. — The  spinal 
nerves  present  near  the  spinal  cord  two  divisions  which  from  their  connection 
with  the  anterior  or  ventral  and  the  posterior  or  dorsal  surfaces  are  known 
as  the  ventral  and  dorsal  roots. 

The  ventral  roots  are  the  axons  of  various  groups  of  nerve-cells  situated 
in  the  anterior  horns  of  the  gray  matter.  From  their  origin  these  axons 
pass  almost  horizontally  forward  through  the  anterior  column  in  three 
distinct  bundles.  After  emerging  from  the  cord  they  curve  downward  and 
backward  to  Join  the  dorsal  roots. 

The  dorsal  roots  are  the  centrally  directed  axons  of  nerve-cells  in  the 
spinal  ganglia.  After  entering  the  cord  they  divide  into  two  main  groups, 
a  lateral  and  a  mesial.  A  portion  of  the  lateral  group  enters  the  posterior 
horn  directly  through  the  caput  comu;  the  other  portion  turns  upward  and 
runs  through  Lissauer's  tract  and  ultimately  enters  the  posterior  horn.  The 
mesial  group  passes  into  the  postero-extemal  column  (Burdach),  where 
the  fibers  divide  into  descending  and  ascending  branches.  The  former 
probably  constitute  the  comma  tract,  the  terminal  branches  of  which  sur- 
round cells  in  the  gray  matter;  the  latter  (ascending)  cross  the  column 
obliquely  and  enter  the  postero-internal  column  (Goll),  in  which  they  pass 
upward  to  terminate  around  the  cells  of  the  nucleus  gracilis  of  the  same  side. 
As  these  root  fibers  pass  up  and  down  the  cord,  collateral  branches  are 
given  off  which  enter  the  gray  matter  at  successive  levels  and  come  into 
physiologic  relation  with  the  cells  of  Clark's  vesicular  column  on  the  same  and 
opposite  sides  and  with  the  cells  of  the  anterior  horn. 

The  peripherally  directed  axons  of  the  nerve-cells  in  the  spinal  nerve 
ganglia  become  associated  with  the  axons  of  the  ventral  roots  and  together 
they  pass  as  a  spinal  nerve  to  peripheral  organs. 

Classification  of  Spinal-nerve  Fibers.— The  ventral  root  axons  have 
been  shown  by  histologic  and  physiologic  methods  of  investigation  to  be  dis- 
tributed to  skeletal  muscles,  glands,  blood-vessels,  and  viscera.  The  dorsal 
root  axons  have  been  shown  by  the  same  methods  to  be  distributed  to  skin, 
mucous  membranes,  and  muscles.  Hence  the  ventral  and  dorsal  root  fibers 
may  be  classified  in  accordance  with  their  distribution,  and  the  character- 
istic modes  of  activity  to  which  they  give  rise  into  several  groups  as  shown 
in  the  tabulation  on  pages  99,  100. 

Though  both  the  efferent  and  afferent  fibers  of  the  spinal  nerves  are 
directly  connected  with  nerve-cells  in  the  spinal  cord,  they  are  also  indirectly 
connected  by  efferent  and  afferent  nerve-tracts  with  the  cerebral  cortex. 

Experimentally,  it  has  been  determined  that  the  anterior  or  ventral 


THE  SPINAL  CORD  527 

roots  contain  all  the  efferent  fibers,  the  posterior  or  dorsal  roots  all  the  afferent 
fibers.     The  proofs  in  support  of  this  view  are  as  follows: 
Stimulation  of  the  ventral  root  fibers  produces : 

1.  Tetanic  contraction  of  skeletal  muscles. 

2.  Discharge  of  secretions  from  glands. 

3.  Increase  in  the  degree  of  the  contraction,  the  tonus,  of  the  muscle 

walls  of  the  peripheral  arteries. 

4.  Variations  in  the  degree  of  the  contraction,  the  tonus,  of  the  muscle 

walls  of  certain  viscera  either  in  the  way  of  augmentation  or  in- 
hibition.^ 
Division  oj  the  ventral  root  fibers  is  followed  by: 

1.  Relaxation  of  skeletal  muscles  and  loss  of  movement. 

2.  Cessation  in  the  discharge  of  secretions  from  glands. 

3.  Temporary  dilatation  and  loss  of  the  tonus  of  blood-vessels. 

4.  Temporary   impairment   of    the    normal    activities   of    the  visceral 

muscles  from  loss  of  central  nerve  control;  the  degree  of  impair- 
ment depending  on  the  nature  of  the  viscus  involved. 
Peripheral  stimulation  of  the  dorsal  root  fibers  produces : 

1.  Reflex  excitation  of  spinal  centers,  in  consequence  of  which  there  is  an 

increased  activity  of  skeletal  muscles,  glands,  blood-vessels,  and 
visceral  walls. 

2.  Reflex  inhibition  of  spinal  nerve-centers,  in  consequence  of  which 

there  may  be  a  decrease  in  the  activities  of  skeletal  muscles,  glands, 
blood-vessels,  and  viscera. 

3.  Sensations  of  touch,  temperature,  pressure,  and  pain. 

4.  Sensations  of  the  duration  and  direction  of  muscle  movements,  of  the 

resistance  offered  and  of  the  position  of  the  body  or  of  its  individual 
parts  (muscle  sensations). 
Division  oj  the  dorsal  root  fibers  is  followed  by: 

1.  Loss  of  the  power  of  exciting  or  inhibiting  reflexly  the  activities  of 

spinal  nerve-centers  and   in  consequence  a  loss  of  the  power  of 
exciting  or  inhibiting  the  activities  of  peripheral  organs. 

2.  Loss  of  sensation  in  all  parts  to  which  they  are  distributed. 

The  ventral  roots  are  therefore  efferent  in  function,  transmitting  nerve 
impulses  from  the  spinal  cord  to  the  peripheral  organs  which  excite  them  to 
activity. 

The  dorsal  roots  are  afferent  in  function,  transmitting  nerve  impulses 
from  the  general  periphery  to  {a)  the  spinal  cord  where  they  excite  its  con- 
tained nerve-centers  to  activity  or  to  a  more  or  less  complete  cessation  of 
activity  (inhibition),  and  (6)  to  the  cerebrum  where  they  excite  its  centers 
to  activity  with  the  development  of  sensations. 

The  Relation  of  the  Encephalic  Nerves  to  the  Encephalon. — ^The 
encephalic  or  cranial  nerves  consist  of  efferent  and  afferent  fibers.  The  two 
groups  of  fibers,  however,  are  not  comprised  in  a  single  nerve  trunk  as  are 
the  corresponding  fibers  of  the  spinal  nerves  but  pass  for  the  most  part  to 
their  destinations  as  independent  and  distinct  nerves.  These  nerves  also 
serve  as  a  means  of  communication  between  the  encephalon  and  the  peripheral 
organs. 

1  These  last  three  phenomena  are  especially  associated  with  the  ventral  roots  of  the  second 
thoracic  to  the  third  or  fourth  lumbar  nerves  inclusive. 


528  TEXT-BOOK  OF  PHYSIOLOGY 

The  efferent  nerves  taken  collgctively  are  also  distributed  to  skeletal 
muscles,  glands,  blood-vessels  and  viscera. 

The  afferent  nerves  taken  collectively  are  distributed  to  the  skin,  mucous 
membrane  and  to  specialized  sense  organs — eye,  ear,  nose,  tongue  and  skin. 
Some  of  the  afferent  nerves  contain  efferent  libers  which  are  distributed  to 
glands,  blood-vessels  and  viscera. 

The  origin  and  distribution  of  both  efferent  and  afferent  encephalic  nerves 
and  the  phenomena  that  follow  their  stimulation  and  divisions,  and  the  func- 
tions attributed  to  them  will  be  fully  considered  in  a  subsequent  chapter. 

THE  SPINAL  CORD 

The  narrow  elongated  portion  of  the  central  nerve  system  contained 
within  the  spinal  canal  is  named,  from  its  situation  and  appearance,  the 
spinal  cord.  It  is  cylindric  in  shape  though  presenting  an  enlargement  in 
both  the  lower  cervical  and  lower  lumbar  regions  corresponding  to  the  origins 
of  the  nerves  distributed  to  the  upper  and  lower  extremities.  The  cord 
varies  in  length  from  40  to  45  cm.,  measures  12  mm.  in  diameter,  weighs 
42  gms.,  and  extends  from  the  atlas  to  the  second  lumbar  vertebra,  beyond 
which  it  is  continued  as  a  narrow  thread,  the  Jilum  terminale.  It  is  divided 
by  the  anterior  and  posterior  longitudinal  fissures  into  halves,  and  is  there- 
fore bilaterally  symmetric.  A  transverse  section  of  the  cord  shows  that 
it  is  composed  of  both  white  and  gray  matter,  the  former  covering  the  surface, 
the  latter  occupying  the  center. 

The  Segmentation  of  the  Spinal  Cord. — ^For  the  elucidation  of  many 
problems  connected  with  the  physiologic  actions  of  the  spinal  cord,  as  well  as 
of  the  symptoms  which  follow  its  pathologic  impairment,  it  will  be  found 
helpful  to  consider  the  cord  as  consisting  physiologically  of  a  series  of  segments 
placed  one  above  the  other,  the  number  of  segments  corresponding  to 
the  number  of  spinal  nerves.  Each  spinal  segment  would  therefore  comprise 
that  portion  of  the  cord  to  which  is  attached  a  pair  of  spinal  nerves.  The 
nerve-cells  in  each  segment  are  in  histologic  and  physiologic  relation  with 
definite  areas  of  the  body,  embracing  muscles,  glands,  blood-vessels, 
skin,  Qtc. 

If  the  exact  distribution  of  the  nerves  of  any  segment  were  known, 
its  function  could  be  readily  stated.  By  virtue  of  this  segmentation  it 
becomes  possible  for  each  segment  to  act  independently  of  or  in  cooperation 
with  other  segments  near  or  remote,  with  which  they  are  associated  by  the 
intrinsic  or  associative  cells  and  their  axons;  and  by  the  same  cooperative 
action  the  spinal  cord  itself  is  enabled  to  act  as  a  unit. 

The  Structure  of  the  Gray  Matter. — The  gray  matter  is  arranged  in 
the  form  of  two  crescents,  united  in  the  median  line  by  a  transverse  band  or 
commissure  forming  a  figure  resembling  the  letter  H.  Though  varying 
in  shape  in  different  regions  of  the  cord,  the  gray  matter  in  all  situations 
presents  on  either  side  an  anterior  or  ventral  and  a  posterior  or  dorsal  horn. 
Between  the  two  horns  there  is  a  portion  termed  the  intermediate  gray  sub- 
stance. The  commissure  presents  in  its  center  a  narrow  canal  which  extends 
throughout  the  entire  length  of  the  cord.  This  canal  is  lined  by  cylindric 
epithelium  and  surrounded  by  gelatinous  material.     (Fig.  227.) 

The  anterior  horn  is  short  and  broad  and  entirely  surrounded  by  white 
matter.     The  posterior  horn  is   narrow   and  elongated  and  extends  quite 


THE  SPINAL  CORD 


529 


up  to  the  surface  of  the  cord,  where  it  is  capped  by  gelatinous  matter,  the 
substantia  gelatinosa.  In  the  lower  cervical  and  thoracic  regions  a  portion 
of  the  intermediate  gray  substance  projects  outward  and  forms  the  so-called 
lateral  horn.  The  gray  matter  fundamentally  consists  of  a  framework  of 
fine  neuroglia  supporting  blood-vessels,  lym- 
phatics, meduUated  and  non-medullated  nerves, 
and  groups  of  nerve-cells. 

The  Nerve-cells.— The  nerve-cells  of  the 
cord  are  very  numerous  and  they  present  a  va- 
riety of  shapes  and  sizes  in  different  regions. 
They  are  usually  arranged  in  groups  which  ex- 
tend for  some  distance  up  and  down  the  gray 
matter,  forming  columns  more  or  less  continuous. 

In  the  anterior  horn  two  well-marked  groups 
are  found,  one  situated  at  the  anterior  and  inner 
angle,  known  as  the  antero-median  group,  the 
other  situated  at  the  posterior  and  lateral  angle 
and  known  as  the  postero-lateral  group.  In  the 
lower  cervical  and  upper  thoracic  regions,  in  the 
region  of  the  lateral  horn,  another  group  of  cells 
is  found,  known  as  the  intermediate  group.  In 
the  central  portion  of  the  horn  there  is  also  a 
central  group. 

The  cells  of  the  anterior  horns  are  of  large 
size,  nucleated  and  multipolar.  They  are  the 
modified  descendants  of  pear-shaped  cells,  the 
neuroblasts,  which  migrated  from  the  medullary 
tube  (see  page  97  ).  In  the  course  of  their  mi- 
gration they  developed  dendrites  which  form 
an  intricate  felt-work  throughout  the  anterior 
horn.  One  of  the  processes,  the  axon,  ap- 
proached the  surface  of  the  cord,  penetrated  it, 
grew  outward,  became  covered  with  myelin  and 
neurilemma,  and  developed  into  an  anterior 
root  fiber.  These  nerve-cells,  with  their  den- 
drites, axons,  and  terminal  branches,  form 
efferent  neurons  of  the  first  order.     The  inti- 


D 


Fig.  227. — Sections  through 
Different  Regions  of  the 
Spinal  Cord.  A.  At  the  level 
of  the  sixth  cervical  nerve.  B. 
At     the     mid-dorsal     region.     C. 

mate  histologic  and  physiologic  relationship  At^  the^  center^  of^  the  lumbar 
existing  between  the  nerve-cell  and  the  axon  is 
revealed  by  the  degenerative  changes  which 
arise  in  the  latter  when  separated  from  the 
former.  The  cell  apparently  determines  the 
nutrition  of  the  axon  and  may  be  regarded  as 
trophic  in  function.  Some  of  the  cells  of  the  anterior  horn  send  their  axons 
into  the  immediately  surrounding  white  matter  of  the  same  side,  after  which 
they  divide  into  two  branches,  one  passing  up,  the  other  down,  the  cord, 
to  reenter  the  gray  matter  at  different  levels.  They  are  probably  asso- 
ciative in  function.  Other  cells  send  their  axons  into  that  portion  of  the 
white  matter  on  the  same  and  opposite  sides  known  as  Gower's  antero- 
lateral tract  (Fig.  228). 
34 


enlargement.  D.  At  the  upper 
part  of  the  conus  medullaris.  i. 
Posterior  roots.  2.  Anterior  roots. 
3.  Posterior  fissure.  4.  Anterior 
fissure.  5.  Central  canal. — {Mor- 
ris'   "Anatomy,"   after  Schwalbe.) 


530 


TEXT-BOOK  OF  PHYSIOLOGY 


In  the  posterior  horn  nerve-cells  are  also  present,  though  they  are  not 
so  numerous  as  in  the  anterior  horn.  At  the  base  of  the  horn  and  on  its 
inner  side  there  is  a  well-marked  group  of  cells  which  extends  from  the 
seventh  or  eighth  cervical  nerves  downward  to  the  second  or  third  lumbar 
nerves,  being  most  prominent  in  the  thoracic  region.  This  column  is  known 
as  Clarke's  vesicular  column.  From  the  nerve-cells  constituting  this  column 
axons  pass  obliquely  outward  into  the  portion  of  the  white  matter  known 
as  the  direct  cerebellar  tract.  Other  nerve-cells  send  their  axons  into  the 
white  matter  in  the  posterior  portion  of  the  cord  bordering  the  posterior 
median  fissure.  Some  of  the  nerve-cells,  their  situation  and  the  distribu- 
tion of  their  axons  are  shown  in  Fig.  228. 


Dona-l 


L/entral 

Fig.  2 28. — Scheme  of  the  Structure  of  the  Cord. — {Howell  after  Lenhossek.)  On  the 
right  the  nerve-cells;  on  the  left  the  entering  nerve-fibers.  Right  side:  i,  Motor  cells,  anterior 
horn,  giving  rise  to  the  fibers  of  the  anterior  root;  2,  tract  cells  whose  axons  pass  into  the  white 
matter  of  the  anterior  and  lateral  columns;  3,  commissural  cells  whose  axons  pass  chiefly  through 
the  anterior  commissure  to  reach  the  anterior  columns  of  the  other  side;  4,  Golgi  cells  (second  type), 
whose  axons  do  not  leave  the  gray  matter;  5,  tract  cells  whose  axons  pass  into  the  white  matter 
of  the  posterior  column.  Left  side:  i,  Entering  fibers  of  the  posterior  root,  ending,  from  within 
outward,  as  follows:  Clarke's  column,  posterior  horn  of  opposite  side,  anterior  horn  same  side  (re- 
flex arc),  lateral  horn  of  same  side,  posterior  horn  of  same  side;  2,  collaterals  from  fibers  in  the 
anterior  and  lateral  columns;  3,  collaterals  of  descending  pyramidal  fibers  ending  around  motor 
cells  in  anterior  horn. 


Classification  of  Nerve-cells. — The  cells  of  the  gray  matter  may  be 
divided  into  three  main  groups:  viz.,  intrinsic  or  associative,  receptive  or 
afferent,  and  emissive  or  efferent. 

The  intrinsic  cells  are  associative  in  function.  The  axons  to  which  these 
cells  give  origin  pass  more  or  less  horizontally  into  the  white  matter,  where 
they  divide  into  two  branches,  one  of  which  passes  upward,  the  other  down- 
ward. At  various  levels  they  reenter  the  gray  matter  and  arborize  around 
other  intrinsic  cells. 

The  receptive  cells  are  largely  sentient  or  afferent  in  function,  inasmuch 
as  the  excitations  brought  to  the  spinal  cord  by  the  afferent  nerves  in  the 
dorsal  roots  from  the  general  periphery  are  received  by  them  and  transmitted 
by  and  through  their  axons  to  the  cortex  of  the  cerebrum,  where  they  are 
translated  into  conscious  sensations.  As  the  nerve-fibers  in  the  dorsal  roots 
of  the  spinal  nerves  are  classified,  in  accordance  with  the  sensations  to  which 


THE  SPINAL  CORD 


531 


they  give  rise,  as  sensor,  thermal,  tactile,  etc.,  so  these  nerve-cells  may  be 
similarly  classified  according  as  they  transmit  their  excitations  to  those 
specialized  areas  in  the  cerebral  cortex  in  which  these  different  sensations 
arise. 

The  emissive  cells  are  efferent  or  motor  in  function,  inasmuch  as  the 
excitation  arising  in  them  is  transmitted  outwardly  through  their  axons  to 
muscles,  glands,  blood-vessels  and  viscera,  imparting  to  them  motion, 
either  molar  or  molecular.  As  the  efferent  fibers  in  the  ventral  roots  of 
the  spinal  nerves  are  classified  (see  page  99)  in  accordance  with  their  physio- 
logic action  into  motor,  secretor,  vaso-motor,  viscero-motor  and  pilo-motor 
nerves,  so  the  nerve-cells  of  which  the  nerves  are  integral  p'arts  may  be 
classified  physiologically  as  motor,  vaso-motor,  secretor,  viscero-motor  and 
pilo-motor.     Collections  or  groups  of  such  cells  are  termed  "centers." 

The  Structure  of  the  White  Matter. — A  transverse  section  of  the  cord 
shows  that  the  white  matter  completely  covers  the  gray  matter  except  where 


Fig.  229. — Transection  of  the  Cervical  Spinal  Cord  showing  Its  Chief  Subdivisions. — 

{After  Mills.) 


the  posterior  horns  reach  the  surface.  Anteriorly  the  white  matter  of  each 
lateral  half  is  connected  by  a  narrow  strip  or  bridge  of  white  matter,  the 
anterior  commissure.  Microscopic  examination  shows  that  the  white  matter 
is  com.posed  of  vertically  disposed  medullated  nerve-fibers  which  are  devoid 
of  a  neurilemma.  These  fibers  are  supported  partly  by  a  framework  of 
connective  tissue,  and  partly  by  neuroglia.  The  white  matter  of  each  side 
of  the  cord  is  anatomically  divided  into  an  anterior,  a  lateral,  and  a  posterior 
column  by  the  anterior  and  posterior  roots  of  the  spinal  nerves. 

Classification  of  the  Nerve-fibers. — From  a  study  of  the  embryologic 
development  of  the  white  matter  and  of  the  degenerative  changes  which  follow 
its  pathologic  and  experimental  destruction,  it  has  been  differentiated  into 
a  number  of  specialized  tracts  which  have  different  origins,  destinations. 


532 


TEXT-BOOK  OF  PHYSIOLOGY 


and  functions.     Some  of  the  more  important  tracts  are  shown  in  Fig,  229. 
They  may  be  divided,  however,  into  efferent,  afferent,  and  associative  fibers. 

1.  The  anterior  column,  comprising  that  portion  between  the  anterior 
longitudinal  fissure  and  the  anterior  roots,  has  been  subdivided  into: 

(a)  The  direct  pyramidal  tract,  or  column  of  Turck.  This  tract  borders 
the  longitudinal  fissure  and  extends  from  the  upper  extremity  of  the  cord  as 
far  down  as  the  mid-thoracic  region.  From  above  downward  this  tract 
diminishes  in  size,  for  the  reason  that  its  fibers  or  their  collaterals  cross  at 
successive  levels  to  the  opposite  side  of  the  cord  by  way  of  the  anterior  com- 
missure to  enter  the  gray  matter  of  the  anterior  horn.  These  fibers  are  the 
continuations  of  fibers  which  take  their  origin  in  cells  which  are  located  in 
the  cortex  of  the  cerebral  hemisphere  of  the  same  side.  The  terminal 
filaments  of  these  fibers  or  axons  are  in  physiologic  relation  either  directly 
or  indirectly  through  intercalated  neuron  cells  with  the  dendrites  of  the 
cornual  cells.  When  divided  in  any  part  of  their  course,  these  fibers  undergo 
descending  degeneration, 

(6)  The  antero -lateral  ground  bundle  or  root  zone.  This  tract  lies  external 
to  the  pyramidal  tract,  surrounds  the  anterior  horn  of  the  gray  matter,  and 
extends  throughout  the  length  of  the  cord.  It  is  composed  of  short  com- 
missural or  associative  fibers  which  come  from  nerve-cells  in  the  gray  matter 
from  the  same  and  opposite  sides  of  the  cord.  After  entering  the  white 
matter  they  divide  into  two  branches,  pursue  opposite  directions,  then  re- 
enter the  gray  matter  at  higher  and  lower  levels  and  come  into  relation  with 
other  nerve-cells: 

2.  The  lateral  column,  comprising  that  portion  between  the  ventral 
and  dorsal  roots,  has  been  di\ided  into: 

{a)  The  antero -lateral  tract  of  Gow^ers.  This  tract  is  somewhat  crescentic 
in  shape  and  situated  on  the  lateral  aspect  of  the  cord  external  to  the  antero- 
lateral root  zone.  It  extends  throughout  the  entire  length  of  the  cord. 
When  divided  it  undergoes  ascending  degeneration,  which  would  indicate 
that  the  axons  originate  in  nerve-cells  in  the  gray  matter.  This  tract  is 
therefore  probably  afferent  in  function. 

The  majority  of  the  fibers  composing  this  tract  on  reaching  the  pons 
turn  backward,  pass  through  the  superior  medullary  velum  to  terminate  in 
the  dorsal  vermis  of  the  cerebellum. 

(&)  The  lateral  limiting  tract.  This  tract,  which  is  quite  narrow,  lies 
close  to  the  external  border  of  the  gray  matter.  It  is  composed  of  fibers 
which  do  not  degenerate  to  any  considerable  extent  after  transverse  section 
and  are  in  all  probability  associative  fibers  which  come  from  nerve- cells  in 
the  gray  matter  to  reenter  at  lower  and  higher  levels.  It  is  also  believed  by 
some  investigators  that  the  anterior  portion  contains  efferent  and  the  pos- 
terior portion  afferent  fibers;  for  this  reason  it  is  frequently  termed  the 
mixed  lateral  tract. 

(c)  The  crossed  pyramidal  tract.  This  tract  occupies  the  posterior  por- 
tion of  the  lateral  column,  though  its  exact  position  varies  somewhat  in 
different  regions  of  the  cord.  In  the  cervical  and  thoracic  regions  it  is 
covered  by  a  layer  of  fibers.  In  the  lumbar  region,  however,  it  comes  to  the 
surface.  From  above  downward  this  tract  gradually  diminishes  in  size, 
for  the  reason  that  its  fibers  and  their  collaterals  enter  the  gray  matter  at 
successive  levels.     The  terminal  branches  of  these  fibers  are  in  close  physio- 


THE  SPINAL  CORD  533 

logic  relation  either  directly  or  indirectly  through  intercalated  neuron  cells 
with  the  dendrites  of  the  cornual  cells.  These  fibers  are  the  continuations 
of  fibers  which  take  their  origin  in  cells  which  are  located  in  the  cortex  of 
the  cerebral  hemispheres  of  the  opposite  side.  When  divided  in  any  part 
of  their  course,  they  undergo  descending  degeneration.  They  are  therefore 
efferent  neurons  and  of  the  second  order. 

(d)  The  direct  cerebellar  tract,  or  column  of  Flechsig.  This  tract  is 
situated  on  the  surface  of  the  lateral  column  external  to  the  crossed  pyramidal 
tract.  It  slightly  increases  in  size  from  below  upward.  It  is  composed  of 
fibers  the  cells  of  which  are  found  on  the  inner  side  and  base  of  the  posterior' 
horn  (Clark's  vesicular  column).  From  this  origin  the  fibers  pass  obliquely 
outward  to  the  surface  and  then  directly  upward  to  terminate,  as  its  name 
implies,  in  the  cerebellum.  Decussation  of  these  fibers  takes  place  in  the 
.  superior  vermiform  lobe  of  the  cerebellum.  When  divided  this  tract  degener- 
ates upward.  It  is  therefore  in  all  probability  an  afferent  tract  and  of  the 
second  order. 

3.  The  posterior  column,  comprising  that  portion  between  the  dorsal 
roots  and  the  posterior  longitudinal  fissure,  has  been  subdivided  into : 

(a)  The  postero-external  tract  of  Burdach.  This  tract  lies  just  within 
the  posterior  horns.  A  portion  of  this  tract  is  composed  of  ground  fibers 
which,  though  vertically  disposed,  have  but  a  short  course.  They  take  their 
origin  in  cells  in  the  gray  matter,  and  after  entering  this  tract  divide  into 
ascending  and  descending  branches,  which  with  their  collaterals  reenter 
the  gray  matter  at  different  levels.  Another  portion  of  this  tract  is  made  up 
of  nerve-fibers  derived  from  the  dorsal  roots  of  the  spinal  nerves,  which 
cross  this  column  toward  the  median  line  in  an  oblique  or  horizontal  direc- 
tion. The  fibers  of  the  upper  portion  of  this  tract  terminate  around  the 
nucleus  cuneatus  at  the  medulla  oblongata.  When  divided,  these  fibers 
degenerate  for  but  a  short  distance.  The  ground  fibers  are  probably  as- 
sociative in  function. 

(b)  The  postero-internal  tract,  or  column  of  Goll.  This  tract  is  separated 
from  the  former  by  a  septum  of  connective  tissue  which  is  most  marked 
above  the  eleventh  thoracic  segment.  The  fibers  which  compose  this  tract 
are  long  and  derived  for  the  most  part  from  the  dorsal  roots  of  the  spinal 
nerves  of  the  same  side.  This  is  shown  by  the  fact  that  division  of  these 
roots  central  to  the  ganglion  is  followed  by  ascending  degeneration  of  the 
column  of  Goll  as  far  as  the  nucleus  gracilis  in  the  medulla  oblongata. 
Fibers  derived  from  cells  in  the  gray  matter  are  also  contained  in  this  column. 
This  tract  is  largely  afferent  in  function. 

(c)  Lissauer's  tract.  This  tract  embraces  the  tip  of  the  posterior  horn 
and  is  composed  principally  of  fibers  from  the  dorsal  roots  of  the  spinal 
nerves.  After  entering  the  tract  the  fibers  divide  into  ascending  and  de- 
scending branches,  which  finally  terminate  around  cells  in  the  posterior 
horn. 

In  addition  to  the  tracts  described  in  foregoing  paragraphs  a  number 
of  small  narrow  tracts  have  been  discovered  in  different  regions  of  the  spinal 
cord  the  functional  significance  of  which,  however,  has  not  been  determined. 
Of  these  may  be  mentioned: 

I.  The  antero -lateral  tract  of  Marchi  and  Lowenthal,  situated  at  the 
anterior  and  inner  angle  of  the  anterior  column,  which  degenerates  down- 
ward after  removal  of  one-half  of  the  cerebellum. 


534  TEXT-BOOK  OF  PHYSIOLOGY 

2.  The  comma  tract,  a  narrow  bundle  of  fibers  situated  in  the  anterior 
portion  of  the  column  of  Burdach.  When  it  is  divided  it  degenerates  down- 
ward. 

3.  The  septo-marginal  tract,  an  oval-shaped  tract  situated  along  the 
margin  of  the  posterior  longitudinal  fissure. 

4.  The  cornu-commissural  tract  found  along  the  border  of  the  anterior 
portion  of  the  posterior  column  as  far  forward  as  the  posterior  commissure. 
Both  of  these  tracts  are  best  developed  in  the  lumbosacral  region.  They 
arise  .from  nerve-cells  in  the  gray  matter.  They  undergo  descending 
degeneration  when  divided,  but  not  after  division  of  the  dorsal  roots. 

THE  FUNCTIONS  OF  THE  SPINAL  CORD 

Anatomic  investigation  has  demonstrated  that  the  segments  of  the  spinal 
cord  are  associated  through  their  related  spinal  nerves  with  the  organs  and 
tissues  of  definite  areas  of  the  body.  Physiologic  investigation  has  also 
demonstrated  that  the  segments  by  reason  of  the  presence  of  nerve-cells 
and  nerve-fibers  may  be  regarded  as  composed  of: 

1.  Nerve-centers,  each  of  which  has  certain  special  functions,  and 

2.  Conduction  paths  by  which  these  centers  are  brought  into  relation  not 

only  with  one  another,  but  with  the  cerebrum  and  its  subordinate  or 
underlying  parts,  e.g.,  the  medulla  oblongata,  pons  varolii  and 
cerebellum. 

A.    THE  SPINAL  CORD  SEGMENTS  AS  LOCAL  NERVE -CENTERS. 

The  efferent  cells  of  the  spinal  segments  are  the  immediate  sources  of 
the  nerve  energy  that  excites  activity  in  skeletal  muscles,  glands,  vascular, 
and  to  some  extent  visceral  muscles. 

The  discharge  of  their  energy  may  be  caused : 

1.  By  variations  in  the  composition  of  the  blood  or  lymph  by  which  they 

are  surrounded  or  as  the  outcome  of  a  reaction  between  the  chemic 
constituents  of  the  lymph  on  the  one  hand  and  the  chemic  constituents 
of  the  nerve-cell  on  the  other  hand.  The  excitation  of  the  cell  thus 
occasioned  is  termed  automatic  or  autochthonic  excitation. 

2.  By  the  arrival  of  nerve  impulses,  coming  through  afferent  nerves  from 

the  general  periphery,  skin,  mucous  membrane,  etc. 

3.  By  the  arrival  of  nerve  impulses  descending  the  spinal  cord  from  cells  in 

the  cortex  of  the  cerebrum  or  subordinate  regions. 

The  excitation  in  the  former  instance  is  said  to  be  reflex  or  peripheral 
in  origin;  in  the  latter  instance  direct  or  cerebral  in  origin.  In  the  direct 
or  cerebral  excitations  the  skeletal  muscle  movements  are  due  to  psychic 
states  of  a  volitional,  or  an  affective  or  emotional  character;  the  gland  dis- 
charges and  vascular  and  visceral  muscle  movements  to  affective  or  emotional 
phases  of  cerebral  activity  only. 

Automatic  Excitation. — By  this  expression  is  meant  a  discharge  of 
energy  from  the  spinal  nerve-cells  occasioned  by  (a)  a  change  in  the  chemic 
composition  of  the  blood  or  lymph  by  which  they  are  surrounded  or  prob- 
ably a  reaction  between  the  constituents  of  the  lymph  and  the  constituents 
of  the  nerve-cell  or  (b)  the  development  within  the  cell  of  a  stimulus,  the 
so-called  "inner  stimulus,"  the  outcome  of  metabolic  activity. 

As  no  effect  arises  without  a  sufficient  cause  the  term  automatic  has  been 


THE  SPIN.\L  CORD  535 

objected  to  and  the  term  autochthonic  has  been  suggested,  as  more  nearly  ex- 
pressing the  facts  stated.  A  center  so  acting  could  not  be  regarded  as  prima- 
rily a  center  for  reflex  activity,  however  much  it  might  be  influenced  second- 
arily by  afferent  nerve  impulses.  If  the  cell  excitation  is  continuous  though 
variable  from  time  to  time,  it  is  said  to  possess  tonus  and  the  organ  or  tissue 
thus  excited  is  also  said  to  possess  tonus  or  to  be  in  a  state  of  tonic  activity. 
If  the  cell  discharge  is  intermittent  in  character  it  imparts  to  certain  muscles, 
e.g.,  the  respiratory  muscles,  a  rhythmic  activity.  It  must,  however,  be  kept 
in  mind  that  the  tonus  of  nerve-centers  as  well  as  of  peripheral  organs  can  also 
be  developed  and  maintained  by  the  inflow  of  nerve  impulses  transmitted 
from  the  periphery.  The  reason  for  the  belief  that  the  cord  and  its  upper 
prolongation,  the  medulla  oblongata,  are  endowed  with  autochthonic  activity 
is  based  on  the  fact  that  certain  peripheral  organs  are  in  a  state  of  continu- 
ous activity  and  apparently  uninfluenced  to  any  marked  extent  except  tem- 
porarily by  nerve  impulses  transmitted  to  the  cord  through  afferent  nerves. 
As  illustrations  of  such  continuous  activity  may  be  mentioned:  (a)  the 
contraction  of  the  abductor  muscle  of  the  larynx  (the  posterior  crico-arytenoid) 
whereby  the  vocal  membranes  are  separated  and  the  glottis  kept  open  under 
all  circumstances  except  during  the  emission  of  a  vocal  sound;  (b)  the  con- 
traction of  the  dilatator  muscle  of  the  iris;  (c)  the  contraction  of  the  anal 
and  vesic  sphincters;  (d)  the  periodic  contraction  of  the  respiratory  muscles 
(see  page  425);  (e)  the  acceleration  of  the  heart-beat  (page  423). 

Though  automatic  activity  of  the  spinal  cord  is  yet  upheld  by  some 
physiologists,  the  fact  must  be  recognized  that  with  increasing  knowledge 
of  reflex  activities  many  phenomena  previously  regarded  as  automatic  have 
been  found  to  be  dependent  on  peripheral  stimulation  and  therefore  reflex 
in  origin.  Whether  this  will  eventually  be  found  true  for  all  instances  of  so- 
called  automatic  or  autochthonic  activity  will  depend  on  the  results  of  future 
investigations.  Among  the  phenomena  removed  from  the  sphere  of  auto- 
matic, to  the  sphere  of  reflex  activity  may  be  mentioned  muscle  tonus,  vascular 
tonus  and,  trophic  tonus. 

Muscle  Tanus. — ^All  the  skeletal  muscles  of  the  body  are  at  all  times  in 
a  state  of  slight  but  continuous  contraction,  termed  tonus,  by  virtue  of  which 
their  efficiency  as  quickly  responsive  organs  is  increased.  That  such  a 
slight  contraction  is  present  even  in  a  state  of  rest  is  shown  by  the  fact  that 
if  a  muscle  be  divided  in  the  living  animal  the  two  portions  will  contract 
and  separate  to  a  certain  distance.  The  condition  of  the  muscle  was 
formerly  attributed  to  an  automatic  and  continuous  discharge  of  energy  from 
the  nerve-cells.  Brondgeest,  however,  showed  that  this  tonus  is  entirely 
reflex  in  origin  and  immediately  disappears  on  division  of  the  posterior 
roots  of  the  spinal  nerves,  which  would  not  be  the  case  if  the  cells  in  the  cord 
were  acting  automatically.  The  afferent  nerves  in  this  reflex  arise  in  the 
muscle  or  its  tendons,  and  the  stimulus  is  the  slight  degree  of  extension  to 
which  the  muscle  is  subjected  in  virtue  of  its  attachments  and  the  ever-varying 
position  of  the  limbs  and  trunk  (see  page  57). 

Vascular  Tmius. — ^The  arteriole  muscles  throughout  the  vascular  appara- 
tus are  also  constantly  in  a  state  of  slight  but  continuous  contraction  which 
assists  in  the  maintenance  of  an  average  arterial  pressure  and  is  due  to  the 
continuous  discharge  of  nerve  energy  from  the  general  or  dominating  vaso- 
tonic (constrictor)  center  in  the  medulla  oblongata.     This  center  it  will  be 


536 


TEXT-BOOK  OF  PHYSIOLOGY 


recalled  has  been  shown  by  Porter  to  consist  of  two  portions,  a  vaso-tonic 
and  a  vaso-rellex.  The  former  is  in  a  state  of  continuous  tonus  or  activity; 
the  latter  is  capable  of  being  influenced  in  its  activity  not  only  by  variations 
in  the  composition  of  the  blood  but  by  nerve  impulses  transmitted  to  it 
from  all  regions  of  the  body  (see  page  383). 

Trophic  Tonus. — The  normal  metabolism  of  muscle,  gland,  and  con- 
nective tissue  which  underlies  the  assimilation  of  food,  the  production  and 
storage  of  energy-holding  compounds,  and  the  production  of  new  compounds, 
is  dependent,  in  the  higher  animals  at  least,  on  the  connection  of  these  tissues 
with  the  central  nerve  system;  for  if  the  efferent  nerves  be  divided,  not  only 
will  they  themselves  undergo  degeneration  in  their  peripheral  portions,  but 
the  muscles,  glands,  and  connective  tissues  to  which  they  are  distributed  will 
also  undergo  similar  changes.  This  is  to  be  attributed  not  merely  to  inac- 
tivity, but  rather  to  a  loss  of  nerve  influence.     It  would  appear  from  facts  of 


,spc 


Fig.  230. — Diagram  Showing  the  Structures  Involved  in  the  Production  of  Reflex 
Actions,  G.  Bachman.  r.s.  Receptive  surface;  a/.n.  afferent  nerve;  e.c.  emissive  or  motor  cells  in 
the  anterior  horn  of  the  gray  matter  of  the  spinal  cord,  sp.c;  e/.n.  efferent  nerves  distributed  to 
responsive  organs,  e.g.,  directly  to  skeletal  muscles,  sk.m.,  and  indirectly  through  the  interme- 
diation of  sympathetic  ganglia,  sym.g.,  to  blood-vessels,  b.v.,  and  to  glands,  g.  The  nerves 
distributed  to  viscera  are  not  represented. 

this  character  that  the  normal  metabolism  is  dependent  for  its  continuance 
on  nerve  influences.  There  is  no  evidence,  however,  as  to  the  existence  of 
special  trophic  nerves,  separate  from  those  which  impart  to  glands  and  mus- 
cles their  customary  activities.  The  trophic  centers  and  the  motor  centers 
are  identical,  though  the  two  modes  of  their  activity  are  separate  and  distinct. 
The  activity  of  the  so-called  trophic  centers  which  was  at  one  time  believed 
to  be  automatic  is  now  regarded  as  due  to  reflex  influences. 

The  tonic  contraction  of  the  visceral  muscles — e.g.,  the  pyloric,  the 
vesical,  the  anal  sphincters — though  regarded  as  automatic  by  some,  is 
probably  reflex  in  origin,  dependent  on  the  arrival  of  afferent  impulses  from 
the  periphery.  It  is  probable  that  future  investigation  will  disclose  the 
existence  and  pathway  of  these  afferent  fibers.  \\ 

Reflex  Excitation. — It  has  already  been  stated  that  the  nerve-cells  in  the 
spinal  cord  are  capable  of  receiving  and  transforming  afferent  nerve  impulses, 


THE  SPINAL  CORD 


537 


the  result  of  peripheral  stimulation,  into  efferent  nerve  impulses,  which  are 
reflected  outward  to  skeletal  muscles,  exciting  contraction;  to  glands,  pro- 
voking secretion;  to  blood-vessels,  changing  their  caliber;  and  to  organs,  in- 
hibiting or  augmenting  their  activity.  All  such  actions  taking  place  through 
the  spinal  cord  and  medulla  oblongata  independently  of  sensation  or  volition 
are  termed  reflex  actions.  The  mechanism  involved  in  every  reflex  action 
consists  of  at  least  the  following  structures  (Fig.  230) : 

1.  A  receptive  surface;  e.g.,  skin,  mucous  membrane,  sense  organ,  etc. 

2.  An  afferent  fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises — 

4.  An  efferent  nerve,  distributed  to — 

5.  A  responsive  organ,  as  muscle,  gland,  blood-vessel,  etc. 

In  this  connection  the  reflex  contractions  of  skeletal  muscles  only  will  be 
considered. 

If  a  stimulus  of  sufficient  intensity  be  applied  to  the  receptive  surface, 
there  will  be  developed  in  the  terminals  of  the  afferent  nerve  a  series  of 
nerve  impulses  which  will  be  transmitted  by  the  afferent  nerve  to,  and  re- 
ceived by,  the  dendrites  of  the  emissive  cell  in  the  anterior  horn  of  the  gray 
matter.  With  the  reception  of  these  impulses  there  will  be  a  disturbance  in 
the  equilibrium  of  the  molecules  of  the  cells, 
a  liberation  of  energy,  and  a  transmission  of 
nerve  impulses  outward  through  the  efferent 
ner\^e  to  the  muscle. 

A  reflex  mechanism  or  arc  of  this  simplicity 
v/ould  subserve  but  a  simple  movement.  The 
majority  of  the  reflexes,"  however,  are  ex- 
tremely complex  and  involve  the  cooperation 
and  coordination  of  a  number  of  centers  at 
different  levels  of  the  spinal  cord  and  medulla, 
on  the  same  and  opposite  sides,  and  of  mus- 
cles situated  at  distances  more  or  less  remote 
from  one  another.  The  transference  of  nerve 
impulses  coming  from  a  localized  area  of  a 
receptive  surface,  to  emissive  cells  situated  at 
different  levels  is  accomplished  by  the  inter- 
mediation of  a  third  neuron  situated  in  the 
gray  matter,  which  is  in  connection,  on  the  one 
hand,  with  the  central  terminals  of  the  afferent 
nerve  and,  on  the  other  hand,  through  colla- 
teral branches  with  the  dendrites  of  the  efferent 
neurons  situated  at  different  levels.  (Fig.  231 .) 
A  histologic  and  physiologic  mechanism  of 
this  character  readily  explains  how  a  localized 
stimulation  can  give  rise  to  reflex  actions  extremely  complex  in  character. 

The  reflex  contractions  of  skeletal  muscles  are  best  studied  after  division 
of  the  central  nerv^e  system  at  the  upper  limit  of  the  spinal  cord.  After 
this  procedure  the  spinal  centers  can  act  independently  of,  and  uninfluenced 
by  either  sensation  or  volitional  efforts  on  the  part  of  the  animal.  Though 
it  is  possible  to  provoke  reflex  contractions  under  such  circumstances  in 
warm-blooded  animals,  they  are,  as  a  rule,  incomplete  and  of  short  duration, 


Fig.  231. — Diagram  Showing 
THE  Relation  of  the  Third 
Neuron  a,  to  the  Afferent 
Neuron  b,  and  to  the  Efferent 
Neurons  c,  c,  c. — (After  Kdlliker.) 


538  TEXT-BOOK  OF  PHYSIOLOGY 

owing  to  disturbances  of  the  circulation  and  respiration  and  the  consequent 
loss  of  tissue  irritability.  In  frogs  and  in  cold-blooded  animals  generally, 
the  spinal  cord  retains  its  irritability  for  a  long  period  of  time  after  removal 
of  the  brain,  and  therefore  is  well  adapted  to  the  study  of  reflex  actions. 

The  separation  of  the  spinal  cord  from  the  brain  is  readily  effected  by 
destroying  the  medulla  oblongata.  This  can  be  done  by  inserting  a  pin 
through  the  skin  and  the  occipito-atlantal  membrane  covering  the  space  be- 
tween the  occipital  bone  and  the  atlas,  until  it  strikes  the  bodies  of  the 
vertebras  below.  If  the  pin  is  properly  directed  it  passes  through  the  med- 
ulla. Care  should  be  taken  to  avoid  injury  to  the  blood-vessels  on  either 
side.  The  brain  itself  should  then  be  destroyed,  so  as  to  remove  all  con- 
sciousness, by  inserting  the  pin  into  the  brain  cavity  through  the  foramen 
magnum,  and  giving  it  a  few  rotatory  movements. 

A  frog  so  prepared,  and  placed  on  the  table  and  allowed  to  remain  at 
rest  for  a  few  moments  until  the  shock  of  the  operation  passes  away,  will 
draw  the  limbs  close  to  the  body  and  assume  a  position  not  unlike  that  of  a 
normal  frog.  If  then  the  posterior  limbs  be  extended,  they  will  immediately 
be  drawn  close  to  the  side  of  the  trunk  in  the  usual  flexed  position.  If  the 
toes  are  pinched  with  forceps,  the  foot  will  execute  a  series  of  movements  as 
if  the  frog  were  trying  to  free  itself  from  the  source  of  irritation. 

If  the  frog  be  suspended,  the  limbs,  through  the  force  of  gravity,  will  be 
gradually  extended  and  hang  down  freely.  In  this,  as  in  the  sitting  position, 
the  animal  will  remain  perfectly  quiet  and  will  not  execute  spontaneous 
movements.  Any  stimulus  applied  to  the  skin,  however,  provided  it  is  of 
sufficient  intensity,  will  be  followed  by  a  more  or  less  pronounced  move- 
ment. Mechanic,  chemic,  or  electric  stimuli  applied  to  any  part  of  the 
skin  will  call  forth  the  characteristic  reflex  movements.  Chemic  stimuli 
such  as  weak  solutions  of  sulphuric  or  acetic  acid  placed  on  the  toes  will  be 
followed  by  feeble  flexion  of  the  corresponding  leg,  to  be  succeeded  in  a 
short  time  by  extension.  Stronger  solutions  will  produce  more  extensive 
and  vigorous  movements,  the  foot  at  the  same  time  being  rubbed  against  the 
thigh,  apparently  for  the  purpose  of  freeing  it  from  the  irritant.  Similar 
phenomena  follow  the  application  of  the  acid  to  the  fingers  or  the  trunk. 
As  a  rule,  the  extent  and  complexity  of  the  movements  is,  within  limits, 
proportional  to  the  strength  of  the  stimulus.  By  limiting  the  sphere  of 
action  of  the  stimulus  to  definite  but  different  areas  of  the  skin  a  great  variety 
of  movements,  more  or  less  complex  and  coordinated  and  apparently  pur- 
posive and  defensive  in  character,  can  be  produced.  The  coordinated  and 
purposive  character  of  the  movements  exhibited  by  a  brainless  frog  led 
Pfliiger  to  the  assumption  that  the  spinal  cord  in  this  as  well  as  in  other  cold- 
blooded animals  is  possessed  of  sensorial  functions,  and  endowed  with 
rudimentary  consciousness.  This  view,  however,  is  not  generally  accepted, 
the  movement  being  attributed  to  specialized  mechanisms  in  the  cord, 
partially  inherited,  which  permit  of  one  and  the  same  movement  with 
mechanic  regularity  and  precision,  so  long  as  the  conditions  of  the  experi- 
ment remain  the  same. 

In  warm-blooded  animals  similar  results  may  be  obtained  for  a  short 
time  after  division  of  the  cord,  especially  if  artificial  respiration  is  maintained 
and  the  circulation  of  the  blood  continued.  The  cord  will  then  retain  its 
irritability  for  some  time.     If  the  conditions  of  experimentation  were  favor- 


THE  SPIN.\L  CORD  539 

able,  it  is  highly  probable  that  the  human  spinal  cord  would  execute  similar 
movements.  Thus  it  was  observed  by  Robin  in  a  man  who  had  been  decapi- 
tated that  reflex  muscle  contractions  could  be  elicited  by  stimulating  the  skin 
after  the  lapse  of  an  hour  after  execution.  "  While  the  right  arm  was  lying 
extended  by  the  side,  with  the  hand  about  25  centimeters  distant  from  the 
upper  part  of  the  thigh,  I  scratched  with  the  point  of  a  scalpel  the  skin  of 
the  chest  at  the  areola  of  the  nipple,  for  a  space  of  10  or  1 1  centimeters  in 
extent,  without  making  any  pressure  on  the  subjacent  muscles.  We  im- 
mediately saw  a  rapid  and  successive  contraction  of  the  great  pectoral 
muscle,  the  biceps,  probably  the  brachialis  anticus,  and  lastly  the  muscles 
covering  the  internal  condyle.  The  result  was  a  movement  by  which  the 
whole  arm  was  made  to  approach  the  trunk;  with  rotation  inward  and  half- 
flexion  of  the  forearm  upon  the  arm.;  a  true  defensive  movement,  which 
.  brought  the  hand  toward  the  chest  as  far  as  the  pit  of  the  stomach.  Neither 
the  thumb,  which  was  partially  bent  toward  the  palm  of  the  hand,  nor  the 
fingers,  which  were  half  bent  over  the  thumb,  presented  any  movements. 
The  arm  being  replaced  in  its  former  position,  we  saw  it  again  execute  a 
similar  movement  on  scratching  the  skin,  in  the  same  manner  as  before,  a 
little  below  the  clavicle.  This  experiment  succeeded  four  times,  but  each 
time  the  movement  was  less  extensive;  and  at  last  scratching  the  skin  over 
the  chest  produced  only  contractions  in  the  great  pectoral  muscle  which 
hardly  stirred  the  limb"  (Dalton). 
Laws  of  Reflex  Action  (Pfliiger). 

1.  Law  of  Unilaterality. — If  a  feeble  irritation  be  applied  to  one  or  more 

sensory  nerves,  movement  takes  place  usually  on  one  side  only,  and 
that  the  same  side  as"  the  irritation. 

2.  Law  of  Symmetry. — If  the  irritation  becomes  sufficiently  intense,  motor 

reaction  is  manifested,  in  addition,  in  corresponding  muscles  of  the 
opposite  side  of  the  body. 

3.  Law  of  Intensity. — Reflex  movements  are  usually  more  intense  on  the  side 

of  irritation;  at  times  the  movements  of  the  opposite  side  equal  them  in 
intensity;  but  they  are  usually  less  pronounced. 

4.  Law  of  Radiation. — If  the  excitation  still  continues  to  increase,  it  is  pro- 

pagated upward,  and  motor  reaction  takes  place  through  centrifugal 
nerves  coming  from  segments  of  the  cord  higher  up. 

5.  Law  of  Generalization. — When  the  irritation  becomes  very  intense,  it  is 

propagated  to  the  medulla  oblongata;  motor  reaction  then  becomes 
general,  and  it  is  propagated  up  and  down  the  cord,  so  that  all  the  mus- 
cles of  the  body  are  thrown  into  action,  the  medulla  oblongata  acting 
as  a  focus  whence  radiate  all  reflex  impulses. 
Reflex  Irritability. — The  general  irritability  or  quickness  of  response  of 
the  mechanism  involved  in  reflex  action  can  be  approximately  determined 
by  observation  of  the  length  of  time  that  elapses  between  the  application  of 
a  minimal  stimulus  and  the  appearance  of  the  muscle  response.     The  method 
of  Tiirck  is  sufficiently  accurate  for  general  purposes.     This  consists  in 
suspending  a  frog,  after  removal  of  the  brain,  and  immersing  the  foot  in 
a  0.2  per  cent,  solution  of  sulphuric  acid.     The  time  is  determined  by  means 
of  a  metronome  beating  one  hundred  times  a  minute.     Stimulation  of  the 
skin  can  also  be  effected  by  the  induced  electric  current,  as  suggested  by 
Gaskell.     A    single    shock   is,    however,    ineffective.     The    currents  must 


540 


TEXT-BOOK  OF  PHYSIOLOGY 


follow  each  other  with  a  rapidity  sufficient  to  give  rise  to  a  summation  of 
effects  in  the  nerve-centers  which  will  then  be  followed  by  a  muscle  response. 
It  is  highly  probably  that  the  chemic  stimulation  gives  rise  to  a  similar  sum- 
mation of  effects. 

The  period  of  time  thus  obtained  is  distributed  over  the  entire  mechan- 
ism. The  true  reflex  time,  however — i.e.,  the  time  occupied  in  the  passage 
of  the  nerve  impulses  through  the  spinal  mechanism — is  shorter  and  is  obtained 
by  subtracting  from  the  whole  period  the  time  occupied  by  the  passage  of  the 
impulses  through  the  afferent  and  efferent  nerves  as  well  as  the  latent  period 

of  muscle  contraction.  This  corrected 
period,  the  true  reflex  time,  has  been 
found  to  be  twelve  times  longer  than  the 
time  occupied  by  the  passage  of  the  nerve 
impulse  through  the  nerves,  including  the 
latent  period  of  the  muscle. 

The  total  reflex  time,  that  is,  the  time 
elapsing  between  the  application  of  the 
stimulus  and  the  response  of  an  organ 
may  be  shortened  by  influences  which 
heighten  the  irritability  of  any  one  or 
more  portions  of  the  reflex  arc:  e.g. 


Kv...- 


7/ied.  ob. 


2. 


Separation  of  the  Brain  from  the  Cord. 
— This  is  at  once  followed  by  an  in- 
crease in  reflex  irritability,  and  is 
taken  as  evidence  that  the  brain  is 
normally  exerting  an  inhibitor  in- 
fluence over  the  reflex  centers  of  the 
cord.  The  same  increase  is  ob- 
served upon  hemisection  of  the  cord, 
though  the  increase  is  limited  to  the 
same  side. 
The  Toxic  Action  of  Drugs. — Many 
drugs  increase  the  irritability  of  the 
spinal  cord,  though  the  most  efficient 
is  strychnin.  This  drug,  even  in 
small  doses,  increases  the  irritability 
to  such  an  extent  that  a  minimal 
stimulus  is  sufficient  to  call  forth 
spasmodic  contractions  of  all  the  skeletal  muscles.  Under  its  influence 
the  usual  coordinated  reflexes  disappear  and  are  succeeded  by  in- 
coordinated  reflexes.  The  explanation  of  this  fact  is  believed  to  be  a 
diminution  in  the  resistance  offered  by  the  cord  to  the  passage  of  the 
afferent  impulses  rather  than  to  a  direct  stimulation  of  the  efferent  cells. 
So  much  is  this  resistance  decreased  that  the  nerve  impulses  instead 
of  being  confined  to  their  accustomed  paths,  are  radiated  in  all  direc- 
tions. Absolute  repose  of  the  animal  and  the  exclusion  of  all  external 
stimuli  greatly  diminish  the  tendency  to  the  occurrence  of  spasms. 
Degeneration  of  the  Pyramidal  Tracts. — In  primary  lateral  sclerosis,  a 
pathologic  condition  characterized  primarily  by  a  degeneration  of  the 
terminal  filaments  of  the  pyramidal  tract  fibers,  the  reflex  activity  of 


Fig.  232. — Diagram  of  the  Brain  of 
THE  Frog.  olj.  n.  olfactory  nerves;  ol J.  I 
olfactory  lobes;  c.  h.  cerebral  hemispheres; 
op.  thl.  optic  thalamus;  op.  I.  optic  lobes; 
c.  cerebellum;  med.  ob.  medulla  oblon- 
gata; IV.  V.  fourth  ventricle. 


THE  SPINAL  CORD  541 

the  cord  becomes  exalted.  As  the  disease  progresses  the  irritability 
increases  to  such  an  extent  that  violent  spasmodic  contractions  of  the 
arms  and  legs  arise  when  the  skin  or  tendons  are  mechanically  stimu- 
lated. The  explanation  offered  is  practically  the  same  as  in  division 
of  the  cord:  viz.,  withdrawal  of  the  inhibitor  and  controlling  influence 
of  the  brain. 
The  total  reflex  time  may  be  lengthened  by  influences  which  lower  the 
irritability  of  any  one  or  more  portions  of  the  reflex  arches. 

1.  Stimulation  of  Certain  Regions  of  the  Brain. — It  was  discovered  by  Setche- 

now  that  when  the  frog  brain  is  divided  just  anterior  to  the  optic  lobes 
(Fig.  232)  and  the  reflex  time  subsequently  determined  according  to  the 
method  of  Tiirck,  the  time  can  be  considerably  lengthened  by  stimula- 
tion of  the  optic  lobes.  This  is  readily  accomplished  by  placing  small 
crystals  of  sodium  chlorid  on  the  optic  lobes.  It  was  concluded  from 
this  fact  that  these  lobes  contain  centers  which  exert  an  inhibitor  in- 
fluence over  centers  in  the  spinal  cord  through  descending  nerve-fibers. 
This  conclusion  is  strengthened  by  the  fact  that  division  of  the  brain 
just  behind  the  optic  lobes  causes  a  temporary  inhibition  of  the 
reflexes  in  consequence  of  a  mechanical  irritation  of  these  fibers.  It  is 
quite  probable  that  the  volitional  inhibition  of  certain  reflexes  is  accom- 
plished through  the  intermediation  of  this  center  localized  by  Setchenow. 

2.  Stimulation  of  Sensor  Nerves. — If   during   the  application  of  a  stimulus 

sufficient  to  call  forth  a  characteristic  reaction  in  a  definite  period  of  time, 
a  sensor  nerve  in  a  distant  region  of  the  body  be  simultaneously  stimu- 
lated, it  will  be  found  that  the  reflex  time  will  be  lengthened  or  the  reac- 
tion completely  inhibited. 

3.  Lesions  of  the  Spinal  Cord;  e.g.,  atrophy  of  the  multipolar  cells  of  the 

anterior  horns  of  the  gray  matter;  degeneration  of  the  terminals  of  the 
dorsal  root- fibers. 

4.  The  Toxic  Action  of  Drugs— e.g.,  chloroform,  chloral — which  are  believed 

to  exert  a  depressing  action  on  the  nerve-cells  themselves. 
Special  Reflex  Actions. — Among  the  reflexes  connected  with  the 
more  superficial  portions  of  the  body  there  are  some  which  are  so  frequently 
either  increased  or  diminished  in  pathologic  conditions  of  the  spinal  cord 
that  their  study  affords  valuable  indications  as  to  the  seat  and  character  of 
the  lesions.     They  may  be  divided  into: 

1.  The  skin  or  superficial  reflexes. 

2.  The  tendon  or  deep  reflexes. 

3.  The  organ  reflexes. 

The  skin  reflexes,  characterized  by  contraction  of  underlying  muscles, 
are  induced  by  stimulation  of  the  afferent  nerve-endings  of  the  skin — e.g., 
by  pricking,  pinching,  scratching,  etc.  The  following  are  the  principal  skin 
reflexes : 

1.  Plantar  reflex,  consistmg  of  contraction  of  the  muscles  of  the  foot,  induced 

by  stimulation  of  the  sole  of  the  foot;  it  takes  place  through  the  seg- 
ments of  the  cord  which  give  rise  to  the  second  and  third  sacral  nerves. 

2.  Gluteal  reflex,  consisting  of  contraction  of  the  glutei  muscles  when  the  skin 

over  the  buttock  is  stimulated;  it  takes  place  through  the  segments 
giving  origin  to  the  fourth  and  fifth  lumbar  nerves. 

3.  Cremasteric  reflex,  consisting  of  a  contraction  of  the  cremaster  muscle 


542  TEXT-BOOK  OF  PHYSIOLOGY 

and  a  retraction  of  the  testicle  toward  the  abdominal  ring  when  the  skin 
on  the  inner  side  of  the  thigh  is  stimulated;  it  takes  place  through  the 
segments  which  give  origin  to  the  first  and  second  lumbar  nerves. . 

4.  Abdominal  reflex,  consisting  of  a  contraction  of  the  abdominal  muscles 

when  the  skin  upon  the  side  of  the  abdomen  is  gently  scratched;  it  takes 
place  through  the  spinal  segments  which  give  origin  to  the  nerves  from 
the  eighth  to  the  twelfth  thoracic. 

5.  Epigastric  reflex,  consisting  of  a  slight  muscular  contraction  in  the  neigh- 

borhood of  the  epigastrium  when  the  skin  between  the  fourth  and 
sixth  ribs  is  stimulated;  it  takes  place  through  the  segments  of  the  cord 
which  give  origin  to  the  nerves  from  the  fourth  to  the  seventh  thoracic 
inclusive. 

6.  Scapular  reflex  consisting  of  a  contraction  of  the  scapular  muscles  when 

the  skin  between  the  scapulas  is  stimulated;  it  takes  place  through  the 
segments  of  the  cord  which  give  rise  to  the  nerves  from  the  fifth  cervical 
to  the  third  thoracic  inclusive. 

The  skin  or  superficial  reflexes,  though  variable,  are  generally  present  in 
health.  They  are  increased  or  exaggerated  when  the  gray  matter  of  the 
cord  is  abnormally  excited,  as  in  tetanus,  strychnin-poisoning,  and  disease 
of  the  lateral  columns. 

The  so-called  "lendon  reflexes,^'  are  characterized  by  a  movement  of  cer- 
tain parts  of  the  body  due  to  the  contraction  of  certain  muscles  and  are 
elicited  by  a  sharp  tap  on  their  tendons.  The  fundamental  condition  for 
the  production  of  the  tendon  reflex  is  a  certain  degree  of  tonus  of  the  muscle, 
which  is  a  true  reflex,  maintained  by  afferent  nerve  impulses  developed  in 
the  muscle  itself  in  consequence  of  its  extension  and  hence  compression  of 
the  end-organs,  the  muscle  spindles,  of  the  afferent  nerves.  When  the  mus- 
cle is  passively  extended,  as  it  must  be  when  the  reflex  is  to  be  elicited,  there 
is  an  exaltation  of  the  tonus  and  an  increase  in  the  irritability.  To  this 
condition  of  the  muscle  due  to  passive  tension,  the  term  myotatic  irritability 
has  been  given.  If  the  muscle  extension  be  now  suddenly  increased,  as  it 
is  when  the  tendon  is  sharply  tapped,  the  increased  compression  of  the  muscle 
spindles  will  develop  additional  afferent  impulses  which  after  transmission 
to  the  spinal  cord  will  give  rise  to  contraction  of  the  corresponding  muscle. 
The  tendon  reflexes  are  of  much  value  in  the  diagnosis  of  certain  lesions  of 
the  spinal  cord. 

The  following  are  the  principal  forms  of  the  tendon  reflexes : 

1.  The  Patellar  tendon  reflex  or  knee-jerk.     This  phenomenon  is  characterized 

by  a  quick  extension  of  the  leg  from  the  knee  downward,  due  to  the 
contraction  of  the  extensor  muscles  of  the  thigh,  when  the  ligamentum 
patellae  is  struck  between  the  patella  and  tibia.  This  reflex  is  best  ob- 
served when  the  legs  are  freely  hanging  over  the  edge  of  a  table.  The 
patella  reflex  is  generally  present  in  health,  being  absent  in  only  2  per 
cent.;  it  is  greatly  exaggerated  in  lateral  sclerosis,  in  descending  degen- 
eration of  the  cord;  it  is  absent  in  locomotor  ataxia  and  in  atrophic 
lesions  of  the  anterior  gray  cornua. 

2.  The  tendo-Achillis  reflex  or  ankle-jerk.     This  phenomenon  is  characterized 

by  a  flexion  of  the  foot  due  to  a  contraction  of  the  gastrocnemius  muscle 
when  the  tendo-Achillis  is  struck.  To  elicit  the  contraction,  the  leg 
should  be  extended  and  the  dorsum  of  the  foot  be  pressed  toward  the 


THE  SPIN.\L  CORD  543 

leg  so  as  to  give  to  the  gastrocnemius  a  slight  degree  of  extension.  If 
the  tendon  be  now  sharply  struck  a  C|uick  flexion  of  the  foot  is  produced. 

3.  Ankle  clonus. — ^This  phenomenon  consists  of  a  series  of  rhythmic  con- 

tractions of  the  gastrocnemius  muscle,  varying  in  frequency  from  six  to 
ten  per  second.  To  elicit  this  reflex,  pressure  is  made  upon  the  sole  of 
the  foot  so  as  to  extend  the  foot  at  the  ankle  suddenly  and  energetically, 
thus  putting  the  tendo-Achillis  and  the  gastrocnemius  muscle  on  the 
stretch.  The  rhythmic  movements  thus  produced  continue  so  long  as 
the  tension  within  limits  is  maintained.  Ankle  clonus  is  never  present 
in  health,  but  is  very  marked  in  lateral  sclerosis  of  the  cord. 

4.  The  Toe  Reflex. — ^This  phenomenon  is  characterized  by  a  flexion  of  the 

foot,  then  of  the  leg  and  perhaps  of  the  thigh  when  the  great  toe  is 
strongly  and  suddenly  flexed.  It  is  present  in  those  diseases  of  the 
spinal  cord  in  which  there  is-  a  pronounced  patellar  reflex. 

5.  The  Wrist  and  Elboiv  Reflex. — These  phenomena  are  characterized  by  an 

extension  movement  of  the  hand  and  arm  when  the  tendons  of  the  ex- 
tensor  muscles   are    sharply   tapped.     These    reflexes   are    especially 
marked  in  primary  lateral  sclerosis  of  the  cord  in  the  upper  portion. 
The  organ  reflexes,  e.g.,  the  activities  of  the  genito-urinary  organs,  the 
stomach,  intestines,   gall-bladder,  etc.,  which  are   induced  by   peripheral 
stimulation  have  been  considered  in  connection  with  the  physiologic  action 
of  these  organs.     The  genito-urinary  center  is  located  in  the  lumbar  region 
of  the  spinal  cord.     In  diseased  conditions  of  this  region  the  genito-urinary 
reflexes  are  sometimes  increased,  at  other  times  decreased  or  even  abolished. 
Direct  or  Cerebral  Excitation.— The  activity  of  the  emissive  cells  of 
the  spinal  cord  segments,  due  to  the  arrival  of  nerve  impulses  descending 
the  spinal  cord  from  the  cerebrum,  in  consequence  of  psychic  states  of  a 
volitional  or  of  an  affective  or  emotional  character,  will  be  considered  in  a 
subsequent  paragraph  entitled  "  encephalo-spinal  conduction." 

B.     THE  SPINAL  CORD  SEGMENTS  AS  CONDUCTORS. 

The  white  matter  of  the  spinal  cord  consists  of  nerve-fibers  the  special 
functions  of  which  are 

1.  To  conduct  nerve  impulses  from  one  segment  of  the  cord  to  another. 

2.  To  conduct  nen'^e  impulses  coming  to  the  cord  through  afferent  nerves, 

directly  or  indirectly  to  various  areas  of  the  encephalon. 

3.  To  conduct  nerve  impulses  from  the  encephalon  to  the  spinal  cord 

segments. 

Intersegmental  or  Associative  Conduction. — The  spinal  cord  con- 
sists of  a  series  of  physiologic  segments  each  of  which  has  specific  functions 
and  is  associated  through  its  related  spinal  nerve  with  a  definite  segment 
of  the  body.  For  the  harmonious  cooperation  and  coordination  of  all  the 
spinal  segments  it  is  essential  that  they  should  be  united  by  commissural 
or  associative  fibers.  This  is,  in  fact,  accomplished  by  the  axons  of  the  intrin- 
sic cells  of  the  gray  matter,  which  constitute  such  a  large  part  of  the  antero- 
lateral and  posterior  root  zones.  In  consequence  of  this  association, 
the  cord  becomes  capable  of  complex  coordinated  and  purposive  reflex 
actions. 

Some  of  the  nerve- fibers  arising  in  the  intrinsic  cells  become  associated 
with  the  cerebellum  through  the  direct  cerebellar  tract  and  the  antero-lateral 


544  '  TEXT-BOOK  OF  PHYSIOLOGY 

cerebellar  tract.  By  this  means  the  cord  and  cerebellum  are  associated  in 
functional  activity. 

Spino-encephalic  or  Sensor  Conduction. — The  spino-encephalic  or 
sensor  pathway  extends  from  different  levels  of  the  spinal  cord  to  the  cortex 
of  the  cerebrum,  and  consists  of  two  or  more  consecutively  arranged  groups 
of  neurons.  The  first  group  of  these  neurons  extends  from  the  spinal  cord 
to  the  thalamus  and  is  known  as  the  spino- thalamic  system;  the  second  group 
of  these  neurons  extends  from  the  thalamus  to  the  cortex  of  the  cerebrum  and 
is  known  as  the  thalamo-corlical  system. 

The  spino-thalamic  system  is  connected  with  the  cutaneous  and  mucous 
surfaces,  and  with  the  tendons,  joints  and  muscles  by  means  of  the  dorsal 
root  fibers  of  the  spinal  nerves.  Histologic  and  clinical  investigations  have 
shown  that  the  central  terminations  of  the  nerve-fibers  coming  from  the  skin, 
become  associated  with  afferent  cells  on  the  same  side  of  the  gray  matter, 
at  the  same  or  at  somewhat  different  levels;  that  the  axons  of  these  cells 
cross  the  median  plane  and  then  pass  upward  through  the  cord,  medulla, 
pons  and  crura  cerebri  to  the  thalamus.  Similar  methods  of  investigation 
have  shown  that  the  nerve-fibers  coming  from  the  tendons,  joints,  and  mus- 
cles ascend  the  white  matter  of  the  spinal  cord  of  the  same  side,  as  far  as  the 
upper  limits  of  the  posterior  column,  where  their  terminal  branches  become 
associated  with  the  cells  of  the  cuneate  and  gracile  nuclei.  From  the  cells 
of  these  nuclei,  axons  arise  which  cross  the  middle  plane  of  the  medulla  to 
join  the  fibers  ascending  the  spinal  cord.  The  former  fibers  are  known  as 
the  internal  arcuate  fibers  and  assist  in  the  formation  of  the  lemniscus  or 
fillet  (Fig.  233).  The  afferent  or  sensor  pathway  thus  decussates  in  part, 
at  different  levels  of  the  spinal  cord,  and  in  part  at  the  level  of  the  gracile 
and  cuneate  nuclei.  The  former  is  often  termed  the  lower,  the  latter  the 
upper  sensor  decussation  or  crossway. 

The  afferent  pathway  on  passing  toward  the  thalamus  receives  additional 
fibers  at  the  level  of  the  medulla  and  pons  from  the  cells  of  the  opposite 
side  with  which  the  terminations  of  the  afferent  cranial  nerves,  the  trigem- 
inal, glossopharyngeal,  and  vagus,  are  associated. 

The  nerve  impulses  that  arise  in  consequence  of  impressions  made  on 
the  terminals  of  the  nerves  in  the  cutaneous  and  mucous  surfaces,  in  the 
viscera  and  in  the  muscles,  are  transmitted  through  the  dorsal  roots  of  the 
spinal  nerves  to  the  cord.  On  reaching  the  cord  the  cutaneous  nerve  im- 
pulses are  received  by  afferent  or  receptive  nerve-cells  in  the  gray  matter, 
and  transmitted  by  their  axons  across  the  median  plane  of  the  spinal  cord 
and  then  upward  to  and  through  the  medulla,  the  posterior  part  of  the  pons, 
the  posterior  part  of  the  crura  cerebri,  and  for  the  most  part  to  the  ventral 
portion  of  the  thalamus  opticus. 

On  reaching  the  thalamus  the  nerve  impulses  are  received  by  nerve-cells, 
and  then  transmitted  by  their  axons  which  pass  by  way  of  the  internal  cap- 
sule to  the  cells  of  the  cortex  of  the  cerebrum.  It  is  probable  however  that 
some  fibers  from  the  cord  and  medulla  pass  direct  to  the  cortex.  When 
thus  transmitted  through  the  cord  to  the  cerebral  hemispheres  directly  or 
indirectly,  they  are  received  by  specialized  nerve-cells  in  the  cortex  and  trans- 
lated into  conscious  sensations.  The  nerve  impulses  developed  in  tendons 
and  muscles  are  received  by  localized  groups  of  nerve-cells,  the  nucleus 
gracilis  and  nucleus  cuneatus,  situated  at  the  top  of  the  posterior  column, 


THE  SPINAL  CORD  545 

which  are  reached  by  the  upward  coursing  of  some  of  the  dorsal  root  fibers 
through  the  columns  of  Goll  and  Burdach.  They  are  then  transmitted  by 
the  internal  arcuate  fibers  and  the  fillet  either  directly  or  indirectly  to  the 
cortex  of  the  cerebrum  and  translated  into  conscious  sensations.  The  sensa- 
tions thus  arising  may  be  divided  into  special  and  general  sensations.  Of 
the  former  may  be  mentioned  the  sensations  of  temperature,  pain,  touch, 
pressure,  passive  position  and  movements  of  parts  due  to  the  activity  of 
skeletal  muscles;  of  the  latter  may  be  mentioned  hunger,  thirst,  fatigue, 
well-being,  etc. 

Though  all  the  impulses  that  give  rise  to  these,  varied  sensations  are 
contained  within  the  fibers  of  the  afferent  peripheral  nerves,  they  are  on 
reaching  the  cord  distributed  by  the  intraspinal  mechanisms  to  different 
tracts  of  nerve-fibers,  each  of  which  transmits  to  localized  areas  of  the  cerebral 
cortex,  the  someslhetic  areas,  a  special  group  of  impulses  which  give  rise  to 
sensations  of  various  kinds  such  as  those  just  mentioned. 

The  current  views  regarding  the  physiologic  activities  of  the  afferent 
portion  of  the  peripheral  nerve  system  and  its  relation  to  the  production  of 
different  forms  of  sensibility  have  been  enlarged  by  the  results  of  the  investi- 
gations that  have  been  made  by  Head.  Thus,  he  has  shown  that  the  afferent 
nerves  consist  of  three  systems,  each  of  which  when  excited  to  activity  evokes 
in  consciousness  a  different  and  distinct  group  of  sensations  as  follows : 

1.  One  system  of  nerves  which  when  stimulated  evoke  sensations  through 

which  is  gained  the  power  of  cutaneous  localization,  of  the  discrimina- 
tion of  two  points  of  a  compass,  of  the  finer  grades  of  temperature,  and 
of  light  touch.  To  this  form  of  cutaneous  sensibility  the  term  epicritic 
has  been  applied. 

2.  A  second  system  of  nerves  which  when  vigorously  stimulated,  as  by  the 

prick  of  a  pin  or  by  the  application  of  a  hot  or  cold  object,  evoke  sensa- 
tions of  pain  or  heat  and  cold.  To  this  form  of  cutaneous  sensibility 
the  term  protopathic  has  been  applied.  This  form  of  sensibility  is 
unaccompanied  by  a  definite  appreciation  of  the  locality  stimulated  for 
the  reason  that  the  stimulus  causes  a  widespread  or  radiating  sensation 
which  at  times  is  referred  to  parts  far  removed  from  the  part  stimulated. 

3.  A  third  system  of  nerves  which  when  stimulated  evoke  sensations  of  pres- 

sure, of  the  passive  position  and  the  movements  of  parts  of  the  body,  and 
sensations  of  pain  as  welL  if  the  stimulus  (pressure)  be  severe,  or  if  the 
underlying  structures  are  injured,  e.g.,  the  rupture  of  a  joint.     The 
nerves  subserving  this  form  of  sensibility  are  contained  in  the  trunks 
of  the  motor  (muscle)  nerves  and  are  distributed  to  muscles,  tendons 
and  joints.     To  this  form  of  sensibility  the  term,  deep  has  been  applied. 
The  pathways  through  the  spinal  cord  that  conduct  these  afferent  im- 
pulses to  the  brain  are  ill-defined  and  imperfectly  known,  and  only  for  a  few 
sensations  can  it  be  said  that  their  pathways  have  been  determined.     The 
reason  for  this  obscurity  lies  partly  in  the  difficulties  of  experimentation, 
partly  in  the  difficulties  of  interpretation.     Clinical  observations  are  for 
special  reasons  more  or  less  untrustworthy. 

As  the  outcome  of  many  investigations  it  may  be  said  that  a  transverse 
section  of  one  lateral  half  of  the  cord  in  the  monkey,  or  a  lesion  involv- 
ing the  one  lateral  half  in  man,  as  a  rule  abolishes  many  if  not  all  forms  of 
cutaneous  sensibility  on  the  opposite  side  below  the  injury.     This  would  seem 

35 


546 


TEXT-BOOK  OF  PHYSIOLOGY 


to  prove  that  the  nerve  impulses  cross  the  median  line  of  the  cord  immedi- 
ately or  very  shortly  after  entering  and  then  ascend  the  corresponding  half 
of  the  cord  on  their  way  to  the  thalamus.     At  the  same  time,  muscle  sen- 


FiG.  233. — Diagram  of  the  Sensor  Pathways  in  the  Spinal  Cord  enlarged  above  by 
Fibers  of  the  Sensor  Cranial  Nerves  and  Nerves  of  Special  Sense. 

sibility  is  abolished  on  the  same  belov^^  the  injury.  This  would  seem  to 
prove  that  the  fibers  of  the  posterior  roots  that  enter  and  cross  the 
column  of  Burdach  and  ascend  in  the  column  of  Goll  to  terminate  around  the 
cells  of  the  gracile  and  cuneate  nuclei  are  derived  mainly  from  the  muscles. 


THE  SPINAL  CORD  547 

It  is,  however,  believed  by  some  investigators  that  those  fibers  which  sub- 
serve the  sense  of  touch  do  not  decussate  at  once,  but  ascend  in  the  column 
of  Goll  as  far  as  the  medulla  oblongata,  where  they,  in  common  with  the 
fibers  coming  from  the  muscles,  arborize  aroimd  the  nerve-cells  in  the  gracile 
and  cuneate  nuclei. 

The  pathways  for  the  impulses  that  give  rise  to  the  different  sensation 
have  been  variously  located  by  different  observers,  e.g.,  in  the  gray  matter, 
in  the  limiting  layer,  and  in  the  antero-lateral  tract  of  Gowers;  the  pathway 
for  the  impulses  that  give  rise  to  temperature  sensations  has  been  located  in 
the  gray  matter;  the  pathway  for  tactile  impressions  has  been  located  in  the 
posterior  columns,  though  this  is  not  beyond  dispute.  The  pathway  for 
pain  sensations  has  been  located  in  Gowers'  tract. 

The  results  on  both  sides  of  the  body,  which  follow  a  transverse  lesion, 
experimental  or  traumatic,  of  one-half  of  the  spinal  cord  in  the  thoracic 
region  for  example,  are  shown  in  the  subjoined  table,  a  grouping  of  results 
which  is  known  as  the  Brown-Sequard  "  Symptom  Complex."  It  is  under- 
stood, of  course,  that  these  results  are  observed  below  the  level  of  the  lesion. 

THE  BROWN-SEQUARD  SYMPTOM  COMPLEX 

Side  Opposite  of  Lesion.  Side  of  Lesion. 

1.  Temperature  sensibilities  abolished,     i.  Temperature     sensibilities     re- 

tained. 

2.  Painful  sensibilities  abolished.  2.  Painful  sensibility  retained. 

3.  Pressure  (painful)  sensibilities  abol-  3.  Pressure    (painful)    sensibilities 
ighed.  retained. 

4.  Passive  position  of  limb  and  direc-  4.  Passive  position  of  limb  and  di- 
tion  of  movement,  retained.  rection  of  movement,  abolished. 

5.  Light  pressure  or  light  touch,  may  or  5.  Light  pressure  or  light  touch, 
may  not  be  abolished,  retained. 

6.  Tactile  discrimination,  retained.  6.  Tactile     discrimination,     abol- 

ished. 

7.  Cutaneous  localization,  abolished.        7.  Cutaneous  localization,  retained. 

8.  Voluntary  motion,  retained.  8.  Voluntary  motion,  abolished. 

From  a  study  of  this  table  it  is  apparent:  (i)  that  some  forms  of  sensor 
impulses  (those  of  pain  and  temperature  sensibility)  cross  soon  after  their 
entrance  and  pass  up  the  opposite  side  of  the  cord;  (2)  that  other  forms  of 
sensor  impulses  (those  of  the  sense  of  passive  position  of  a  limb  and  of  the 
direction  of  movement  and  tactile  discrimination  (Head))  do  not  cross,  but 
pass  up  on  the  same  side  as  the  entering  dorsal  nerve  roots;  (3)  that  tactile 
sensibility  may  or  may  not  be  abolished  on  the  side  opposite  the  lesion;  and 
(4)  that  the  sense  of  cutaneous  localization  may  be  dissociated  from  the  sense 
of  passive  position,  and  remain  intact  when  the  latter  is  absent  (Head). 

Encephalo-spinal  or  Motor  Conduction. — The  encephalo-spinal  or 
motor  pathway  extends  from  the  cerebral  cortex  to  different  levels  of  the 
gray  matter  of  the  aqueduct  of  Sylvius,  of  the  pons,  medulla  and  spinal  cord. 
It  is  connected  at  these  different  levels  with  the  efferent  nerve-cells  which 
give  origin  to  the  nerve-fibers  which  collectively  constitute  the  cranial  or 
encephalic  motor  nerves  and  the  ventral  spinal  nerves  all  of  which  are  dis- 
tributed to  skeletal  muscles. 


548  TEXT-BOOK  OF  PHYSIOLOGY 

The  reason  for  the  development  of  this  pathway  is  apparent  from  the 
following  considerations: 

At  birth  the  child  is  capable  of  performing  all  the  functions  of  organic 
life,  such  as  sucking,  swallowing,  breathing,  etc.  It  is,  however,  deficient 
in  psychic  activity  and  in  volitional  control  of  its  muscles.  Its  movements 
are  therefore  largely,  if  not  entirely,  reflex  in  character. 

Embryologic  and  histologic  examination  of  the  spinal  cord  and  medulla 
show  that  so  far  as  their  mechanisms  for  independent  physiologic  activities 
are  concerned  both  are  fully  developed.  Similar  investigations  of  the  cere- 
bral hemispheres  and  of  the  nerve-fibers  which  bring  their  nerve-cells  into 
relation  with  the  spinal  segments  show  that  the  cells  of  the  cortex  are  not  only 
immature,  but  that  their  descending  axons  are  incompletely  invested  with 
myelin.  With  the  growth  of  the  child,  psychic  life  unfolds  and  volitional 
control  of  muscles  is  acquired.  Coincidently  the  cells  of  the  cerebral  cortex 
grow  and  develop  and  the  fibers  become  covered  with  myelin. 

The  nerve-fibers,  which  have  their  origin  in  the  cells  of  the  cerebral  cortex, 
and  which  terminate  in  tufts  around  the  cells  in  the  anterior  horns  of  the 
gray  matter  of  the  spinal  segments,  are  to  be  regarded  as  long  commissural 
tracts  uniting  and  associating  these  two  portions  of  the  central  nerve  system. 

Experimental  investigations  and  observations  of  pathologic  lesions 
accord  with  the  view  that  physiologically  these  fibers  are  efferent  pathways 
for  the  transmission  of  motor  or  volitional  impulses  from  the  cortex  to  the 
spinal  segments.  The  nerve-cells  in  which  the  motor  impulses  originate 
are  located  for  the  most  part,  as  will  be  fully  stated  later,  in  the  central 
portion  of  the  cortex  of  the  cerebral  hemispheres  in  the  neighborhood  of  the 
central  or  Rolandic  fissure.  The  axons  of  these  cells  from  each  hemisphere 
descend  through  the  corona  radiata  to  and  through  the  internal  capsule, 
along  the  inferior  surface  of  the  crura  cerebri,  behind  the  pons  to  the  medulla, 
of  which  they  constitute  the  anterior  pyramids  (Fig.  234).  At  this  point 
the  pyramidal  tract^  of  each  side  divides  into  two  portions,  viz. : 

1.  A  large  portion,  containing  from  80  to  90  per  cent,  of  the  fibers,  which 

decussates  at  the  lower  border  of  the  medulla  and  passes  downward  in 
the  posterior  part  of  the  lateral  column  of  the  opposite  side,  constituting 
the  crossed  pyramidal  tract;  as  it  descends  it  gradually  diminishes  in  size 
as  its  fibers  or  their  collaterals  enter  the  gray  matter  of  each  successive 
segment. 

2,  A  small  portion,  containing  from  20  to  10  per  cent,  of  the  fibers,  which 

does  not  decussate  at  the  medulla  but  passes  downward  on  the  inner 
side  of  the  anterior  column  of  the  same  side,  constituting  the  direct 
pyramidal  tract  or  column  Turck.  This  tract  can  be  traced  down,  as 
a  rule,  only  as  far  as  the  mid-dorsal  region.  As  it  descends  it  becomes 
smaller  as  its  fibers  cross  the  anterior  commissure  to  enter  the  gray 
matter  of  the  opposite  side.  Thus  all  the  fibers  of  the  pyramidal  tract 
from  each  cerebral  hemisphere  eventually  are  brought  into  relation  with 
the  cells  of  the  gray  matter  of  the  opposite  side  of  the  cord. 

*  From  the  fact  that  the  region  included  between  the  origin  of  these  fibers  and  the  internal 
capsule  presents  somewhat  the  form  of  a  pyramid  with  four  sides,  Charcot  designated  it  the 
pyramidal  region  and  the  fibers  composing  it  the  pyramidal  tract.  The  base  of  the  pyramid  in- 
cludes the  convolutions  of  the  cortex  in  front  of  the  Rolandic  fissure.  The  summit  of  the 
pyramid  is  truncated  and  covers  the  pyramidal  region  of  the  internal  capsule. 


THE  SPIK\L  CORD 


549 


That  the  pyramidal  tracts  are  the  conductors  of  volitional  impulses 
throughout  the  length  of  the  cord  to  its  various  segments  has  been  made 


Fig.  234. — Diagram  OF  THE  Pyramidal  Tract  OR  Motor  Path.  III.  Common  oculo-motor 
nerve.  IV.  Pathetic  nerve.  V.  Motor  division  of  the  trigeminal  nerve.  VI.  The  abducens 
nerve.  VII.  Facial  nerve.  IX.  and  X.  Motor  divisions  of  the  glosso-pharyngeal  and  pneumogas- 
tric  nerves.     XI.  Spinal  accessory  nerve.     XII.     Hypoglossal  nerve. — (Van  Gehuchten.) 

evident  by  the  results  of  section,  electric  stimulation,  and  disease.  Division 
of  the  anterior  and  lateral  columns  of  one  side  of  the  cord  in  any  part  of  its 
extent  is  invariably  followed  by  a  loss  of  motion  or  paralysis  of  the  muscles 


550  TEXT-BOOK  OF  PHYSIOLOGY 

below  the  section,  while  electric  stimulation  of  the  peripheral  end  of  the 
isolated  crossed  pyramidal  tract  is  followed  by  marked  characteristic  move- 
ments of  the  muscles.  Similar  results  follow  division  of  the  pyramidal  tract 
in  any  part  of  its  course  from  the  cerebral  cortex  downward.  Electric 
stimulation  of  the  cortical  cells  which  give  origin  to  the  pyramidal  tract  is 
also  followed  by  contraction  of  the  muscles  of  the  opposite  side,  while  their 
destruction  is  attended  by  paralysis  of  the  same  muscles.  As  the  nutrition  of 
the  fibers  is  governed  by  the  cells,  it  follows  that  when  the  axon  is  separated 
from  its  cell-body  it  degenerates.  It  has  been  found  that  a  lesion  of  the 
pyramidal  tract  in  any  part  of  its  course  is  followed  by  descending  degenera- 
tion, which  is  taken  in  evidence  that  it  conducts  nerve  impulses  from  above 
downward.  Thus  experimental  investigation  and  pathologic  observation  are 
in  accord  in  the  view  that  physiologically  these  nerve-fibers  are  the  pathways 
for  the  transmission  of  motor  or  volitional  impulses  from  the  encephalon 
to  the  spinal  cord. 

The  pyramidal  tracts  are  also  the  conductors  of  the  nerve  impulses  dis- 
charged by  the  cells  of  the  cerebrum  during  the  occurrence  of  the  affective 
or  emotional  states,  that  excite  to  activity  the  lower  nerve  centers;  those  that 
give  origin  to  the  autonomic  nerve  fibers  which  excite  to  activity  the  epithelium 
of  glands,  the  non-striated  muscles  in  the  walls  of  the  blood-vessels  and  viscera. 


CHAPTER  XXII 

THE  ANATOMIC  RELATIONS  OF  THE  MEDULLA  OBLONGATA; 

THE  ISTHMUS  OF  THE  ENCEPHALON;  THE  CORPORA 

QUADRIGEMINA;  THE  BASAL  GANGLIA 

THE  MEDULLA  OBLONGATA 

The  medulla  oblongata  is  that  portion  of  the  central  nerve  system  im- 
m-cdiately  superior  to  and  continuous  with  the  spinal  cord.  It  has  the  shape 
of  a  truncated  cone,  the  base  of  which  is  directed  upward,  the  truncated 
apex  downward.  It  is  ^8  mm.  in  length,  i8  mm.  in  breadth,  and  12  mm. 
in  thickness.  By  the  continuation  upward  of  the  anterior  and  posterior 
median  fissures,  the  medulla  is  divided  into  symmetric  halves  (Figs.  235  and 
236).  Like  the  cord,  of  which  it  is  a  continuation,  it  is  composed  of  white 
matter  externally  and  gray  matter  internally. 

Structure  of  the  Gray  Matter. — The  gray  matter  of  the  medulla  is 
continuous  with  that  of  the  cord,  though  owing  to  the  shifting  of  position  of 
the  different  tracts  of  the  white  matter  it  is  arranged  with  much  less  regular- 
ity. The  appearance  which  the  gray  matter  presents  on  transverse  section 
varies  also  at  different  levels. 

At  the  level  of  the  first  cervical  nerve  the  posterior  horns  are  narrow, 
elongated,  and  directed  outward.  The  lateral  horns  are  well  developed  and 
present  a  collection  of  cells  near  their  bases  which  can  be  traced  upw^ard  and 
downward  for  some  distance.  At  the  level  of  the  decussation  of  the  py- 
ramidal tracts  the  head  of  the  anterior  horn  becomes  detached  from  the  rest  of 
the  gray  matter  and  is  pushed  backward  toward  the  posterior  horn;  the  bases 
of  the  anterior  horns  become  spread  out  to  form  a  layer  of  gray  matter  near  the 
dorsal  aspect  of  the  medulla.  Transverse  sections  of  the  medulla  at  all 
levels  show  a  more  or  less  extensive  network  of  nerve-fibers  known  as  the 
reticular  formation.  In  its  meshes  are  found  collections  of  nerve-cells  of 
varying  size.  Toward  the  dorsal  aspect  of  the  medulla  special  groups 
of  cells  are  found  from  which  axons  arise  to  become  the  fibers  of  various 
efferent  cranial  nerves,  e.g.,  hypoglossal,  efferent  fibers  of  the  vagus,  and 
glossopharyngeal. 

Structure  of  the  White  Matter. — The  white  matter  is  composed  of 
nerve-fibers  supported  by  connective  tissue  and  neuroglia.  It  is  subdivided 
on  either  side  by  grooves  into  three  main  columns:  viz.,  an  anterior  column 
or  pyramid,  a  lateral  column,  and  a  posterior  column. 

The  anterior  column  or  pyramid  is  composed  partly  of  fibers  continuous 
with  those  of  the  anterior  column  of  the  spinal  cord  (the  direct  pyramidal 
tract),  and  partly  of  fibers  continuous  with  those  of  the  lateral  column  of  the 
cord  of  the  opposite  side  (the  crossed  pyramidal  tract) ,  which  decussate  at 
the  anterior  portion  of  the  medulla.  The  united  fibers  can  be  traced  up- 
ward to  the  pons,  where  they  disappear  from  view. 

The  lateral  column  is  composed  of  fibers  continuous  with  those  of  the  lateral 


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column  of  the  cord.  As  the  fibers  pass  upward,  however,  they  diverge  in  several 
directions.  The  fibers  of  the  crossed  pyramidal  tract  cross  the  median  line, 
as  previously  stated,  to  enter  into  the  formation  of  the  anterior  column; 
the  fibers  of  the  direct  cerebellar  tract  gradually  curve  backward,  and  in  so 


Fig.  235, — Anterior  or  Ventral 
View  of  the  Medulla  Oblongata 
AND  Isthmus.  1.  Infundibulum.  2. 
Tuber  cinereum.  3.  Corpora  albi- 
cantia.  4.  Cerebral  peduncle.  5. 
Tuber  annulare.  6.  Origin  of  the 
middle  peduncle  of  the  cerebellum. 
7.  Anterior  pyramids  of  the  medulla 
oblongata.  8.  Decussation  of  the  an- 
terior pyramids.  9.  Olivary  bodies. 
10.  Restiform  bodies.  11.  Arciform 
fibers.  12.  Upper  extremity  of  the 
spinal  cord.  13.  Ligamentum  dentic- 
ulatum.  14,  14.  Dura  mater  of 
the  cord.  15.  Optic  tracts.  16. 
Chiasm  of  the  optic  nerves.  17. 
Motor  oculi  communis.  18.  Patheti- 
cus.  19.  Fifth  nerve.  20.  Motor 
oculi  externus.  21.  Facial  nerve.  22. 
Auditory  nerve.  23.  Nerve  of  Wris- 
b  e  r  g.  24.  Glosso-pharyngeal  nerve. 
25.  Pneumogastric.  26,  26.  Spinal 
accessory.  27.  Sublingual  nerve.  28, 
29,  30.  Cervical  nerves. — (Sappey.) 


Fig.  236. — Posterior  or  Dorsal  View 
OF  the  Medulla  Oblongata,  Isthmus, 
AND  Basal  Ganglia,  i.  Corpora  quad- 
rigemina.  2.  Corpus  quadrigeminum  an- 
terius  (pregeminum).  3.  Corpus  quadri- 
geminum posterius  (post-geminum).  4. 
Tract  of  fibers  (brachium)  passing  to  the 
corpus  geniculatum  externum.  5.  Tract  of 
fibers  (brachium)  passing  to  6,  the  corpus 
geniculatum  internum.  7.  Posterior  com- 
missure. 8.  Pineal  gland.  9.  Superior  cere- 
bellar peduncle.  10,  11,  12.  The  valve  of 
Vieussens.  13.  The  pathetic  nerve.  14. 
Lateral  groove  of  the  isthmus.  15.  Triangu- 
lar bundle  of  the  isthmus.  16.  Superior 
cerebellar  ])eduncle.  17.  Middle  cerebellar 
peduncle.  18.  Inferior  cerebellar  peduncle. 
19.  Antero-inferior  wall  of  the  fourth  ventri- 
cle. 20.  Acoustic  nerve.  21.  Spinal  cord. 
22.  The  postero-median  column.  23.  The 
posterior  pyramids. — (Sappey.) 


doing  unite  with  other  fibers  to  form  the  restiform  body,  after  which  they 
enter  the  cerebellum  by  way  of  the  inferior  peduncle.  Situated  between  the 
anterior  pyramid  and  the  restiform  body  is  a  small  oval  mass,  the  olivary 
body,  composed  of  both  white  and  gray  matter. 


ISTHMUS  OF  THE  ENCEPHALON  553 

The  posterior  column  is  composed  largely  of  fibers  continuous  with  those 
of  the  posterior  column  of  the  cord.  The  subdivision  of  this  column  into  a 
postero-external  (Burdach)  and  a  postero-internal  (Goll)  is  more  marked 
in  the  medulla  than  in  the  cord.  The  former  is  here  known  as  the  funiculus 
cuneatus,  the  latter  as  the  funiculus  gracilis.  These  two  strands  of  fibers 
are  apparently  continued  into  the  restiform  body.  Owing  to  the  divergence 
of  the  restiform  bodies  a  V-shaped  space  is  formed,  the  floor  of  which  is 
covered  with  epithelium  resting  on  the  ependyma.  At  the  upper  extremity 
of  the  funiculus  cuneatus  and  funiculus  gracilis,  two  collections  of  gray 
matter  are  found,  known  respectively  as  the  nucleus  cuneatus  and  nucleus 
gracilis.  Around  the  cells  of  these  nuclei  many  of  the  fibers  of  the  posterior 
column  end  in  brush-like  expansions  (see  Fig?  233). 

The  Fillet  or  Lemniscus. — From_  the  ventral  surface  of  the  cuneate  and 
gracile  nuclei  axons  emerge  which  pass  forward  and  upward  through  the 
gray  matter  and  decussate  with  corresponding  fibers  coming  from  the  op- 
posite nuclei.  They  then  assume  a  position  just  posterior  to  the  pyramids 
and  between  the  olivary  bodies.  These  fibers  thus  form  a  new  tract,  termed 
the  fillet  or  lemniscus.  As  this  tract  ascends  toward  the  cerebrum  it  receives 
additional  axons  from  the  sensor  end-nuclei  of  all  the  afferent  cranial 
nerves  of  the  opposite  side  with  the  exception  of  the  auditory.  From  the 
end-nuclei  of  the  auditory  nerve,  new  axons  ascend  as  a  distinct  tract  situated 
near  the  lateral  aspect  of  the  pons.  From  their  position,  these  two  separate 
tracts  have  been  termed  the  mesial  and  lateral  fillets  respectively. 

THE  ISTHMUS  OF  THE  ENCEPHALON 

The  isthmus  of  the  encephalon  comprises  that  portion  of  the  central 
nerve  system  connecting  the  cerebrum  above,  the  cerebellum  behind,  and  the 
medulla  below.  Its  ventral  surface  presents  below  an  enlargement,  convex 
from  side  to  side,  the  pons  Varolii.  On  each  side  the  fibers  of  which  the 
pons  consists  converge  to  form  a  compact  bundle,  the  middle  peduncle, 
which  enters  the  corresponding  half  of  the  cerebellum.  Above  the  pons, 
this  surface  presents  tv/o  large  columns  of  white  matter  which  diverge  some- 
what from  below  upward,  enter  the  base  of  the  cerebrum  and  are  known  as 
crura  cerebri.  Embracing  the  crura  above  are  two  large  bands  of  white 
matter,  the  optic  tracts  (Fig.  235). 

The  dorsal  surface  presents  below  two  diverging  columns  of  white  matter, 
the  inferior  peduncles;  above,  two  converging  columns,  the  superior  peduncles 
of  the  cerebellum  (Fig.  236).  At  the  extreme  upper  part  of  this  surface 
there  are  four  small  grayish  eminences,  the  corpora  quadrigemina.  From 
the  disposition  of  the  white  matter  on  the  dorsal  surface  of  the  isthmus  and 
medulla,  there  is  formed  a  lozenge-shaped  space,  the  fourth  ventricle.  The 
space  is  an  expansion  of  the  central  cavity  of  the  cord,  the  result  of  the 
changed  relations  of  the  white  and  gray  matter  in  this  region  of  the  central 
nerve  system.  Above,  this  ventricle  communicates  by  a  narrow  canal,  the 
aqueduct  of  Sylvius,  with  the  third  ventricle.  The  floor  of  the  fourth 
ventricle  is  covered  with  a  layer  of  epithelium  resting  on  the  ependyma  con- 
tinuous with  that  lining  the  central  canal  of  the  cord.  Beneath  this  is  a 
layer  of  gray  matter. 

The  pons  varolii  comprises  in  a  general  way  that  portion  of  the  central 


554  TEXT-BOOK  OF  PHYSIOLOGY 

nerve  system  situated  between  the  medulla  oblongata  and  the  crura  cerebri. 
The  ventral  surface  is  convex  from  side  to  side;  the  lateral  surface,  owing  to 
the  convergence  of  the  fibers  of  which  it  is  composed,  is  contracted  to  form  the 
middle  peduncle  of  the  cerebellum;  the  posterior  surface  is  fiat  and  forms 
the  upper  half  of  the  floor  of  the  fourth  ventricle.  The  .pons  consists  of 
white  fibers  and  gray  matter  supported  by  connective  tissue  and  neuroglia. 
Transverse  sections  of  the  pons  show  that  it  is  divided  into  an  anterior 
or  ventral,  and  a  posterior  or  dorsal  portion,  the  latter  being  usually  termed 
the  tegmentum. 

The  ventral  portion  consists  for  the  most  part  of  white  fibers,  arranged 
longitudinally  and  transversely  (Fig.  237).  The  longitudinal  fibers  are 
largely  continuations  of  the  pyramidal  tracts,  or  the  fibers  composing  in  part 
the  anterior  pyramid  of  the  medulla.     In  the  lower  part  of  the  pons  these 

fibers  are  compactly  arranged,  but  at  higher 
levels  they  are  separated  into  a  number  of 
bundles  by  the  interlacing  of  the  transverse 
fibers.  The  transverse  fibers  are  divided  into 
a  superficial  and  a  deep  set.  Among  these 
fibers  are  groups  of  nerve-cells  which  collec- 
tively are  known  as  the  nucleus  pontis.  Some 
of  the  transverse  fibers,  especially  the  super- 
ficial ones,  are  commissural  in  character — i.e.., 

T7  '^^..,^^ZZZ..^^^„^     they  connect  corresponding  parts  of  the  gray 

Fig.  237. — Transection  OF  THE  ^  r   ^      ^  i  ^     ^  K   1  in 

Pons  through  its  Middle  For-  matter  of  the  lateral  halves  of  the  cerebellum; 
TioN,  Showing  the  Relation  others  coming  from  the  gray  matter  of  the 
^t'  iTco^mTosed^'' p'\/  DorS  Cerebellum  cross  the  median  line  and  terminate 
longitudinal  fasciculus,  i.e.  and  around  the  cells  of  the  nucleus  pontis;  others 
c.  Locus    ceruleus.    L.f.  Lateral     again  are  Connected  with  the  gray  cells  of  the 

same  side.  Through  the  intermediation  of 
the  nucleus  pontis  and  certain  of  the  longitudinal  fibers  of  the  pons,  the 
cerebellum  is  brought  into  relation  with  the  cerebrum. 

The  dorsal  or  tegmental  portion  consists  of:  (i)  The  fillet;  (2)  the  formatio 
reticularis;  (3)  the  medial  longitudinal  bundle;  (4)  groups  of  efferent  and 
afferent  nerve-cells. 

The  fillet  or  lemniscus  in  this  region  is  divided  into  a  mesial  and  a  lateral 
portion.  The  fibers  of  the  mesial  portion  are  partly  the  axons  of  the  nerve- 
cells  of  the  gracile  and  cuneate  nuclei  of  the  opposite  side  of  the  medulla,  and 
partly  of  the  axons  of  the  sensor  nerve-cells  of  the  afferent  cranial  nerves  with 
the  exception  of  the  auditory.  The  fibers  of  the  lateral  portion  are  mainly 
the  axons  of  the  cells  in  the  floor  of  the  fourth  ventricle  around  which  the 
auditory  nerve-fibers  end.  They  are  therefore  a  continuation  of  the  acoustic 
tract. 

The  formatio  reticularis  is  a  continuation  of  that  of  the  medulla. 
The  medial  longitudinal  bundle  is  a  band  of  nerve-fibers,  triangular  in 
shape,  placed  on  either  side  of  the  median  line  just  beneath  the  floor  of  the 
fourth  ventricle  and  the  aqueduct  of  Sylvius.  It  consists  of  both  afferent 
(ascending)  and  efferent  (descending)  fibers.  The  afferent  fibers  are  the 
axons  of  sensor  end-nuclei  located  in  the  upper  segments  of  the  spinal  cord 
as  well  as  of  axons  of  sensor  end-nuclei  of  cranial  nerves.  As  they  pass 
upward  some  of  the  fibers,  as  well  as  collateral  branches,  arborize  around 


ISTHMUS  OF  THE  ENCEPHALON 


555 


the  nuclei  of  origin  of  the  various  motor  cranial  nerves  of  the  same  and 
opposite  sides.  The  ascending  fibers  thus  associate  anatomically  sensor 
end-nuclei  of  both  spinal  and  cranial  nerves  with  the  nuclei  of  the  motor 
cranial  nerves.  The  efferent  fibers  are  the  axons  of  nerve-cells  located  in 
the  corpora  quadrigemina  and  in  a  special  nucleus  in  the  floor  of  the  third 
ventricle.  From  this  origin  the  fibers  soon  cross  the  median  line,  pursue  a 
downward  course  and  come  into  close  relation  with  the  ascending  fibers. 
In  their  descent  fibers  pass  successively  to  the  nuclei  of  origin  of  the  motor 
cranial  nerves  and  to  nuclei  in  the  upper  segments  of  the  spinal  cord.  The 
efferent  fibers  thus  associate  anatomically  the  nuclei  from  which  they  arise 
with  the  nuclei  just  alluded  to. 

The  superior  olive  is  a  cylindric  mass  of  gray  matter  situated  in  the  pons 
in  the  anterior  part  of  the  formatio  reticularis.  It  consists  of  nerve-cells 
the  axons  of  which  pass  dorso-later- 
ally,  decussate  in  the  median  line, 
and  form  the  lateral  fillet  of  the  oppo- 
site side.  Some  few  axons  go  to  the 
lateral  fillet  of  the  same  side. 

The  groups  of  efferent  nerve-cells 
lying  just  beneath  the  floor  of  the 
fourth  ventricle  give  origin  to  axons 
composing  the  motor  portion  of  the 
fifth,  the  sixth,  the  seventh  cranial 
nerves.  The  groups  of  afferent  cells 
are  the  sensor  end-nuclei  of  the  fifth 
and  eighth  cranial  nerves  from  which 
new  axons  pass  as  a  part  of  the 
mesial  and  lateral  fillets  toward  the 
cerebrum. 

The  crura  cerebri  comprise  that 
portion  of  the  central  nerve  system 
situated  between  the  pons  below  and 
the  cerebrum  above.  They  are  com- 
posed of  strands  of  nerv^e-fibers  which  are  divided,  as  shown  on  cross-section, 
into  a  ventral  and  a  dorsal  portion  by  a  crescentic-shaped  layer  of  gray  matter, 
the  substantia  nigra  (Fig.  238).  Of  the  fibers  which  compose  the  ventral 
portion  of  each  crus,  the  crusta  or  pes,  the  larger  part  is  continuous  below, 
through  the  longitudinal  fibers  of  the  pons,  with  the  pyramid  of  the  medulla 
and  the  pyramidal  tract;  above  they  assist  in  the  formation  of  the  internal 
capsule.  On  the  inner  and  on  the  outer  side  of  each  crusta  there  is  a  bundle 
of  fibers  derived  from  the  frontal,  and  from  the  temporal  and  occipital  por- 
tions of  the  cerebrum  respectively.  These  fibers  are  connected  directly 
with  the  nuclei  pontis  and  indirectly  with  the  cerebellum  of  the  same  and 
opposite  sides.  The  fibers  which  compose  the  dorsal  portion,  the  tegmentum^ 
are  continuous  with  those  which  pass  upward  from  the  medulla  and  pons, 
e.g.,  the  fillet,  both  mesial  and  lateral,  the  formatio  reticularis,  the  medial 
longitudinal  bundle,  and,  in  addition,  the  fibers  of  the  superior  peduncles  of 
the  cerebellum.  Above,  the  fibers  terminate  largely  in  collections  of  gray 
matter  at  the  base  of  the  cerebrum. 

The  aqueduct  of  Sylvius  is  a  short  narrow  canal  which  connects  the  cavity 


Fig.  238- — Scheme  of  Transverse  Sec- 
tion OF  the  Cerebral  Peduncles.  CQ- 
Corpora  quadrigemina.  Aq.  Aqueduct- 
p.l.b.  Posterior  longitudinal  bundle.  F- 
Fillet  or  lemniscus.  RN.  Red  nucleus.  SN. 
Substantia  nigra.  III.  Third  nerve.  Py. 
Pvramidal  tracts.  Fc.  Fronto-cerebellar; 
and  TOC,  temporo-occipital  fibers  of  the 
crusta.  CC.  Caudato-cerebellar  fibers  in 
upper  part  of  crusta. — {After  Wernicke  and 
Gowers.) 


556  TEXT-BOOK  OF  PHYSIOLOGY 

of  the  fourth  with  the  cavity  of  the  third  ventricle.  It  is  Hned  by  the  epen- 
dyma  and  surrounded  by  a  layer  of  gray  matter  continuous  with  that  forming 
the  floor  of  the  fourth  ventricle.  In  that  portion  of  the  gray  matter  lying 
beneath  or  ventral  to  the  aqueduct  there  are  groups  of  nerve-cells  which 
give  origin  to  axons  which  unite  to  form  the  third  and  fourth  cranial  nerves. 

THE   CORPORA  QUADRIGEMINA 

The  corpora  quadrigemina  are  four  small  grayish  eminences  situated 
beneath  the  posterior  border  of  the  corpus  callosum  and  behind  the  third 
ventricle.  They  rest  upon  the  lamina  quadrigemina,  which  forms  the 
roof  of  the  aqueduct  of  Sylvius.  The  superior  pair  are  the  larger  and  are 
known  as  the  superior  quadrigeminal  bodies,  the  superior  colliculi  or  the  pre- 
gemina;  the  inferior  pair  are  the  smaller  and  are  known  as  the  inferior  quad- 
rigeminal bodies,  the  inferior  colliculi,  or  the  post-gemina. 

External  and  somewhat  inferior  to  the  corpora  quadrigemina  are  two 
small  collections  of  gray  matter  the  more  external  of  which  has  been  termed 
the  external  geniculate  body  or  the  pregeniculum,  the  more  internal  of  which 
has  been  termed  the  internal  geniculate  body  or  the  post-geniculum. 

Though  these  bodies  are  closely  associated  anatomically,  they  differ  in 
origin,  in  their  relations,  and  in  their  functions. 

On  either  side  the  fibers  composing  the  optic  tract  pass  to  and  through 
the  geniculate  bodies  in  which  some  of  the  fibers  terminate,  while  others 
pass  onward  to  the  superior  and  inferior  quadrigeminal  bodies  and  there 
terminate.  The  bands  of  white  matter  associating  the  superior  or  external 
and  the  inferior  or  internal  geniculate  bodies,  with  the  corresponding  quad- 
rigeminal bodies  are  known  as  the  superior  and  inferior  brachia  respectively. 
The  internal  geniculate  body  gives  origin  to  and  receives  fibers  from  the 
mesial  portion  of  the  optic  tract  which  is  in  reality  not  a  portion  of  the  optic 
tract  proper,  but  a  commisural  band  (Gudden)  which  associates  the  body 
from  which  it  arises  with  that  of  the  opposite  side.  The  point  of  decussa- 
tion is  in  the  posterior  part  of  the  optic  chiasm. 

The  external  geniculate  body  is  a  terminal  station  for  a  portion  of  the 
fine  visual  fibers  coming  from  the  retina.  From  the  cells  of  this  body 
new  axons  arise  which  course  forward  and  upward,  enter  the  internal 
capsule  and  pass  by  way  of  the  optic  radiation  to  the  cortex  of  the  occipital 
region  of  the  cerebrum. 

The  corpora  quadrigemina  show  on  microscopic  examination  that  they 
are  composed  of  nerve-cells  and  nerve-fibers,  both  of  which  are  so  intricately 
arranged  that  it  is  difficult  to  trace  their  relation  one  to  another  and  to  ad- 
joining structures.  Some  of  the  cells  of  the  superior  quadrigeminal  body  give 
origin  to  axons  which  pass  downward  and  forward  and  terminate  in  brush- 
like expansions  around  the  nuclei  of  origin  of  the  oculo-motor,  trochlear,  and 
abducent  nuclei;  other  cells  are  surrounded  by  the  terminal  branches  of  some 
of  the  fibers  of  the  optic  tract,  though  it  is  not  probable  that  they  are  true  visual 
fibers.  Still  other  cells  receive  the  terminal  branches  of  axons  the  cells  of 
origin  of  which  are  located  in  the  occipital  cortex  of  the  cerebrum  and  which 
reach  the  superior  quadrigeminal  body  by  way  of  the  optic  radiation  and 
internal  capsule. 

The  cells  of  the  post-geminum  give  origin  to  axons  which  pass  upward, 


BASAL  GANGLIA 


557 


forward,  and  outward,  enter  the  internal  capsule,  and  pass  by  way  of  the 
auditory  tract  to  the  cortex  of  the  temporo-sphenoidal  region  of  the  cerebrum. 
Many  of  the  fibers  of  the  lateral  fillet,  a  portion  of  the  auditory  tract,  termi- 
nate in  brush-like  expansions  around  these  same  cells.  There  is  thus  es- 
tablished a  connected  pathway  between  the  cochlea  and  the  temporo-sphe- 
noidal cortex.  The  cells  of  the  temporal  cortex,  how^ever,  send  axons  in  the 
reverse  direction  by  way  of  the  auditory  tract  to  the  cells  of  the  post-geminum. 
There  is  thus  established  a  double  communication  between  the  occipital 
and  temporal  region  of  the  cerebral  cortex,  and  the  pre-geminal  and  post- 
geminal  bodies  respectively. 


THE  BASAL  GANGLIA:  THE  CORPORA  STRIATA  AND  OPTIC 

THALAMI 

The  basal  gangha  are  collections  of  ganglionic  matter,  situated  at  the 
base  of  the  cerebrum  along  the  course  of  the  nerve-fibers  that  pass  to  and 


Fig.  239. — Corpora  Striata,  Optic  Thalami,  Corpora  Quadrigemina,  Cerebellum  and 
Associated  Structures,  t,  Corpora  quadrigemina;  2,  valve  of  Vieussens;  3,  pre-peduncle; 
4,  upper  part  of  medi-peduncle;  5,  upper  part  of  crus;  6,  lateral  fillet;  7,  band  of  Reil;  8  post- 
brachium;  9,  frenulum;  10,  gray  matter  of  valve  of  Vieussens;  11,  medi-commissure;  12,  pre-com- 
missure;  13,  14,  center  of  cerebellum;  15,  post-commissure;  16,  peduncles  of  the  pineal  gland;  17, 
pineal  gland;  18,  19,  posterior  and  anterior  tubercles  of  the  thalamus;  20,  tenia semicircularis;  21, 
vessels  of  the  corpus  striatum;  22,  fornicolumn;  23,  corpus  striatum;  24,  septum  lucidum. — 
{Sappey.) 

from  its  cortical  expansion.  Among  these  ganglia  the  more  important  are 
the  corpora  striata  and  the  optic  thalami.  They  are  made  visible  upon 
removal  of  the  cerebrum.  The  general  relations  of  these  ganglia  are  shown 
in  Fig.  239. 

The  corpus  striatum,  the  more  anterior  of  the  two,  is  an  ovoid  col- 


558 


TEXT-BOOK  OF  PHYSIOLOGY 


lection  of  gray  and  white  matter  and  receives  its  name  from  the  fact  that  it 
presents  on  cross-section  a  striated  appearance.  The  larger  portion  of  this 
body  is  embedded  in  the  cerebral  white  matter,  while  the  smaller  portion 
projects  into  the  anterior  part  of  the  lateral  ventricle.  A  dissection  of  this 
nucleus  shows  that  it  is  subdivided  by  a  band  of  white  matter  into  two 
smaller  nuclei,  viz. :  the  caudate  and  the  lenticular  nuclei. 
I.  The  caudate  nucleus  is  a  pyriform  body  which  corresponds  with  the 
intra-ventricular  portion  of  the  corpus  striatum.  It  consists  of  a  head, 
an  arching  body  and  a  tail.     The  head,  which  is  thick  and  large,  projects 

into  the  anterior  cornu  of  the  ventri- 


AHYGDA I A    ^gt**---*^ 
A. 


cle;  the  body  arches  across  the  ven- 
tricle   from    before    backward  and 
from  within  outward,  while  the  tail 
is  directed  downward  and  forward 
to  become  associated  with  the  collec- 
tion of  gray  matter  situated  beneath 
the  lenticular  nucleus  and  known  as 
the  amygdaline  nucleus.    Anteriorly, 
the  caudate  nucleus  is  united  with 
the  lenticular  nucleus  by  a  narrow 
bridge  of  gray  matter,  partially  sub- 
divided by  small  bands  or  strands  of 
nerve-fibers  passing  through  it. 
The  lenticular  nucleus  is  an  irregularly 
triangular    pyramidal- shaped    body 
.  and  corresponds  with  the  extra-ven- 
tricular portion  of  the  corpus  stria- 
tum, the  portion  embedded  in  the 
cerebral  white  matter.     The  apical 
extremity  of  the  nucleus  is  directed 
toward    the  median   line  while  its 
convex  base  is  directed  toward  and 
runs  almost  parallel  with  the  gray 
matter  of  the  Island  of  Reil.     The 
general  appearance  and  relation  of 
these  nuclei,  are  shown  in  Figs.  240 
and  241.     A  horizontal  section  of  the  lenticular  nucleus  shows  that  it  is 
divided  by  two  lamina  of  white  matter  into  three  portions.     The  two  in- 
ner, from  their  pale  yellow  color,  form  the  globus  pallidus,  the  outer, 
somewhat  darker  in  color,  is  termed  the  putamen.     External  to  the  len- 
ticular nucleus  is  a  thin  stratum  of  gray  matter,  arranged  more  or  less 
vertically,  and  placed  between  the  outer  surface  of  the  lenticular  nucleus 
and  the  cortex  of  the  Island  of  Reil,  and  known  as  the  claustrum. 
The  optic  thalamus  is  an  oblong  mass  of  gray  matter  situated  between 
the  sensor,  afferent  pathway  and  the  cortex  of  the  cerebrum.     The  anterior 
and  posterior  extremities  of  each  thalamus  present  enlargements  known 
respectively  as  the  anterior  tubercle  and  the  posterior  tubercle  or  pulvinar. 
The  mesial  surface  of  the  thalamus  forms  the  lateral  wall  of  the  third  ventri- 
cle and  is  covered  by  epithelium  resting  on  a  thin  layer  of  ependyma. 

A  transection  of  the  thalamus  shows  that  it  is  not  only  covered  externally 


.-    AHYVDALA 


Fig.  240. — Two  Views  of  a  Model 
OF  THE  Striatum.  A,  Lateral  aspect;  B, 
mesial  aspect. — {Spitzka.) 


BASAL  GANGLIA  559 

but  penetrated  by  white  matter,  which  subdivides  its  contained  gray  cells 
into  four  more  or  less  distinct  masses  termed  nuclei,  viz.:  an  anterior,  a 
lateral,  occupying  the  external  part  of  the  thalamus,  a  -ventral,  close  to  the 
entire  ventral  surface,  and  a  posterior,  situated  beneath  the  pulvinar.     Be 


Fig.  241. — Horizontal  Section  through  the  Cerebrum  showing  the  Natural  Relations 

OF  THE  Various  Structures. 

neath  and  somewhat  internal  to  each  optic  thalamus  there  is  a  region,  the 
subthalamic,  consisting  of  an  intricate  network  of  nerve-fibers  and  several 
nuclei  of  gray  matter,  e.g.,  the  red  or  tegmental  nucleus,  the  subthalamic 
nucleus,  or  Luys'  body,  and  the  substantia  nigra. 

Though  the  thalamus  has  extensive  connections  with  many  portions  of 


56o  TEXT-BOOK  OF  PHYSIOLOGY 

the  central  nerve  system,  the  most  important  are  with  the  cortex,  the  teg- 
mentum, and  the  optic  tracts. 

From  the  cells  of  these  various  nuclei  axons  emerge  which  pass  into  the 
internal  capsule,  and  through  the  corona  radiata  to  various  portions  of  the 
cortex.  Those  which  come  from  the  pulvinar  and  pass  to  the  occipital  lobe 
constitute  a  part  of  the  optic  radiation;  those  from  the  lateral  and  ventral 
nuclei  ultimately  reach  the  parietal  lobe;  those  from  the  anterior  nucleus 
pass  to  the  hippocampal  and  uncinate  convolutions.  In  a  similar  manner 
various  portions  of  the  cortex  are  brought  into  relation  with  the  thalamus, 
axons  from  the  cortical  cells  passing  downward  to  terminate  in  tufts  around 
the  thalamic  nuclei. 

The  tegmentum  is  intimately  related  to  the  thalamus,  though  the  exact 
distribution  of  various  strands  of  fibers  is  a  subject  of  much  discussion. 
Most  of  the  fibers  of  the  mesial  fillet  end  in  tufts  around  the  cells  of  the 
ventral  and  lateral  nuclei;  other  fibers  pass  directly  to  the  cortex. 

The  optic  tract  sends  fibers  directly  into  the  pulvinar,  the  external 
geniculate  body,  and  the  superior  corpus  quadrigeminum,  around  the  cells 
of  which  they  terminate  in  brush-like  expansions. 

The  Internal  Capsule. — The  lenticular  nucleus  is  enclosed  on  all  sides 
by  ascending  and  descending  nerve-fibers.  From  the  manner  in  which 
they  surround  and  enclose  the  nucleus  they  have  collectively  been  called  the 
lenticular  capsule.  If  a  horizontal  section  of  the  cerebrum  be  made  at  a 
certain  level  so  as  to  cut  across  the  capsule  and  the  enclosed  nucleus  an 
appearance  similar  to  that  shown  in  Fig.  241  will  be  presented.  That  portion 
of  the  capsule  that  lies  between  the  caudate  nucleus  and  the  optic  thalamus 
internally  and  the  lenticular  nucleus  externally  is  known  as  the  internal 
portion  of  the  lenticular  capsule  or  in  its  abbreviated  form  as  the  internal 
capsule,  while  that  portion  between  the  external  convex  border  of  the  len- 
ticular nucleus  and  the  claustrum  is  known  as  the  external  portion  of  the 
lenticular  capsule  or  in  its  abbreviated  form  as  the  external  capsule.  At  a 
given  level  the  internal  capsule  may  be  said  to  consist  of  two  segments  or 
limbs,  an  anterior,  situated  between  the  caudate  nucleus  and  the  anterior 
extremity  of  the  lenticular  nucleus,  and  a  posterior,  situated  between  the 
optic  thalamus  and  the  posterior  extremity  of  the  lenticular  nucleus.  The 
two  segments  unite  at  an  obtuse  angle,  termed  the  knee,  which  is  directed 
toward  the  median  line.  The  appearance  which  is  presented  at  different 
levels  varies  however  considerably. 

SUMMARY  OF  THE  STRUCTURE  OF  THE  MEDULLA,  ISTHMUS,  AND 

BASAL  GANGLIA 

Structure  of  the  Central  Gray  Matter. — Though  the  general  arrange- 
ment of  the  central  gray  matter  has  been  incidentally  alluded  to  in  the  fore- 
going presentation  of  the  anatomic  features  of  the  medulla  and  isthmus,  it 
will  be  convenient  to  summarize  its  arrangement  and  structure  at  this  point. 

The  gray  matter  of  the  cord,  of  the  dorsal  aspect  of  the  medulla  and  pons, 
of  the  region  surrounding  the  aqueduct  of  Sylvius,  and  of  the  lining  of  the 
third  ventricle,  constitute  practically  a  continuous  system,  though  presenting 
modifications  in  various  parts  of  its  extent.  In  the  transition  region  of  the 
spinal  cord  and  medulla  the  gray  matter  of  the  former  becomes  much  changed 
in  shape  owing  to  the  shifting  of  position  of  the  various  tracts  of  white  matter, 


MEDULLA  AND  BASAL  GANGLIA  561 

until  in  the  medulla  and  pons  it  is  spread  out  in  the  form  of  a  thin  layer  near 
their  dorsal  surfaces,  where,  together  with  the  ependyma,  it  forms  the  floor 
of  the  fourth  ventricle. 

In  the  region  of  the  aqueduct  of  Sylvius  the  gray  matter  again  converges 
and  ultimately  surrounds  the  canal,  to  again  expand  at  its  anterior  extremity 
to  form  the  lining  of  the  third  ventricle. 

The  Nerve-cells.^ — The  nerve-cells  in  these  different  regions  do  not 
differ  morphologically  from  those  in  the  gray  matter  of  the  spinal  cord.  The 
corpus,  or  body  of  the  cell,  presents  a  number  of  dendrites  as  well  as  the 
sharply  defined  axon.  As  a  rule,  the  cells  are  arranged  in  groups,  or  clusters, 
or  nests,  partially  surrounded  and  enclosed  by  supporting  tissue,  and  situ- 
ated beneath  the  floor  of  the  fourth  ventricle  and  the  floor  of  the  aqueduct  of 
Sylvius. 

Classification  of  Nerve-cells. — I'he  cells  of  the  gray  matter  may  be 
divided  into  three  main  groups;  viz.,  intrinsic  or  associative,  receptive  or 
aft'erent  and  emissive  or  efferent. 

The  iiitrmsic  cells  are  associative  in  function.  The  axons  to  which  these 
cells  give  origin  pass  more  or  less  horizontally  into  the  white  matter,  where 
they  divide  into  two  branches,  one  of  which  passes  upward,  the  other  down- 
ward. At  various  levels  they  re-enter  the  gray  matter  and  arborize  around 
other  intrinsic  cells. 

The  receptive  cells  are  largely  sentient  or  afferent  in  function,  inasmuch  as 
they  receive  the  nerve  impulses  transmitted  to  them  by  afferent  cranial 
nerves.  As  the  aft'erent  nerve  fibers  are  classified,  in  accordance  with  the  sensa- 
tions to  which  they  give  rise,  as  sensor,  thermal,  tactile,  gustatory,  auditory, 
etc.,  so  these  cells  may  be  similarly  classified,  according  as  they  transmit  their 
excitations  to  those  specialized  areas  in  the  cerebral  cortex  in  which  these 
different  sensations  arise. 

The  emissive  cells  are  efferent  or  motor  in  function,  inasmuch  as  the 
excitation  arising  in  them  is  transmitted  outwardly  through  their  axons  to 
muscles,  glands,  blood-vessels,  and  viscera,  imparting  to  them  motion, 
either  molar  or  molecular.  As  the  efferent  fibers  are  classified  (see  page  99) 
in  accordance  with  their  physiologic  action  into  motor,  vaso-motor,  secretor, 
viscero-motor,  so  the  nerve-cells  of  which  the  nerves  are  integral  parts  may 
be  classified  physiologically  as  motor,  vaso-motor,  secretor,  and  viscero- 
motor.    Collections  or  groups  of  such  cells  are  termed  "centers." 

Structure  of  the  White  Matter. — The  white  matter  is  composed  of 
medullated  nerve-fibers,  and  though  arranged  in  a  very  complex  manner 
may  be  di\dded  into  longitudinal  and  transverse  fibers. 

The  Icnigittidinal  fibers  which  compose  the  main  portion  of  the  isthmus 
may  be  subdi\ided  into  (i)  a  ventral  or  pedal  portion  and  (2)  a  dorsal  or 
tegmental  portion.  The  fibers  constituting  the  ventral  or  pedal  portion  may 
for  convenience  be  said  to  extend  from  the  cerebral  cortex  through  the  crus 
cerebri  to  the  pons,  medulla,  and  spinal  cord.  They  may  be  divided  into 
three  distinct  tracts:  e.g.,  the  pyramidal  tract,  the  fronto-cerebellar  tract, 
and  the  occipito-temporo-cerebellar  tract  (Fig.  242). 

The  Pyramidal  or  motor  tract  descends  from  the  cortex  of  the  cerebrum 

mainly  from  the  gyrus  anterior  to  the  central  fissure,  passes  through  the 

posterior  one-third  of  the  anterior  segment  and  the  anterior  two-thirds  of  the 

posterior  segment  of  the  internal  capsule,  the  middle  two-fifths  of  the  crusta, 

36 


562 


TEXT-BOOK  OF  PHYSIOLOGY 


behind  the  transverse  fibers  of  the  pons,  to  become  the  anterior  pyramid  of 
the  medulla,  beyond  which  it  divides  into  the  direct  and  crossed  pyramidal 
tracts  of  the  cord.  In  its  course  some  of  the  fibers  and  their  collaterals 
arborize  around  efferent  cells  from  the  anterior  extremity  of  the  aqueduct  of 
Sylvius  to  the  termination  of  the  spinal  cord. 

The  fronto-cerehellar  tract  descends  from  the  cortex  of  the  frontal  gyri 
of  the  anterior  lobe,  passes  through  the  anterior  portion  of  the  anterior 
segment  of  the  internal  capsule,  the  inner  fifth  of  the  crusta  to  the  pons, 
where  its  fibers  terminate  or  arborize  around  the  nucleus  pontis  of  the  same 
and  opposite  sides. 

The  occipito-temporo-cerehellar  tract  descends   from   the  occipital   and 


Fig.  242. — Schema  of  the  Projection  Fibers  of  the  Cerebrum  and  of  the  Peduncles  of 
THE  Cerebellxjm;  Lateral  View  of  the  Internal  Capsule.  A,  Tract  from  the  frontal  gyri 
to  the  pons  nuclei,  and  so  to  the  cerebellum  (frontal  cerebro-cortico-pontal  tract);  B,  the  motor 
(pyramidal)  tract;  C,  the  sensory  (body  sense)  tract;  D,  the  visual  tract;  E,  the  auditory  tract;  i^, 
the  fibers  of  the  superior  peduncle  of  the  cerebellum;  G,  fibers  of  the  middle  peduncle  uniting  with 
A  in  the  pons;  H,  fibers  of  the  inferior  peduncle  of  the  cerebellum;  /,  fibers  between  the  auditory 
nucleus  and  the  inferior  quadrigeminal  body;  K,  motor  (pyramidal)  decussation  in  the  bulb;  Vt, 
fourth  ventricle.     The  numerals  refer  to  the  cranial  nerves. — (Modified  from  Starr.) 

temporal  lobes,  passes  to  the  inner  side  of  the  lenticular  nucleus,  and  con- 
tinues downward  on  the  outer  side  of  the  crusta,  occupying  about  one-fifth  of 
its  bulk,  to  the  pons,  where  its  fibers  also  arborize  around  the  nucleus  pontis 
of  the  same  and  opposite  sides.  By  means  of  fibers  in  the  middle  peduncle 
these  descending  fibers  are  brought  into  relation  with  the  cerebellum. 
In  this  tract,  not  shown  in  the  figure,  are  to  be  found  the  afferent  fibers 
constituting  the  visual  tract,  D  and  the  auditory  tract  E. 

The  fibers  constituting  the  dorsal  or  tegmental  portion  of  the  longitudinal 
system  may  be  said  for  convenience  to  extend  from  the  posterior  portion  of 
the  medulla  and  pons  to  the  optic  thalamus  and  cerebrum.  They  may  be 
subdivided  into  several  tracts,  the  more  important  of  which  are  the  fillet  and 
the  dorsal  longitudinal  bundle. 


FUNCTIONS  OF  THE  MEDULLA  OBLONGATA  563 

The  fillet  or  lemniscus,  consisting  oi  fibers  having  their  origin  partly  from 
the  cells  of  the  cuneate  and  gracile  nuclei  and  partly  from  the  cells  of  the 
sensor  end-nuclei  of  the  spinal  and  various  sensor  cranial  nerves,  occupies 
a  region  in  the  ventral  and  mesial  portion  of  the  tegmentum  throughout  its 
entire  extent.  Superiorly  this  mesial  fillet  terminates  for  the  most  part 
around  nerve  cells  in  the  nuclei  of  the  thalamus.  From  these  nuclei  new 
fibers  arise  which  pass  for  the  most  part  to  the  cortex  of  the  post-central 
and  parietal  gyri.  The  fibers  coming  from  the  sensor  end-nucleus  of  the 
auditory  nen'e  (the  lateral  fillet)  lie  on  the  lateral  aspect  of  the  pons  and 
cms.  Superiorly  they  terminate  in  the  post-geminum  (the  inferior  quadri- 
geminal  body)  and  in  the  internal  geniculate  body.  From  these  nuclei  the 
fibers  composing  the  auditory  tract  pass  to  the  super-temporal  convolution. 

The  dorsal  longitudinal  bundle,  an  upward  extension  of  the  fibers  com- 
posing a  portion  of  the  ground  bundle  of  the  spinal  cord,  is  located  on 
either  side  of  the  median  line  just  beneath  the  floor  of  the  fourth  ventricle  and 
the  aqueduct  of  Syhius.  As  it  passes  upward  collateral  branches  are  given 
off,  some  of  which  arborize  around  the  cell  nuclei  of  the  third,  fourth,  and 
sixth  cranial  nerves  of  the  same  side,  while  others  cross  the  median  line  and 
arborize  around  the  corresponding  cell  nuclei  of  the  opposite  side.  Supe- 
riorly some  of  the  fibers  become  related  to  cells  in  the  thalamus  and  sub- 
thalamic region.  This  bundle  of  fibers  appears  to  be  mainly  commissural  in 
character. 

The  transverse  fibers  of  the  isthmus  are  found  in  the  pons.  The  fibers 
of  the  ventral  as  well  as  those  of  the  more  dorsal  regions  have  their  origin 
in  nerve-cells  in  the  cortex  of  the  cerebellum.  From  their  origin  they  pass 
through  the  cerebellar  white  matter,  and  through  the  middle  peduncle  as 
far  as  the  median  line,  where  they  decussate  with  fibers  coming  from  the  op- 
posite side.  Beyond  this  point  they  pass  to  the  cerebellar  cortex.  From 
their  anatomic  relations  it  is  probable  that  these  transverse  fibers  are  com- 
missural in  character,  bringing  into  relation  opposite  but  corresponding 
regions  of  the  cerebellar  cortex.  In  addition  to  the  commissural  fibers  other 
transverse  fibers  associate  the  cerebellar  cortex  with  the  gray  matter  in  the 
pons  on  both  the  same  and  opposite  sides.  In  this  way  the  cerebellum  is 
brought  into  relation  with  longitudinal  fibers  coming  from  and  going  to  the 
cerebrum. 

FUNCTIONS   OF  THE   MEDULLA  OBLONGATA,   ISTHMUS,  CORPORA 
QUADRIGEMINA,  AND  BASAL  GANGLIA 

Microscopic  examination  of  the  white  and  gray  matter  of  these  various 
parts  of  the  central  nerv^e  system  show^s  that  they  are  composed  of  ner\-e- 
cells  and  nerve-fibers  which  morphologically  do  not  differ  in  essential  respects 
from  those  found  in  the  spinal  cord,  though  their  arrangement  is  far  more 
complicated  and  involved.  The  functions  of  these  closely  related  structures 
are  in  consequence  equally  complex  and  involved  and  but  imperfectly  known. 

In  a  general  way  it  may  be  said  that  by  virtue  of  the  presence  of  nerve- 
cells  and  definite  tracts  of  nerve-fibers  these  structures  collectively  may  be 
regarded  as  consisting: 

T.  Of  nerve  centers  each  of  which  has  a  special  function,  and 
2.  Of  conducting  paths  by  which  these  centers  are  brought  into  relation 


564  TEXT-BOOK  OF  PHYSIOLOGY 

not  only  with  one  another  but  with  the  cerebrum,  the  cerebellum  and 
the  spinal  cord. 
A.  The  Medulla  and  Isthmus,  as  Local  Nerve-centers. — The  emis- 
sive or  efferent  cells  situated  in  the  gray  matter  beneath  the  floor  of  the 
fourth  ventricle  and  beneath  the  aqueduct  of  Sylvius  and  arranged  in  more 
or  less  well-delmed  groups,  are  the  sources  of  the  nerve  energy  that  excites 
activity  in  the  skeletal  muscles,  glands,  vascular  and  to  some  extent  visceral 
muscles  of  the  head,  neck,  thorax  and  abdomen. 

The  efferent  fibers  emerging  from  these  cells  constitute  in  part,  the  trunks 
of  the  efferent  encephalic  or  cranial  nerves,  the  origin,  course,  distribution 
and  functions  of  which  will  be  considered  in  a  subsequent  chapter. 
The  discharge  of  their  energy  may  be  caused: 

1.  By  variations  in  the  composition  of  the  blood  or  lymph  by  which  they 

are  surrounded  or  as  the  outcome  of  a  reaction  between  the  chemic 
constituents  of  the  lymph  on  the  one  hand  and  the  chemic  constituents 
of  the  nerve-cell  on  the  other  hand.  The  excitation  of  the  cell  thus 
occasioned  is  termed  automatic  or  aiitochthonic  excitation. 

2.  By  the  arrival  of  nerve  impulses,  coming  through  afferent  cranial  and 

spinal  nerves  from  the  general  periphery,  skin,  mucous  membrane, 
etc. 

3.  By  the  arrival  of  nerve  impulses  descending  from  cells  in  the  cortex  of 

the  cerebrum  or  subordinate  regions. 
The  excitation  in  the  former  instance  is  said  to  be  reflex  or  peripheral 
in  origin;  in  the  latter  instance  direct  or  cerebral  in  origin.  In  the  direct  or 
cerebral  excitations  the  skeletal  muscle  movements  are  due  to  psychic  states 
of  a  volitional  or  an  affective  or  emotional  character;  the  gland  discharges  and 
vascular  and  visceral  muscle  movements  to  emotional  phases  of  cerebral 
activity  only. 

The  activities  of  these  centers  are  thus  called  forth  by  autochthonic,  reflex 
or  peripheral,  and  direct  or  cerebral  excitations.  The  forms  of  activity  excited 
in  peripheral  organs  resemble  those  excited  by  the  spinal  nerve-centers. 
See  page  451. 

Special  Nerve-centers. — Since  it  is  by  means  of  nerve-cells  and  their 
associated  fibers  that  many  important  functions  of  organic  life  are  initiated 
and  maintained,  it  would  naturally  be  expected  from  its  extensive  nerve 
connections  that  this  region  of  the  nerve  system  plays  an  extensive  role  in 
this  respect.  As  the  accomplishment  of  these  functions  requires  the  co- 
operation and  coordination  of  a  number  of  separate  but  related  structures, 
it  is  evident  that  there  must  exist  in  the  medulla  and  pons  a  number  of  co- 
ordinating mechanisms  consisting  of  nerve-cells  and  nerve-fibers  which  are 
associated  in  various  ways  for  the  accomplishment  of  definite  functions. 
To  such  a  coordinating  mechanism  the  term  "center"  has  been  given: 
e.g.,  respiratory,  cardiac,  deglutitory,  etc.^ 

Experimentation   has   shown   that   the  medulla  and   pons  contain   a 
number  of  such  centers,  the  more  important  of  which  are  as  follows: 
I.  The  cardiac  centers,  which  exert  (i)  an  accelerator  action  over  the  heart's 
pulsations  through  nerve-fibers  emerging  from  the  spinal  cord  in  the 

*  By  the  term  center  as  here  employed  is  meant  a  collection  of  nerve-cells  and  nerve-fibers 
occupying  an  area  of  greater  or  less  extent,  though  its  exact  anatomic  limits  may  not  be  accurately 
defined.  That  an  area  may  merit  the  term  center,  it  is  necessary  that  its  stimulation  should 
increase,  its  destruction  should  abolish  or  impair,  functional  activity. 


FUNCTIONS  OF  THE  MEDULLA  AND  ISTHMUS  ^6. 


5^:> 


roots  of  the  first  and  second  dorsal  nerves  and  reaching  the  heart  through 
the  sympathetic  nerve;  (2)  an  inhibitor  or  retarding  action  on  the  rate 
of  the  heart-beat  through  efferent  fibers  in  the  trunk  of  the  pneumogastric 
or  vagus  nerve.     (See  pages  322  and  323.) 

2.  A  vaso-motor  center,  which  regulates  the  caliber  of  the  blood-vessels 

throughout  the  body  in  accordance  with  the  needs  of  the  organs  and 
tissues  for  blood,  through  nerve-fibers  passing  by  way  of  the  spinal 
nerv^es  to  the  walls  of  the  blood-vessels.     (See  page  382.) 

3.  A  respiratory  center,  which  coordinates  the  muscles  concerned  in  the 

production  of  the  respiratory  movements.     (See  page  425.) 

4.  A  mastication  center,  which  excites  to  activity  and  coordinates  the  muscles 

of  mastication.     (See  page  144.) 

5.  A  deglutition  center,  which  excites  and  coordinates  the  muscles  concerned 

in  the  transference  of  the  food  from  the  mouth  to  the  stomach.  (See 
page  1 64. )_ 

6.  An  articulation  center,  which  coordinates  the  muscles  necessary  to  the 

production  of  articulate  speech. 

7.  A  diabetic  center  stimulation  of  which  gives  rise  to  glycosuria. 

8.  A  salivary  center,  stimulation  of  which  excites  the  discharge  of  saliva. 

B.  The  Medulla  and  Isthmus  as  Conductors. — The  anterior  pyra- 
mids of  the  medulla  and  their  continuations  through  the  more  ventral  por- 
tions of  the  pons,  being  portions  of  the  general  pyramidal  tract,  serve  to 
conduct  volitional  efferent  nerve  impulses  from  higher  portions  of  the  brain 
to  the  spinal  cord.  Division  of  these  pathways  is  at  once  followed  by  a  loss 
of  volitional  control  of  the  muscles  below  the  section. 

The  dorsal  or  tegmental  portion,  containing  the  fillet,  serves  to  transmit 
afferent  nerve  impulses  from  the  spinal  cord  to  higher  portions  of  the  brain. 
Transverse  division  of  one-half  of  the  dorsal  portion  of  the  pons  is  followed 
by  complete  anesthesia  of  the  opposite  half  of  the  body  without  any  im- 
pairment of  motion. 

The  restiform  bodies  constitute  a  pathway  between  the  spinal  cord  and 
the  cerebellum.  The  transverse  fibers  of  the  pons  associate  opposite  but 
corresponding  portions  of  the  cerebellar  hemispheres. 

The  crura  cerebri  consists  ventrally  of  fibers  which  are  largely  derived 
from  the  pyramidal  tracts  and  are  continuous  with  the  longitudinal  fibers 
of  the  ventral  portion  of  the  pons  and  medulla;  and  dorsally  of  fibers  continu- 
ous with  those  coming  through  the  lower  portions  of  the  tegmentum.  Hence 
they  are  conductors  of  motor  impulses  in  the  former  and  of  sensor  impulses 
in  the  latter  region.  It  is  not  definitely  known  as  to  whether  reflex  actions 
take  place  through  the  gray  matter,  the  locus  niger,  or  not. 

The  Corpora  Quadrigemina. — From  the  anatomic  relation  of  the 
superior  quadrigeminal  body  (the  pre-geminum)  to  the  optic  tract,  the 
inference  can  be  drawn  that  it  is  in  some  way  essential  to  the  performance 
of  various  reflex  ocular  movements  and  perhaps  to  the  variations  in  size  of 
the  pupil.  Experimental  investigations  and  pathologic  changes  support  the 
inference. 

Irritation  of  the  pre-geminum  in  monkeys  on  one  side  is  followed  by 
diminution  of  the  pupils  first  on  the  opposite  side  and  then  almost  immedi- 
ately on  the  same  side.  The  eyes  at  the  same  time  are  also  widely  opened  and 
the  eyeballs  turned  upward  and  to  the  opposite  side.     If  the  irritation  be 


566  TEXT-BOOK  OF  PHYSIOLOGY 

continued,  motor  reactions  are  exhibited  in  various  parts  of  the  body. 
Destruction  of  the  pre-geminum  in  both  monkeys  and  rabbits  is  followed  by 
blindness,  dilatation  and  immobility  of  the  pupils,  with  marked  disturbance 
of  equilibrium  and  locomotion  (Ferrier). 

From  the  anatomic  relation  of  the  inferior  quadrigeminal  body  (the 
post-geminum)  to  the  lateral  fillet,  the  basal  tract  for  hearing,  the  inference 
may  be  drawn  that  it  is  in  some  way  connected  with  the  auditory  process. 

Stimulation  of  the  post-geminum  gives  rise  to  cries  and  various  forms 
of  vocalization.  Pathologic  states  of  this  body  are  also  attended  by  impair- 
ment of  hearing  and  disorders  of  the  equilibrium. 

From  the  foregoing  facts  it  is  probable  that  the  corpora  quadrigemina  are 
associated  with  station  and  locomotion.  Ferrier  assumes  that  in  these 
bodies  "sensory  impressions,  retinal  and  others,  are  coordinated  with  adapt- 
ive motor  reactions  such  as  are  involved  in  equilibration  and  locomotion." 

The  Corpora  Striata. — The  relation  of  these  bodies  to  the  pyramidal 
motor  tract  would  indicate  that  they  are  in  some  way  connected  with  motor 
activities.  Their  function,  however,  is  obscure.  While  stimulation  of  one 
corpus  produces  convulsion  of  the  muscles  of  the  opposite  side  of  the  body, 
and  destruction  gives  rise  to  paralysis  of  the  corresponding  muscles,  it  is 
difficult,  owing  to  the  intimate  association  of  the  white  and  the  gray  matter, 
to  state  to  which  the  phenomena  are  to  be  attributed.  The  evidence  at 
hand  points  to  the  conclusion  that  if  a  lesion  is  limited  to  the  gray  matter  the 
paralysis  which  might  result  would  be  but  temporary  and  of  short  duration. 
The  pathologic  evidence  is  of  a  similar  character.  Gowers  is  of  the  opinion, 
that  if  the  lesion  is  small  and  at  a  sufficient  distance  from  the  white  fibers  of 
the  capsule,  there  may  even  be  no  initial  hemiplegia;  neither  motor  nor 
sensory  paralysis  will  arise  if  the  lesion  is  confined  to  the  gray  matter. 

It  is  stated  by  some  experimenters  that  localized  injuries,  both  experi- 
mental and  pathologic,  are  followed  by  a  persistent  rise  of  temperature, 
varying  from  i°  to  2.6°C. 

The  Optic  Thalami. — From  the  anatomic  relation  of  the  optic  thalami 
to  the  general  and  special  sense  nerve-tracts,  on  the  one  hand,  and  to  the 
cerebral  cortex,  on  the  other  hand,  it  is  assumed  that  they  are  connected 
with  the  production  of  sensations  both  general  and  special,  and  act  as 
intermediaries  between  the  peripheral  sense-organs  and  the  cortex. 

The  results  of  experimentar  stimulation  and  destruction  of  the  thalami 
are  extremely  contradictory  and  fail  to  throw  much  light  on  their  functions. 
Ferrier  states  that  destruction  of  the  posterior  part  of  one  thalamus  pro- 
duced blindness  in  the  opposite  eye  and  impairment  of  the  sense  of  touch 
and  pain  in  the  opposite  side  of  the  body.  In  a  patient  under  the  care  of 
Hughlings  Jackson  there  was  blindness  in  the  right  half  of  each  eye,  loss  of 
hearing  in  the  left  ear,  impairment  of  taste  on  the  left  side  of  the  tongue, 
and  a  diminution  of  the  sense  of  touch  on  the  left  side  of  the  body.  Post- 
mortem examination  showed  a  patch  of  softening  in  the  posterior  part  of 
the  right  thalamus,  the  remainder  of  the  organ  being  normal. 

It  is  probable  that  in  the  thalamus  visual,  tactile,  and  labyrinthine  im- 
pressions are  received,  coordinated,  and  reflected  outward,  with  the  result 
of  producing  various  adaptive  motor  reactions  connected  with  station  and 
equilibrium.  The  thalamus  is  believed  by  some  investigators  to  act  also  as 
an  intermediary  between  emotional  states  and  their  expression  in  the  muscles 


FUNCTIONS  OF  THE  INTERNAL  CAPSULE 


567 


of  the  face,  this  power  being  lost  in  certain  pathologic  conditions.  The 
power  of  regulating  the  temperature  of  the  body  has  been  also  assigned  to 
the  thalamus,  as  destruction  of  its  anterior  extremity  is  usually  followed  by 
a  rise  in  temperature. 

The  Internal  Capsule. — The  internal  capsule  has  been  shown  by  the 
results  both  of  experiment  and  of  pathologic  processes  to  be,  jBjst,  a  pathway 
for  the  transmission  of  nerve  impulses  from  the  cerebral  cortex  to  the  pons, 
medulla,  and  spinal  cord,  which  give  rise  to  contraction  of  the  muscles  of  the 
opposite  side  of  the  body;  and,  second,  a  pathway  for  the  transmission  of 
nerve  impulses  coming  from  skin,  mucous  membrane,  muscles,  and  special 
sense-organs  to  the  cortex,  where  they  give  rise  to  sensations  general  and 
special.     It  is  therefore  the  common  motor  and  sensor  pathway.     For  the 


w  Afcut/i 


'  U.T 
AT. 


Fig.  243. — Horizontal  Section  of  the  Internal  Capsule  Showing  the  Position  and 
Relation  of  the  Motor  Tracts  for  the  Eye,  Head,  Tongue,  Mouth,  Shoulder  (Shi.), 
Elbow  (Elb.),  Digits  of  Hand  (Dig.),  Abdomen  (Abd.),  Hip,  Knee  (Kn.),  Digits  of  Foot 
(Dig.).     S.  Sensor  tract.     O.  T.  Optic  tract.     A.  T.  Auditory  tract. 

reason  that  it  transmits  both  motor  and  sensor  impulses,  and  for  the  further 
reason  that  it  is  frequently  the  seat  of  pathologic  lesions  which  are  followed 
by  either  a  loss  of  motion  or  sensation  or  both,  the  internal  capsule  is  one 
of  the  most  interesting  parts  of  the  central  ner\'e  system.  As  shown  in  Fig. 
243,  it  consists  of  two  segments  or  limbs  united  at  an  obtuse  angle,  the  knee 
or  elbow,  which  is  directed  tov/ard  the  median  line.  The  motor  tract  is 
confined  to  the  posterior  one-third  of  the  anterior  segment  and  the  anterior 
two-thirds  of  the  posterior  segment.  The  sensor  tract  is  confined  to  the 
posterior  one-third  of  the  posterior  segment,  the  extreme  end  of  which  also 
contains  the  optic  and  auditory  tracts. 

The  region  of  the  anterior  segment  in  front  of  the  motor  tract  contains 
the  fibers  of  the  fronto-cerebellar  tract,  the  function  of  which  is  unknown. 

The  motor  region  contains  fibers  which  descend  from  the  cerebral  cortex 


568 


TEXT-BOOK  OF  PHYSIOLOGY 


to  nerve-centers  situated  in  the  gray  matter  beneath  the  aqueduct  of  Sylvius, 
in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle,  and  in  the  anterior 
horns  of  the  gray  matter  of  the  spinal  cord,  and  which  in  turn  are  connected 
by  the  cranial  and  spinal  nerves  with  the  muscles  of  the  eye,  head,  face, 
trunk,  and  limbs.  The  positions  occupied  by  these  different  tracts  are 
shown  in  Fig.  243.  The  relation  of  the  internal  capsule  to  the  caudate 
nucleus  and  the  optic  thalamus  internally,  and  to  the  lenticular  nucleus  ex- 
ternally, is  also  shown  in  a  vertical  section  of  the  cerebrum  made  in  front  of 
the  gray  commissure  (Fig.  244). 

From  the  fact  that  the  internal  capsule  contains  efferent  or  motor  tracts, 
and  afferent  or  sensor  tracts,  it  is  evident  that  a  destructive  lesion  of  the 
motor  tract  would  be  followed  by  a  loss  of  motion;  and  of  the  sensor  tract, 
by  a  loss  of  sensation  on  the  opposite  side  of  the  body. 


Fig.  244. — A  Transverse  Section  of  One-half  of  the  Cerebrum.  Arteries  supplying 
the  lenticular  nucleusLAT",  the  internal  capsule  IC,  and  the  caudate  nucleus  CN.  i.  The  middle 
cerebral.  2,  2.  Lenticular  arteries.  3.  The  lenticulo-striate  artery  passing  through  the  ex- 
ternal capsule. 


This  condition,  to  a  greater  or  less  extent,  frequently  arises  in  consequence 
of  a  rupture  of  the  blood-vessels  which  supply  the  caudate  nucleus,  the  len- 
ticular nucleus  and  the  internal  capsule.  These  arteries,  branches  of  the 
middle  cerebral,  penetrate  the  brain  in  the  anterior  and  posterior  perforated 
spaces  (Fig.  244).  By  reason  of  a  diseased  condition  of  their  walls, 
especially  of  the  lenticulo-striate  artery,  the  vessel  ruptures  and  the  blood 
is  extravasated  into  the  surrounding  tissues. 

If  the  extravasation  occurs  in  the  anterior  two-thirds  of  the  posterior 
limb  of  the  internal  capsule,  there  will  be  a  destruction  of  the  efferent  or 
motor  fibers,  and  a  separation  of  the  cortical  motor  area  from  the  medullary 
and  spinal  motor  nuclei  and,  therefore,  a  loss  of  volitional  control  of  the  mus- 
cles of  the  face,  arm  and  leg  of  the  opposite  side.  The  extent  of  this  paraly- 
Bis  will  depend,  of  course,  on  the  extent  of  the  extravasation.     The  muscles 


FUNCTIONS  OF  THE  INTERNAL  CAPSULE  569 

of  the  larynx,  thorax  and  abdomen,  those  associated  with  symmetrical  move- 
ments escape  paralysis  for  the  reason  that  in  all  such  movements  each  side  of 
the  brain  controls  the  muscles  of  both  sides  of  the  body.  If  the  extravasation 
of  blood  also  destroys  the  fibers  of  the  posterior,  one-third  of  the  internal 
capsule  in  the  region  between  the  optic  thalamus  and  the  lenticular  nucleus, 
there  will  be  a  destruction  of  the  afferent  or  sensor  fibers  and  a  separation  of 
the  cutaneous  areas  from  the  cortical  sensor  areas  and,  therefore,  a  loss  of 
sensation  on  the  opposite  side  of  the  body.  If  the  extravasation  is  confined 
to  the  external  limits  of  the  posterior  one-third  of  the  capsule,  there  would  be 
a  loss  of  sensation  without  impairment  of  motion,  a  rare  condition  however. 
In  these  hemorrhages  the  optic  and  auditory  tracts  are  seldom  involved. 
Coincident  with  the  rupture  of  these  vessels  there  is  a  loss  of  consciousness 
as  a  rule,  and  the  patient  falls  as  if  struck  and  hence  this  condition  received 
the  name  of  apoplexy.  With  the  recovery  of  consciousness  the  paralysis  of 
motion  becomes  apparent  and  is  usually  permanent.  Not  infrequently,  how- 
ever, partial  recovery  of  motion  and  sensation  takes  place  after  the  blood 
coagulates  and  the  serum  is  absorbed,  thus  relieving  pressure  on  adjoining 
nerve-tracts  which  were  not  otherwise  injured. 


CHAPTER  XXIII 
THE  CEREBRUM 

The  cerebrum  is  the  largest  portion  of  the  encephalon,  constituting 
about  85  per  cent,  of  its  total  weight.  In  shape  it  is  ovate,  convex  on  its 
outer  surface,  narrow  in  front  and  broad  behind.  It  is  divided  by  a  deep 
longitudinal  cleft  or  fissure  into  halves,  known  as  the  cerebral  hemispheres. 
The  hemispheres  are  completely  separated  anteriorly  and  posteriorly  by  this 
fissure,  but  in  their  middle  portions  are  united  by  a  broad  white  band  of 
nerve-fibers,  the  corpus  callosum.  Each  hemisphere  or  hemi-cerebrum  is 
convex  on  its  outer  aspect,  and  corresponds  in  a  general  way  with  each  side 
of  the  cavity  of  the  skull;  the  inner  or  mesial  surface  is  flat  and  forms  the 
lateral  boundary  of  the  longitudinal  fissure. 

The  surface  of  each  hemi-cerebrum  presents  a  series  of  alternate  indenta- 
tions and  elevations,  known  respectively  as  fissures  or  sulci,  and  convolutions 
or  gyri.  A  knowledge  of  the  situation  and  extent  of  the  principal  fissures 
and  convolutions,  as  well  as  of  their  relation  one  to  another,  is  essential  to  a 
clear  understanding  of  many  physiologic  processes,  clinical  phenomena, 
and  surgical  procedures.  The  general  arrangement  of  the  primary  fissures 
and  convolutions  is  represented  in  Figs.  245  and  246. 

Fissures. — 

1.  The  Sylvian  fissure,  one  of  the  most  important  of  the  primary  fissures,  is 

found  on  the  side  of  the  cerebrum.  It  begins  at  the  base  and  extends 
upward,  outward,  and  backward  to  a  point  corresponding  to  the  emi- 
nence of  the  parietal  bone,  where  it  usually  terminates  in  a  more  or  less 
vertically  directed  branch,  the  epi-sylvian  branch.  Anteriorly  a  short 
branch  is  given  off  which  passes  upward  and  forward  into  the  frontal 
lobe  and  known  as  the  pre-sylvian;  a  horizontal  branch  is  known  as  the 
sub-sylvian.  The  Sylvian  fissure  is  the  first  to  appear  in  the  develop- 
ment of  the  fetal  brain,  becoming  visible  at  the  third  month.  In  the 
adult  it  is  deep  and  well  marked  and  divides  the  hemi-cerebrum  into  a 
frontal  and  a  temporo-sphenoidal  lobe. 

2.  The  Rolandic  or  central  fissure,  equally  important,  is  found  on  the  superior 

and  lateral  aspects  of  the  cerebrum.  It  runs  from  a  point  on  the  con- 
vexity of  the  hemisphere  near  the  median  line  transversely  outward  and 
downward  toward  the  fissure  of  Sylvius,  but  as  a  rule  does  not  pass  into 
it.  It  divides  the  frontal  from  the  parietal  lobe.  The  inclination  of 
the  central  fissure  is  such  as  to  form  with  the  longitudinal  fissure  an  angle 
of  about  70  degrees. 

3.  The  intra-parietal  fissure  arises  a  short  distance  behind  the  central  fissure. 

It  then  runs  upward,  backward,  and  downward  to  terminate  near  the 
posterior  extremity  of  the  parietal  lobe.  It  divides  the  parietal  lobe 
into  a  superior  and  an  inferior  portion. 

570 


THE  CEREBRUM 


571 


4.  The  parieto-occipital  fissure,  situated  on  the  mesial  surface  of  the  hemi- 
spheres, divides  the  latter  into  a  parietal  and  an  occipital  lobe.  It  begins 
as  a  deep  notch  on  the  surface  of  the  hemisphere,  and  is  then  continued 
downward  and  forward  until  it  enters  the  calcarine  fissure.     (Fig.  246.) 


SVPfRlf/HT/iAL    f. 


Fig.  245. — ^Fissures  and  Gyri  on  the  Lateral  Surface  of  the  Left  Hemi-cerebrum.- 
F.  Fissure.     G.  Gyrus.     R.  Ramus. — {Spitzka.) 


Fig.  246. — Fissures  and  Gyri  of  the  Mesial  Surface  of  the  Left  Hemi-cerebrum. 

—{Spitzka.) 


5.  The  calcarine  fissure  begins  on  the  posterior  extremity  of  the  mesial  surface 
of  the  occipital  lobe.  From  this  point  it  passes  downward  and  forward 
to  unite  with  the  parieto-occipital  fissure. 


572  TEXT-BOOK  OF  PHYSIOLOGY 

6.  The  para-central  fissure  begins  at  the  supero-mesial  border  of  the  hemis- 

phere. It  then  passes  downward  and  forward  for  a  variable  distance 
and  then  turns  upward  enveloping  a  lobule  known  as  the  para- 
central lobule. 

7.  The  super-callosal  fissure  extends  from  a  point  just  anterior  to  the  para- 

central lobule  downward  and  forward  below  the  rostrum  of  the  corpus 
callosum. 

Secondary  fissures  of  more  or  less  importance  are  present  in  the  different 
lobes,  subdividing  the  surface  into  convolutions:  e.g.,  in  the  frontal  lobe  are 
found  the  pre-central,  the  super-frontal,  medi-frontal  and  sub-frontal  fissures; 
in  the  temporal  lobe  the  super-temporal  and  medi-temporal  fissures. 

Convolutions. — The  convolutions  or  gyri  are  the  portions  of  the  cere- 
bral surface  comprised  between  the  fissures.  The  arrangement  of  the  sur- 
face is  such  that  only  the  more  superficial  portions  are  visible.  The  depth 
of  the  convolution,  the  portion  bordering  the  fissure,  is  concealed  from  view. 
Each  lobe  presents  a  series  of  such  convolutions  which  differ  considerably 
in  their  relative  physiologic  importance. 

The  Frontal  Lobe. — The  frontal  lobe  presents  on  its  convex  surface 
four  convolutions,  viz. :  the  anterior  or  pre-central  convolution,  and  the 
super-,  medi-,  and  sub-frontal  convolutions. 

1.  The  anterior  or  pre-central  convolution  or  gyrus  is  situated  just  in  front  of 

the  Rolandic  or  central  fissure,  with  which  it  corresponds  in  direction. 
It  is  continuous  above  with  the  super-frontal  and  below  with  the  sub- 
frontal  convolution. 

2.  The  super -frontal  convolution  or  gyrus  is  bounded  internally  by  the  longi- 

tudinal fissure  and  externally  by  the  super-frontal  fissure.  From  the 
upper  end  of  the  pre-central  convolution,  with  which  it  is  continuous, 
it  runs  forward  and  downward  to  the  anterior  extremity  of  the  frontal 
lobe,  where  it  turns  backward  and  rests  on  the  orbital  plate  of  the 
frontal  bone. 

3.  The  medi-frontal  convolution  or  gyrus  is  situated  on  the  side  of  the  lobe, 

between  the  super-frontal  fissure  above  and  the  medi-frontal  fissure 
below.     Its  general  direction  is  downward  and  forward. 

4.  The  sub-frontal  convolution  or  gyrus  winds  around  the  pre-sylvian  branch 

of  the  fissure  of  Sylvius  in  the  anterior  and  inferior  portion  of  the  frontal 

lobe.     It  is  continuous  posteriorly  with  the  lower  end  of  the  pre-central 

convolution. 

The  Parietal  Lobe. — The  parietal  lobe  presents  three  well-marked 

convolutions,  viz. :  the  posterior  or  post-central  convolution,  and  the  super- 

and  sub-parietal.     The  latter  is  again  subdivided  into  the  marginal  and  the 

angular  convolution. 

1.  The  posterior  or  post-central  convolution  or  gyrus  is  situated  just  behind  the 

Rolandic  or  central  fissure,  with  which  it  corresponds  in  direction. 
Above,  it  is  continuous  with  the  super-parietal  convolution;  below,  with 
the  marginal  and  the  pre-central  convolutions. 

2.  The  super-parietal  convolution  or  gyrus  is  bounded  internally  by  the  longi- 

tudinal fissure  and  externally  by  the  intra-parietal  fissure.  From  the 
upper  end  of  the  post-central  convolution,  with  which  it  is  connected, 
it  runs  downward  and  backward  as  far  as  the  parieto-occipital  fissure. 

3.  The  sub-parietal  convolution  or  gyrus  is  connected  anteriorly  with  the 


THE  CEREBRUM  573 

post-central    convolution.     Passing   backward,    it   winds   around   the 
superior  extremity  of  the  fissure  of  Sylvius,  in  which  situation  it  is  known 
as  the  supra-marginal  convolution.     Beyond  this  point  it  divides  into 
two  portions,  one  of  which  runs  forward  into  the  temporal  lobe  above 
the  super-temporal  fissure,  while  the  other  runs  downward  and  back- 
ward, following  the  intra-parietal  fissure  to  its  termination.     At  this 
point  it  makes  a  sharp  bend  and  runs  forward  into  the  temporal  lobe 
just  beneath  the  super-temporal  fissure.     In  the  neighborhood  of  the 
bend  it  is  generally  known  as  the  angular  convolution  or  gyrus. 
The  Temporo-sphenoidal  Lobe. — The  temporo-sphenoidal  lobe  presents 
on  its  external  surface  three  well-marked  convolutions:  viz.,  the  super-,  the 
medi-,  and  the  sub-temporal,  separated  by  the  super-  and  medi-temporal 
fissures.     These  three  convolutions  are  in  a  general  way  parallel  with  each 
other,  and  pursue  a  direction  from  before  backward  and  upward.     Ante- 
riorly, they  are  fused  together,  but  posteriorly  their  connections  are  some- 
what different.     The  super-temporal  is  continuous  behind  and  above  with 
the  supra-marginal  convolution,  and  behind  and  below  with  the  angular 
convolution  or  gyrus.     The  medi-temporal  blends  with  the  preceding  and 
with  the  middle  occipital.     The  sub-temporal  is  continuous  with  the  in- 
ferior occipital. 

The  Occipital  Lobe. — The  occipital  lobe  is  triangular  in  shape  and 
forms  the  posterior  apex  of  the  hemisphere.  Its  base  on  the  external 
surface  is  formed  by  an  imaginary  line  drawn  from  the  parieto-occipital 
fissure  to  the  pre-occipital  notch  on  the  inferior  and  lateral  border.  The 
external  surface  presents  three  convolutions — the  superior,  middle,  and  in- 
ferior occipital. 

The  inner  or  mesial  surface  of  the  hemisphere,  formed  in  part  by  the 
frontal,  the  parietal,  the  occipital,  and  the  temporal  lobes,  presents  several 
convolutions  of  much  physiologic  interest,  viz. : 

1.  The  callosal  convolution,  or  gyrus,  situated  between  the  super-callosal 

fissure  and  the  corpus  callosum.  From  its  origin  anteriorly  at  the  base 
of  the  brain  this  convolution  passes  backward,  gradually  increasing  in 
width  as  it  approaches  the  posterior  extremity  of  the  corpus  callosum. 
At  this  point  it  again  narrows  and  descends  between  the  calcarine  and 
hippocampal   fissures   to   blend   with   the   hippocampal   convolution. 

2.  The  hippocampal  gyrus,  formed  by  the  union  of  the  posterior  extremity 

of  the  callosal  convolution  and  the  sub-calcarine  convolution  is  situated 
just  below  the  dentate  or  hippocampal  fissure.  Anteriorly  it  becomes 
enlarged,  and  just  behind  the  apex  of  the  temporal  lobe  turns  backward 
and  inward  to  form  a  hook-shaped  eminence,  the  uncinate  gyrus  or 
uncus. 

The  limbic  lobe  is  the  name  given  to  an  area  of  the  brain  which 
includes,  among  other  structures,  the  callosal  convolution,  the  gyrus 
hippocampus,  and  the  uncus.  As  forming  a  part  of  this  general 
lobe  may  be  mentioned  the  dentate  fascia,  the  striae  and  peduncle  of 
the  corpus  callosum,  the  septum  lucidum,  the  fornix,  and  the  in- 
fra-callosal  gyrus. 

3.  The  sub-collateral  cmivolutimi  or  gyrus  is  bounded  by  the  collateral  fissure 

above,  and  its  inferior  border  extends  from  the  occipital  lobe  to  the 
anterior  pole  of  the  temporal  lobe. 


574 


TEXT-BOOK  OF  PHYSIOLOGY 


4.  The  quadrate  lobule,  or  precuneus,  a  square-shaped  convolution,  is  situated 

between  the  posterior  termination  of  the  para-central  fissure  and  the 
parieto-occipital  fissure.  It  blends  with  the  callosal  convolution,  on  the 
one  hand,  and  with  the  parietal  lobule  on  the  other. 

5.  The  cuneus,  a  triangular  or  wedge-shaped  convolution  or  lobule,  is  situated 

on  the  mesial  surface  of  the  occipital  lobe 
between  the  parieto-occipital  and  calca- 
rine  fissures. 

The  Insula  or  Island  of  Reil. — This 
anatomic  structure  consists  of  a  triangular- 
shaped  cluster  of  six  small  convolutions  situa- 
ted at  the  bifurcation  of  the  Sylvian  fissure 
and  concealed  from  view  by  the  convolutions 
bordering  it,  spoken  of  collectively  as  the  oper- 
culum. These  convolutions  are  connected 
with  the  frontal,  the  parietal,  and  the  tem- 
poral lobes. 

Structure  of  the  Gray  Matter  of  the 
Cortex. — The  gray  matter,  the  cortex  of  the 
cerebrum,  varies  from  two  to  four  millime- 
ters in  thickness.  When  examined  with  a  lens 
of  low  power,  it  presents  a  laminated  appear- 
ance, due  to  differences  in  color  and  arrange- 
ment of  its  constituent  elements.  With 
higher  magnification  the  cortex  is  seen  to  con- 
sist of  neuroglia  cells,  nerve-cells  with  special- 
ized dendrites  and  axons,  medullated  and 
non-medullated  nerve-fibers,  blood-vessels, 
connective  tissue,  etc.,  all  of  which  are  arranged 
and  interblended  in  a  most  intricate  man- 
ner. Notwithstanding  the  complexity  of  its 
structure,  modern  histologic  methods  have 
enabled  Cajal  to  divide  it  into  four  fairly  dis- 
tinct layers  or  zones,  from  without  inward,  as 
follows  (Fig,  247): 

I .   The  Molecular  Layer. — The  most  superficial 
portion  of  this  layer  consists  mainly  of 
neuroglia   or  glia  cells,  the  processes  of 
which  interlace  in  all  directions,  forming 
a   distinct  sheath  just  beneath  the  pia. 
The  deeper  portions  of   this  layer  con- 
tain   a    specialized   type  of  nerve-cells    (Cajal  cells),  of  which  there 
are  several  varieties.     These  cells  give  off  nerve-fibers   which   pur- 
sue   a    horizontal    direction    for   a   variable   distance,   but   in   their 
course  give  off  collateral  branches  which  ascend  to  the  outer  surface 
of  the  layer.     Among  these  structures  are  to  be  found,   also,   den- 
dritic processes  of  cells  situated  in  the  subjacent  layer.     The  terminal 
filaments  of  medullated  nerve-fibers  coming  from  nerve-cells  in  lower 
regions  of  the  encephalo-spinal  axis  are  also  present,  but  for  the  most 
part  they  pursue  a  tangential  direction. 


Fig.  247.— Section  of  the 
Cerebral  Cortex  (Motor  Area) 
OF  Child,  Stained  by  Golgi's 
Silver  Method.  A.  Layer  of 
neuroglia  cells.  B.  Layer  of  small 
pyramidal  ganglion  cells.  C 
Layer  of  large  pyramidal  cells.  D. 
Layer  of  irregular  smaller  cells. — 
{Piersol.) 


THE  CEREBRUM  575 

2.  The  Layer  of  Small  Pyramidal  Cells. — This  layer   consists   mainly   of 

nerve-cells,  the  majority  of  which  are  pyramidal  in  shape  and  of  small 
size.  Other  cells,  however,  are  present,  which  present  a  variety  of 
shapes,  for  which  reason  the  layer  was  at  one  time  termed  the  ambiguous 
layer.  The  apical  process  of  the  pyramidal  cells  is  broad  at  the  base, 
but  narrows  rapidly  as  it  passes  upward.  It  frequently  divides  into  sev- 
eral branches,  each  of  which  develops  club-shaped  processes  or  gem- 
mules,  which  give  to  it  a  feathery  appearance.  Dendrites  are  also  given 
off  from  the  sides  and  base  of  the  cell-body.  From  the  base  a  single 
axon  descends,  which  ultimately  becomes  the  axis-cylinder  of  a  medu- 
lated  nerve. 

3.  The  Layer  of  Large  Pyramidal  Cells. — The  nerve-cells  of  this  layer,  as  the 

name  implies,  are  also  pyramidal  in  shape,  but  of  large  size.  Each 
cell  presents  the  same  features  as  the  cells  of  the  preceding  layer,  with 
the  exception  that  the  apical  process  is  larger,  better  developed,  and 
branches  more  freely.  All  the  dendrites  are  extensively  provided  with 
gemmules.  The  axon  is  well  developed,  sharply  defined,  and  smooth. 
After  giving  off  collateral  branches,  the  axon  descends  into  the  cere- 
brum and  becomes  a  medullated  nerve-fiber. 

4.  The  Layer  of  Polymorphous  Cells. — In  this  layer  the  nerve-cells  present  a 

variety  of  forms:  e.g.,  spindle,  polygonal,  pyramidal,  etc.     The  spindle 
form  is  the  most  common.     From  either  end  of  the  spindle  a  large 
dendrite  emerges,  soon  branches,  and  becomes  gemmulated.     The  axon 
is  well  defined  and  it  soon  descends  into  the  white  matter. 
The  Ntimber  of  Cortical  Cells. — Attempts  have  been  made  by  various 
histologists  to  estimate  the  total  number  of  functional  nerve-cells  in  the  cere- 
bral cortex  of  man.     Though  the  estimates  are  widely  different,  the  lowest 
presents  numbers  which   are  beyond   comprehension.     Thus,   Meynert's 
estimate  is  612  milHons;  Donaldson's  estimate  for  the  entire  brain  is  12,000 
millions;  and  Thompson's  9283  millions. 

Structure  of  the  White  Matter. — The  white  matter  of  the  cerebrum 
consists  of  medullated  nerv'e-fibers  which,  though  intricately  arranged,  may 
be  divided  into  three  systems:  viz.,  the  commissural,  the  association,  and  the 
projection. 

1.  The  commissural  system. — The  fibers  which  compose  this  system  unite 

corresponding  areas  of  the  cortex  of  each  hemisphere.  The  fibers 
from  the  frontal,  parietal,  and  occipital  lobes  cross  in  the  median  line 
and  form  a  band  of  transversely  arranged  fibers,  the  corpus  callosum. 
The  fibers  which  unite  the  corresponding  areas  of  the  temporo-sphe- 
noidal  lobes  cross  in  the  anterior  commissure.  All  the  commissural 
fibers  are  the  axons  of  nerve-cells  in  the  cortex,  the  terminals  of  which 
are  to  be  found  in  the  cortex  of  the  opposite  side. 

2.  The  association  system. — The  fibers  which  compose  this  system  unite 

neighboring  as  well  as  distant  parts  of  the  same  hemisphere,  and  may 
therefore  be  divided  into  long  and  short  fibers.  They  associate  the  in- 
excitable  or  association  areas  with  the  excitable  or  projection  areas, 

3.  The  projection  system. — The  fibers  composing  this  system  unite  certain 

areas  of  the  cortex  of  the  cerebrum  with  the  basal  ganglia,  the  pons, 
medulla  oblongata,  and  spinal  cord.  They  may  be  divided  into:  (i) 
afferent  fibers  which  have  their  origin  in  the  lower  nerve-centers  at  dif- 


576  TEXT-BOOK  OF  PHYSIOLOGY 

ferent  levels  and  thence  pass  to  the  cortex;  and  (2)  efferent  fibers  which 
have  their  origin  in  the  cortex  and  thence  pass  to  the  lower  nerve-centers, 
terminating  at  different  levels.     The  former  are  also  termed  the  cortico- 
afferent  or  cortico-petal;  the  latter,  cortico-efferent  or  cortico-fugal. 
The  afferent  fibers,  the  so-called  sensor  tract,  which  transmit  nerve  im- 
pulses coming  from  the  general  periphery  and  the  sense-organs,  pass  through 
the  tegmentum  as  the  mesial  and  lateral  fillets,  and  thence  to  the  cortex 
directly  by  way  of  the  internal  capsule,  or  indirectly  through  the  intermedia- 
tion of  the  thalamic  and  subthalamic  nuclei.     (See Fig.  242,  page  562.)     The 
distribution  of  these  fibers  to  the  various  areas  of  the  cortex  will  be  stated  in 
subsequent  paragraphs. 

The  efferent  fibers  of  the  so-called  motor  tract  which  transmit  motor  or 
volitional  nerve  impulses  from  the  cortex  to  the  pons,  medulla,  and  spinal 
cord,  emerge  from  the  layer  of  pyramidal  cells  of  the  gray  matter  of  the  an- 
terior or  the  pre-central  convolution,  the  para-central  lobule,  and  immedi- 
ately adjacent  areas.  From  this  origin  the  axons  descend  through  the 
white  matter  of  the  corona  radiata,  converging  toward  the  internal  cap- 
sule, into  and  through  which  they  pass,  occupying  the  anterior  two-thirds 
of  the  posterior  limb  or  segment.  Beyond  the  capsule  they  continue  to 
descend,  occupying  the  middle  three-fifths  of  the  pes  or  crusta  of  the  crus 
cerebri,  the  ventral  portion  of  the  pons,  and  eventually  the  anterior 
pyramid  of  the  medulla  oblongata.  At  this  point  the  tract  divides  into 
two  portions,  viz. : 

1.  A  large  portion,  containing  from  ninety-one  to  ninety-seven  per  cent,  of 

the  fibers,  which  decussates  at  the  lower  border  of  the  medulla  and 
passes  down  the  lateral  column  of  the  cord,  constituting  the  crossed 
Pyramidal  tract. 

2.  A  small  portion,  containing  from  three  to  nine  per  cent,  of  the  fibers, 

wJiich  does  not  decussate  at  the  medulla,  but  passes  down  the  inner  side 

of  the  anterior  column  of  the  same  side,  constituting  the  direct  pyramidal 

tract  or  column  of  Tiirck. 

After  passing  through  the  internal  capsule,  and  as  it  descends  through 

the  crus,  pons,  and  medulla,  the  cortico-efferent  tract  gives  off  a  number  of 

fibers  which  cross  the  median  line  and  arborize  around  the  nerve-cells  of 

the  gray  matter  beneath  the  aqueduct  of  Sylvius  (the  nuclei  of  origin  of  the 

third  and  fourth  cranial  nerves),  and  around  the  nerve-cells  in  the  gray 

matter  beneath  the  floor  of  the  fourth  ventricle  (the  nuclei  of  origin  of  the 

remainder  of  the  motor  cranial  nerves).     The  remaining  fibers  go  to  form 

the  crossed  and  direct  pyramidal  tracts  and  arborize  around  the  cells  in  the 

anterior  horn  of  the  gray  matter  of  the  opposite  side  of  the  cord  at  successive 

levels.     B^  this  means  the  cortex  is  brought  into  anatomic  and  physiologic 

relation  with  the  general  musculature  of  the  body  through  the  various  cranial 

and  spinal  motor  nerves.     (See  Fig.  234,  page  549.) 

The  fronto-cerebellar  and  the  occipito-temporo-cerebellar  tracts  are  also 
efferent  tracts  and  parts  of  the  projection  system.  The  fronto-cerebellar, 
originating  in  the  nerve-cells  of  the  cortex  of  the  frontal  lobe,  passes  down  to 
and  through  the  internal  capsule,  occupying  the  anterior  one-third  of  the 
anterior  segment.  It  then  descends  along  the  inner  side  of  the  crus  cerebri 
to  the  pons,  where  its  fibers  arborize  around  the  cells  of  the  nucleus  pontis. 
Through  the  intermediation  of  these  cells  this  tract  is  brought  into  relation 


THE  CEREBRUM  577 

with  the  cerebellum  of  the  same  but  chiefly  of  the  opposite  side.  The 
occipito-temporal  tract,  originating  in  the  cells  of  the  cortex  of  both  the 
occipital  and  temporal  lobes,  passes  downward  and  inward  toward  the 
lenticular  nucleus,  beneath  which  it  passes  to  enter  the  outer  one-fifth  of  the 
cnista.  It  then  enters  the  pons,  and  through  the  nucleus  pontis  also  comes 
into  relation  with  the  cerebellum  of  both  sides.     (See  Fig.  242,  page  562,) 

THE  FUNCTIONS  OF  THE  CEREBRUM 

The  functions  of  the  cerebrum  comprehend,  in  man  at  least,  all  that 
pertains  to  sensation,  cognition,  feeling,  and  volition.  All  subjective  experi- 
ences, which  in  their  totality  constitute  mind,  are  dependent  on  and  asso- 
ciated with  the  anatomic  integrity  and  the  physiologic  activity  of  the  cere- 
brum and  its  related  sense-organs,  the  eye,  ear,  nose,  tongue,  etc. 

From  an  examination  of  the  anatomic  development  of  the  brain  in  different 
classes  of  animals,  in  different  men  and  races  of  men,  and  from  a  study  of  the 
pathologic  lesions  and  the  results  of  experimental  lesions  of  the  brain,  evi- 
dence has  been  obtained  which  reveals  in  a  striking  manner  the  intimate 
connection  of  the  cerebrum  and  all  phases  of  mental  activity. 

1.  Comparative  anatornic  investigations  show  that  there  is  a  general  connec- 

tion between  the  size  of  the  brain,  its  texture,  the  depth  and  number  of 
convolutions,  and  the  exhibition  of  mental  power.  Throughout  the 
entire  animal  series  an  increase  in  intelligence  goes  hand  in  hand  with 
an  increase  in  the  development  of  the  brain.  In  man  there  is  an  enor- 
mous increase  in  size  over  that  of  the  highest  animals,  the  anthropoid 
apes.  The  most  cultivated  races  of  men  have  the  greatest  cranial 
capacity,  that  of  the  educated  European  or  American  being  approxi- 
mately 92.1  cubic  inches  (1835  c.c);  w^hile  that  of  the  Australian  is  but 

81.7  cubic  inches  (1628  c.c).  Men  distinguished  for  great  mental 
power  usually  have  large  and  well-developed  brains;  e.g.,  that  of  Cuvier 
weighed  64.4  ounces  (1830  grams);  that  of  Abercrombie,  63  ounces 
(1786  grams).  A  large  intelligence,  however,  is  not  incompatible  with 
a  much  smaller  brain  weight;  thus,  the  brain  of  Helmholtz  weighed  but 

50.8  ounces  (1440  grams);  that  of  Leidy,  49.9  ounces  (141 5  grams); 
that  of  Liebig,  47.7  ounces  (1352  grams).  The  average  brain 
weight  of  96  distinguished  men  has  been  found  to  be  51.9  ounces 
(1473  grams)  (Spitzka). 

2.  Pathologic  lesions  and  mechanic  injuries  which  disorganize  the  cerebrum 

are  at  once  followed  by  a  disturbance  or  an  entire  suspension  of  mental 
activity.  Concussion  of  the  brain  or  sudden  compression  from  a  hem- 
orrhage destroys  consciousness.  Physical  and  chemic  alterations  of 
the  gray  matter  of  the  cerebrum  have  been  shown  to  coexist  with  insan- 
ity, loss  of  memory,  loss  of  articulate  speech,  etc.  Congenital  defects  of 
organization  are  accompanied  by  a  deficiency  in  mental  capacity  and 
the  higher  instincts.  Under  such  circumstances  no  great  advance  in 
brain  development  is  possible  and  the  intelligence  remains  at  a  low 
level.  In  congenital  idiocy  the  brain  is  small,  imperfectly  developed, 
and  wanting  in  proper  chemic  composition. 

3.  Experimental  lesions  of  the  brain  in  lower  animals  are  attended  by  results 

similar  to  those  observed  in  disease  or  after  injury  in  man.     Removal 
37 


578  TEXT-BOOK  OF  PHYSIOLOGY 

of  the  cerebrum  in  the  pigeon  completely  aboHshes  intelligence  and 
destroys  the  capability  of  performing  volitional  movements.  The 
pigeon  remains  in  a  state  of  profound  stupor,  though  retaining  the  cap- 
ability of  executing  reflex  or  instinctive  movements.  It  can  temporarily 
be  aroused  by  loud  noises,  light  placed  before  the  eyes,  pinching  of  the 
toes,  etc.,  but  it  soon  relapses  into  a  condition  pf  quietude.  Coincident 
with  the  destruction  of  the  cerebrum  there  occurs  a  loss  of  memory, 
reason,  and  judgment,  and  the  animal  fails  to  associate  the  impressions 
with  any  previous  train  of  ideas.  The  higher  the  animal  in  the  scale  of 
development,  the  more  striking  is  the  loss  of  mentality  after  removal 
of  the  cerebrum. 
4.  Experimental  interference  with  the  blood-supply  to  the  cerebrum  is  followed 
by  a  diminished  or  complete  cessation  of  its  activities.  There  is  per- 
haps no  organ  of  the  body  that  is  so  directly  dependent  upon  its  blood- 
supply  for  the  continuance  of  its  activities  as  the  cerebrum.  The 
supply  of  blood  is  furnished  by  four  large  blood-vessels:  viz.,  the  two 
carotid  and  the  two  vertebral  arteries.  These  vessels,  after  entering 
the  cavity  of  the  skull,  give  off  branches  which  unite  to  form  the  "circle 
of  Willis."  From  this  circle,  large  branches  are  given  off  which  enter 
the  cerebrum  and  distribute  blood  to  all  its  parts.  After  passing  through 
the  capillaries  the  blood,  greatly  altered  in  chemic  composition,  is 
returned  by  large  veins.  The  large  volume  of  blood  that  is  present  in 
the  brain  and  the  marked  changes  in  composition  that  it  undergoes  while 
passing  through  the  brain  indicate  a  very  active  and  complex  metab- 
olism in  this  organ.  By  means  of  the  anatomic  arrangement  of  the 
blood-vessels  at  the  base  of  the  brain,  the  blood-supply  is  equalized. 
It  also  explains  why,  when  one,  or  even  two,  of  the  four  large  vessels 
are  occluded  by  pathologic  deposits  or  surgical  procedures,  brain 
activity  continues,  though  perhaps  diminished  in  degree.  Occlusion 
of  all  four  vessels,  however,  is  at  once  followed  by  a  complete  abolition 
of  all  forms  of  cerebral  activity.  An  experiment  performed  by  Brown- 
Sdquard  illustrates  the  dependence  of  cerebral  activity  on  the  blood- 
supply.  A  dog  was  beheaded  at  the  junction  of  the  neck  and  chest. 
After  a  period  of  ten  minutes  all  evidences  of  life  had  entirely  ceased. 
Four  tubes  connected  with  a  reservoir  of  warm  defibrinated  blood 
were  then  connected  with  the  four  arteries  of  the  head.  By  means  of  a 
pumping  apparatus  imitating  the  action  of  the  heart  the  blood  was 
driven  into  and  through  the  brain.  After  a  few  minutes  cerebral  activ- 
ity returned,  as  shown  by  contractions  of  the  muscles  of  the  face  and 
eyes.  The  character  of  the  contractions  were  such  as  to  convey  the 
idea  that  they  were  directed  by  the  will.  These  vital  manifestations 
continued  for  a  period  of  fifteen  minutes,  when  on  the  cessation  of  the 
artificial  circulation  they  disappeared,  and  the  head  exhibited  once 
more  the  usual  phenomena  observed  in  dying:  viz.,  contraction  and 
then  dilatation  of  the  pupils  and  convulsive  movements  of  the  muscles 
of  the  face. 
The  Localization  of  Functions  in  the  Cerebrum. — By  the  term, 
localization  of  functions,  is  meant  the  assignment  of  definite  physiologic 
functions  to  definite  anatomic  areas  of  the  cerebral  cortex.  From  experi- 
ments made  on  the  brains  of  animals,  by  the  observation  and  association  of 


THE  CEREBRUM  579 

clinical  symptoms  with  pathologic  lesions  of  the  central  nerve  system,  and 
from  observation  of  the  developmental  stages  of  the  embryonic  brain,  it  has 
been  established  in  recent  years: 

1.  That  the  general  and  special  sense-organs  of  the  body  are  associated 

through  afferent  nerve-tracts  with  definite  though  perhaps  not  sharply 
delimited  areas  of  the  cerebral  cortex;  and — 

2.  That  certain  areas  of  the  cortex  are  associated  through  efferent  nerve- 

tracts  with  special  groups  of  skeletal  or  voluntary  muscles. 

Experimental  excitation  of  a  cortical  area  associated  with  a  sense-organ  is 
undoubtedly  attended  by  the  production  of  a  sensation  at  least  similar  to 
that  produced  by  peripheral  excitation  of  the  sense-organ  itself;  destruction 
of  the  area  is  followed  by  an  aboHtion  of  all  the  sensations  associated  with  the 
sense-organ.     For  these  reasons  such  areas  are  termed  sensor. 

Experimental  excitation  of  a  cortical  area  associated  with  a  group  of  skele- 
tal muscles  is  attended  by  their  contraction ;  destruction  of  the  area  is  followed 
by  their  relaxation  or  paralysis.  For  these  reasons  such  areas  are  termed 
motor. 

Since  the  sense-organs  are  remote  from  the  brain  and  the  impressions 
made  upon  them  by  the  objective  world  can  be  utilized  in  the  mental  life 
only  after  they  have  been  reproduced  in  the  cortical  areas,  it  may  be  said  that 
each  sense-organ  has  its  special  area  in  the  cortex  by  which  it  is  represented, 
or,  in  other  words,  each  sense-organ  has  a  cortical  area  of  representation. 

Since  the  muscles  are  remote  from  the  brain  and  since  they  contract  in 
response  to  the  discharge  of  nerve  impulses  from  the  cells  of  the  cortical  motor 
areas,  it  may  be  said  that  the  mental  activities  associated  with  the  motor 
areas  are  represented  by  the  contractions  of  the  muscles;  in  other  words, 
that  the  cortical  motor  areas  have  areas  of  representation  in  the  general 
skeletal  musculature.  It  is  usually  stated,  however,  in  the  reverse  way:  viz., 
that  the  muscle  movements  have  areas  of  representation  in  the  cortex. 

The  cortex  of  the  cerebrum  may  therefore  be  compared  to  a  mosaic 
made  up,  partially  at  least,  of  sensor  and  motor  areas  which  respectively 
represent  sense-organs  and  motor  organs,  and  which  bear  a  definite  anatomic 
and  physiologic  relation  one  to  the  other.  Their  cooperation  is  essential  to 
the  normal  performance  of  many  forms  of  cerebral  activity. 

A  knowledge  of  the  situation  of  these  areas,  the  order  of  their  develop- 
ment, the  effects  that  arise  from  their  stimulation  or  follow  their  destruction, 
are  matters  of  the  highest  importance  in  the  study  of  cerebral  activity  and 
indispensable  to  the  physician  in  the  localization  of  lesions  which  manifest 
themselves  in  perversions  or  abolition  of  sensations  and  in  convulsive  seizures 
or  paralyses. 

The  Sensor  Areas. — The  sensor  areas  which  should  theoretically  be 
present  in  the  cortex  are  primarily  those  which  receive  and  translate  into  con- 
scious sensations  ner\'e  impulses,  developed  by  changes  going  on  in  the  body 
itself;  and  secondarily  those  which  receive  and  translate  into  conscious  sensa- 
tions the  nerve  impulses  developed  in  the  special  sense-organs  by  the  impact  of 
the  external  or  objective  world.  In  the  former  areas,  are  received  the  nerve 
impulses  that  come  from  the  mucous  membranes,  muscles,  joints,  viscera,  etc., 
and  give  rise  to  muscle,  and  visceral  sensations.  In  the  latter  areas  are 
received  the  nerve  impulses  that  come  from  the  sense-organs  and  give  rise 
to  cutaneous,  gustatory,  olfactory,  auditory,  and  visual  sensations.     A  num- 


58o  TEXT-BOOK  OF  PHYSIOLOGY 

ber  of  such  sense  areas  may  be  predicated:  e.g.,  areas  of  cutaneous  and  muscle 
sensibility,  of  gustatory,  olfactory,  auditory,  and  visual  sensibility. 

The  Motor  Areas. — The  motor  areas  which  should  theoretically  be 
present  in  the  cortex  are  those  which  in  consequence  of  the  discharge  of  nerve 
impulses  excite  contraction  of  special  groups  of  muscles  and  which,  from 
their  coordinate  and  purposive  character,  are  conventionally  termed  voli- 
tional. Five  such  general  motor  areas  may  be  predicated:  e.g.,  one  for  the 
muscles  of  the  head  and  eyes,  one  for  the  muscles  of  the  face  and  associated 
organs,  and  others  for  the  muscles  of  the  arm,  leg,  and  trunk.  They  are 
usually  designated  as  head  and  eye,  face,  arm,  leg,  and  trunk  motor  areas. 

The  existence  and  anatomic  location  of  these  areas  in  the  cortex  of 
animals  have  been  determined  by  the  employment  of  two  methods  of  ex- 
perimentation: viz.,  stimulation  and  destruction  or  extirpation;  the  first 
by  means  of  the  rapidly  repeated  induced  electric  currents,  the  seqond  by 
the  electric  cautery  and  the  knife. 

If  the  stimulation  of  a  given  area  is  attended  by  phenomena  which  indi- 
cate that  the  animal  is  experiencing  sensation,  and  its  destruction  by  a  loss 
of  this  capability  or  the  loss  of  a  special  sense,  it  is  assumed  that  the  area  is 
sensor  in  function — is  an  area  of  special  sense.  If  the  stimulation  or  excitation 
of  any  given  area  is  followed  by  contraction,  and  its  destruction  by  paralysis 
of  muscles,  it  is  assumed  that  the  area  is  motor  in  function — is  an  area  of 
motion. 

The  animals  generally  employed  for  experiments  of  this  character  are 
dogs  and  monkeys,  though  other  animals  have  frequently  been  employed  by 
different  investigators.  Of  all  animals,  the  monkey  is  the  most  frequently 
selected,  as  the  configuration  of  the  brain  in  its  general  outlines  more  closely 
resembles  that  of  man  than  does  the  brain  of  any  other  animal.  The  results 
therefore  which  are  obtained,  there  is  every  reason  to  believe,  are  the  results, 
in  their  general  outlines,  that  would  follow  stimulation  of  the  human  brain 
if  this  were  possible  under  the  same  conditions.  Indeed,  the  clinical  symp- 
toms which  arise  during  the  development  of  pathologic  processes,  and  the 
phenomena  which  occur  during  surgical  procedures  for  the  removal  of 
growths  and  pathologic  cortical  areas,  justify  the  conclusion  that  the  chart 
of  the  sensor  and  motor  areas  of  the  monkey  brain  may  be  transferred  to  the 
human  brain  without  introducing  any  serious  errors. 

The  Sensor  and  Motor  Areas  of  the  Monkey  Brain.— From  experi- 
ments made  on  the  brains  of  monkeys,  Ferrier,  Schafer,  Horsley,  and  many 
others  have  mapped  out,  though  not  with  a  high  degree  of  definiteness  and 
certainty,  a  number  of  areas,  stimulation  of  which  gives  rise  to  sensation, 
while  their  destruction  is  followed  by  a  loss  of  sensation  on  the  opposite 
side  of  the  body.  Collectively  these  areas  are  known  as  sensor  areas  and 
are  as  follows : 

1.  A  tactile  area  or  an  area  for  tactile  sensibility. 

2.  An  olfactory  and  a  gustatory  area  or  areas  for  olfactory  and  gustatory 

sensibility. 

3.  An  auditory  area  or  an  area  for  auditory  sensibility. 

4.  A  visual  area  or  an  area  for  visual  sensibility. 

These  same  investigators  have  mapped  out  a  number  of  other  areas,  stimu- 
lation of  which  gives  rise  to  contraction  of  muscles  on  the  opposite  side  of 
the  body  producing  movements  which  are  so  coordinate  and  purposive  in 


THE  CEREBRUM 


581 


character  that  they  may  be  regarded  as  identical  with  those  produced  by 
voHtional  effort,  while  destruction  of  these  areas  is  followed  by  a  loss  of 
motion  or  paralysis  of  these  muscles.  Collectively  these  areas  are  known  as 
motor  areas  and  are  as  follows : 

1.  A  head  and  eye  area. 

2.  A  face  area. 

3.  An  arm  area. 

4.  A  trunk  area. 

5.  A  leg  area. 

The  location  of  these  areas  will  be  apparent  from  an  examination  of  Figs. 
248  and  249. 


Fig.  248.- 


-DlAGRAM    OF   THE   MOTOR  AND    SeNSOR   AREAS  ON  THE   LATERAL   SURFACE   OF 

THE  Monkey  Brain. — {After  Horsley  and  Schdfer.) 


Motor  Reactions  Following  Electric  Stimulation  of  Sensor  Areas.— 

Electric  stimulation  of  the  sensor  areas  is  attended  by  certain  motor  reac- 
tions which  vary  in  accordance  with  the  area  stimulated.  Thus,  when  the 
electrodes  are  applied  to  different  portions  of  the  occipital  lobe  the  eyeballs 
are  conjugately  turned  upward,  downward,  or  laterally  and  to  the  opposite 
side;  when  placed  on  the  upper  portion  of  the  superior  temporal  convolu- 
tion, the  ear  is  pricked  up  or  retracted,  the  head  is  turned  to  the  opposite 
side  and  the  pupils  are  dilated;  when  placed  on  the  hippocampal  convolu- 
tion, there  is  movement  of  torsion  of  the  nostril  and  lips  of  the  same  side. 
It  was  then  assumed  that  the  movements  were  reflex  in  character  inasmuch 
as  they  resembled  the  movements  caused  by  strong  impressions  made  on  the 
sense-organs,  and  due  to  the  development  of  sensations  by  the  electric  cur- 
rents and  not  an  evidence  that  the  area  in  question  is  a  motor  area  in  the 
sense  that  this  term  is  applied  to  the  area  along  the  Rolandic  fissure  especially 
as  their  destruction  is  not  followed  by  paralysis  of  any  of  the  corresponding 
muscles.  The  view  that  these  reactions  are  reflex  in  character  has  been 
substantiated  by  the  anatomic  fact  that  in  the  immediate  neighborhood  of 
each  sense  area,  though  not  overla])ping  it,  motor  cells  are  present  from 
which  efl'erent  nerve-fibers  pass  in  close  relation  to  the  afferent  libers,  toward 
and  to  lower  nerve-centers  and  presumably  to  the  centers  of  origin  of  related 


582  TEXT-BOOK  OF  PHYSIOLOGY 

cranial  nerves.  This  interpretation  is  supported  by  the  experiments  of  Schafer, 
which  showed  that  the  contraction  of  the  eye  muscles  which  followed  stimula- 
tion of  the  occipital  lobe  took  place  between  0.2  and  0.3  second  later  than  when 
the  frontal  lobe  was  stimulated;  and  that  as  the  motor  reaction  takes  place 
after  extirpation  of  the  frontal  region,  the  route  of  the  efferent  impulse  can- 
not be  to  and  through  the  frontal  lobe,  but  probably  through  some  lower 
center.  The  same  facts  hold  true  for  the  reactions  of  the  ear  muscles  follow- 
ing stimulation  of  the  temporal  lobe. 

f^  For  these  reasons  it  came  to  be  believed  that  each  sense  area  is  associated 
with  a  motor  area,  though  the  two  are  not  identical  but  separate  in  their 
distribution.  The  associated  motor  areas  assist  in  the  formation  of  a 
mechanism  by  which  reflex  movements  are  executed  when  sense  organs 
are  stimulated. 

The  view  that  the  cortex  of  the  cerebrum  can  be  divided  into  separate  and 
independent  though  physiologically  related  motor  and  sensor  areas  has,  how- 


FiG.  249. — Diagram  of  the  Motor  and  Sensor  Areas  on  the  Mesial  Surface  of 
THE  Monkey  Brain. — {AJter  Horsley  and  Schafer.) 

ever,  been  questioned  in  recent  years,  and  a  somewhat  different  interpreta- 
tion given  to  the  facts.  It  is  believed  by  many  physiologists  and  neurologists 
that  the  so-called  motor  and  sensor  areas  are  so  closely  related  that  it  is 
almost  impossible  to  distinguish  one  from  the  other  either  anatomically  or 
physiologically.  Thus  the  Rolandic  region  is  believed  to  be  both  motor  and 
sensor  in  function,  the  former,  however,  being  more  predominant  in  the  pre- 
central,  the  latter  in  the  post-central,  convolution.  As  these  two  functions 
are  so  intimately  blended  and  their  anatomic  substrata  so  diflficult  of  separa- 
tion, it  is  thought  the  term  sensori-motor  should  be  employed  as  more  descrip- 
tive and  more  in  accordance  with  the  facts  to  the  entire  Rolandic  region. 

This  view  has  been  strengthened  by  the  results  of  the  embryologic 
investigation  of  Flechsig,  which  show  that  different  nerve-tracts  become 
medullated  or  receive  their  myelin  investment  at  successively  later  periods  and 
that  the  tracts  which  first  become  myelinated  and  are  hence  first  functionally 
active,  belong  to  the  afferent  system.     Among  the  first  to  undergo  myelini- 


THE  CEREBRUM  583 

zation  are  three  tracts  numbered  by  Flechsig  i,  2  and  3,  which  arise  largely 
from  the  median  nucleus  of  the  thalamus  and  the  medial  lemniscus  and  pass 
to  the  anterior  and  posterior  convolutions,  to  the  para-central  lobule  and 
foot  of  the  superior  frontal  convolution,  and  to  the  foot  of  the  third  frontal 
convolution  respectively.  It  is  these  fibers  which  convey  nerve  impulses  to 
the  cortex  and  furnish  information  regarding  changes  taking  place  in  the 
body  itself  and  thus  lead  to  the  performance  of  muscle  movements.  This 
area  is  therefore  primarily  a  sensor  area,  an  area  for  body-feelings,  cutaneous, 
tactile,  muscle,  and  visceral,  and  secondarily  a  motor  area.  The  afferent 
fibers  to  this  region  become  myelinated  during  the  ninth  month  of  intra- 
uterine life,  the  efferent  fibers  from  it  become  myelinated  during  the  third 
month  of  extra-uterine  life. 

By  the  same  method  of  reasoning  the  gustatory,  olfactory,  auditory,  and 
visual  sense  areas  are  to  be  regarded  assensori-motor  in  character,  for  embryo- 
logic  investigations  show  that  subsequently  to  the  myelinization  of  the 
afferent  tracts  connecting  the  sense-organs  with  the  cortex,  efferent  nerve- 
tracts  arise  from  or  near  to  the  same  centers  and  undergo  myelinization.  In 
other  words,  these  areas  are  primarily  sensor  and  secondarily  motor,  and 
therefore  should  be  termed  sensori-motor.  In  Flechsig's  own  terminology 
each  corticopetal  or  afferent  tract  is  accompanied  by  a  corticofugal  or  efferent 
tract. 

The  view,  viz.:  the  coincidence  of  sensor  and  motor  areas  has  had  general 
acceptance  for  the  reason  that  it  seemed  more  in  accordance  with  the  facts 
than  the  earlier  view.  Nevertheless  there  were  many  facts  both  of  a  physio- 
logic and  pathologic  character  which  were  difl&cult  to  harmonize  with  it,  and 
in  recent  years  the  accumulation  of  facts  and  the  weight  of  evidence  inclines 
toward  the  view  that  the  areas  are  anatomically  separate  and  distinct  though 
associated  functionally. 

The  Motor  Area  of  the  Chimpanzee  Brain. — In  a  series  of  experi- 
ments made  by  Sherrington  and  Griinbaum  on  the  brain  of  the  chimpanzee 
it  was  discovered  that  the  so-called  motor  area  was  not  so  widely  distributed 
as  in  the  monkeys  generally,  but  was  confined  almost  exclusively  to  the 
convolution  just  in  front  of  the  fissure  of  Rolando,  as  it  was  impossible  to 
obtain  any  movement  on  direct  stimulation  of  the  convolution  just  behind  it. 
All  points  on  the  surface  of  the  pre-central  convolution,  including  the  portion 
forming  the  wall  of  the  Rolandic  fissure  itself,  were  found  to  be  excitable 
and  productive  of  movement  when  stimulated.  The  sequence  of  representa- 
tion from  below  upward  is  similar  to  that  obsen-ed  in  the  monkey.  One 
peculiarity,  however,  was  the  location  of  the  area  for  conjugate  de\'iation  of 
the  eyeballs  to  the  opposite  side.  This  is  situated  far  forward  in  the  middle 
and  inferior  frontal  convolutions,  and  separated  from  the  areas  in  the  pre- 
central  convolution  by  a  region  apparently  inexcitable.  These  facts  are  of 
great  interest  and  value  in  the  assignment  of  the  motor  areas  in  the  cortex 
of  the  human  brain,  as  in  its  development  and  configuration  the  chimpanzee 
brain  more  closely  resembles  the  human  brain  than  docs  the  monkey's. 

The  Localization  of  Sensor  and  Motor  Areas  in  the  Human  Brain. — 
The  observation  of  clinical  symptoms  and  their  interpretation  by  post-mortem 
findings,  the  phenomena  observed  during  surgical  procedures,  and  the  results 
of  embryologic  investigations,  point  to  the  conclusion  that  corresponding 
areas  both  for  sensations  and  movements  exist  in  the  cerebral  cortex  of  the 


S84 


TEXT-BOOK  OF  PHYSIOLOGY 


human  brain,  though  it  is  probable  that  their  locations  do  not  in  all  respects 
coincide  with  those  characteristic  of  the  monkey  or  even  the  ape  brain.  In 
the  following  diagrams  (Figs.  250  and  251),  the  sensor  and  motor  areas  are 
at  least  provisionally  located,  in  accordance  with  recent  observations.  They 
are  represented  as  limited  or  bounded  by  a  serrated  line  to  indicate,  as 
suggested  by  Mills,  that  they  are  not  sharply  delimited,  but  that  they  inter- 
fuse or  interdigitate  with  surrounding  regions. 

In  the  following  paragraphs  these  areas  are  considered  in  the  order  of 
their  development  and  physiologic  activities. 

The  Sensor  Areas. — The  sensor  areas  occupy  regions  corresponding  in 
a  general  way  with  those  of  the  monkey  brain. 

I.  The  Cutaneous  Area.—T]\t  area  of  cutaneous  or  tactile  sensibility  has  been 
assigned  to  the  post-central  convolution  on  the  lateral  aspect,  and  to  a 
portion  of  the  super-frontal  convolution  and  the  lower  half  of  the  para- 


CONCRLTE   CONCEPT 

Fig.  250. — The  Areas  and  Centers  of  the  Lateral  Aspect  of  the  Human  Hemicerebrum. — 

(C.  K.  Mills.) 

central  lobule  on  the  mesial  aspect  of  the  hemicerebrum.     It  has  been 
stated  by  Flechsig,  who  bases  his  statements  on  the  results  of  embryo- 
logic  investigations  as  to  the  course,  time  of  myelinization  and  termina- 
tion of  certain  afferent  tracts,  that  the  cutaneous  area  must  also  be 
assigned  though  perhaps  in  less  degree  to  the  pre-central  convolution 
as  well. 
Clinic  observations  and  post-mortem  findings,  together  with  the  results 
of  more  recent  experimental  investigations  make  it  extremely  probable  that 
the  cutaneous  area  is  confined  entirely  to  the  regions  posterior  to  the  central 
fissure  or  fissure  of  Rolando.     Dr.  Charles  K.  Mills,  whose  skill  in  interpret- 
ing the  phenomena  of  the  diseases  of  the  brain  is  well  known,  states  in  this 
connection,  that  "innumerable  cases  have  been  reported  of  lesions  of  the 
motor  (the  pre-central  convolution)  without  the  slightest  impairment  of  sen- 
sibility."    In  many  instances  portions  of  the  motor  cortex  of  the  human 
brain  have  been  excised  by  necessary  surgical  procedures.     In  these  cases 


THE  CEREBRUM 


585 


also,  careful  studies  have  failed  to  show  any  impairment  of  sensibility.  On 
the  other  hand,  destruction  of  the  post-central  convolution  in  monkeys  by 
the  electro-cautery  and  in  man  by  disease  has  invariably  led  to  a  loss  of  sen- 
sibility, hemianesthesia,  on  the  opposite  side  of  the  body  without  at  the 
same  time  causing  any  loss  of  motion.  The  location  and  extent  of  the  an- 
esthesia corresponds,  of  course,  with  the  location  and  extent  of  the  lesion  of 
the  cortex. 

Gushing  has  recently  recorded  the  results  of  stimulation  with  the  electrical 
current  of  the  post-central  convolution  in  two  conscious  patients.  The  local- 
ized stimulus  gives  rise  to  definite  sensations  which  were  likened  in  one 
case  to  a  sensation  of  numbness,  and  in  the  other  to  a  definite  tactual  im- 
pression. From  the  foregoing  facts  it  may  be  stated  in  a  general  way  that 
the  post-central  convolution  is  the  region  for  tactile  (light  touch)  and  asso- 
ciated forms  of  sensation  such  as  tactile  discrimination  and  localization  and 


Fig.  251. — The  Areas  and  Centers  of  the  Mesial  Aspect  of  the  Human  Hemicerebrum. — 

(C.  K.  Mills.) 


perhaps  of  the  finer  shades  of  temperature,  in  other  words  of  epi-critic 
sensibility. 

2.   TJie  Muscle  Sense  Area.—Th&  area  of  muscle  sensibility  has  been  assigned 
to  the  region  posterior  to  but  adjoining  the  post-central  convolution  and 
includes  the  anterior  part  of  the  super-parietal  and  sub-parietal  con- 
volutions and  perhaps  the  supra-marginal  convolution  on  the  lateral 
aspect  and  a  portion  of  the  callosal  convolution  on  the  mesial  aspect  of 
the  hemicerebrum. 
The  sensations  which  are  evoked  in  response  to  the  action  of  nerve 
impulses  coming  from  tendons,  muscles,  etc.,  are  those  of  passive  position 
and  the  direction  and  duration  of  movements  of  parts  of  the  body.     Clinic 
observations  and  post-mortem  findings  indicate  that  lesions  of  this  area  are 
followed  by  a  loss  of  the  muscle  sense.     Owing  to  the  close  juxtaposition 
of  the  areas  of  cutaneous  and  muscle  sensibility  it  is  unusual  to  find  a  loss 


586  TEXT-BOOK  OF  PHYSIOLOGY 

of  muscle  sense  without  impairment  of  the  tactile  sense,  not  infrequently 
they  coexist.  Surgical  removal  of  a  small  angioma  posterior  to  the  post- 
central convolution  and  about  the  junction  of  the  super-  and  sub-parietal 
convolutions  was  followed  by  a  loss  of  the  muscle  sense  in  the  opposite  hand 
and  forearm  without  any  disturbance  of  other  sensations  (Starr  and  McCosh). 
In  addition  to  sensations  of  passive  position  and  direction  of  movements, 
the  sensations  of  temperature  and  deep  pressure  are  also  associated  with 
the  physiologic  activities  of  this  region  of  the  parietal  lobe.  There  is  much 
obscurity  as  to  the  location,  however,  of  the  area  in  which  sensations  of  pain 
are  evoked.  These  two  areas  together  constitute  the  area  of  the  body-feelings 
or  the  someaesthetic  area. 

Subdivisions  of  the  Cutaneous  and  Muscle  Sense  Areas. — Clinic  observa- 
tions and  post-mortem  findings  also  warrant  the  deduction  that  the  general 
areas  of  cutaneous  and  muscle  sense  areas  are  physiologically  subdivided 
as  is  the  general  motor  area  (see  page  590)  into  areas  for  the  skin  and 
muscles  of  the  face,  arm,  trunk,  and  leg  which  occupy  respectively  areas 
that  adjoin  the  corresponding  subdivisions  of  the  motor  area  representing 
these  parts  of  the  body  (Mills).  On  the  mesial  surface  of  the  cerebrum  there 
are  sensor  areas  for  a  portion  of  the  leg,  anus,  genitalia  and  viscera. 

The  afferent  pathway  through  which  the  nerve  impulses,  developed  in 
the  sense-organs  of  the  skin,  tendons  and  muscles,  pass  to  the  cortical  areas 
and  evoke  the  characteristic  sensations  has  been  described  on  pages  544 
and  545. 

3.  The  Stereognostic  Area. — The  area  of  stereognostic  perception.     Stereog- 

nosis  is  the  recognition  of  an  object  when  placed  in  the  hands,  through 
its  form,  density,  temperature,  etc.     The  area  associated  with  stereog- 
nostic perception  has  been  assigned  to  a  portion  of  the  super-parietal 
convolution  and  to  the  precuneus. 
This  perception  depends  on  the  integrity  and  cooperation  of  the  tactile, 
the  pressure,  the  temperature  and  muscle  senses  as  well  as  the  power  of  dis- 
criminating points  in  contact  with  the  skin.     A  lesion  of  this  area  impairs  or 
destroys  the  power  of  recognition  of  objects  and  establishes  the  condition  of 
astereo gnosis.     This  judgment,  however,  would  also  be  impaired  or  abolished 
if  either  the  tactile  or  muscle  sense  were  impaired  or  abolished,  since  both  are 
necessary  factors  in  the  series  of  events  that  lead  to  the  formation  of  the 
judgment.     The  existence  of  such  a  center  has  been  made  highly  probable 
by  clinical  cases  in  which  astereognosis  existed  without  impairment  of  either 
the  cutaneous  or  muscle  sensibility.     Inasmuch  as  focal  lesions  of  the  parietal 
cortex  give  rise  to  localized  impairment  of  stereognostic  perception  as  well  as 
impairment  of  the  cutaneous  and  muscle  senses,  it  is  stated  that  the  area  is 
also  capable  of  subdivision  into  smaller  areas  for  the  face,  arm,  trunk  and 
leg  (Mills). 

4.  The  Gustatory  Area. — ^The  area  for  gustatory  sensibility  has  been  assigned 

to  the  sub-collateral  convolution  on  the  mesial  aspect  of  the  temporo- 
sphenoidal  lobe. 
Disease  processes  involving  this  area  give  rise  frequently  to  subjective 
sensations  of  taste.     Electric  stimulation  of  this  area  in  mammals  causes 
movements  of  the  lips,  tongue,  etc.,  which  are  usually  associated  with  sen- 
sations of  taste. 

The  aflferent  pathway  by  which  nerve  impulses,  developed  by  action  of 


THE  CEREBRUM  587 

organic  matter  in  solution  on  the  terminal  portions  of  the  gustatory  nerves, 
are  transmitted  to  the  cortical  area  is  considered  in  connection  with  the 
sense  of  taste. 

5.  The  Olfactory  Area. — The  area  for  olfactory  sensibility  has  been  assigned 

to  the  anterior  portion  of  the  hippo-campal  convolution  (the  uncinate 
region)  and  the  anterior  portion  of  the  callosal  convolution  or  gyrus 
fornicatus. 

Disease  processes  in  this  region  give  rise  frequently  to  subjective  sensa- 
tions of  odors  which  as  a  rule  are  of  an  unpleasant  character.  Destruction 
of  this  area  is  followed  by  a  loss  of  odor  sensations.  Electric  stimulation 
of  this  area  in  mammals  was  found  by  Ferrier  to  be  attended  with  a  peculiar 
torsion  of  the  nostril  and  lips  on  the  same  side  similar  to  the  reaction  brought 
about  by  the  application  of  an  odorous  substance  more  or  less  disagreeable 
to  the  nostril. 

The  afferent  pathway,  by  which  nerve  impulses  developed  in  the  ter- 
minal portion  of  the  olfactory  nerve  by  the  contact  of  odorous  particles  are 
transmitted  to  the  cortical  areas,  is  considered  in  connection  with  the 
olfactory  nerve. 

6.  The  Auditory  Area. — The  area  of  auditory  sensibility  has  been  assigned 

to  portions  of  the  temporal  lobe  and  may  be  divided  into  primary  and 
secondary  areas. 

The  primary  area  is  located   in  the  posterior  portion   of  the   super- 
temporal  convolution,  and  perhaps  the  posterior  portion  of  the  insula. 
The  secondary  areas  are  located  one  below  and  in  advance,  and  the  other 
below  and  some  what  behind  the  primary  area,  both  extending  into  the 
medi-temporal  convolution. 
Unilateral  destruction  of  the  primary  area  is  followed,  however,  only 
by  a  partial  loss  of  hearing  in  the  opposite  ear,  owing  to  partial  decussation 
of  the  auditory  nerve,  which,  however,  may  be  recovered  from,  after  a  time, 
owing   probably  to  a   compensatory  activity  of  the   insular   convolution. 
Bilateral  destruction  of  this  region  is  followed  by  complete  deafness.     The 
primary  area  is  connected  on  the  one  hand  with  the  basal  auditory  center 
(the  internal  geniculate  body)  by  the  auditory  radiation,  and  on  the  other 
hand  with  the  secondary  areas  by  association  fibers. 

In  the  first  of  these  areas  there  are  cells  in  which  the  sounds  of  objects 
are  registered  (object  hearing);  in  the  second  of  these  areas  there  are  cells 
in  which  the  sounds  of  words,  letters,  etc.,  are  registered  or  memorized. 
If  these  areas  are  destroyed  by  disease  the  condition  of  object-deafness  and 
word-deafness  is  established.  If  w^ord-deafness  alone  exists,  the  patient 
though  able  to  recognize  sounds  is  unable  to  understand  spoken  language  and 
is  in  the  condition  of  a  man  who  is  hearing  a  language  of  which  he  has  not 
the  slightest  idea.  The  same  holds  true  for  the  perception  of  sensations  of 
sound  produced  by  objects. 

The  aft'erent  pathway  by  which  nerve  impulses,  developed  in  the  terminal 
portions  of  the  auditory  nerve  by  the  impact  of  atmospheric  vibrations  are 
transmitted  to  the  auditory  area,  is  considered  in  connection  with  the  con- 
sideration of  the  auditory  nerve  and  tracts. 

In  the  temporal  lobe  there  are  other  areas,  some  of  which  are  more  or 
less  associated  with  the  auditory  nerve,  such  as  intonation,  equilibratory 
and  orientation  areas.     In  the   subtemporal  and  probably   in   the  medi- 


588  TEXT-BOOK  OF  PHYSIOLOGY 

temporal  convolution  as  well,  there  is  an  area  in  which,  it  is  believed  by 
some  clinicians,  words  are  associated  with  concrete  ideas  of  objects  recog- 
nized by  one  or  more  of  the  senses.  To  this  area  the  term  naming  area 
has  been  given.  It  is  connected  by  association  fibers  with  the  areas  for 
word  hearing  and  word  seeing. 

7.  The  Visual  Area. — The  area  for  visual  sensibility  has  been  assigned  to 
portions  of  the  occipital  and  parietal  lobes  and  may  be  divided  into 
primary  and  secondary  areas. 

The  primary  area  is  located  in  a  triangular-shaped  area  on  the  mesial 
surface  of  the  occipital  lobe,  which  includes  the  gray  matter  between 
the  parieto-occipital  and  the  calcarine  fissures   (the  cuneus),  and  in 
the  gray  matter  of  the  first  occipital  convolution  on  the  lateral  aspect 
of  the  occipital  lobe.     The  secondary  areas  are  located  partly  on  the 
lateral  aspect  of  the  occipital  lobe  and  partly  in  the  supra-marginal 
and  angular  convolutions  of  the  parietal  lobe. 
Focal  lesions  of  the  primary  area  on  one  side  are  followed  by  lateral 
homonymous  hemianopsia,  which,  however,  does  not  involve,  as  a  rule, 
the  fovea  or  macula.^      It  is,  therefore,  the  area  of  homonymous  half- retinal 
representation.     The  location  of  the  area  for  macular  or  central  vision  is 
uncertain.     Henschen  locates  it  in  the  anterior  part  of  the  area  near  the  ex- 
tremity of  the  calcarine  fissure,  and  asserts  that  in  each  area  both  maculae 
are  represented.     From  experiments  made  on  monkeys  Schafer  locates  it  in 
the  same  region. 

The  primary  area  is  connected,  on  the  one  hand,  with  the  basal  visual 
centers  (the  external  geniculate  body  and  the  thalamus)  by  the  optic  radiation 
and,  on  the  other  hand,  with  the  secondary  areas  by  association  libers. 

The  secondary  areas  on  the  lateral  aspect  of  the  occipital  lobe  are  rather 
extensive,  reaching  down  as  far  as  the  third  and  fourth  occipital  convolu- 
tions. Clinical  evidence  indicates  that  the  cortex  of  this  entire  area  is 
associated  with  the  registration  or  memorization  of  the  visual  sensations 
and  perceptions  of  objects,  though  it  may  be  subdivided  into  smaller  areas 
for  the  registration  of  the  visual  sensations  of  different  groups  of  objects 
such  as  geometric  and  architectonic  forms,  of  persons,  places  and  natural 
objects.  Diseased  processes  in  this  region  of  the  brain  may  result  in  the 
condition  known  as  object  blindness.  The  area  on  the  lateral  aspect  of  the 
parietal  lobe  (the  supramarginal  and  angular  convolutions)  are  associated 
with  the  memorization  of  the  visual  sensations  and  perceptions  of  words, 

'  In  a  consideration  of  this  subject  certain  facts  connected  with  visual  perception,  both  in 
physiologic  and  pathologic  conditions,  must  be  kept  in  mind.  Thus,  visual  sensation  may  arise 
from  stimulation  of  either  the  central  portion,  the  macula,  or  the  peripheral  portion  of  the  retina 
or  both.  In  the  first  instance  the  vision  is  termed  central  or  macular;  in  the  second  instance, 
peripheral  or  retinal.  Macular  vision  is  clear,  sharp,  and  distinct;  retinal  vision  progressively 
indistinct  from  the  center  toward  the  periphery.  Division  of  one  optic  tract  is  followed,  in  con- 
sequence of  the  partial  decussation  of  the  optic  nerve-fibers  at  the  chiasma,  by  a  loss  of  function 
in  the  outer  two-thirds  of  the  retina  of  the  same  side,  both  in  the  central  (macular)  as  well  as  in 
its  peripheral  portions,  and  the  inner  one-third  of  the  retina  of  the  opposite  side.  To  this  condition 
the  term  hemianopsia  has  been  applied.  As  a  result  of  this  want  of  functional  activity  of  these  reti- 
nal portions  on  the  side  of  the  lesion,  rays  of  hght  emanating  from  objects  situated  in  the  opposite 
side  of  the  field  of  vision  will  not  be  perceived  when  both  eyes  are  directed  to  the  fixation  point. 
To  this  "  bhndness  "  in  the  opposite  half  of  the  field  of  vision  the  name  hemianopsia  is  given.  In 
the  lesion  under  consideration  (division  of  one  optic  tract)  the  hemianopsia  is  bilateral,  and  as 
it  affects  the  corresponding  portions  associated  in  normal  vision  it  is  of  the  hotnonymous  variety. 
Division  of  the  right  optic  tract  is  followed  by  left  lateral  homonymous  hemianopsia,  indicative  of  the 
fact  that  objects  in  the  field  of  vision  to  the  left  of  the  binocular  fixation  point  are  invisible. 


THE  CEREBRUM  589 

letters,  numbers,  and  perhaps  objects.  If  the  visual  word  area  is  destroyed 
by  disease,  word  blindness  is  established,  and  the  patient  is  unable  to  under- 
stand written  or  printed  language  because  of  his  inability  to  revive  memory 
images  of  words.  Letter  and  number  blindness  may  or  may  not  be  present 
according  to  the  extent  of  the  lesion. 

All  the  special  sense  areas  may,  therefore,  be  said  to  consist  of  two  smaller 
areas,  a  sensor  and  a  psychic,  e.g.,  a  cutaneous  sensor  and  a  cutaneous  psychic, 
a  visual  sensor  and  a  visual  psychic,  etc. ;  the  former  is  for  the  development 
of  crude  sensation;  the  latter,  more  complex  in  character,  is  for  the  correla- 
tion and  formation  of  sensations  whereby  judgments  or  definite  conceptions 
are  formed  (Campbell). 

The  Sensori-motor  Areas. — It  will  be  observed  on  examination  of  dia- 
grams 250  and  251,  that  each  sense  area  has  closely  associated  with  it  a 
sense  motor  area,  each  of  which  is  to  be  regarded,  from  one  point  of  view  at 
least,  as  a  constituent  part  of  the  motor  area  of  the  cortex. 

On  a  previous  page  it  was  stated  that  electric  stimulation  of  sensor  areas 
is  attended  with  certain  motor  reactions  in  the  muscle  groups  associated  with 
the  activities  of  the  sense-organs,  reactions  which  resemble  those  caused  by 
strong  impressions  made  on  the  sense-organs  by  external  agents;  that  some 
investigators,  therefore,  believed  that  the  areas  in  question  were  motor  and 
sensor  in  function  and  that  their  anatomic  substrata  were  so  intermingled 
and  so  difficult  of  separation  that  the  term  sensori-motor  should  be  employed 
as  more  descriptive  and  more  in  accordance  with  the  facts;  but  that  the 
progress  of  experimental  work,  together  with  the  accumulation  of  facts  from 
the  clinical  and  pathologic  fields,  incline  to  the  view  that  the  groupings  of  the 
motor  and  sensor  cells  are-  anatomically  and  physiologically  separate  and 
distinct  though  related  functionally.  Corresponding  areas  are  now  believed 
to  be  present  in  the  human  brain.  For  this  reason  each  of  the  special  sense 
areas,  with  the  exception  of  the  cutaneous,  is  represented  in  Fig.  250,  as  asso- 
ciated with  a  motor  area,  viz.:  a  gustatory,  an  olfactory,  an  auditory  and 
a  visual  motor  area.  The  cutaneous  sensor  area  is,  however,  also  associated 
with  a  cutaneous  motor  area  in  the  posterior  portion  of  the  pre-central  con- 
volution. These  associated  motor  and  sensor  areas  are  here  represented, 
not  as  overlapping,  but  contiguous,  though  perhaps  interdigitating  with 
each  other.  Their  location  and  relations  are  apparent  from  an  examina- 
tion of  the  diagrams.  From  the  results  of  experimental  investigations  it  is 
generally  believed  the  sense  motor  areas  are  more  particularly  related  to 
sensori-motor  reflex  actions  rather  than  to  voluntary  actions  and  that  they 
constitute  the  eft'erent  element  of  a  reflex  arc,  since  destruction  of  the  area 
interferes  with  the  former  rather  than  the  latter. 

It  will  be  recalled  that  the  investigations  of  Flechsig  demonstrated  that 
from  these  areas  efferent  or  motor  nerves  pass  through  the  sense  tracts  or 
radiations  in  the  reverse  way,  to  the  mid-brain,  where  they  become  related 
to  the  nuclei  of  origin  of  the  motor  cranial  nerves  which  excite  to  action  those 
muscles  associated  with  the  sense. organs  in  their  adjustments  to  the  stimulus. 
Thus  electric  stimulation  of  the  gustatory  motor  area  gives  rise  to  movements 
of  the  lips  and  tongue;  of  the  olfactory  motor  area,  to  movements  of  the  nos- 
trils, etc.,  as  in  sniffing;  of  the  auditory  motor  area  to  movements  of  the  head 
and  eyes  to  the  opposite  side  and  to  pricking  of  the  ears;  of  the  visual  motor 
area  to  movements  of  the  eyeballs  and  head  in  different  directions,  in  accord- 


59° 


TEXT-BOOK  OF  PHYSIOLOGY 


ance  with  the  part  stimulated.  All  these  movements  are  similar  to  those 
which  follow  gustatory,  olfactory,  auditory,  and  visual  sensations,  evoked 
by  unexpected  stimulation  of  the  peripheral  sense-organs  themselves. 

The  Motor  Areas. — -The  motor  areas  especially  those  associated  with 
the  execution  of  volitional  movements  have  been  assigned  to  the  pre-central 
convolution,  the  contiguous  portions  of  the  base  of  the  medi-  and  sub-frontal 
convolutions  and  the  para-central  lobule. 

The  exclusion  of  the  post-central  convolution  from  the  motor  area,  to 
which  it  belongs  in  the  monkey  brain,  is  in  accordance  with  the  embryologic 
researches  of  Flechsig,  which  indicate  that  the  efferent  fibers  which  compose 
the  pyramidal  tract  come  from  the  region  anterior  to  the  central  fissure,  and 
with  the  experiments  of  Sherrington  and  Griinbaum  on  the  brain  of  the 
chimpanzee,  which  demonstrate  that  the  post-central  convolution  is  abso- 
lutely inexcitable  to  electric  stimulation.     It  is  quite  probable  that  with  the 


Fig.  252. 


-Scheme  of  the  Motor  Area  of  the  Hxjman  Brain  and  its  Subdivisions.- 

Mills.) 


-(After 


growth  of  the  brain  in  size  and  complexity,  the  motor  area  has  come  to  occupy 
a  position  somewhat  farther  forward  in  the  human  brain  than  in  the  monkey 
brain. 

This  general  area  is  capable  of  subdivision  into  areas  of  variable  size,  e.g., 
head  and  eyes,  face,  arm,  trunk,  and  leg  areas  which  are  related  through  the 
efferent  nerve-fibers  composing  in  part  the  pyramidal  tract  to  groups  of 
muscles,  e.g.,  head  and  eye  muscles,  face,  arm,  trunk  and  leg  muscles,  on 
the  opposite  side  of  the  body  and  the  activities  of  which  they  initiate  and 
control.     (See  Figs.  250  and  252.) 

These  large  subdivisions  of  the  cortical  motor  area  are  in  turn  capable 
of  a  further  subdivision  into  smaller  areas  which  are  in  relation  with  one  or 
more  muscles  of  the  larger  groups.  These  smaller  areas  indicated  in  the 
diagram  by  compressed  italics  contain  groups  of  nerve-cells  which  excite  to 
action,  through  their  efferent  axons  and  their  medullary  and  spinal  connec- 


THE  CEREBRUM  591 

tions,  the  muscles  which  unpart  movements  to  the  regions  corresponding  to 
the  words  in  compressed  itaUcs. 

The  main  motor  areas  are  as  follows: 

1.  The  Head  and  Eye  Area. — ^This  area  has  been  assigned  to  the  contiguous 

portions  of  the  medi-  and  sub-frontal  convolutions  just  anterior  to  the 
pre-central  convolution.  It  is  subdivided  into  smaller  areas  which  initiate 
and  govern  the  movements  of  the  head  and  eyeballs.  Stimulation  of 
this  area,  in  the  chimpanzee  at  least,  produces  turning  of  the  head  to 
the  opposite  side  with  conjugate  deviation  of  the  eyes  to  that  side. 

2.  The  Face  Area. — ^This  area  has  been  assigned  to  the  lower  portion  of  the 

pre-central  convolution  and  extends  from  below  upward  to  about  the 
level  of  the  genu  of  the  central  fissure.  This  rather  large  area  may  be 
subdivided  into  (a)  an  upper  portion  including  about  one-third  of  the 
whole  and  {h)  a  lower  portion  including  the  remaining  two-thirds.  In 
both  the  upper  and  lower  portions,  there  are  groups  of  nerve-cells  which 
excite  to  action  the  muscles  imparting  movements  to  {a)  the  angle  of 
the  mouth,  the  eyelids  and  jaws  and  {h)  the  movements  of  the  vocal 
bands  or  cords,  the  opening  and  closing  of  the  mouth,  the  protrusion 
and  retraction  of  the  tongue.  All  of  these  movements  have  their  areas 
of  representation  in  the  face  area. 

3.  The  Arm  Area. — ^This  area  has  been  assigned  to  the  pre-central  convolu- 

tion just  above  and  contiguous  to  the  face  area  which  it  exceeds  some- 
what in  extent.  It  is  the  largest  of  all  the  subdivisions  of  the  general 
area.  It  may  be  divided  into  at  least  five  smaller  areas,  the  cells  of 
which  excite  to  action  the  muscles  imparting  movements  to  the  thumb, 
the  fingers,  the  wrist,  the  elbow  and  the  shoulder. 

4.  The  Trunk  Area. — This  area  has  been   assigned   by   Sherrington   and 

Greenbaum  to  the  pre-central  convolution  just  superior  to  the  arm  area 
and  is  rather  limited  in  extent.  Horsley  located  a  portion  of  this  area 
on  the  mesial  and  lateral  edges  of  the  hemisphere  in  front  of  the  leg 
area.  The  nerve-cells  of  this  area  when  electrically  stimulated  excite 
to  action  the  muscles  which  impart  movements  to  the  spinal  column, 
such  as  arching  rotation,  etc. 

5.  The  Leg  Area. — This  area  has  been  assigned  to  the  extreme  upper  portion 

of  the  pre-central  convolution  and  to  the  adjoining  mesial  surface,  the 
upper  portion  of  the  para-central  lobule.  The  area  on  the  lateral  aspect 
of  the  cerebrum  may  be  subdivided  into  at  least  four  smaller  areas  con- 
taining groups  of  nerve-cells  which  excite  to  action  the  muscles  impart- 
ing movements  to  the  toes,  ankle,  knee  and  hip. 

Evidence  from  the  clinical  side  has  demonstrated  the  fact  that  a 
localized  irritative  lesion  of  any  one  of  these  areas  gives  rise  to  convulsive 
movements  of  the  muscles  of  the  opposite  side  of  the  body,  similar  in 
character  to  those  resulting  from  electric  simulation  of  the  correspond- 
ing areas  of  the  monkey  and  ape  brains.  Destruction  of  these  areas 
from  the  growth  of  tumors,  softening,  etc.,  is  followed  by  paralysis  of  the 
muscles.  Electric  stimulation  of  these  areas  of  the  human  brain  for 
the  purpose  of  localizing  obscure  irritative  lesions  prior  to  surgical  pro- 
cedures on  the  brain  gives  rise  to  similar  convulsive  movements. 

The  m.otor  speech  and  the  motor  writing  areas  will  be  considered 
the  following  paragraphs. 


592  TEXT-BOOK  OF  PHYSIOLOGY 

Language. — Language  may  be  defined  as  the  expression  of  ideas  by  a 
succession  of  motor  acts  and  may  be  divided  into  (i)  spoken  language  (ar- 
ticulate speech),  and  (2)  written  language.  The  expression  of  ideas  by 
words  (speech)  and  by  verbal  signs  (writing)  depends  primarily  on  the  power 
of  reviving  images  or  memories  of  objects,  words,  letters,  numbers,  etc.,  seen 
and  heard,  and  secondarily  on  the  power  of  reviving  the  images  or  memories 
of  the  muscle  movements  which  were  previously  employed  in  an  effort  to 
imitate  or  reproduce  the  sounds  of  objects,  words,  letters,  etc.  (speech)  or 
the  visual  impressions  by  verbal  signs  (writing). 

For  both  spoken  and  written  language  two  phases  of  activity  are  to  be 
recognized,  viz.:  a  receptive  or  sensor  and  an  emissive  or  motor. 
I.  The  Receptive  or  Sensor  Phase.— Before  language  can  be  acquired  by 
a  developing  child,  impressions  of  external  objects  must  be  made  on  the 
visual,  auditory  and  other  sense-organs  as  well,  which  in  consequence 
lead  to  the  development  in  the  corresponding  cortical  areas  of  sensations 
and  ultimately  of  conceptions  regarding  the  nature  of  the  objects  im- 
pressing themselves  on  the  peripheral  sense-organs. 

Thus,  through  the  visual  apparatus  in  its  entirety,  definite  concrete 
conceptions  regarding  the  shape,  size,  color,  etc.,  of  an  object  or  word, 
letters  and  numbers,  etc.,  are  obtained.     For  future  use  the  sensations 
must  not  only  be  perceived  but  registered  or  memorized.     The  memor- 
ization of  the  sensations  produced  by  luminous  objects,  is  supposed  to 
be  associated  with  the  acti\dties  of  the  cortex  on  the  lateral  aspect  of  the 
occipital  lobe;  the  sensations  produced  by  words,  letters,  and  numbers 
with  the  activities  of  the  cortex  of  the  supra-marginal  and  angular  con- 
volutions  of   the   parietal   lobe.     This  is  made  possible  by  means  of 
association  fibers  which  unite  the  primary  visual  area  in  and  around 
the  cuneus,  with  the  secondary  visual  areas.     That  the  visual  memories 
are  associated  with  the  previously  mentioned  convolutions  of  the  parietal 
and  occipital  lobes  is  apparent  from  the  fact,  that  if  they  are  destroyed 
by  disease  there  is  a  loss  of  these  memories  even  though  the  individual 
sees  the  objects  as  formerly;  but  though  seeing  them  they  cannot  be 
remembered  or  recalled.     To  this  condition  the  terms  word  and  object 
blindness  have  been  given. 
Coincidently  with  the  development  of  visual  sensations  and  their  memori- 
zation, the  child  receives  through  the  auditory  apparatus  in    its   entirety, 
definite  concrete  conceptions  regarding  the  intensity,  pitch  and  timbre  or 
quality  of  sounds  produced  by  atmospheric  vibrations,  due  to  vibrating 
bodies;  sounds  which  are  subsequently  associated  with  the  vocal  and  articu- 
lating apparatus  of  human  beings  when  pronouncing  words,  letters  and 
numbers,  or  with    objects   such    as    bells,  musical   instruments,  etc.     For 
future  use  the  sensations  thus  aroused  must  not  only  be  perceived  but  like- 
wise registered  or  memorized.     The  memorization  of  the  sensations  pro- 
duced  by  atmospheric  vibrations  is  supposed   to  be  associated    with  the 
activities  of  the  temporal  lobe  and  more  particularly  of  the  posterior  two- 
thirds  of  the  super-temporal  and  medi-temporal  convolutions.     This  is  made 
possible  by  means  of  association  fibers  which  unite  the  primary  auditory  area 
with  the  secondary  auditory  areas.     That  the  auditory  memories  are  associ- 
ated with  the  previously  mentioned  convolutions  is  apparent  from  the  fact 
that  if  they  are  destroyed  by  disease,  there  is  a  loss  of  these  memories 


THE  CEREBRUM  593 

even  though  the  individual  hears  the  sounds  as  formerly,  but  though  hear- 
ing them,  they  cannot  be  remembered  or  recalled.  To  this  condition  the 
terms  ivord  deafness  and  object  deafness  have  been  given. 

From  the  concrete  conceptions  formed  of  individual  objects  of  a  similar 
character  or  of  a  class,  there  are  developed  abstract  conceptions,  by  the 
uniting  into  a  single  idea  the  elements  which  are  common  to  all  the  objects 
of  the  class.  This  involves  an  analysis  and  a  synthesis,  a  recognition  of 
the  points  of  similarity  and  dissimilarity.  The  development  of  abstract 
conceptions  is  believed  to  be  associated  with  the  activities  of  the  cortex  of 
one  or  the  other  of  the  association  areas  (see  page  596). 

For  the  emission  of  the  sounds  of  words  which  constitute  spoken 
language  it  is  essential  that  the  sensations  produced  by  the  muscle  move- 
ments shall  not  only  be  perceived  but  registered  or  memorized.  Without 
this  the  capacity  to  express  words  would  not  be  possible.  This  registration 
is  believed  to  be  associated  with  the  lower  portion  of  the  area  for  muscle 
sensibility. 

2.  The  Emissive  or  Motor  Phase. — ^In  an  attempt  to  express  ideas  by 
spoken  or  written  language  the  muscles  determining  the  action  of  the 
larynx,  jaws  and  teeth,  tongue,  lips,  hands,  etc.,  are  excited  to  action 
by  nerve  impulses  descending  through  nerves,  the  nuclei  of  origin  of 
which  lie  in  the  gray  matter  beneath  the  floor  of  the  medulla  oblongata 
and  in  the  gray  matter  of  the  spinal  cord.  The  nuclei  of  these  nerves 
are  in  turn  excited  to  action  by  nerve  impulses  descending  by  way  of 
the  internal  capsule  from  the  cortical  areas  for  the  face  and  arm 
respectively. 

The  Motor  Speech  Area. — By  this  term  is  meant  an  area  of  the  cortex, 
the  function  of  which  is  to  arrange  language  for  outward  expression;  for  the 
use  of  the  executive  centers  concerned  with  speech,  viz. :  the  laryngeal,  lingual 
and  facial  centers  located  at  the  foot  of  the  pre-central  convolution.  This 
area,  i.e.,  the  motor  speech  area,  has  been  assigned  to  the  posterior  part  of 
the  subfrontal  convolution  (Broca's  convolution)  on  the  left  side  in  those  who 
are  right-handed  and  on  the  right  side  in  those  who  are  congenitally  left- 
handed,  and  in  the  anterior  part  of  the  insular  or  perhaps  the  pre-insular 
convolutions.  Unipolar  faradic  stimulation  of  this  area  fails  to  call  forth 
any  motor  response;  its  destruction  by  disease,  however,  is  followed  by  a 
more  or  less  extensive  loss  of  the  faculty  of  articulate  speech  or  the  faculty 
of  expressing  ideas  with  words,  a  condition  usually  spoken  of  as  motor 
aphasia  or  aphemia.  This  area  and  the  area  at  the  foot  of  the  pre-central 
convolution  are  united  by  association  fibers. 

The  Motor  Writing  Area. — By  this  term  is  meant  an  area  of  the  cortex, 
the  function  of  which  is  to  arrange  language  for  outward  projection;  for  the 
use  of  the  executive  centers  concerned  with  writing,  viz.,  the  arm  centers 
located  in  the  middle  portion  of  the  pre-cental  convolution.  This  area,  i.e., 
the  motor  writing  area,  has  been  assigned  to  the  posterior  half  or  third  of  the 
medi-frontal  convolution.  Unipolar  faradic  stimulation  of  this  area  fails  to 
call  forth  any  motor  response;  its  destruction  by  disease,  however,  is  followed 
by  an  inability  to  express  ideas  by  writing,  a  condition  usually  spoken 
of  as  agraphia.  This  area  and  the  general  arm  center  in  the  pre-central 
convolution  are  united  by  association  fibers. 
38 


594 


TEXT-BOOK  OF  PHYSIOLOGY 


The  Language  Zone.— These  different  areas  are  connected  with  one 
another  by  association  fibers,  and,  taken  collectively,  constitute  the  language 
zone,  which  in  people  who  are  congenitally  right-handed  is  located  on  the 
left  side  of  the  brain  only.  Their  situation  and  relations  are  shown  in  Fig. 
253.  In  this  figure  the  dotted  lines  coming  from  the  eye  (v)  and  ear  (a) 
represent  the  visual  and  auditory  tracts  through  which  nerve  impulses  pass 
to  the  visual  centers  (V)  and  the  auditory  (A)  respectively.     Similar  lines 

coming  from  the  muscles  involved  in 
speech  and  writing  might  also  be  repre- 
sented to  indicate  the  paths  of  the 
nerve  impulses  to  the  muscle  sense 
areas  not  shown  in  the  diagram.  The 
single  continuous  lines  on  the  surface 
of  the  cortex  represent  nerve-fibers 
which  associate  the  visual  and  audi- 
tory centers  with  the  speech  and  writ- 
ing centers.  The  double  lines  asso- 
ciate the  visual  and  the  auditory  areas 
with  the  frontal  association  area  on  the 
one  hand  and  the  association  area  with 
the  motor  speech  and  the  motor  writ- 
ing area  respectively.  The  dotted  lines 
coming  from  the  speech  and  writing 
centers  represent  the  tracts  coming 
from  the  areas  in  the  pre-central  con- 
volutions, and  through  which  nerve  im- 
pulses pass  to  the  muscle  of  the  larynx, 
tongue,  mouth,  and  lips,  and  to  the 
muscles  of  the  hand.  The  anatomic 
and  physiologic  association  of  the  vari- 
ous areas  is  essential  to  the  registration 
of  the  impressions  made  on  the  ear 
and  eye  and  for  the  expression  of  the 
ideas  evolved  from  them  by  words 
(speech)  and  signs  (writing).  Their 
collective  action  is  essential  to  the  ac- 
quisition of  language.  Destruction  of 
any  part  of  this  cerebral  mechanism 
is  attended  by  an  impairment  of  or  a 
total  loss  in  either  the  power  of  ob- 
taining auditory  images  of  words  heard 
and  visual  images  of  words  seen,  or  the 
power  of  expressing  ideas  by  speech  and  writing.  To  this  pathologic  con- 
dition the  term  aphasia  has  been  given. 

Aphasias  are  of  many  degrees  and  kinds,  though  they  may  be  included 
in  the  two  general  divisions,  motor  and  sensor. 

Motor  aphasia  may  be  either  aphemic  or  agraphic.  In  aphemic  aphasia 
the  patient  is  unable  to  express  or  communicate  his  thoughts  by  spoken 
words,  owing  to  an  inabihty  to  arrange  words  for  outward  expression  and 
hence  to  execute  those  movements  of  the  mouth,  tongue,  etc.,  necessary  for 


Fig.  253. — Diagram  Showing  the  Re- 
lation OF  THE  Centers  of  Language  and 
THEIR  Principal  Associations.  A.  Audi- 
tory center.  V.  Visual  center.  M.  Motor 
speech  center.  E.  Motor  writing  center. 
O   O.  Intellectual  center. — {After  Grasset.) 


THE  CEREBRUM  595 

speech  without  there  being  any  paralysis  of  these  muscles.  The  lesion  is 
usually  in  the  third  frontal  convolution  and  most  frequently  associated  with 
right  hemiplegia.  In  agraphic  aphasia  the  patient  is  unable  to  communicate 
his  ideas  by  writing  through  an  inability  to  arrange  verbal  signs  for  outward 
expression  and  hence  to  execute  the  movements  of  the  hand  and  arm  neces- 
sary for  writing.  In  this  form  of  aphasia  the  lesion  is  in  the  writing  area, 
in  the  posterior  half  or  third  of  the  medi-frontal  convolution.  These  two 
forms  of  motor  aphasia  are  not  infrequently  associated. 

Sensor  aphasia  or  amnesia  may  be  either  visual  or  auditory.  In  visual 
aphasia  or  amnesia  the  patient  is  unable  to  recognize  a  letter  or  word,  printed 
or  written  (though  capable  of  seeing  other  objects),  a  condition  known  as 
letter  or  word  blindness.  It  is  usually  associated  with  lesions  in  the  neighbor- 
hood of  the  supra-marginal  convolution.  In  auditory  aphasia  or  amnesia 
the  patient  cannot  understand  articulate  or  vocal  speech,  though  capable  of 
hearing  and  understanding  other  sounds,  through  an  inability  to  distinguish 
the  associations  of  words  and  letters — a  condition  known  as  word  deafness. 
It  is  associated  with  lesions  of  the  auditory  area. 

Paraphasia  is  an  inability  to  recall  the  proper  words  to  associate  with 
ideas  and  necessary  to  their  expression. 

Concept  aphasia  is  the  inability  to  recall  the  names  of  objects.  It  is 
associated  with  lesions  of  the  cortex  of  the  mid-temporal  or  third  temporal 
convolution  (Mills).     This  area  is  known  as  the  concept  or  naming  area. 

Many  other  forms  of  aphasia  have  been  observed  and  described  by 
clinicians  which  are  fully  considered  in  works  relating  to  diseases  of  the 
nerve  system. 

The  statements  regarding  the  mechanism  of  speech,  the  functions  as- 
signed to  the  motor  speech  area  (Broca's  convolution)  and  the  motor  writing 
area,  though  very  generally  accepted,  have  been  questioned  in  recent  years 
by  Marie  who  on  the  basis  of  clinico-pathologic  facts  has  presented  a  some- 
what different  view  which  has  found  many  adherents. 

Marie's  Theory  of  Aphasia. — Marie  states  that  there  is  but  one  aphasia 
and  but  one  speech  center,  which  he  locates  somewhere  in  the  left  temporo- 
parietal lobe,  and  which  he  designates  as  a  specialized  intellectual  center  for 
speech.  Motor  aphasia  in  the  accepted  sense,  he  states,  is  a  combination  of 
word  blindness  and  word  deafness  (both,  however,  being  defects  of  the  speech 
center)  and  defective  articulation  (anarthria)  the  result  of  a  lesion  of  the 
motor  tracts  necessary  to  the  excitation  of  the  muscles  for  articulation.  The 
lesion  causing  the  anarthria  is  in  the  lenticular  zone,  in  close  relation  to  the 
lenticular  nucleus.  This  zone,  in  a  horizontal  section  of  the  brain,  is  limited 
anteriorly  by  a  vertical  plane  level  with  the  anterior  sulcus  of  the  insula, 
posteriorly  by  a  similar  plane  level  with  the  posterior  sulcus  of  the  insula; 
internally  by  the  lateral  ventricle  and  externally  by  the  surface  of  the  insula. 
This  zone  is  anatomically  associated  with  the  supra-marginal  and  angular 
convolutions  and  with  the  posterior  portions  of  the  super-  and  medi-temporal 
convolutions  which  together  constitute  Wernicke's  zone.  In  this  view  the 
distinction  between  motor  and  sensor  aphasia,  the  functions  assigned  to  the 
sub-frontal  and  medi-frontal  convolutions  and  the  existence  of  four  speech 
centers  are  all  rejected.  Many  published  facts  support  this  view  though 
many  others  are  at  variance  with  it. 


596 


TEXT-BOOK  OF  PHYSIOLOGY 


Bilateral  Representation. — Though  highly  speciaHzed  movements, 
such  as  those  performed  by  the  arms  and  hands,  legs  and  feet,  have  their 
areas  of  representation  on  one  side  of  the  cerebrum  only,  and  that  opposite 
to  the  side  of  the  movement,  less  highly  specialized  movements,  such  as  the 
masticatory,  phonatory,  respiratory  and  various  trunk  movements,  which 
require  for  their  performance  the  cooperation  of  muscles  on  both  sides  of 

Motor  and  tactile  area. 


Parietal  association  area 


Vis 


Frontal 

association 

area. 


i  ^Island  of  Reil. 


Occipito-temporal 
association  area. 


Auditory  area. 


Motor  and  tactile  area. 


Parietal  association  area 
(Precuneus). 


Frontal 

association 

area. 


Occipito-tempor., 
association  area. 


Olfactory  lobe. 
Olfactory  tract. 

Olfactory  area. 


Gyrus  hippocampus. 

Fig.  254. — Diagrams  to  show  the  Position  and  the  Relation  of  the  Association  and 
Projection  Areas.     The  Projection  Areas  are  Dotted. — {After  Flechsig.) 

the  body,  have  their  areas  of  representation  on  both  sides  of  the  cerebrum; 
the  area  of  either  side  exciting  to  action  the  muscles  on  both  sides  of  the 
body.  In  the  case  of  specialized  movements  the  representation  is  unilat- 
eral; in  the  case  of  the  more  general  movements  the  representation  is 
bilateral. 

Association  Centers.^ — The  sensor  and  motor  areas  to  which  specific 
functions  have  been  assigned  do  not  constitute  more  than  one-third  of  the 


THE  CEREBRUM  597 

total  cerebral  cortex.  There  yet  remain  large  regions  to  which  it  has  been 
impossible  to  assign  specific  functions  based  on  physiologic  experiments. 
Three  or  four  such  regions  separated  by  the  sensor  and  motor  centers  are  to 
be  recognized  on  the  lateral  and  mesial  aspects  of  the  hemisphere.  In  Fig. 
254  the  location,  extent,  and  names  of  these  regions  are  represented.  The 
fibers  which  are  found  in  these  regions  belong  almost  exclusively  to  the 
association  system,  and  become  medullated  at  a  later  period  than  do  the  fi- 
bers of  the  projection  system;  moreover,  from  the  method  of  their  medulliza- 
tion  it  would  appear  that  many  of  these  fibers  grow  out  directly  from  the 
sensor  centers  into  these  regions  and  become  related  to  the  nerve-cells  of 
their  convolutions,  while  others  grow  out  from  adjacent  as  well  as  distant 
convolutions.  From  histologic  and  pathologic  evidence  these  regions  were 
termed  by  Flechsig  association  centers  or  areas,  implying  the  idea  that 
through  the  interv^ention  of  their  cell  mechanisms  the  sense  areas  are  in- 
directly associated  anatomically  and  physiologically,  and  together  constitute 
a  mechanism  by  which  sensations  are  associated  and  elaborated  into  concrete 
forms  of  knowledge  or  related  to  definite  forms  of  movement. 

It  has  been  assumed  by  Flechsig  that  the  frontal  association  center,  from 
its  connections  with  the  sensor  and  motor  areas  of  the  Rolandic  region,  the 
olfactory,  and  perhaps  other  regions,  is  engaged  in  associating  and  registering 
body  sensations  and  volitional  acts,  and  that  the  knowledge  thus  gained  has 
reference  largely  to  the  personality  of  the  individual ;  that  the  parieto-occipital 
association  area,  from  its  relation  to  the  visual,  auditory,  and  tactile  sense 
areas,  is  engaged  in  associating  and  registering  visual,  auditory,  and  tactile 
sensations,  and  that  the  knowledge  thus  gained  has  reference  mainly  to  the 
external  world.  These  assumptions  in  a  general  way  are  supported  by  the 
phenomena  of  disease.  In  certain  lesions  of  the  frontal  lobe  the  symptoms 
indicate  a  loss  or  change  of  ideas  regarding  personality  rather  than  of  the 
objective  world,  while  the  reverse  is  true  in  disease  of  the  parieto-occipital 
lobe. 

The  Intra-cranial  Circulation. — The  circulation  within  the  cranium 
presents  certain  peculiarities  which  distinguish  it  from  that  in  other  parts  of 
the  body.  These  peculiarities  reside  in  part  in  the  anatomic  arrangement  of 
the  blood-vessels,  in  the  probable  absence  of  vaso-motor  nerves  to  the  blood- 
vessels, and  in  greater  part  in  the  fact  that  the  brain  and  its  blood-vessels 
are  contained  in  a  case  with  rigid,  unyielding,  and  closed  walls. 

The  Blood-supply. — As  stated  in  a  previous  paragraph  the  arteries 
supplying  the  brain  with  blood  are  four  in  number,  viz. :  the  two  internal 
carotids  and  the  two  vertebrals.  These  four  arteries  anastomose  very  freely 
at  the  base  of  the  brain,  the  anastomosis  constituting  the  circle  of  Willis. 
From  this  circle  there  arise  the  anterior,  middle  and  posterior  cerebral  arteries 
which  are  distributed  to  the  cortex  and  the  underlying  white  matter.  The 
basal  ganglia,  the  capsule  and  adjacent  white  matter  are  supplied  by  a  num- 
ber of  branches  which  arise  from  the  circle  of  Willis  or  from  the  three  cerebral 
arteries  immediately  after  their  origin.  From  the  distribution  of  these  two 
sets  of  vessels  they  have  been  named  the  cortical  and  the  central  ganglionic 
respectively. 

The  venous  blood  is  returned  by  a  system  of  vessels  which  present  charac- 
teristics of  physiologic  interest.  These  vessels  consist  of  large,  sinuses  formed 
by  folds  of  the  dura  mater  or,  as  at  the  base  of  the  cranium,  by  the  dura  mater 


598  TEXT-BOOK  OF  PHYSIOLOGY 

and  the  bone.  These  sinuses,  from  the  very  nature  of  the  tissues  which 
enter  into  their  formation,  have  rigid  walls  and  will  therefore  withstand  any 
pressure  to  which  they  may  be  subjected  under  physiologic  conditions.  The 
same  obtains  at  their  points  of  exit  from  the  cranium  where  a  free  outflow 
is  in  consequence  always  assured. 

The  various  sinuses  have  opening  into  them,  the  veins  which  return  the 
blood  from  the  cortex  and  subjacent  white  matter,  and  from  the  inner  struc- 
tures of  the  brain.  Neither  sinuses  nor  veins  have  valves  and  most  of  the 
veins  which  empty  into  the  superior  longitudinal  sinus  have  their  mouths 
directed  forward,  hence  the  blood  discharged  from  these  veins  must  flow 
against  the  current  in  the  sinus.  The  venous  blood  leaves  the  cranium 
mainly  by  way  of  the  internal  jugular  veins  which  are  direct  continuations  of 
the  lateral  sinuses. 

The  Intra-cranial  Lymph-spaces.— In  order  to  understand  the  phe- 
nomena attending  the  circulation  of  blood  through  the  cranium  it  is  necessary 
to  take  into  consideration  an  important  fact,  viz. :  that  the  brain  and  spinal 
cord  are  surrounded  on  all  sides  by  a  relatively  large  and  continuous  lymph- 
space.  This  space  which  is  found  between  the  arachnoid  and  the  pia  mater 
is  filled  with  a  liquid,  the  so-called  cerebrospinal  fluid,  which  being  interposed 
between  the  brain  and  the  skull  on  the  one  hand  and  the  spinal  cord  and  the 
vertebrae  on  the  other  hand,  acts  as  a  water  cushion  protecting  these  delicate 
organs  from  the  injury  which  might  result  from  sudden  jars.  The  ventricles 
of  the  brain  are  also  filled  with  cerebrospinal  fluid  which  is  in  communication 
with  that  in  the  subarachnoid  space  through  the  foramen  of  Magendie  and  the 
foramina  of  Key  and  Retzius.  The  cerebrospinal  fluid  may  also  penetrate 
into  the  perineural  lymph-spaces  surrounding  the  cranial  and  spinal  nerves. 
The  quantity  of  the  cerebrospinal  fluid  is  relatively  small,  amounting  to 
from  60  to  80  c.c. 

The  Mechanism  of  the  Intra-cranial  Circulation. — As  previously 
stated,  by  virtue  of  the  physical  relations  existing  between  the  blood,  the 
brain,  the  cerebrospinal  fluid  and  rigid  walls  of  the  cranium,  the  flow  of  the 
blood  through  the  brain  and  cranial  cavity,  is  attended  by  certain  phenomena 
which  are  peculiar  to  this  region  and  present  in  no  other  situation. 

Taking  as  a  point  of  departure  the  condition  of  the  arteries  during  the 
cardiac  diastole,  the  relations  of  these  structures  are  somewhat  as  follows:  the 
cerebrospinal  fluid  occupies  all  the  available  lymph-space,  but  under  a  pres- 
sure approximately  equal  to  that  in  the  large  veins  and  hence  not  materially 
above  that  of  the  atmosphere;  the  pressure  in  the  arteries,  capillaries  and 
veins  presents  the  usual  values  in  these  different  regions  of  the  vascular 
apparatus;  the  brain  presents  a  volume  which  may  be  termed  diastolic. 

With  the  occurrence  of  the  succeeding  cardiac  systole,  the  cerebral 
vessels,  receiving  an  additional  volume  of  blood,  expand  and  occasion  a 
corresponding  increase  in  the  volume  of  the  brain,  which  is  accomplished  by 
a  partial  displacement  of  the  cerebrospinal  fluid  into  extra-cranial  lymph- 
spaces.  Because  of  the  fact  that  the  displacement  of  the  cerebrospinal 
fluid  is  insufficient  to  permit  of  the  complete  expansion  of  the  brain,  there  is 
developed  in  the  intra-cranial  lymph-spaces  a  counter-pressure  (the  so-called 
intra-cranial  pressure)  which  would  keep  pace  with  and  finally  equalize  the 
rising  pressure  in  the  arteries.  In  consequence  of  this,  the  brain  tissue,  it  is 
believed,  would  be  subjected  to  a  pressure  sufficiently  great  to  interfere  with 


THE  CEREBRUM  599 

its  activities,  even  to  the  point  of  unconsciousness.  If  this  is  not  to  occur 
the  maximum  expansion  of  the  arteries,  and  hence  the  brain,  must  be  checked 
and  controlled.  This  is  accomplished  in  the  following  way:  As  the  brain 
approaches  that  degree  of  expansion  permitted  by  the  displacement  of  the 
cerebrospinal  fluid,  it  begins  to  exert  a  compression  of  the  pial  veins.  This 
compression  by  narrowing  the  lumen  of  the  veins  diminishes  their  capacity 
and  hence  increases  the  pressure  of  their  contained  blood  until  it  is  equiva- 
lent to  the  pressure  exerted  by  the  brain  against  the  veins.  At  this  moment 
the  pressures  in  the  arterioles,  capillaries  and  veins  approximate  each  other 
in  value. 

From  these  factors  it  will  be  seen  that  the  circulation  through  the  brain 
approximates  a  circulation  through  a  system  of  rigid  tubes.  The  result  is  an 
increase  in  the  velocity  of  the  outflow  and  a  diminution  of  the  blood-pressure. 
As  an  additional  result  the  pulse-wave  of  the  arterial  system  is  transmitted 
to  the  blood  of  the  large  veins  and  sinuses  which  therefore  exhibit  normally 
pulsations  synchronous  with  those  of  the  arteries.  The  rise  of  the  pressure 
in  the  cerebral  veins  is  regarded  therefore  as  the  factor  which,  by  limiting 
brain  expansion,  checks  the  rise  of  the  intra-cranial  pressure  beyond  physio- 
logic limits.  With  the  diastole  of  the  heart  and  the  recoil  of  the  arteries, 
the  former  relation  of  the  blood,  brain,  cerebrospinal  fluid  and  cranial  walls 
is  regained.  Because  of  this  change  of  relation  with  each  heart-beat, 
the  brain  pulsates  synchronously  with  the  arteries. 

The  brain  differs  from  other  organs,  also,  in  that  normally  its  volume  is 
more  influenced  in  a  positive  direction  by  the  expiratory  rise  of  venous  pres- 
sure than  by  the  inspiratory  rise  of  general  arterial  pressure.  Thus  the  rise 
of  pressure  in  the  thoracic  veins  which  occurs  with  each  expiratory  act, 
causes  a  damming  back  of  the  venous  blood  in  the  sinuses  and  pial  veins, 
resulting  in  a  further  increase  in  the  volume  of  the  brain  and  in  the  intra- 
cranial pressure.     The  reverse  takes  place  in  inspiration. 

It  has  been  ascertained  experimentally  that  the  intra-cranial  pressure 
may  vary  considerably  and  consciousness  still  be  preserved.  Hill  found  it 
to  be  40  to  50  mm.  of  Hg.  in  the  convulsions  of  strychnin  poisoning  and  a 
little  less  than  zero  in  a  patient  standing  erect. 

The  Regulation  of  the  Volume  of  Blood  Entering  the  Brain. — It  is 
generally  believed  that  the  cerebral  vessels  are  not  provided  with  vaso-motor 
nerves.  Every  attempt  to  prove  their  existence  either  by  physiologic  or 
histologic  methods  has  thus  far  failed  of  convincing  proof.  In  the  absence 
of  vaso-motor  nerves,  the  regulation  of  the  circulation  in  the  brain  must  neces- 
sarily be  dependent  on  changes  affecting  the  arterial  and  venous  pressures  in 
other  regions  of  the  body.  • 

The  most  effective  factor  in  increasing  or  decreasing  the  blood-supply 
to  the  brain  resides  in  the  power  of  the  vaso-motor  center  to  cause  a  contrac- 
tion or  dilatation  of  the  cutaneous  and  splanchnic  vessels.  Thus  if  the 
vaso-motor  center  declines  in  its  tonus  from  any  cause  whatever,  there  is  a 
relaxation  of  the  blood-vessels  in  one  or  both  of  these  regions,  an  increase  in 
the  volume  of  the  blood  flowing  into  them,  and  in  consequence,  a  decrease 
in  the  volume  of  the  blood  flowing  through  the  brain.  If  on  the  contrary  the 
vaso-motor  center  is  increased  in  its  tonus,  the  reverse  conditions  prevail  in 
the  cutaneous  and  splanchnic  vessels  and  the  quantity  of  blood  flowing  into 
the  brain  is  increased.     Thus  in  an  indirect  way  the  vaso-motor  center,  by 


6oo  TEXT-BOOK  OF  PHYSIOLOGY 

bringing  about  a  rise  or  a  fall  in  the  general  arterial  pressure,  regulates  the 
blood-supply  to  the  brain,  and  controls  its  amount  in  accordance  with  its 
needs. 

Brain  Activity. — Brain  activity  is  characterized  by  an  active  conscious- 
ness, the  development  of  sensations,  ideas,  feelings,  and  the  exercise  of 
volitional  power  (which  manifests  in  muscle  movement)  and  is  the  result  of  a 
physiologic  condition  of  the  body  at  large.  For  the  manifestation  of  brain 
activity  it  is  essential  that  the  irritability  of  the  brain  cells  and  more  especially 
of  those  composing  in  large  measure  the  cerebral  cortex  be  maintained  at  a 
normal  physiologic  level,  so  that  they  may  respond  in  the  manner  peculiar 
to  them  to  the  action  of  nerve  impulses  transmitted  through  afferent  nerves 
from  all  regions  of  the  body.  Here  as  elsewhere  throughout  the  body,  the 
irritability  depends  on,  and  is  maintained  by,  the  presence  of  blood  flowing 
into  and  out  of  the  brain  in  varying  quantity  from  moment  to  moment,  with 
a  given  velocity  and  under  a  definite  pressure.  So  long  as  these  conditions 
are  maintained  in  the  strictly  physiologic  condition,  so  long  will  the  brain 
respond  to  stimuli  by  the  development  of  sensations.  The  avenues  through 
which  nerve  impulses  pass  to  the  cortical  cells  are  those  beginning  in  the 
special  and  general  sense  organs  of  the  body  in  contact  with  the  external 
world,  viz.:  the  eyes,  ears,  nose,  tongue,  and  skin.  The  maintenance  of 
these  structures  in  a  strictly  physiologic  condition  is  also  one  of  the  essential 
conditions  for  brain  activity. 

Judging  from  the  changes  in  the  character  and  composition  of  the  blood 
which  occur  during  its  passage  through  the  brain  capillaries,  there  is  coin- 
cidently  with  brain  activity  an  active  metabolism,  which  eventuates,  at  the 
end  of  a  variable  number  of  hours,  in  the  decline  of  the  irritability,  a  reduc- 
tion of  functional  activity,  and  the  establishment  of  the  condition  of  fatigue. 
The  irritability  of  the  sense  organs,  especially  of  the  eyes  and  ears,  in  all 
probability  declines  in  a  similar  manner.  These  structures  pass  into  the 
condition  of  fatigue  and  become  less  responsive  to  external  stimuli.  The 
results  of  all  these  conditions  is  a  less  active  stimulation  of  the  brain  cells, 
which  in  connection  with  other  factors  predisposes  to — 

Brain  Repose  or  Sleep. — Brain  repose  or  sleep  is  characterized  by  a 
greater  or  less  degree  of  unconsciousness,  the  non-development  of  sensations, 
ideas,  feelings  and  volitional  acts,  and  is  the  result  of  a  diminution  in  the 
physiologic  activities  of  the  body  at  large  and  more  especially  of  the  brain, 
sense  organs,  and  spinal  cord.  Coincident  with  the  cessation  of  brain  activity 
and  the  onset  of  sleep,  there  is  a  diminution  in  the  rate  and  force  of  the  heart- 
beat, and  in  the  frequency  and  depth  of  the  respiratory  movements,  and  a 
relaxation  of  the  skeletal  muscles,  especially  those  employed  in  voluntary 
movements. 

The  sense-organs  are  in  part  protected  from  the  action  of  external  stimuli. 
The  eyeball  is  so  turned  that  its  anterior  pole  is  directed  far  upward  under 
the  eyelid,  while  the  pupil  is  markedly  diminished  in  size,  and  in  consequence 
the  entrance  of  light  largely  prevented.  The  ear  is  protected  against  the 
reception  of  sounds  of  ordinary  pitch  by  an  increased  tension  of  the  tympanic 
membrane.  The  nose  and  mouth  are  less  responsive  to  various  stimuli 
because  of  the  dryness  of  their  mucous  membranes  from  diminished  secre- 
tion. The  skin  appears  to  be  less  sensitive  to  mechanic  pressure  and  other 
forms  of  stimulation. 


THE  CEREBRUM  6oi 

In  addition  to  the  foregoing  phenomena,  experimental  investigations 
have  shown  that  there  is  a  shunting  of  a  portion  of  the  blood-stream  from 
the  brain  to  other  regions  of  the  body,  especially  to  the  skin  and  perhaps  to 
the  abdominal  \dscera  as  well,  whereby  it  becomes  incapable  of  functionating 
physiologically.  The  fact  that  the  brain  receives  a  lessened  quantity  of 
blood  during  sleep  has  been  shown  by  trephining  the  skull  and  inserting  in 
the  orifice  a  glass  plate  through  which  the  circulatory  conditions  of  the  brain 
can  be  observed.  In  the  waking  condition  the  blood-vessels  on  the  surface 
of  the  brain  are  prominent,  and  turgid  with  blood  and  the  w^hole  organ 
completely  fills  the  cranial  cavity,  indicating  that  the  blood-vessels  in  the 
interior  of  the  brain  are  in  a  similar  condition.  With  the  onset  of  sleep  the 
larger  blood-vessels  begin  to  diminish  in  size,  the  smaller  vessels  disappear 
from  view,  the  brain  tissues  become  pale  and  the  volume  of  the  brain  shrinks. 
During  the  continuance  of  deep  sleep,  this  anemic  condition  persists.  As 
the  period  of  sleep  approaches  its  termination,  the  smaller  blood-vessels 
again  fill  with  blood,  the  surface  of  the  brain  flushes,  and  in  a  very  short 
time  the  former  circulatory  conditions  return,  the  volume  of  the  brain 
increases  and  the  waking  state  is  reestablished. 

The  fact  that  the  skin  receives  an  increased  volume  of  blood  during  sleep, 
has  been  shown  by  inserting  an  arm  or  leg  in  a  plethysmograph  by  which 
means  a  record  of  any  change  in  volume  can  be  obtained.  Howell  thus 
succeeded  in  obtaining  graphic  records  in  the  variations  of  the  volume  of  the 
arm  during  sleep.  These  records  disclosed  the  fact  that  with  the  onset  of 
sleep  the  volume  of  the  arm  gradually  increased  in  size  until  it  attained  a 
maximum  which  was  from  one  to  two  hours  after  the  beginning  of  sleep. 
After  this  period  the  volume  remains  practically  the  same  for  several  hours, 
diminishing  as  the  intensity  of  sleep  diminishes  and  the  waking  state  is 
approached.  Just  previous  to  the  return  of  consciousness  there  is  a  rapid 
diminution  in  the  volume  of  the  arm.  If  it  be  accepted  that  the  enlargement 
of  the  cutaneous  vessels  is  followed  by  a  diminution  in  size  of  the  cerebral 
vessels,  it  follows  that  the  former  condition  stands  to  the  latter  in  the  relation 
of  cause  and  effect,  whereby  a  portion  of  ttie  blood  is  diverted  from  the  brain 
to  the  skin.  It  also  naturally  follows  that  the  withdraw^al  of  the  blood  from 
the  brain  to  the  skin  and  possibly  other  regions  as  Well,  is  the  fundamental 
condition  for  brain  repose. 

The  Intensity  of  Sleep.^Observations  of  individuals  during  sleep 
show  that  the  intensity  or  the  depth  of  sleep  varies  from  hour  to  hour. 
Attempts  have  been  made  to  estimate  the  intensity  by  measuring  the 
loudness  of  a  sound  caused  in  several  ways  that  is  necessary  to  awaken  the 
sleeper.  Accepting  this  criterion  it  may  be  stated  from  the  results  of  many 
experiments,  that  sleep  increases  in  intensity  or  depth  and  reaches  its  maxi- 
mum between  the  first  and  second  hours,  after  which  it  rapidly  decreases  un- 
til the  end  of  the  third  hour,  when  consciousness  is  so  nearly  restored,  that 
but  a  very  slight  stimulus  is  required  to  awaken  the  sleeper.  It  is  during 
the  latter  period  when  the  brain  is  reviving  that  dreams  arise,  the  elements 
of  which  are  formed  of  previous  sensations. 

The  Causes  of  Sleep. ^ — Different  theories  have  been  proposed  to  account 
for  the  causes  of  sleep,  none  of  which  have  been  wholly  satisfactory.  From 
all  the  facts  which  have  been  presented  it  would  appear  that  one  cause  is  a 
decline  in  the  irritability  of  the  nerve-cells  of  the  brain  and  associated  sense- 


6o2  TEXT-BOOK  OF  PHYSIOLOGY 

organs,  and  the  development  of  fatigue  conditions,  the  result  of  prolonged 
activity. 

A  second  cause  is  the  withdrawal  of  a  large  portion  of  the  blood  from  the 
brain,  on  the  presence  of  which,  here  as  elsewhere,  normal  activity  depends. 
As  to  whether  the  diminished  activity  of  the  brain  is  the  cause  of,  or  the  result 
of  the  withdrawal  of  the  blood  there  has  been  much  difference  of  opinion. 
Howell  has  offered  a  plausible  explanation  for  the  withdrawal  of  the  blood 
from  the  brain  to  the  cutaneous  vessels,  based  on  the  activity  of  the  vaso- 
motor center.  He  assumes  that  for  a  variable  number  of  hours,  correspond- 
ing to  the  usual  waking  state,  this  center  possesses  a  certain  average  tonus, 
due  in  all  probability  to  reflex  influences,  by  virtue  of  which  it  maintains  a 
certain  average  contraction  of  the  cutaneous  vessels.  But  at  the  end  of 
this  period  it  too  becomes  fatigued,  declines  in  irritability,  becomes  less 
responsive  to  reflex  influences,  and  hence  loses  its  control  over  the  vessels. 
As  a  result  they  dilate  and  thus  reduce  the  amount  of  blood  flowing  to  the  brain 
to  a  level  insufi&cient  to  maintain  its  activity,  after  which  sleep  supervenes. 
During  sleep  the  irritability  and  tonus  of  the  center  are  restored,  when  its 
control  of  the  blood-vessels  is  regained.  Unless  the  brain  in  its  functional 
activities  differs  from  all  other  organs  of  the  body,  it  may  be  inferred  that 
cessation  of  activity  or  repose  is  the  result  partly  of  fatigue  and  partly  of  a 
diminution  of  the  blood-supply. 


CHAPTER  XXIV 
THE  CEREBELLUM 

The  cerebellum  is  situated  in  the  inferior  fossae  of  the  occipital  bone 
beneath  the  posterior  lobes  of  the  cerebrum,  from  which  it  is  separated  by  the 
tentorium  cerebelli,  a  semilunar  fold  of  the  dura  mater.  It  is  partially 
divided  into  hemispheres  by  a  longitudinal  fissure,  more  apparent  on  the 
inferior  surface,  though  united  by  a  central  lobe,  the  vermiform  process. 
Each  hemisphere  is  connected  with  the  cerebrum,  the  pons,  medulla,  and 
spinal  cord  by  three  bundles  of  nerve-fibers  known  respectively  as  the  superior, 
middle,  and  inferior  peduncles.  The  surface  of  the  cerebellum  presents  a 
series  of  lobes  and  fissures  of  which  the  former  have  received  more  or  less 
fanciful  names.  A  section  of  the  cerebellum  shows  that  it  is  composed 
of  gray  matter  externally  and  white  matter  internally.  The  general 
appearance  presented  on  section  is  shown  in  Fig.  255. 

Structure  of  the  Gray  Matter. — The  gray  matter  consists  mainly  of 
nerve-cells  of  varying  size  and  shape,  which  are  arranged  in  two  layers: 
viz.,  an  outer  or  molecular  and  an  inner  or  granular. 

The  molecular  layer  consists  of  stellate  and  multipolar  cells  of  small 
size,  from  which  dendrites  and  axons  pass  horizontally  and  vertically.  The 
granular  layer  consists,  as  its  name  implies,  of  granular-shaped  cells  and 
large  stellate  cells.  These  cells  are  characterized  by  the  possession  of  den- 
drites and  axons,  the  course  and  relation  of  which  have  not  been  clearly 
determined. 

The  inner  border  of  the  molecular  layer  presents  a  series  of  large  cells 
originally  described  by  Purkinje  and  known  by  his  name.  From  the  outer 
end  of  the  cell-body  one  or  more  dendrities  emerge  which  soon  divide  and 
subdivide  into  a  number  of  branches  which  pass  toward  the  cerebellar  sur- 
face. The  general  arrangement  of  these  dendrities  gives  to  the  entire  cell  a 
tree-like  appearance  (Fig.  256).  From  the  inner  end  of  the  cell  an  axon 
emerges  which  passes  centrally  into  the  white  matter. 

Structure  of  the  White  Matter. — The  white  matter  consists  of  nerve- 
fibers  which  are  arranged  in  association  and  projection  systems. 

The  Association  System. — The  fibers  which  compose  this  system  are  of 
variable  lengths  and  unite  adjacent  as  well  as  distant  regions  of  the  cere- 
bellar cortex.  They  doubtless  associate  them  both  anatomically  and  physio- 
logically. 

The  Projection  System. — The  fibers  composing  this  system  connect  the 
cerebellar  cortex  with  certain  structures  in  the  cerebrum,  pons,  medulla, 
and  spinal  cord.     They  may  be  divided  into  efferent  and  afferent  systems. 

The  efferent  fibers  have  their  origin  in  the  cells  of  Purkinje  and  the 
dentate  nucleus.  Some  of  these  fibers  emerge  from  the  cerebellum  in  the 
superior  peduncles  through  which  they  pass  toward  and  beneath  the  corpora 
quadrigemina  to  terminate  around  the  cells  of  the  red  nucleus.  As  they 
approach  this  nucleus  some  of  the  fibers  cross  the  median  line  and  decussate 

603 


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TEXT-BOOK  OF  PHYSIOLOGY 


with  those  coming  from  the  opposite  side,  while  others  pursue  a  straight 
direction,  terminating  on  the  same  side.  Through  the  intervention  of  fibers 
which  arise  in  the  red  nucleus  and  ascend  to  the  cerebral  cortex,  the  hemi- 
sphere is  thus  connected  with  both  sides  of  the  cerebellum,  though  chiefly 
with  the  opposite  side. 

Efferent  fibers  also  leave  the  cerebellum  by  the  middle  peduncle  and  pass 
directly  to  the  nucleus  pontis,  around  the  cells  of  which  their  terminals 
arborize.  Efferent  fibers  also  descend  the  inferior  peduncles  and  constitute 
the  tract  known  as  the  Lowenthal  and  Marchi  tract,  situated  in  the  antero- 
lateral region  of  the  spinal  cord  in  its  upper  part. 

The  afferent  fibers  come  from  a  variety  of  sources.     Those  found  in  the 


^11  \'U.,  «jii#x\'E\    k\ 


tJJ 


&m^ 


Fig.  256. — Section  OF  Cerebellar 
Cortex.  A.  Outer  or  molecular 
layer.  B.  Inner  or  granular  layen 
C.  White  matter,  a.  Cell  of  Purk- 
inje.  b.  Small  cells  of  inner  layer. 
c.  Dendrites  of  these  cells,  d.  A 
similar  cell  King  in  the  Avhite  matter. 
— {Stirling.) 


Fig.  2  55.^\'"ie\v  of  Cerebellum  in  Section  and 
OF  Fourth  Ventricle,  with  the  Neighboring  Parts. 
— (From  Sappey.)  i.  Median  groove  fourth  ventricle, 
ending  below  in  the  calamus  scriptorius,  with  the  longi- 
tudinal eminences  formed  by  the  fascicuh  teretes, 
one  on  each  side.  2.  The  same  groove,  at  the  place 
where  the  white  streaks  of  the  auditory  nerve  emerge 
from  it  to  cross  the  floor  of  the  ventricle.  3.  Inferior 
peduncle  of  the  cerebellum,  formed  by  the  restiform 
body  4.  Posterior  pyramid;  above  this  is  the  calamus 
scriptorius.  5,  5.  Superior  peduncle  of  cerebellum,  or 
processus  e  cerebello  ad  testes.  6,  6.  Fillet  to  the  side 
of  the  crura  cerebri.  7,7.  Lateral  grooves  of  the  crura 
cerebri.  8.  Corpora  quadrigemina. — {After  Hirschfeld 
and  Leveille.) 

superior  peduncles  come  from  the  red  nucleus;  those  in  the  middle  peduncles 
from  the  nucleus  pontis  of  the  opposite  side,  having  crossed  or  decussated  at 
the  raph^  near  the  anterior  surface  of  the  pons;  those  contained  in  the  in- 
ferior peduncles  are  the  most  abundant  and  important,  and  are  represented 
by  (i)  the  direct  cerebellar  tract,  which  terminates  in  the  superior  vermis 
after  decussation;  (2)  the  anterior  and  posterior  arcuate  fibers,  the  former 
coming  from  the  gracile  and  cuneate  nuclei  of  the  opposite  side,  the  latter 
from  the  same  side,  which  also  pass  to  the  superior  vermis;  (3)  the  acoustico- 
cerebellar  tract,  composed  of  fibers  which  are  the  axons  of  the  sensory 
end-nuclei  (Deiters)  of  the  vestibular  portion  of  the  auditory  nerve.  It  is 
probable  that  all  these  fibers  decussate  prior  to  their  final  termination. 


THE  CEREBELLUM  605 

The  cerebellum  through  this  system  of  efferent  and  afferent  fibers  is 
brought  into  relation  with  many  different  regions  of  the  cerebrum,  pons, 
medulla,  and  spinal  cord.  Each  half  of  the  cerebellum  is  connected  with  the 
foregoing  structures  of  the  same  side,  and  of  the  opposite  side. 

THE  FUNCTIONS  OF  THE  CEREBELLUM. 

From  the  obser\'ations  of  the  results  of  experimental  lesions,  from  the 
analysis  of  clinico-pathologic  facts,  and  from  the  comparative  anatomic 
development  in  different  animals,  the  deduction  has  been  drawn  that  the 
cerebellum  coordinates  and  harmonizes  the  action  of  those  muscles  the 
activities  of  which  are  necessary  to  the  maintenance  of  body  equilibrium 
both  during  station  and  progression. 

By  eqiiilihrium  of  the  body  is  understood  a  condition  which  may  be  main- 
tained for  a  variable  length  of  time  without  displacement,  and  is  possible  only 
so  long  as  a  vertical  line  passing  through  the  center  of  gra\dty  falls  within  the 
base  of  support.  The  support  offered  by  the  earth  to  the  feet  neutralizes  and 
counteracts  the  force  of  gravity.  In  standing,  when  the  body  is  in  the  erect  or 
military  position,  the  arms  by  the  side,  the  center  of  gravity  lies  between  the 
sacrum  and  the  last  lumbar  vertebra,  and  the  line  of  gravity  falls  between  the 
feet  and  within  the  base  of  support.  The  entire  skeleton  for  the  time  being 
is  rendered  fixed  and  rigid  at  all  its  joints  by  the  combined  action  of  the 
muscles  connected  with  it.  That  this  position  may  be  maintained  all  the 
different  groups  of  antagonistic  but  cooperative  muscles  must  be  accurately 
coordinated  in  their  actions.  Any  failure  in  this  respect  is  at  once  attended 
by  a  disturbance  of  the  equilibrium  and  displacement. 

In  progression,  walking,  running,  dancing,  etc.,  the  body  is  translated 
from  point  to  point  by  the  alternate  action  of  the  legs.  Whether  the  direction 
of  the  translation  be  rectilinear  or  curvilinear,  as  the  legs  change  their  position 
from  moment  to  moment,  the  center  of  gravity  also  changes,  and  at  once  the 
equihbriura  is  menaced.  If  it  is  to  be  maintained  and  displacement  prevented 
there  must  be  a  prompt  readjustment  in  the  relation  of  all  parts  of  the 
body  so  that  the  line  of  gravity  falls  again  within  the  base  of  support.  The 
more  complicated  the  moments  of  progression,  or  the  narrower  the  base  of 
support,  the  greater  is  the  danger  to  the  equilibrium,  and  hence  the  necessity 
for  rapid  and  compensatory  changes  in  coordinated  muscle  activity.  All 
movements  of  this  character,  in  man  at  least,  are  primarily  volitional  and 
require  for  their  performance  the  constant  exercise  of  the  attention.  With 
frequent  repetition  they  gradually  come  to  be  performed  independently  of 
consciousness  and  fall  into  the  category  of  secondary  or  acquired  reflexes. 

Though  coordinating  power  is  exhibited  by  the  spinal  cord,  medulla, 
and  basal  ganglia,  it  is  only  in  the  cerebellum  that  this  power  attains  its 
highest  development  and  differentiation.  To  it  is  assigned  the  power  of 
selecting  and  grouping  muscles,  not  in  any  restricted  part,  but  in  all  parts  of 
the  body,  and  coordinating  their  actions  in  such  a  manner  as  to  preserve  the 
equilibrium. 

The  Results  of  Experimental  Lesions. — If  the  cerebellum  in  its  totality 
coordinates  and  harmonizes  the  action  of  the  muscles  on  the  opposite  sides  of 
the  body,  any  derangement  of  its  structure  or  its  connections  with  the  cord, 
medulla,  pons,  or  basal  ganglia  should  at  once  be  followed  by  incoordination 
of  muscles  and  a  want  of  harmony  in  their  action.     Experimental  lesions  of 


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TEXT-BOOK  OF  PHYSIOLOGY 


the  cerebellum  are  attended  by  such  results.  The  phenomena  observed  are 
many  and  complex.  They  differ  in  extent  and  character  in  different  animals 
and  in  accordance  with  the  extent  and  location  of  the  lesion,  though  the  note 
of  incoordination  runs  through  them  all. 


Fig.  257. — Attitude  Assumed  After  Destruction  of  the  Left  Half  of  the  Cerebellum. — 
(Moral  and  Doyon,  after  Thomas.) 

Removal  of  one  lateral  half  of  the  cerebellum  in  the  dog  is  followed  by  an 
inability  to  maintain  the  equilibrium  necessary  to  the  erect  position.  On 
attempting  to  stand,  the  animal  at  once  falls  toward  the  side  of  the  lesion,  the 
muscles  of  which  at  the  same  time  contract  and  give  to  the  body  a  distinctly 
curved  condition  (Fig.  257).  The  anterior  limbs  are  extended  to  the  opposite 
side.     On  making  efforts  to  regain  the  standing  position,  the  animal  may 

roll  over  around  the  long  axis  of  its 
body.  Conjugate  deviation  of  the 
eyes  is  frequently  observed  as  well  as 
nystagmus. 

After  a  few  days  the  symptoms  par- 
tially subside  and  the  animal  acquires 
the  power  of  sitting  on  the  abdomen 
when  the  anterior  limbs  are  widely 
extended  (Fig.  258).  As  the  days  go 
by  the  improvement  continues,  and  the 
animal  recovers  the  power  of  walking, 
though  each  step  is  attended  with 
tremor  and  oscillations  of  the  body. 
Any  change  in  the  center  of  gravity 
such  as  results  when  one  leg  is  lifted  may  result  in  a  fall  toward  the  side  of 
the  lesion,  owing  to  an  inability  to  promptly  bring  about  the  necessary 
compensatory  muscle  actions.  With  time  the  animal  continues  to  improve 
in  its  power  of  adjustment,  though  it  never  completely  recovers  it.  Move- 
ments of  progression  are  apt  to  be  characterized  by  stiffness  and  accom- 
panied by  tremor  suggestive  of  volitional  efforts. 

Total  removal  of  the  cerebellum  is  followed  by  a  different  train  of  symp- 
toms. The  extensor  muscles  apparently  preponderate  in  their  action,  for 
the  limbs  are  extended  and  abducted,  the  head  and  neck  are  retracted,  and 
opisthotonos  is  established.  In  time  these  effects  also  partially  subside, 
though  all  attempts  at  walking  are  permanently  accompanied  by  tremor  and 
oscillations.  The  characteristic  effect  which  follows  section  of  the  peduncles 
is  again  incoordination,  manifesting  itself  in  deviation  of  the  head,  eyes,  in- 
ability to  walk,  tremor  on  exertion,  etc.  The  effects  vary,  however,  according 
to  the  peduncle  divided.     Section  of  the  middle  peduncle  gives  rise  to  the 


Fig.  258. — Attitude  in  Repose  after 
the  Complete  Removal  of  the  Cere- 
bellum but  during  the  Period  of  Res- 
toration OF  Function. — {Marat  and  Doyon, 
after  Thomas.) 


THE  CEREBELLUM  607 

most  pronounced  effects.  The  head  and  the  anterior  part  of  the  body  are 
at  once  drawn  toward  the  pelvis  on  the  side  of  the  section.  A  voluntary 
effort  on  the  part  of  the  animal  causes  it  to  lose  all  control  of  its  muscles  and 
the  body  is  rotated  around  its  longitudinal  axis  from  40  to  60  times  a 
minute  before  it  comes  to  rest.  According  as  the  lesion  is  made  from  be- 
hind or  before,  the  rotation  is  from  or  to  the  side  of  the  section.  In  time 
these  symptoms  subside,  though  the  animal  never  completely  recovers. 

The  partial  recovery  of  the  power  of  coordination,  observ^ed  after  removal 
of  a  portion  or  the  whole  of  the  cerebellum,  indicates  that  the  centers  in  the 
cord,  medulla,  pons,  and  cerebrum 
endowed  with  corresponding 
though  less  developed  power,  de- 
velop compensatory  activity  and 
acquire  to  some  extent  the  capa- 
bilities of  the  cerebellum  itself 
(Fig.  259). 

Clinico-pathologic  facts  partly 
corroborate  the  results  of  physio- 
logic investigations.     In    various      ^  ^  ^ 
.  °           .         °        ,.          J           ,     ,,            Fig.    259. — Progression    after    Destruction 
forms  of  uncomplicated  cerebellar   of  the  Vermis.— (Mora^  and  Doyon,  ajter  Thomas.) 

disease,  vertigo,  tremor  on  making 

voluntary  efforts,  difficulty  in  maintaining  the  erect  position,  unsteadiness 

in  walking,  opisthotonos,  pleurothotonos,  are  among  the  symptoms  generally 

observ^ed. 

Comparative  anatomic  investigations  reveal  a  remarkable  correspondence 
between  the  development  of  the  cerebellum  and  the  complexity  of  the  move- 
ments exhibited  by  animals.  In  those  animals  whose  movements  are  com- 
plex and  require  for  their  performance  the  cooperation  of  many  groups  of 
muscles  the  cerebellum  attains  a  much  greater  development  in  reference  to 
the  rest  of  the  brain  than  in  animals  whose  movements  are  relatively  simple 
in  character.  This  relative  increase  in  the  development  of  the  cerebellum 
is  found  in  many  animals,  as  the  kangaroo,  the  shark,  the  swallow,  and  the 
predaceous  birds  generally. 

The  Coordinating  Mechanism.^ — Though  it  is  not  known  how  the 
cerebellum  selects  and  coordinates  groups  of  muscles  for  the  performance 
of  any  complex  movement,  it  is  known  that  its  activity  is  largely  reflex  in 
origin  and  excited  by  impulses  which  come  to  it  from  peripheral  organs.  In 
this  as  in  other  forms  of  reflex  activity  the  mechanism  involves  (i)  afferent 
nerves,  e.g.,  cutaneous,  muscle,  optic,  and  vestibular,  and  their  related  end- 
organs,  tactile  corpuscles,  muscle  spindles,  retina,  and  semicircular  canals, 
all  indirectly  connected  with  (2)  the  cerebellar  centers;  (3)  efferent  nerves 
indirectly  connected  with  (4)  the  general  musculature  of  the  body.  Both 
station  and  progression  are  directly  dependent  on  the  development  and 
transmission  of  afferent  impulses  from  the  pre\dously  mentioned  peripheral 
sense-organs  to  the  cerebellum.  Tactile,  muscle,  visual,  and  labyrinthine 
impressions  and  sensations  not  only  cooperate  in  the  development  and  or- 
ganization of  the  motor  adjustments  necessary  to  the  maintenance  of  the 
equilibrium  and  locomotive  coordination,  but  even  after  their  organization 
they  are  necessary  to  the  excitation  of  cerebellar  activity.  The  manner  in 
which  they  lead  to  the  development  of  this  capability  on  the  part  of  the  cere- 


6o8  TEXT-BOOK  OF  PHYSIOLOGY 

bellum  is  conjectural.  Their  ever-present  influence  is  shown  by  the  efifects 
which  follow  their  removal,  as  the  following  facts  indicate. 

The  prevention  of  the  development  of  tactile  impulses  by  freezing  or 
anesthetizing  the  soles  of  the  feet,  and  the  blocking  of  normally  developed 
impulses  through  destruction  of  afferent  pathways  in  diseases  of  the  spinal 
cord  lead  at  once  to  make  impairment  in  the  coordinating  power.  The 
removal  of  the  skin  from  the  hind  legs  of  the  frog,  previously  deprived  of  its 
cerebrum,  destroys  its  coordinating  power,  which  it  would  otherwise  possess 
in  a  high  degree. 

The  blocking,  in  consequence  of  destructive  lesions  of  the  spinal  cord, 
of  the  impulses,  which  come  from  the  muscles,  tendons,  etc.,  and  which  inform 
us  of  the  activity  and  the  degree  of  activity  of  our  muscles,  the  location  of 
limbs,  the  amount  of  effort  necessary  to  produce  a  given  movement,  etc., 
also  gives  rise  to  much  incoordination.  A  blocking  of  both  tactile  and 
muscle  impulses  frequently  exists  in  degeneration  or  sclerosis  of  the  posterior 
columns  of  the  spinal  cord.  The  coordinating  power  is  so  much  impaired 
in  this  disease  that  the  patient  is  unable  to  maintain,  without  strained  effort, 
the  erect  position  and  especially  if  the  directive  power  of  the  eyes  be  removed 
by  closure  of  the  lids.  Walking  becomes  extremely  difiicult;  the  gait  is 
irregular  and  jerky,  and  equilibrium  is  maintained  only  by  keeping  the 
eyes  fixed  on  the  ground  in  front  and  by  artificially  increasing  the  basis  of 
support  by  the  use  of  canes. 

An  interference  with  the  development  of  the  customary  visual  impres- 
sions which  in  a  measure  maintain  the  sense  of  relation  of  the  individual 
to  surrounding  objects  also  gives  rise  to  equilibratory  disturbances.  A 
rapid  change  in  the  relation  of  the  individual  to  surrounding  objects  or  the 
reverse;  a  change  in  the  direction  of  one  optic  axis  from  the  use  of  a  prism 
or  from  paralysis  of  an  eye  muscle;  the  destruction  of  an  eye — these  and 
similar  conditions  frequently  give  rise  to  such  marked  disturbances  of  the 
equilibratory  power  that  displacement  is  difficult  to  prevent. 

An  interference  with  the  development  of  the  so-called  labyrinthine  im- 
pressions by  destruction  of  the  semicircular  canals  gives  rise  to  the  most 
remarkable  disturbances  in  this  respect.  Section  of  one  horizontal  canaP 
in  the  pigeon  is  followed  by  oscillations  of  the  head  in  a  horizontal  plane 
around  a  vertical  axis.  Bilateral  section  so  increases  these  oscillations  that 
the  pigeon  is  unable  to  maintain  equilibrium  and  forced  to  fall  and  turn  con- 
tinuously around  the  vertical  axis.  Bilateral  section  of  the  posterior  vertical 
canals  gives  rise  to  oscillations  around  a  horizontal  axis  which  frequently  be- 
come so  exaggerated  as  to  eventuate  in  the  turning  of  backward  somersaults, 
head  over  heels.  Similar  phenomena  follow  division  of  the  superior  vertical 
canals. 

Bilateral  destruction  of  both  sets  of  canals  is  attended  by  extraordinary 
disturbances  in  the  equilibrium.  From  the  moment  of  the  operation  the 
animal,  the  pigeon,  loses  all  control  of  its  motor  mechanisms.  It  can  neither 
maintain  a  fixed  attitude  nor  execute  orderly  movements  of  progression;  its 
activity,  continuous  and  uncontrollable,  is  characterized  by  spinning  around 
a  vertical  axis,  turning  somersaults,  dashing  itself  against  surrounding  objects 
until  life  is  endangered.     If  the  animal  be  protected  from  injury,  these  dis- 

*  The  physiologic  anatomy  of  the  semicircular  canals  is  described  in  the  chapter   devoted 
to  the  ear,  to  which  the  reader  is  referred. 


THE  CEREBELLUM  609 

turbances  gradually  subside,  and  in  the  course  of  a  few  months  the  equilibra- 
tory  power  is  so  far  regained  that  standing  and  walking  at  least  become 
possible.  In  this  condition,  however,  the  coordinating  power  is  directly  de- 
pendent on  visual  impulses,  for  with  the  closure  of  the  eyes  all  the  previous 
motor  disturbances  at  once  recur.  These  and  similar  facts  indicate  that  the 
semicircular  canals  are  the  peripheral  sense-organs  from  which  come  the 
nerve  impulses  most  essential  to  the  excitation  of  the  cerebellar  coordina- 
tive  centers  in  their  control  of  equilibrium  and  of  progression. 

The  cerebellum  may  therefore  be  regarded  as  the  essential,  most  highly 
differentiated  portion  of  the  coordinating  mechanism  concerned  in  the  main- 
tenance of  equilibrium,  during  both  station  and  progression.  The  manner 
in  which  the  cerebellum  accomplishes  this  result  is  unknown,  though  it  is 
certain,  from  the  foregoing  facts,  that  its  special  mode  of  activity  is  dependent 
on  the  excitatory  action  of  nerve  impulses  transmitted  from  a  variety  of 
peripheral  sense-organs. 


39 


CHAPTER  XXV 
THE  ENCEPHALIC  OR  CRANIAL  NERVES 

The  nerve-trunks  which  serve  as  channels  of  communication  between  the 
encephalon  and  the  structures  of  the  head,  the  face,  and  in  part  the  organs 
of  the  thorax  and  abdomen,  pass  through  foramina  in  the  walls  of  the  cranium, 
and  for  this  reason  are  termed  cranial  nerves. 

According  to  the  classification  now  generally  adopted,  there  are  twelve 
cranial  nerves  on  either  side  of  the  median  line,  which,  enumerated  from 
before  backward,  are  as  follows  (Fig.  260) : 

The  First  or  Olfactory.  The  Seventh  or  Facial. 

The  Second  or  Optic.  The  Eighth  or  Acoustic. 

The  Third  or  Oculo-motor.  The  Ninth  or  Glosso-pharyngeal. 

The  Fourth  or  Trochlear.  '  The  Tenth  or  Pneumogastric  or  Vagus. 

The  Fifth  or  Trigeminal.  The  Eleventh  or  Spinal  Accessory. 

The  Sixth  or  Abducent.  The  Twelfth  or  Hypoglossal. 

The  cranial  nerves  may  be  classified  physiologically  in  accordance  with  their 
functional  manifestations  into  three  groups,  \'iz. : 

1.  The  Nerves  of  Special  Sense:  e.g.,  Olfactory,  Optic,  Acoustic,  Gustatory  (Glosso-phar- 
yngeal and  Chorda  tympani). 

2.  The  Nerves  of  General  Sensibility:  e.g.,  Large  root  of  the  Trigeminal,  Glosso-pharyngeal, 
and  Pneumogastric. 

3.  The  Nerves  of  Motion:  e.g.,  Oculo-motor,  Trochlear,  the  small  root  of  the  Trigeminal, 
Abducent,  Facial,  Spinal  Accessory,  and  Hypoglossal. 

Though  this  classification  in  the  main  holds  true,  it  must  be  borne  in  mind 
that  modern  investigations  have  demonstrated  that  the  glosso-pharyngeal 
and  pneumogastric  nerves  contain  even  at  their  junction  with  the  medulla 
oblongata  a  number  of  efferent  or  motor  fibers,  and  to  this  extent  are  mixed 
nerves. 

The  Origins  of  the  Cranial  Nerves. — In  accordance  with  modern 
views  as  to  the  origins  of  nerves  in  general,  it  may  be  stated  that — 

The  nerves  of  special  sense  have  their  origin  respectively  in  the  neuro- 
epithelial cells  in  the  mucous  membrane  of  the  olfactory  region  of  the  nose, 
in  the  ganglion  cells  of  the  retina,  in  the  cells  of  the  spiral  ganglion  of  the 
cochlea  and  the  ganglion  of  Scarpa,  and  in  the  cells  of  the  petrous  and  jugular 
ganglia.  From  the  cells  of  these  ganglia  dendrites  pass  peripherally  to 
become  associated  with  specialized  end-organs,  while  axons  pass  centrally 
in  well-defined  bundles  to  become  related  by  means  of  their  end-tufts  with 
primary  basal  ganglia. 

The  nerves  of  general  sensibility  have  their  origin  in  the  ganglia  on  their 
trunks,  and  in  this  respect  resemble  the  spinal  nerves.  From  the  ganglion 
cell  there  emerges  a  short  axon  process  which  soon  divides  into  a  central  and  a 
peripheral  branch.  The  former  passes  toward  and  into  the  gray  matter 
located  beneath  the  floor  of  the  fourth  ventricle,  where  its  end-tufts  arborize 

610 


THE  ENCEPHALIC  OR  CRANIAL  NERVES 


6ii 


about  nerve-cells.     The  latter  (the  peripheral  branch)  passes  toward  the 
general  periphery  to  be  distributed  to  skin  and  mucous  membranes. 

The  nerves  of  motion  have  their  origin  in  the  nerve-cells  in  the  gray  matter 
beneath  the  aqueduct  of  Syhdus  and  beneath  the  floor  of  the  fourth  ventricle. 
The  axons  emerging  from  these  cells  course  peripherally  to  be  distributed 
to  skeletal  muscles.  In  some  of  the  motor  nerves,  and  in  some  sensor 
nerves  as  well,  there  are  to  be  found  efferent  fibers  of  smaller  size  which 
have  a  similar  origin  and  which  become  related  through  the  intervention 
of  sympathetic  ganglia  (peripheral  neurons)  with  visceral  and  vascular 
muscles  and  glands.     These  nerves  have  been  termed  autonomic  nerves. 

The  Cortical  Connections  of  the  Cranial  Nerves. — Each  of  these  three 
groups  of  cranial  nerves  has  special  connections  with  the  cerebral  cortex. 

The  nerves  of  special  sense  for  the 
most  part  terminate  in  primary  basal 
ganglia,  around  the  cells  of  which  their 
central  end-tufts  arborize.  From  these 
cells  axons  arise  which  pass  upward  and 
directly  or  indirectly  come  into  physio- 
logic relation  with  sensor  nerve-cells  in 
the  cerebral  cortex. 

The  nerves  of  general  sensibility  ter- 
minate in  the  gray  matter  beneath  the 
floor  of  the  fourth  ventricle,  around  the 
nerve-cells  of  which  their  end-tufts  arbor- 
ize. These  groups  of  nerv^e-cells  are 
known  as  sensor  end-niiclei.  Though 
once  regarded  as  the  centers  of  origin  of 
the  sensor  nerves,  they  are  now  regarded 
as  the  centers  of  origin  of  axons  which 
pass  upward  to  the  cortex  of  the  cere- 
brum, where  they  also  come  into  physio- 
logic relation  with  sensor  nerve-cells. 

The  axons  in  both  of  these  classes  of 
nerves  thus  originate  in  the  cells  of  the 
central  nerve  system  and  continue  up- 
ward to  the  cerebrum,  the  primary  affer- 
ent path. 

The  motor  nerves  which  have  their 
origin  in  the  cells  of  the  gray  matter 
beneath  the  aqueduct  of  Sylvius  and  beneath  the  floor  of  the  fourth  ventricle 
are  in  physiologic  relation  with  nerve-cells  in  the  motor  region  of  the 
cortex  through  descending  axons  contained  in  the  pyramidal  tract,  the  end- 
tufts  of  which  arborize  around  the  ner\^e-cells.  The  efferent  path  be- 
ginning in  the  cerebral  cortex  is  thus  continued  by  the  motor  nerves  to  the 
general  periphery. 

The  three  groups  of  nerves,  those  of  special  sense,  of  general  sensibility, 
and  the  motor  nerves,  are  neurons  of  the  first  order;  the  nerve-cells  and 
fibers  which  constitute  the  cerebral  connections  are  neurons  of  the  second 
order.  It  is  probable  that  the  sensor  cells  in  the  cerebral  cortex  are  neurons 
of  a  third  order. 


Fig.  260. — Superficial  Origin  of 
THE  Cranial  Nerves  from  the  Base 
OF  THE  Encephalon.  I.  Olfactox)'.  2. 
Optic.  3.  ^Motor  oculi.  4.  Trochlear. 
5.  Trigeminal.  6.  .\bducent  7.  Facial. 
7'.  Xerv-e  of  ^^'l•isberg.  8.  Acoustic.  9. 
Glosso-phar}'ngeal.  10.  Pneumogastric. 
ir.  Spinal  accessory.  12.  Hypoglossal. — 
{Moral  and  Doyon.) 


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THE  FIRST  NERVE.     THE  OLFACTORY 

The  first  cranial  nerve,  the  olfactory,  is  situated  in  the  upper  third  of  the 
nasal  fossa,  in  the  regio  olfactoria.  It  consists  of  from  20  to  30  branches,  the 
fibers  of  which  are  non-medullated. 

Origin. — The  olfactory  nerve  is  composed  of  centrally  coursing  axons 
which  have  their  origin  in  the  central  ends  of  bipolar,  rod-shaped,  or  spindle- 
shaped  nerve-cells  interspersed  among  the  epithelial  cells  covering  the  mucous 
membrane  in  the  regio  olfactoria;  the  peripheral  ends  of  these  cells  give  oft 
a  number  of  dendrites  which  are  spread  out  to  form  a  delicate  feltwork  over 
the  surface  of  the  mucous  membrane.     From  their  origin  the  axons  gradually 

converge  to  form  bundles  which 
ascend  to  the  cribriform  plate  of 
the  ethmoid  bone,  through  the 
foramina  of  which  they  pass  to  be- 
come related  by  their  end-tufts  with 
structures  in  the  gray  matter  of 
the  olfactory  bulb  (Fig,  261). 

Cortical  Connections. — The 
olfactory  bulb  and  olfactory  tract, 
formerly  called  the  olfactory  nerve, 
are  portions  of  the  cerebrum  (the 
olfactory  lobe)  which  arise  em- 
bryologically  by  a  protrusion  of 
the  walls  of  the  cerebral  cavity. 
The  bulb  is  oval-shaped  and  con- 
sists of  both  gray  and  white  matter. 
It  rests  on  the  cribriform  plate  of 
the  ethmoid  bone  and  is  embraced 
by  the  olfactory  nerves.  As  seen 
on  sagittal  section,  there  is  just  be- 
neath the  surface  a  layer  of  large 
pyramidal  and  spindle-shaped 
cells  (termed  also  mitral  cells), 
each  provided  with  an  apical  and 
two  lateral  dendrites.  The  apical  dendrite  passes  toward  the  surface  and 
ends  in  a  brush-  or  basket-like  expansion  which  interlaces  with  the  end- 
tufts  of  the  olfactory  nerves,  forming  what  are  known  as  the  olfactory  glom- 
erules.     The  lateral  dendrites  end  free. 

The  axons  of  the  pyramidal  cells  pass  toward  the  center  of  the  bulb  and 
bend  at  right  angles,  after  which  they  pursue  a  horizontal  direction  toward 
and  into  the  olfactory  tract.  This  tract  is  about  five  centimeters  in  length, 
prismatic  in  shape  on  cross-section  and  divisible  into  a  ventral  and  a  dorsal 
portion.  It  emerges  from  the  posterior  extremity  of  the  bulb,  passes  back- 
ward to  the  posterior  part  of  the  anterior  lobe,  where  it  divides  into  three 
roots:  viz.,  a  lateral  or  external,  a  mesial  or  internal,  a  middle  or  dorsal. 
The  fibers  of  the  lateral  and  mesial  roots  are  derived  almost  exclusively 
from  the  ventral  portion  of  the  tract,  the  fibers  of  which  come  from  the 
mitral  cells  in  the  bulb.  The  lateral  root-fibers  pass  outward  into  the  fossa  of 
Sylvius  and  come  into  relation  with  nerve-cells  in  the  inferior  extremity  of 


Fig.  261.— The  Relation  of  the  Olfactory 
Nerves  to  the  Olfactory  Tract,  i.  Ol- 
factory nerve-cell.  2.  Axon  process.  3.  Epi- 
thelial cells.  4.  Glomerulus.  5.  Mitral  cells. 
6.  Centrally  coursing  axons  of  the  olfactory 
tract. — {Morat  and  Doyon.) 


THE  ENCEPH.ALIC  OR  CRANIAL  NERVES 


613 


the  gyrus  hippocampus  and  the  gyrus  uncinatus.  The  mesial  fibers  pass 
inward  and  come  into  relation  with  nerve-cells  in  the  pre-callosal  part  at 
least  of  the  gyrus  fornicatus.  The  fibers  thus  far  considered  are  undoubt- 
edly true  olfactor}^  fibers,  pursuing  a  centripetal  direction,  carrying  nerve 
impulses  from  the  olfactory  cells  to  the  cerebrum  (Fig.  262). 

Histologic  and  embryologic  methods  of  research  have  shown  that  some 
of  the  fibers  in  the  olfactory  tract  are  centrifugal  in  function.  They  originate 
in  the  olfactory  cortical  areas,  pass  toward  the  periphery  as  far  as  the  an- 
terior commissure,  where  they 
cross  to  become  the  dorsal 
root,  enter  the  olfactory  tract, 
and  finally  terminate  in  the 
bulb.  This  tract  serves  to 
connect  the  cortex  with  the 
bulb  of  the  opposite  side,  and 
carries  impulses  from  the  cor- 
tex to  the  bulb.  The  two  op- 
posite cerebral  olfactory  areas 
are  also  united  by  commissural 
fibers  which  decussate  at  the 
anterior  commissure. 

Function. — The  function 
of  the  olfactory  system  in  its 
entirety  is  the  transmission  of 
nerve  impulses  from  its  origin 
in  the  olfactory  region  of  the 
nose  to  the  cerebral  cortex, 
where  they  evoke  sensations 
of  odor.  The  stimulus  to  its 
excitation  is  the  impact  and 
chemic  action  of  gaseous  or 
volatile  organic  matter  on  the 
dendrites  of  the  olfactory  cells. 
The  sensitiveness  of  the  olfac- 
tory end-organ  to  the  action 
of  many  substances  is  remark- 
able, responding,  for  example,  to  the  j ToioTo  ^^  ^  gram  of  oil  of  roses  and  to 
the  o-y-g-o Too"  of  3.  gram  of  mercaptan. 

Division  or  destruction  of  the  olfactory  path  at  any  point  is  followed 
by  an  abolition  of  the  sense  of  smell  on  the  corresponding  side.  Destructive 
lesions  of  the  hippocampal  and  uncinate  gyri  are  followed  by  similar  results. 


Fig.  262. — Olfactory  Lobe  of  the  Human  Br.'^in. 
—Bu.  Olfacton-  bulb.  T.  Tract.  Tr.o.  Trigone.  R. 
Rostrum  of  corpus  callosum.  p.  Peduncle  of  corpus 
callosum,  passing  into  G.  s.,  gyrus  subcallosus  (diagonal 
tract,  Broca).  Br.  Broca's  area.  i^./>.  Fissura  prima. 
F.s.  Fissura  serotina.  C.a.  Position  of  anterior  com- 
missure. L.t.  Lamina  terminalis.  Ch.  Optic  chiasma. 
T.o.  Optic  tract,  p.  olf.  Posterior  olfactors'  lobule  (or 
anterior  perforated  space),  m.r.  Mesial  root,  l.r. 
Lateral  root  of  tract. — (His.) — A/ler  Qtiain.) 


THE  SECOND  NERVE.    THE  OPTIC 

The  second  cranial  nerve,  the  optic,  consists  of  centrally  coursing  axons 
of  neurons  which  connect  the  essential  part  of  the  organ  of  vision,  the  retina, 
with  sensory  end-nuclei  or  ganglia  situated  at  the  base  of  the  cerebrum. 

Origin. — The  axons  which  constitute  the  optic  nerve  have  their  origin 
in  the  ganglion  cells  in  the  anterior  part  of  the  retina.  Through  their 
dendrites  these  cells  are  brought  into  relation  posteriorly  with  successive 


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TEXT-BOOK  OF  PHYSIOLOGY 


layers  of  cells  which  collectively  constitute  the  retina.  Though  the  retina  is 
said  to  consist  of  teii  or  eleven  layers,  it  may  be  reduced  practically  to  three, 
viz.  (Fig.  263): 

1.  The  layer  of  visual  cells. 

2.  The  layer  of  bipolar  cells. 

3.  The  layer  of  ganglionic  cells. 

The  visual  cells  present  peripherally  modified  dendrites,  known  as  the 
rods  and  cones;  centrally  they  give  off  an  axon  which  after  a  short  course 
terminates  in  an  end-tuft.  The  bipolar  cells  also  possess  dendrites  and  an 
axon;  the  former  interlace  with  the  end-tufts  of  the  visual  cell  axon,  the  latter 
with  the  dendrites  of  the  ganglion  cell.  The  retina  may 
be  regarded  therefore  as  the  peripheral  end-organ  in 
which  the  optic  nerve  originates.  From  their  origin  the 
axons  turn  backward,  at  the  same  time  converging  to 
form  a  distinct  bundle  which  passes  through  the  cho- 
rioid  coat  and  sclera.  After  emerging  from  the  eyeball 
the  nerve-bundle  (the  optic  nerve)  passes  backward  as 
far  as  the  sella  turcica,  traversing  in  its  course  the  orbit 
cavity  and  the  optic  foramen.  At  the  sella  turcica  there 
is  a  union  and  partial  decussation  in  man  and  other 
mammals  of  the  two  nerves,  forming  the  optic  chiasm.^ 

The  Decussation  of  the  Optic  Nerves. — ^The  extent 
to  which  the  fibers  from  each  eye  decussate  at  the  chiasm 
is  a  subject  of  dispute,  but  the  results  of  various  methods 
of  research  would  seem  to  indicate  that  the  fibers  from 
the  nasal  third  of  the  retina  of  the  left  eye  cross  in  the 
chiasm,  to  unite  with  the  fibers  from  the  temporal  two- 
thirds  of  the  retina  of  the  right  eye.  In  a  similar  man- 
ner the  fibers  from  the  nasal  third  of  the  retina  of  the 
right  eye  cross  in  the  chiasm,  and  unite  with  the  fibers 
from  the  temporal  two-thirds  of  the  retina  of  the  left 
eye  (Fig.  264).  Posterior  to  the  chiasm  the  crossed  and 
uncrossed  fibers  form  the  so-called  optic  tracts,  which  after 
winding  around  the  crura  cerebri  enter  the  optic  basal  ganglia.  Tran- 
section of  the  optic  nerve  shows  that  it  is  composed  of  an  enormous  number 
of  non-medullated  nerve-fibers,  estimated  by  Salzer  at  from  450,000  to 
800,000,  enclosed  in  a  sheath  of  the  dura  mater. 

The  visual  fibers  comprising  the  optic  nerve  may  be  physiologically 
divided  into  two  clases,  (a)  those  coming  from  the  peripheral  portion  of  the 
retina,  and  (b)  those  coming  from  that  central  area  known  as  the  macula 
lutea.  The  retinal  fibers  are  by  far  the  more  abundant,  and  make  up  the 
major  portion  of  the  nerve;  the  macular  fibers  are  less  abundant.  An  ex- 
amination of  a  cross-section  of  the  optic  nerve  shows  the  presence  of  a  wedge- 
shaped   tract   occupying   the   center   of   the   nerve   which  is  regarded  as 

'  Though  the  foregoing  is  the  usual  method  of  stating  the  origin  and  course  of  the  optic  nerve, 
nevertheless  morphologically  the  true  optic  nerve  Hes  wholly  within  the  retina  and  is  composed 
of  the  visual  cells  there  found.  The  remainder  of  the  visual  system  from  and  including  the 
ganglion  cells  of  the  retina  to  the  optic  basal  gangUa,  is  the  optic  tract,  there  being  no  anatomic 
or  physiologic  distinction  between  the  optic  nerve  so  called  and  the  optic  trad.  Both  are  out- 
growths from  the  brain  and  hence  possess  properties  which  differentiate  them  from  other  cranial 
nerves. 


Fig.  263. — Reti- 
nal Cells,  s',  z'. 
Visual  cells  with 
their  peripheral  ter- 
minations. 5.  Rods. 
2.  Cones,  b.  B  i- 
polar  cells,  g.  Gan- 
glion cells  from  which 
arise  the  axons  of  the 
optic  nerve. 


THE  ENCEPHALIC  OR  CRANIAL  NERVES 


615 


VISUAL  miD  VISUAL  HEID 

Mas./ u..... AM       ^^ 


^^H^iSL 


composed  of  the  macular  fibers.  At  the  chiasm  this  bundle  of  fibers  un- 
dergoes a  partial  decussation  similar  to  that  of  the  fibers  coming  from  the 
more  peripheral  portions  of  the  retina.  In  the  left  optic  tract,  therefore,  fibers 
from  at  least  four  different  regions  are  to  be  found:  viz.,  the  two-thirds  of 
the  temporal  side  of  the  left  retina,  the  temporal  half  of  the  left  macula, 
the  nasal  third  of  the  right  retina,  and  the  nasal  half  of  the  right  macula. 
Corresponding  fibers  are  to  be  found  in  the  right  optic  tract.  As  the  optic 
tract  passes  around  the  cms  cerebri 
it  divides  into  a  lateral  or  outer,  and 
a  mesial  or  inner  bundle,  which  then 
terminate  in  the  optic  basal  ganglia. 
The  fibers  of  the  lateral  bundle  are 
traceable  into  the  lateral  or  external 
geniculate  body  (the  pre-geniculum) , 
the  pulvinar  of  the  optic  thalamus, 
and  the  anterior  quadrigeminal  body 
(thepre-geminum).  With  the  excep- 
tion of  the  fibers  passing  to  the  ante- 
rior quadrigeminal  body,  these  are 
in  all  probability  the  true  visual  fibers. 
The  fibers  of  the  mesial  bundle  are 
traceable  into  the  internal  geniculate 
body  (the  post-geniculum)  and  the 
posterior  quadrigeminal  body  (the 
post-geminum).  These  ^  fibers  are 
not  a  part  of  the  optic  nerve  proper, 
but  commissural  fibers  associating 
the  internal  geniculate  bodies  of  the 
two  sides.  {Gudden's  Commissure.) 
Cortical  Connections. — After 
entering  the  pulvinar  and  the  lateral 
or  external  geniculate  body  the  visual 
fibers  terminate  in  end-tufts  which 
arborize  around  nerve-cells.  From 
these    cells    new    axons  arise  which  Visual  Fields  Indicate  the  Darkened  Akea. 

ascend  through  the  posterior  part  of  the  internal  capsule,  at  the  same  time 
curving  backward  to  form  the  optic  radiation  of  Gratiolet,  and  terminate 
finally  around  nerve-cells  in  the  gray  matter  of  the  cuneus  and  in  the  gray 
matter  bordering  the  calcarine  fissure,  both  situated  on  the  mesial  aspect 
of  the  occipital  lobe. 

Centrifugal  Fibers  of  the  Optic  Nerve. — All  the  fibers  previously  alluded 
to  have  been  afferent  or  centripetal  in  direction;  but  the  optic  nerve  also 
contains  efferent  or  centrifugal  fibers  which  come  from  nerve-cells  in  the 
basal  ganglia  and  ramify  around  special  cells,  the  amacrine  cells,  in  the 
retina.  Their  function  is  unknown.  It  has  been  suggested  that  they 
regulate  the  vascular  supply  to  the  retina.  Centrifugally  coursing  fibers  also 
connect  the  visual  areas  of  the  cortex  with  the  superior  quadrigeminal  body. 

Function. — The  function  of  the  optic  nerve  and  tract  is  the  transmission 
of  nerve  impulses  from  the  retina  to  the  cerebral  cortex  where  they  evoke  the 
sensations  of  light  and  its  different  qualities — colors.     The  specific  physio- 


FiG.  264. — Diagram  Illustrating  Left 
Homonymous  Lateral  Hemunopsia  from  a 
Lesion  of  the  Right  Optic  Tract  or  the 
Right  Cuneus.    The  Shaded  Lines  in  the 


6i6  TEXT-BOOK  OF  PHYSIOLOGY 

logic  stimulus  to  the  retinal  visual  cells  is  the  impact  of  the  undulations  of 
the  ether.  In  general  it  may  be  said  that,  at  least  for  the  same  color,  the 
intensity  of  the  objective  undulation  or  vibration  determines  the  intensity  of 
the  sensation. 

Owing  to  the  partial  decussation  of  the  optic  nerve-fibers  at  the  chiasma, 
the  impressions  made  on  the  temporal  side  of  the  retina  are  received  by  the 
cuneus  of  the  same  side,  while  the  impressions  made  on  the  nasal  side  are  re- 
ceived by  the  cuneus  of  the  opposite  side.  Each  eye  is,  therefore,  associated 
with  the  cuneus  of  each  cerebral  hemisphere. 

Pupillary  Fibers. — The  optic  nerve  also  contains  nerve-fibers  some- 
what larger  in  caliber  than  the  usual  visual  fibers,  which  are  sup- 
posed to  form  the  afferent  path  for  those  nerve  impulses  which  excite 
reflexly  a  contraction  of  the  sphincter  pupillcB  muscle,  thus  varying  the  size 
of  the  pupil.  These  fibers,  termed  pupillary  fibers,  come  from  all  portions 
of  the  retina  but  most  abundantly  from  the  posterior  pole  in  and  around 
the  macula.  The  existence  of  these  fibers  is  confirmed  by  pathologic  find- 
ings. In  a  manner  similar  to  that  of  the  visual  fibers  they,  too,  undergo  a 
decussation  in  the  optic  chiasm,  so  that  in  the  optic  tract  there  are  pupil- 
lary fibers  which  come  from  the  temporal  side  of  the  eye  of  the  corresponding 
side,  and  fibers  which  come  from  the  nasal  side  of  the  eye  of  the  opposite 
side  (Fig.  268,  page  620).  The  central  termination  of  these  fibers  is  not 
positively  known. 

Hemiopia  and  Hemianopsia. — Division  of  the  optic  nerve  between  the 
eyeball  and  the  optic  chiasm  is  followed  by  complete  blindness  in  the  eye  of 
the  corresponding  side.  Owing  to  the  partial  decussation  of  the  fibers  in 
the  chiasm,  division  of  an  optic  tract  is  followed  by  a  loss  of  sight  in  the  outer 
two-thirds  of  the  eye  of  the  same  side  and  in  the  inner  third  of  the  eye  of  the 
opposite  side.  To  this  loss  of  visual  power  in  the  retina  the  term  hemiopia 
is  given.  In  consequence  of  this  loss  of  visual  power  in  the  retina  there  is 
a  corresponding  obscuration  or  total  obliteration  of  nearly  one-half  of  the 
visual  field/  to  which  the  term  hemianopsia  is  given.  If,  for  example,  the 
right  optic  tract  is  divided  there  will  be  hemiopia  in  the  outer  two-thirds  of 
the  right  eye  and  the  inner  third  of  the  left  eye,  with  left  lateral  hemianopsia, 
and  as  the  portions  of  the  retina  which  are  affected  are  associated  in  vision 
the  loss  of  the  visual  fields  is  spoken  of  as  homonymous  hemianopsia  (Fig. 
266).  A  destructive  lesion  of  the  cerebral  visual  area,  the  cuneus  and 
the  adjacent  gray  matter  on  the  right  side,  is  also  followed  by  left  lateral 
hemianopsia.^ 

*  The  visual  field  comprises  that  portion  of  the  external  world  from  which,  with  the  eyes 
stationary,  rays  of  Ught  pass  to  the  retinae  and  is  the  area  included  between  the  extremes  of 
the  visual  lines  entering  the  pupil.  The  center  of  the  visual  field  is  the  area  the  rays  of  hght  from 
which  are  focahzed  on  the  fovea  centralis.  The  visual  field  is  somewhat  irregular  in  outUne  by 
reason  of  the  position  of  the  eyeball  in  the  orbit  cavity,  and  the  consequent  interference  with 
the  entrance  of  light  by  the  bridge  of  the  nose,  the  cheek  bones,  and  the  eyebrows.  The  hori- 
zontal diameter  of  the  visual  field  for  the  right  eye  is  about  150°,  of  which  90°  pertain  to  the 
temporal  and  60°  to  the  nasal  portion.  The  vertical  diameter  is  about  115°,  of  which  45°  pertain 
to  the  superior  and  70°  to  the  inferior  portion.  By  reason  of  the  position  of  the  eyes  in  the  orbit 
cavity  the  two  visual  fields,  viz.:  that  of  the  right  and  of  the  left  eye,  overlap  to  a  variable  extent  in 
their  nasal  divisions. 

*  It  should  be  borne  in  mind  that  in  both  instances  the  retina  itself  is  unaffected.  The  impact 
of  light  generates,  as  usual,  nerve  impulses  which  proceed  as  far  backward  as  the  point  of  division 
or  destruction.  In  consequence  those  portions  of  the  cerebral  cortex  stimulation  of  which  evokes 
the  sensation  of  Ught  remain  unaffected  and  the  individual  does  not  become  aware,  through 
sensation,  of  the  presence  of  a  luminous  body  in  the  left  side  of  the  visual  field. 


THE  ENCEPHALIC  OR  CRANIAL  NERVES 


617 


The  existence  of  a  homonymous  hemianopsia  becomes  evident  when 
the  individual  is  directed  to  focus  the  vision  on  an  object  placed  directly  in 
front  and  with  its  center  in  the  median  plane  of  the  body,  when  if  the  lesion 
be  on  the  right  side,  the  left  half  of  the  object  will  be  invisible.  The  reason 
for  this  will  be  apparent  on  reference  to  Fig.  265.  All  the  light  rays  emanating 
from  the  left  half  of  the  object  fall  on  the  retina  on  the  side  of  the  injury, 
and  hence  there  will  be  no  sensation.  If,  however,  the  object  be  moved  to 
the  right  without  change  in  the  position  of  the  head,  the  entire  object  will  be 
visible,  as  all  the  rays  fall  on  the  normal  side.  If,  on  the  contrary,  the 
object  be  moved  to  the  left,  it  will  be  invisible  for  the  opposite  reason. 

Hemianopsia  may  be  the  result  of 
either  destruction  of  the  optic  tract  or 
of  the  cortical  visual  area.  The  seat 
of  lesion  in  any  given  case  is  indicated 
by  a  peculiarity  of  the  iris  reflex 
pointed  out  by  Wernicke,  which  will 
be  referred  to  in  connection  with  the 
considei*ation  of  the  oculo-motor 
nerve. 

THE  THIRD  NERVE.     THE 
OCULO-MOTOR 

The  third  cranial  nerve,  the  oculo- 
motor, consists  of  some  15,000  pe- 
ripherally coursing  nerve-fibers  which 
serve  to  bring  the  nerve-cells  from 
which  they  arise  into  relation  with  a 
large  portion  of  the  general  muscula- 
ture of  the  eye. 

Origin. — The  axons  composing 
the  third  nerve  arise  from  a  series  of 
seven  or  eight  groups  of  nerve-cells, 
located  in  the  gray  matter  beneath  the  floor  of  the  aqueduct  of  Sylvius. 
From  each  of  these  groups  or  nuclei,  bundles  of  axons  emerge,  which  after 
a  short  course  unite  to  form  the  common  trunk.  The  large  majority  of 
the  fibers  in  the  nerve  come  directly  from  the  nuclei  of  the  same  side;  the 
remainder  come  from  a  group  of  cells  on  the  opposite  side  of  the  median 
line.     There  is  thus  a  partial  decussation  of  its  fibers  (Fig.  266). 

The  different  groups  of  cells,  the  nuclei  of  origin,  are  arranged  in  a  serial 
manner.  The  anatomic  arrangement  of  these  nuclei  would  indicate  that 
each  nucleus  is  related  to  an  individual  member  of  the  eye-group  of  muscles. 
Clinical  observation  and  the  investigation  of  the  results  of  pathologic  processes 
have  not  only  shown  that  this  is  the  case,  but  also  succeeded  in  locating  the 
position  of  the  nucleus  for  any  given  muscle.  Though  there  is  some  differ- 
ence of  opinion  in  regard  to  the  exact  location  of  one  or  two  of  the  nuclei, 
the  tabulation  subjoined  is  approximately  correct. 

Enumerating  them  from  before  backward,  the  nuclei  occur  in  the  follow- 
ing order: 

1.  The  sphincter  pupillae. 

2.  The  tensor  chorioideas  (the  accommodation  nucleus). 


Fig.  265. — Diagram  to  Show  the  Exist- 
ence OF  Hemianopsia.  The  lesion  is  sup- 
posed to  be  in  the  right  optic  tract. 


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TEXT-BOOK  OF  PHYSIOLOGY 


The  convergence  nucleus,  a  common  nucleus  for  the  conjoint  action  of 

the  two  internal  recti  muscles. 
The  superior  rectus. 
The  inferior  rectus. 
The  levator  palpebrae. 
The  inferior  oblique. 
Cortical  Connections. — The  oculo-motor  nuclei  are  in  histologic  and 
physiologic  relation  with  the  motor  area  of  the  cerebrum.     Nerve-cells  in  the 

cortex  give  off  axons  which,  entering  the 
pyramidal  tract,  descend  through  the  inter- 
nal capsule,  and  the  crus  cerebri,  from  which 
they  cross  to  the  opposite  side.  The  end- 
tufts  arborize  around  the  nuclei  of  the 
oculo-motor  nerve  with  the  exception  of  the 
nucleus  for  the  iris  sphincter. 

Distribution. — After  their  origin  the 
axons  converge  to  form  a  common  trunk, 
which  emerges  from  the  base  of  the  enceph- 
alon,  on  the  inner  side  of  the  crus  cerebri, 
in  front  of  the  pons  Varolii.  The  nerve  then 
passes  forward  through  the  sphenoid  fissure 
into  the  orbit  cavity,  where  it  divides  into 
a  superior  and  an  inferior  branch.  The 
former  is  distributed  to  the  superior  rectus 
and  the  levator  palpebrcB  muscles;  the  latter 
is  distributed  to  the  internal  and  inferior 
recti  and  inferior  oblique  muscle  (Fig.  267). 
From  the  inferior  branch  a  short  bundle 
of  fibers  passes  to  the  ciliary  or  ophthalmic 
ganglion,  where  they  terminate,  arborizing 
around  the  ganglion  cells.  These  fibers  are 
smaller  in  size  than  those  constituting  the 
bulk  of  the  nerve  and  belong  to  the  system 
known  as  the  autonomic.  These  cells  give 
origin  to  new  axons,  the  ciliary  nerves,  which 
enter  the  eyeball,  pass  forward  between 
the  sclera  and  chorioid  coat,  and  terminate 
in  the  ciliary  muscle  and  the  sphincter  of 
the  pupil.  The  ciliary  nerves  are  not  por- 
tions of  the  third  nerve  proper,  but  periph- 
eral sympathetic  neurons.  As  the  ciliary 
ganglion  receives  filaments  from  the  caver- 
nous plexus  of  the  sympathetic  and  fila- 
ments which  become  a  part  of  the  trigeminal  nerve,  it  is  probable  that  the 
ciliary  nerves  contain  not  only  motor,  but  vaso-motor  and  sensor  fibers 
as  well. 

Properties. — Stimulation  of  the  nerve  near  its  exit  from  the  encepha- 
lon  is  followed  by  contraction  of  the  muscles  to  which  it  is  distributed 
with  the  following  results,  viz. : 
I.  Diminution  in  the  size  of  the  pupil. 


Fig.  266. — Diagrammatic  View  of 
THE  Situation  and  Relation  of 
THE  Nuclei  of  Origin  of  the 
Oculo-motor  and  Patheticus 
(Trochlearis)  Nerves.  The  oculo- 
motor nuclei  consist  of  an  anterior 
nucleus,  the  Edinger-Westphal  nucleus 
(a  and  h),  and  a  posterior  nucleus; 
the  posterior  nucleus  has  a  dorsal,  a 
ventral,  and  a  mesial  portion;  the  de- 
cussation of  fibers  from  the  dorsal 
portion  of  the  posterior  nucleus  is  also 
shown.  The  decussation  of  the  fibers 
of  the  fourth  nerve  is  also  represented. 
— i^Edinger.) 


THE  ENCEPHALIC  OR  CRANIAL  NERVES 


619 


2.  Accommodation  of  the  eye  for  near  vision. 

3.  Elevation  of  the  upper  eyelid. 

4.  Internal  deviation  and  rotation  upward  and  inward  of  the  anterior  pole 

of  the  eye,  combined  with  a  small  amount  of  torsion  toward  the  mesial 
line,  due  to  preponderating  action  of  the  internal  rectus  and  inferior 
oblique  muscles. 
Division  of  the  nerve  either  experimentally  or  as  a  result  of  compression 

from  a  pathologic  cause  is  followed  by  a  relaxation  of  the  muscles,  with  the 

following  effects,  viz.: 

1.  Dilatation  of  the  pupil,  the  iris 

responding  neither  to  light  nor 
to  efforts  of  accommodation. 

2.  Loss     of     the    accommodative 


the    upper    eyelid 


Fig.  267. — Intra-orbital  Portion  of  the 
Third  Nervk.  i.  Optic  nerve.  2.  Third 
nerve.  3.  Superior  branch.  4.  Injerior  branch. 
5.  Abducens.  6.  Trifacial.  7.  Ophthalmic 
branch  divided.  8.  Nasal  branch.  9.  Ciliary 
ganglion.  10.  Motor  branch  to  this  ganglion 
from  the  inferior  branch  of  the  third  nerve.  1 1 . 
Sensory  fibers.  12.  Sympathetic  fibers.  13. 
Ciliary  nerves. — {Sappey.) 


power. 

3.  Falling     of 

(ptosis). 

4.  External  deviation  and  rotation 

downward  and  outward  of  the 
anterior  pole  of  the  eyeball 
combined  vvdth  a  small  amount 
of  torsion  toward  the  mesial 
line  due  to  the  unopposed 
action  of  external  rectus  and 
the  superior  oblique  muscles. 

5.  Double  vision  or  diplopia.     The 

image  of  the  eye  of  the  para- 
lyzed side  is  projected  to  the 
opposite  side  of  the  true  image  and  to  the  upper  part  of  the  visual  field. 
Owing  to  the  slight  mesial  torsion  the  false  image  is  inclined  away 
from  the  true  image. 

6.  Immobility  and  slight  protrusion  of  the  eyeball. 

Function. — The  function  of  the  third  nerve  is  to  transmit  nerve  im- 
pulses from  the  nuclei  of  origin  to  all  the  muscles  of  the  eye  except  the  ex- 
ternal rectus  and  superior  oblique  and  excite  them  to  activity.  The  majority 
of  the  ocular  movements,  the  power  of  accommodation,  the  variations  in 
the  size  of  the  pupil  in  accordance  with  variations  in  the  intensity  of  the  light, 
the  power  of  convergence  of  the  visual  axes,  are  all  excited  by  the  trans- 
mission of  nerve  impulses  by  the  constituent  fibers  of  the  nerve  from  their 
related  nuclei.  This  is  made  evident  by  the  effects  which  follow  stimula- 
tion and  division  of  the  nerve  or  lesions  of  the  nuclei  themselves. 

The  central  nuclei  can  be  excited  to  activity  (i)  by  nerve  impulses  de- 
scending the  motor  tract,  from  the  cerebral  cortex,  (2)  by  nerve  impulses 
coming  through  various  afferent  nerves.  This  holds  true  more  especially 
for  the  sphincter  pupillae  nucleus. 

The  Iris  Reflex  or  the  Pupillary  Reflex. — These  are  terms  applied 
to  the  variations  in  the  size  of  the  pupil  that  follow  variations  in  the  inten- 
sity of  the  light.  In  the  absence  of  light  the  pupil  widely  dilates,  due 
largely  to  the  relaxation  of  the  sphir^ter  pupillcB  muscle  and  partly  to  a  con- 
traction of  the  radiating  fibers  of  tne  iris,  which  collectively  constitute  the 
dilatator  piipillcB  muscle.     With  the  entrance  of  light  into  the  eye,  the  pupil 


620 


TEXT-BOOK  OF  PHYSIOLOGY 


diminishes  in  size,  in  consequence  of  the  contraction  of  the  sphincter  pupillcB 
caused  by  a  stimulation  of  the  peripheral  ends  of  the  pupillary  fibers  of  the 
retina,  the  degree  of  contraction  depending  within  limits  on  the  intensity 

of  the  light. 

The  action  of  the  sphincter  pupillae  muscle  is,  therefore,  a  reflex  action 
and  involves  the  usual  mechanism,  viz.:     A  receptive  surface,  the  retina; 

afferent  nerves,  the  pupil- 
lary fibers  in  the  optic 
nerve;  an  emissive  center, 
the  sphincter  nucleus  of 
the  motor  oculi  center;  effer- 
ent nerves,  including  fibers 
in  the  trunk  of  the  motor 
oculi  and  in  the  cihary 
nerves;  and  a  responsive 
organ,  the  muscle.  (See 
Fig.  268).  That  this  is  the 
mechanism  involved  in  this 
reflex,  is  shown  by  the  fact 
that  when  any  portion 
of  it  is  destroyed,  the 
reflex  contractions  of  the 
sphincter  are  impaired  or 
abolished. 

As  stated  in  a  preceding 
paragraph  the  central  ter- 
mination of  the  afferent 
pupillary  fibers  concerned 
in  this  reflex  is  not  posi- 
tively known.  No  one  has 
as  yet  succeeded  in  tracing 
these  fibers  directly  to  the 
sphincter  nucleus.  It  has 
been  shown,  however,  that 
as  the  optic  tract  ap- 
proaches its  termination 
the  visual  and  the  pupil- 
lary fibers  separate  and  it 
has  been  assumed  that  the 
latter  come  into  anatomic 
relation  with  some  inter- 
calated system  which  in  turn  is  connected  with  the  sphincter  nucleus.  As 
to  the  situation,  origin  and  course  of  this  system  nothing  positively  is  known. 
There  is  some  evidence  for  the  view  that  this  intercalated  system  has  its 
origin  in  the  anterior  corpora  quadrigemina  as  shown  in  the  accompanying 

diagram.  •        r  u  -i 

The  contraction  of  the  sphincter  and  a  diminution  in  the  size  of  the  pupil 
may  be  direct,  as  when  the  light  which  enters  one  eye  causes  a  reflex  contrac- 
tion of  the  sphincter  of  one  and  the  same  side;  or  it  may  be  indirect  or 
consensual,  as  when  the  light,  which  enters  one  eye  only,  causes  a  contraction 


Fostffanffliomc  fibers. 
Sup.  Cervical  Ga/i^lion-L 
B'e^a/f.g'liomc  fibers, 

2"^1horacic  Nent 

Ni 

[Transect ion  of  Spinal  Cord 


Fig.  268. — Diagram  Designed  to  Show  the  Mechan- 
ism OF  THE  Iris  Reflex.  The  central  termination  of  the 
pupillary  fibers  is  hypothetical. 


THE  ENCEPHALIC  OR  CRANIAL  NERVES  621 

of  the  sphincter  not  only  in  the  eye  of  the  same,  but  in  the  eye  of  the  opposite 
side  also.  It  is,  however,  highly  probable  that  all  reflex  contractions  of 
the  sphincter  muscles  are  consensual,  that  is,  bilateral  reflex  actions  because 
of  the  decussation  of  the  pupillary  fibers  at  the  chiasm.  Contraction  of  both 
pupils  also  occurs  as  an  associated  movement  in  the  convergence  of  the  eyes 
during  accommodation. 

The  dilatation  of  the  pupil  is,  however,  not  due  exclusively  to  the 
relaxation  of  the  sphincter  pupillge  muscle,  but  partly  to  the  contraction  of 
the  dilatator  pupillse  muscle,  which  is  kept  normally  in  a  state  of  tonic  con- 
traction by  impulses  emanating  from  a  nerve-center  in  the  medulla 
oblongata. 

The  axons  which  arise  in  this  center  pass  down  the  cord,  emerge  through 
the  first  thoracic  nerve,  and  then  ascend  to  the  superior  cervical  ganglion 
(see  Fig.  268),  in  which  their  terminal  branches  arborize  around  its  nerve- 
cells.  From  these  cells  new  axons  of  the  sympathetic  system  arise  which  pass 
successively  to  the  ophthalmic  division  of  the  fifth  nerve,  the  nasal  nerve, 
the  long  ciliary  nerve  and  the  iris. 

Experimental  research  renders  it  highly  probable  that  the  dilatator 
center  is  in  a  state  of  continuous  activity  and  the  dilatator  muscle  in  a  state 
of  tonic  contraction.  Whatever  the  normal  stimulus  may  be,  the  center 
is  increased  in  activity  by  dyspneic  blood,  by  severe  muscle  exercise,  by 
emotional  excitement,  and  by  stimulation  of  various  sensor  nerves.  That 
the  efferent  pathway  just  alluded  to  transmits  the  impulses  to  the  iris  is  shown 
by  the  fact  that  division  in  any  part  of  the  course  is  followed  by  narrowing, 
stimulation,  by  active  dilatation  of  the  pupil. 

The  variations  in  size  of  the  pupil,  though  largely  a  reflex  act  under  the 
control  of  the  oculo-motor  nerve,  are  nevertheless  partly  due  to  the  active 
cooperation  of  the  dilatator  nerves  and  their  related  muscle.  The  size  of  the 
pupil  necessary  from  moment  to  moment  for  the  admission  of  just  that 
amount  of  light  essential  to  the  formation  and  perception  of  a  distinct  image 
is  the  result  of  two  nicely  adjusted  and  delicately  balanced  forces. 

Wernicke's  Hemianopic  Pupillary  Reaction. — It  was  stated  on 
page  617  that  a  modification  of  the  pupillary  reaction  is  observed  in  some 
cases  of  hemianopsia,  which  indicates  approximately  the  seat  of  the  lesion. 
This  reaction,  or  inaction  as  it  is  sometimes  called,  is  present  when  the  lesion 
is  along  the  course  of  the  optic  tract  between  the  chiasma  and  the  anterior 
quadrigeminal  body.  In  a  case  of  left  lateral  hemianopsia,  the  lesion  being 
in  the  right  optic  tract,  the  method  of  testing  for  the  reaction  is  as  follows: 
The  eye  of  the  left  side  is  first  carefully  shielded  from  the  light.  A  fine  ray 
of  light  is  then  projected  into  the  right  eye  in  such  a  manner  that  it  falls  en- 
tirely on  the  non-sensitive  (the  temporal)  side  of  the  retina.  There  will  be  an 
absence  of  the  usual  pupillary  response,  or  rather  the  pupil  remains  inactive; 
but  if  the  light  is  gradually  directed  toward  the  sensitive  (the  nasal)  side  of 
the  retina,  there  will  come  a  moment,  as  the  central  line  is  crossed  and  the 
light  falls  on  the  sensitive  side,  when  the  usual  pupillary  response  manifests 
itself,  viz.:  a  contraction  of  the  sphincter  pupillae  and  a  diminution  in  the 
size  of  the  pupil.  The  explanation  of  these  facts  will  become  apparent 
from  an  examination  of  Fig.  268  in  which  the  course  of  the  pupillary  fibers 
is  shown  and  especially  if  it   be  accepted  that  these  fibers  at  their  central 


62  2  TEXT-BOOK  OF  PHYSIOLOGY 

terminations  decussate  or  are  in  relation  either  directly  or  indirectly  with  the 
sphincter  centers. 

The  eye  of  the  right  side  is  then  in  turn  shielded  from  the  light  and  the 
same  method  of  examination  is  carried  out.  In  this  case,  however,  the  light 
is  projected  first  on  the  nasal,  which  is  the  non-sensitive  side  of  the  retina; 
there  will  again  be  no  response  in  the  pupil.  But  if  the  light  is  gradually 
directed  toward  the  sensitive  (the  temporal)  side,  there  will  come  a  moment, 
as  the  central  line  is  crossed  and  the  light  falls  on  the  sensitive  portion  of 
the  retina,  when  the  usual  pupillary  response  manifests  itself.  The  course 
of  the  pupillary  fibers  in  this  instance  will  also  become  apparent  from  an 
examination  of  Fig.  268.  It  is  evident,  however,  that  in  either  case  a  bilateral 
pupillary  reaction  will  follow  stimulation  of  the  sensitive  side  of  either  eye 
because  of  the  central  decussation  of  the  pupillary  fibers. 


THE  FOURTH  NERVE.  THE  TROCHLEAR 

The  fourth  cranial  nerve,  the  trochlear,  consists  of  peripherally  coursing 
axons  which  serve  to  bring  the  cells  from  which  they  arise  into  relation  with 
the  superior  oblique  muscle. 

Origin. — The  axons  of  this  nerve  arise  from  a  group  of  cells  located 
beneath  the  aqueduct  of  Sylvius  just  posterior  to  the  last  nucleus  of  the 
third  nerve.  After  emerging  from  the  nucleus  the  nerve-fibers  pass  down- 
ward for  a  short  distance,  then  curve  dorsally  around  the  aqueduct  of 
Sylvius,  and  enter  the  valve  of  Vieussens,  where  they  completely  decussate 
with  the  nerve-fibers  of  the  opposite  side. 

Cortical  Connections. — The  nucleus  of  the  trochlear  nerve  is  in  his- 
tologic and  physiologic  connection  with  the  motor  area  of  the  cerebral  cor- 
tex. Nerve-cells  in  this  region  give  off  axons  which  enter  the  pyramidal 
tract  and  descend  through  the  internal  capsule  and  the  crus  cerebri,  after 
which  they  cross  to  the  opposite  side.  Their  end-tufts  arborize  around  the 
cells  of  the  nuclei  already  described. 

Distribution. — After  its  decussation  the  nerve-trunk  emerges  just  be- 
low the  posterior  quadrigeminal  body,  crosses  the  superior  cerebellar  pe- 
duncle, and  winds  around  the  crus  cerebri  to  the  anterior  border  of  the  pons 
Varolii.  It  then  enters  the  orbit  cavity  through  the  sphenoid  fissure  and 
finally  terminates  in  the  superior  oblique  muscle.  In  its  course  the  nerve 
receives  filaments  from  the  cavernous  plexus  of  the  sympathetic  and  the 
ophthalmic  division  of  the  trigeminal. 

Properties. — Stimulation  of  the  nerve-trunk  is  followed  by  spasmodic 
contraction  of  the  superior  oblique  muscle,  the  anterior  pole  of  the  eyeball 
being  turned  downward  and  outward,  combined  with  slight  torsion  away 
from  the  middle  line. 

Division  of  the  nerve  is  followed  by  a  relaxation  or  paralysis  of  the 
muscle.  In  consequence  of  the  now  unopposed  action  of  the  inferior 
oblique  muscle,  the  anterior  pole  of  the  eyeball  is  turned  upward  and  in- 
ward with  slight  torsion  toward  the  middle  line.  The  diplopia  consequent 
upon  this  paralysis  is  homonymous,  the  images  appearing  one  above  the 
other.  The  image  of  the  paralyzed  eye  is  below  that  of  the  normal  eye  and 
its  upper  end  inclined  toward  that  of  the  normal  eye. 


THE  ENCEPHALIC  OR  CRANIAL  NERVES 


623 


Function. — The  function  of  the  trochlear  nerve  is  to  transmit  nerve 
impulses  to  the  superior  obUque  muscle  and  to  excite  it  to  contraction. 

THE  FIFTH  NERVE.    THE  TRIGEMINAL 

The  fifth  cranial  nerve,  the  trigeminal,  consists  of  both  afferent  and 
efferent  axons  which  for  the  most  part  are  separate  and  distinct.  The 
afferent  axons  constitute 
by  far  the  major  portion, 
the  efferent  fibers  the 
minor  portion,  of  the 
nerve. 

Origin  of  the  Affer- 
ent Axons. — The  afferent 
axons  have  their  origin 
in  the  monaxonic  cells  in 
the  ganglion  of  Gasser, 
which  rests  on  the  apex  of 
the  petrous  portion  of  the 
temporal  bone.  The  cells 
of  this  ganglion  give  origin 
to  a  short  process  which 
soon  divides  into  two 
branches,  one  of  which 
passes  centrally,  the  other 
peripherally  (Fig.  269). 
The  centrally  directed 
branches  collectively  form  MotoT'root 
the  so-called  large  or  sen 
sorroot;  the  peripherally  directed  branches  collectively  constitute  the  three 
main  divisions  of  the  nerve:  viz.,  the  ophthalmic,  the  superior  maxillary, 
and  the  inferior  maxillary.  Branches  of  the  carotid  plexus  of  the  sym- 
pathetic enter  the  nerve  in  the  neighborhood  of  the  ganglion  of  Gasser  and 
accompany  some  of  its  branches  to  their  terminations. 

Distribution. — i.  The  Central  Branches. — The  axons  of  the  large  root 
pass  backward  into  the  pons  Varolii  on  its  lateral  aspect.  After  entering 
the  pons  each  axon  divides  into  two  branches,  one  of  which  passes  upward 
a  short  distance,  the  other  passes  downward,  descending  as  far  as  the 
second  cervical  segment.  Both  branches  give  off  a  number  of  collaterals, 
some  of  which  terminate  in  fine  end-tufts  around  nerve-cells  in  the  sub- 
stantia gelatinosa. 

2.  The  Peripheral  Branches. — The  peripheral  axons  emerge  from  the 

peripheral  end  of  the  ganglion  of  Gasser  in  three  distinct  and  separate 

branches,  each  of  which  is  distributed  to  a  different  region  of  the  face 

and  head. 

I.  The  ophthalmic  branch  passes  forward  and  subdivides  into  three  large 

branches,   the  frontal,  the  lachrymal,  and  the  nasal.     The  ultimate 

termination  of  the  branches  of  these  nerves  is  as  follows:  viz.,  the 

conjunctiva  and  skin  of  the  upper  eyelid,  the  cornea,  the  skin  of  the 

forehead  and  the  nose,   the  lachrymal  gland  and  caruncle,  and  the 

mucous  membrane  of  the  nose. 


3  4 

Fig.  269. — Scheme  of  Origin  and  Constitution  of  the 

Trigeminal    Nerve,     i.  Centrally    coursing    fibers.     2,    3, 

4.  Peripherally  coursing  fibers  of  the  cells  of  the  ganghon  of 

Gasser.     R,  N.  Nuclei  of  origin  of  the  efferent  fibers.     6. 

Central  terminations  of  the  large  root. 


624  .  TEXT-BOOK  OF  PHYSIOLOGY 

2.  The   superior  maxillary   branch   passes   forward   through    the   foramen 

rotundum,  crosses  the  spheno-maxillary  fossa,  enters  the  infra-orbital 
canal,  and  emerges  at  the  infra-orbital  foramen.  In  its  course  it 
gives  off  a  number  of  branches  which  are  distributed  as  follows:  viz., 
to  the  integument  and  conjunctiva  of  the  lower  lid,  the  nose,  cheek, 
and  upper  lip,  the  palate,  the  teeth  of  the  upper  jaw,  and  the  alveolar 
processes. 

3.  The  inferior  maxillary  branch  passes  through  the  foramen  ovale,  after 

which  it  subdivides  into  three  branches — the  aurlculo-temporal,  the 
lingual,  and  the  inferior  dental.     The  ultimate  branches  are  distributed 
as  follows:  viz.,  the  external  auditory  meatus,  the  side  of  the  head,  the 
mucous  membrane  of  the  mouth,  the  anterior  portion  of  the  tongue,  the 
arches  of  the  palate,  the  teeth  and  alveolar  process  of  the  lower  jaw  and 
the  integument  of  the  lower  part  of  the  face. 
The  afferent  axons  thus  serve  to  bring  into  relation  the  skin,  mucous 
membranes  of  the  head  and  face,  and  other  sentient  structures,  with  certain 
sensor  end-nuclei  in  the  pons,  medulla  oblongata,  and  adjoining  structures. 
Cortical  Connections. — The  afferent  portion  of  the  trigeminal  nerve 
is  brought  into  physiologic  relation  with  the  sensor  portion  of  the  cerebral 
cortex  by  means  of  nerve-fibers  which  have  their  origin  in  the  cells  around 
which  the  terminal  branches  of  the  centrally  coursing  fibers  arborize.     The 
cells  situated  in  the  substantia  gelatinosa  give  off  axons,  which  after  a  short 
course  cross  the  median  line,  enter  the  fillet  and  then  ascend  in  the  general 
sensor  tract  to  the  cortex  where  they  in  turn  arborize  around  sensor  nerve- 
cells. 

Properties. — Irritative  pathologic  lesions,  e.g.,  pressure  by  tumors, 
aneurysms,  neuritis,  degenerative  changes  in  the  ganghon  cells,  or  lesions 
which  in  any  way  gradually  impair  the  physical  or  chemic  integrity  of  the 
nerve-fibers,  give  rise  to  a  variety  of  painful  sensations  referable  to  the  seat 
of  the  lesion  or  to  one  or  more  regions  in  the  peripheral  distribution  of  the 
nerve.  Many  of  the  various  forms  of  trigeminal  neuralgia  are  caused  by 
lesions  of  this  character.  Exposure  of  the  dental  nerves  from  caries  of  the 
teeth,  the  presence  of  minute  foreign  bodies  in  the  conjunctiva,  operative 
procedures  in  the  nasal  chambers,  all  testify  to  the  extreme  sensibility  of  the 
nerve.  Division  of  the  large  root  within  the  cranium  is  followed  at  once  by 
complete  abolition  of  all  sensibility  in  the  head  and  face  to  which  its  branches 
are  distributed.  The  skin  and  mucous  membranes,  the  eye,  nose,  or  teeth 
may  be  experimentally  injured  without  any  evidences  of  pain  on  the  part  of 
the  animal.  Various  reflexes,  e.g.,  those  of  mastication,  insalivation,  degluti- 
tion, the  afferent  paths  of  which  are  formed  in  part  by  the  fifth  nerve,  are 
often  seriously  impaired.  At  the  same  time  the  lachrymal  secretion  dimin- 
ishes and  the  pupil  contracts.  The  same  results  are  observed  in  human 
beings  in  whom  the  nerve  has  been  divided  for  relief  from  severe  neuralgia. 
Anesthesia  or  a  loss  of  sensibility  may  also  be  caused  by  pathologic  lesions  of 
the  nerve-trunks  or  of  the  sensor  end-nuclei. 

Division  of  the  large  root  at  or  near  the  ganglion  of  Gasser  has  not  infre- 
quently been  followed  by  an  alteration  in  the  nutrition  of  the  eye  and  nose. 
In  the  course  of  twenty-four  hours  the  eye  becomes  vascular  and  inflamed; 
the  cornea  becomes  opaque;  and  ulceration  sets  in,  which  may  lead  to  com- 
plete destruction  of  the  eyeball.     The  mucous  membrane  of  the  nose  be- 


THE  ENCEPHALIC  OR  CRANIAL  NERVES  625 

comes  swollen,  vascular,  and  liable  to  hemorrhage  on  the  slightest  irritation. 
The  degenerative  changes  may  lead  to  a  complete  loss  of  smell.  These  results 
were  formerly  attributed  to  a  loss  of  trophic  influence  which  it  was  believed 
the  nerve  exercised  over  these  structures.  Modern  experimentation  and 
various  surgical  procedures  have  demonstrated  that  the  nutritive  disorders 
are  septic  in  origin,  made  possible  by  the  anesthetic  condition  and  by  the 
changed  vascular  supply  from  division  of  the  vasomotor  fibers  which  join  the 
nerve  at  or  near  the  ganglion. 

Origin  of  the  Efferent  Axons. — The  efferent  axons  arise  for  the 
most  part  from  nerve-cells  located  in  the  gray  matter  beneath  the  upper 
half  of  the  floor  of  the  fourth  ventricle.  A  group  of  cells  known  as  the 
superior  or  accessory  nucleus,  situated  posterior  to  the  corpora  quadrigem- 
ina,  gives  origin  to  axons  which  descend  and  join  the  axons  from  the  chief 
motor  nucleus  (Fig.  269). 

Distribution. — From  their  origin  the  fibers  pass  forward  through  the 
pons  and  emerge  on  its  lateral  aspect,  forming  the  so-called  small  root  of 
the  fifth  nerve.  This  then  passes  forward  beneath  the  ganglion  of  Gasser, 
leaves  the  cavity  of  the  skull  through  the  foramen  ovale,  and  joins  the 
inferior  maxillary  division  already  described.  Its  axons  are  ultimately 
distributed  to  the  muscles  of  mastication:  viz.,  the  masseter.  the  temporal, 
the  external  and  internal  pterygoids,  the  mylohyoid,  and  the  anterior  portion 
of  the  digastric.  A  few  axons  are  also  distributed  to  the  tensor  tympani  and 
tensor  palati  muscles.  The  efferent  or  peripherally  coursing  axons  thus  serve 
to  bring  the  nerve-cells  from  which  they  arise  into  relation  with  the  muscles 
of  mastication. 

Cortical  Connections. — The  nuclei  of  origin  of  the  small  root  are  in 
histologic  and  physiologic  relation  with  the  lower  third  of  the  motor  area 
of  the  cerebral  cortex.  Nerve-cells  in  this  region  give  off  axons  which  enter 
the  pyramidal  tract,  descend  through  the  internal  capsule  and  the  crus 
cerebri,  after  which  they  cross  to  the  opposite  side.  Their  end-tufts  arbor- 
ize around  the  cells  of  nuclei  in  the  medulla  oblongata. 

Properties. — Stimulation  of  the  small  root  gives  rise  to  convulsive  move- 
ments of  the  muscles  of  mastication.  Division  of  the  nerve  is  followed  by 
a  paralysis  of  these  muscles.  Contraction  or  paralysis  of  the  tensor  tympani 
and  tensor  palati  muscles  would  also  be  observed  under  the  same  conditions. 

Functions. — The  function  of  the  afferent  fibers  of  the  fifth  nerve  is 
the  transmission  of  nerve  impulses  from  its  peripheral  distribution  to  (a) 
the  medulla  oblongata;  (b)  through  its  afferent  cortical  tracts  to  the  cerebral 
cortex  where  they  evoke  sensations.  The  nerve  therefore  endows  all  the 
parts  to  which  it  is  distributed  with  sensibility. 

The  function  of  the  efferent  fibers  is  the  transmission  of  nerve  impulses 
from  the  cells  from  which  they  take  their  origin,  to  the  muscles  of  mastication, 
which  are  excited  to  activity  by  them.  The  afferent  nerves  are  in  relation 
centrally  with  the  nuclei  of  origin  of  the  efferent  nerves,  hence  the  latter 
can  be  excited  not  only  voluntarily  but  reflexly  as  in  the  usual  acts  of  masti- 
cation. The  afferent  fibers  from  the  mouth  doubtless  assist  in  the  reflex 
secretion  of  saliva. 

Peripheral  stimulation  of  different  areas  in  the  distribution  of  the 
afferent  fibers,  e.g.,  conjunctiva,  nasal  and  oral  mucous  membranes,  teeth, 
etc.,  causes  a  variety  of  reflex  activities  in  the  muscles  associated  with  the 
40 


626  TEXT-BOOK  OF  PHYSIOLOGY 

eyes,  face,  the  respiratory  and  cardiac  mechanisms,  which  indicate  that  the 
afferent  fibers  are  centrally  in  relation  with  a  number  of  motor  nerve- 
centers. 

THE  SIXTH  NERVE.    THE  ABDUCENT 

The  sixth  cranial  nerve,  the  abducent,  consists  of  peripherally  coursing 
axons  which  serve  to  bring  the  nerve-cells  from  which  they  arise  into  rela- 
tion with  the  external  rectus  muscle. 

Origin. — The  axons  arise  from  a  group  of  cells  located  in  the  gray  matter 
beneath  the  upper  half  of  the  floor  of  the  fourth  ventricle.  It  is  quite  prob- 
able that  a  few  fibers  in  each  nerve-trunk  come  from  the  nucleus  on  the 
opposite  side  of  the  middle  line. 

Distribution. — The  nerve-fibers  pass  forward  from  their  origin  through 
the  gray  and  white  matter  and  emerge  through  the  groove  between  the  med- 
ulla oblongata  and  the  pons  Varolii  just  external  to  the  anterior  pyramid. 
The  nerve  then  passes  through  the  sphenoid  fissure  into  the  orbit  cavity, 
where  it  is  distributed  to  the  external  rectus  muscle.  In  its  course  the  nerve 
receives  filaments  from  the  carotid  plexus  of  the  sympathetic. 

Cortical  Connections. — The  nucleus  of  the  sixth  nerve  is  in  histologic 
and  physiologic  connection  with  the  motor  area  of  the  cerebral  cortex.  From 
nerve-cells  in  this  region  axons  are  given  off  which  enter  the  pyramidal 
tract,  descend  through  the  internal  capsule  and  crus  cerebri,  after  which 
they  cross  to  the  opposite  side,  where  their  end-tufts  arborize  around  the 
cells  of  the  nucleus  already  described. 

Properties. — Stimulation  of  the  nerve  is  followed  by  spasmodic  con- 
traction of  the  external  rectus  muscle  and  external  deviation  of  the  eyeball. 
Division  of  the  nerve  is  followed  by  paralysis  or  relaxation  of  the  muscle. 
As  a  result  of  the  unopposed  action  of  the  internal  rectus  the  anterior  pole 
of  the  eyeball  is  turned  toward  the, middle  line  (internal  strabismus).  In 
consequence  of  this  deviation  there  is  homonymous  diplopia.  The  images 
are  on  the  same  level  and  parallel.  The  image  of  the  paralyzed  eye  lies 
external  to  that  of  the  normal  eye. 

Function. — -The  function  of  this  nerve  is  to  transmit  nerve  impulses  to 
the  external  rectus  muscle  and  excite  it  to  contraction. 

THE  SEVENTH  NERVE.  THE  FACIAL 

The  seventh  cranial  nerve,  the  facial,  consists  of  peripherally  coursing 
nerve-fibers,  which  serve  to  bring  the  nerve-cells  from  which  they  arise 
into  relation  with  most  of  the  superficial  muscles  of  the  head  and  face. 

The  muscles  supplied  by  this  nerve,  as  stated  by  the  general  anatomists, 
are  as  follows:  The  occipito-fron talis,  corrugator  supercilli,  orbicularis 
palpebrarum,  levator  labii  superioris  alseque  nasi,  zygomatici,  the  pyramidalis 
nasi,  compressor  nasi,  depressor  alae  nasi,  levator  anguli  oris,  buccinator, 
orbicularis  oris,  depressor  anguli  oris,  depressor  labii  inferioris,  levator 
menti,  posterior  belly  of  the  digastric,  stylohyoid,  and  platysma  myoides. 
These  muscles  by  their  individual  and  cooperative  contraction  express  ideas 
and  feelings  and  are  therefore  termed  muscles  of  expression. 

Origin. — The  nerve-fibers  or  axons  composing  the  seventh  nerve  arise 
for  the  most  part  from  a  nucleus  of  large  multipolar  nerve-cells  situated 


THE  ENCEPHALIC  OR  CRANIAL  NERVES  627 

about  five  millimeters  beneath  the  upper  half  of  the  floor  of  the  fourth  ven- 
tricle toward  the  middle  line. 

From  this  nucleus,  which  is  about  four  millimeters  long,  axons  -emerge 
which  at  first  pass  inward  and  backward  as  far  as  the  ependyma  of  the  ven- 
tricle; they  then  turn  on  themselves,  forming  an  arch  that  encloses  the  nu- 
cleus of  the  sixth  nerve;  they  then  course  downward  and  outward,  emerging 
from  the  pons  at  its  lower  border  between  the  olivary  and  restiform  bodies. 
As  the  axons  approach  the  floor  of  the  ventricle  collateral  branches  are 
given  off  which,  crossing  the  median  line,  arborize  around  the  nerve-cells 
of  the  opposite  facial  nucleus. 

Clinic  observations  and  histologic  investigations,  however,  render  it 
probable  that  the  fibers  distributed  to  the  occipito-frontalis,  the  corrugator 
supercilii,  and  the  upper  half  of  the  orbicularis  palpebrarum,  are  derived 
from  the  oculo-motor  nucleus,  and,  descending  the  posterior  longitudinal 
bundle,  enter  the  trunk  of  the  facial  as  it  turns  to  pass  forward  through  the 
pons.  It  is  also  probable,  for  similar  reasons,  that  the  fibers  distributed  to 
the  orbicularis  oris  are  derived  from  the  hypoglossal  nucleus. 

Cortical  Connections. — The  nucleus  of  the  facial  nerve  is  in  histologic 
and  physiologic  connection  with  the  facial  region  of  the  general  motor  area 
of  the  cerebral  cortex.  From  the  cells  of  this  region  axons  descend  through 
the  pyramidal  tract,  the  internal  capsule,  and  the  crus  cerebri,  beyond  which 
they  cross  to  the  opposite  side  and  arborize  around  the  cells  of  the  nucleus 
already  described. 

Distribution. — From  its  superficial  origin  the  trunk  of  the  nerve  passes 
into  the  internal  auditory  meatus  beside  the  auditory  nerve.  After  passing 
forward  and  outward  for  a  short  distance  through  the  bone  above  and  be- 
tween the  cochlea  and  vestibule,  the  nerve  makes  a  sharp  bend,  forming  the 
genu  facialis,  turns  backward  and  enters  the  aqueduct  of  Fallopius,  the  gen- 
eral course  of  which  it  follows. as  far  as  the  stylo-mastoid  foramen.  After 
emerging  from  this  foramen  the  nerve  passes  downward  and  forward  as  far 
as  the  parotid  gland,  within  which  it  terminates  by  dividing  into  two  main 
branches,  the  temporo-facial  and  the  cervico-facial,  the  ultimate  branches 
of  which  are  distributed  as  previously  stated  to  the  superficial  muscles  of  the 
head  and  face. 

Properties. — Electric  stimulation  of  the  trunk  of  the  nerve  after  its  emer- 
gence from  the  stylo-mastoid  foramen  produces  convulsive  movements  in  all 
the  muscles  to  which  its  branches  are  distributed.  The  same  results  follow 
stimulation  of  the  intra -cranial  portion  of  the  nerve  in  an  animal  recently 
killed. 

Irritative  pathologic  lesions — e.g.,  tumors,  aneurysms,  etc. — situated 
along  the  course  of  the  nerve  or  at  its  nuclear  origin,  or  in  the  cortical  or 
sub-cortical  regions,  frequently  give  rise  to  spasmodic  movements  of  the 
facial  muscles  on  one  side,  which  may  be  tonic  or  clonic  in  character. 

Division  of  the  facial  nerve  after  its  emergence  from  the  stylo-mastoid 
foramen  is  followed  by  a  complete  relaxation  or  paralysis  of  the  superficial 
facial  muscles  of  the  corresponding  side.  The  same  result  follows  compres- 
sion of  the  nerve-trunk  in  any  part  of  its  intra-cranial  course. 

The  phenomena  presented  by  an  individual  suffering  from  division  or 
compression  of  the  facial  nerve,  and  which  collectively  constitute  facial  paraly- 
sis, are  as  follows:  A  relaxed  and  immobile  condition  of  the  side  of  the 


628  TEXT-BOOK  OF  PHYSIOLOGY 

face  corresponding  to  the  lesion;  separation  of  the  eyelids  from  paralysis  of 
the  orbicularis  palpebrarum  and  the  unopposed  contraction  of  the  levator 
palpebrae  muscle;  abohtion  of  the  ability  to  wink;  drooping  of  the  angle  of 
the  mouth;  an  escape  of  saliva  from  the  mouth;  contraction  of  the  muscles 
and  distortion  of  the  opposite  side  of  the  face;  on  attempting  to  laugh 
or  talk  the  distortion  of  the  face  is  increased;  during  mastication  the  food 
accumulates  between  the  teeth  and  cheek,  from  paralysis  of  the  buccinator; 
articulation  is  impaired  from  paralysis  of  the  orbicularis  oris  muscle,  the 
labial  sounds  especially  being  imperfectly  produced. 

Functions. — The  function  of  the  facial  nerve  is  the  transmission  of 
nerve  impulses  from  the  nerve-cells  in  which  it  arises  to  the  superficial 
muscles  of  the  face,  those  more  especially  concerned  in  the  expression  of  the 
feelings.  By  reason  of  the  association  of  the  cortical  facial  area  and  the 
nucleus  of  origin  of  the  facial  nerve  the  latter  becomes  the  medium  of 
communication  between  the  cortical  area  and  the  facial  muscles  and  serves 
for  the  transmission  to  the  muscles  of  those  nerve  impulses  developed  by 
and  associated  with  psychic  states.  The  muscles  thus  excited  to  action 
individually  and  collectively  express  in  a  general  way  the  character  of 
the  psychic  state.  For  this  reason  the  facial  nerve  is  termed  the  nerve  of 
expression. 

Branches  of  the  Facial  Nerve ;  Their  Origin,  Properties  and  Func- 
tions.— ^The  branches  usually  described  in  works  on  anatomy  as  coming 
from  the  facial  nerve  are  as  a  matter  of  fact,  not  branches  of  the  facial  nerve 
proper  (with  one  exception,  viz. :  the  tympanic)  but  are  branches  of  autonomic 
nerves  (vaso-motor  and  secretor)  which  after  leaving  the  central  nerve  sys- 
tem enter  the  sheath  of  the  facial.  After  a  short  distance,  however,  they 
leave  it  to  be  distributed  around  sympathetic  ganglia.  In  one  of  these 
branches  (the  chorda  tympani)  afferent  nerve-fibers  are  present  which  minis- 
ter to  the  sense  of  taste.  These  fibers  very  naturally  cannot  be  regarded  as 
integral  parts  of  the  facial  proper. 

Between  the  facial  and  the  acoustic  nerve  there  is  a  small  nerve  known 
as  the  pars  intermedia,  the  nervus  intermedins  or  the  nerve  of  Wrisberg.  The 
true  nature  of  this  nerve  has  long  been  a  subject  of  investigation.  The 
results  of  histologic  investigation  and  physiologic  experimentation  would 
indicate  that  it  is  composed  of  both  efferent  and  afferent  fibers.  The  efferent 
fibers  which  constitute  in  part  the  nerve  of  Wrisberg  have  their  origin  in  a 
group  of  cells  situated  beneath  the  floor  of  the  fourth  ventricle  near  the 
median  line  between  the  nucleus  of  the  facial  and  the  nucleus  of  the  motor 
root  of  the  trigeminal  nerve  and  known  as  the  nucleus  salivatorius.  The 
afferent  fibers  arise  from  nerve-cells  composing  in  large  part  the  ganglionic 
enlargement  found  on  the  genu  of  the  facial  nerve  at  the  point  where  it  turns 
backward  to  enter  the  aqueduct  of  Fallopius.  The  cells  of  this  geniculate 
ganglion,  originally  bipolar  present  single  axons  which  soon  divide  into 
centrally  and  peripherally  coursing  branches.  The  centrally  coursing 
branches  constitute  in  part  the  nerve  of  Wrisberg,  which  entering  and  pass- 
ing through  the  pons  terminates  directly  or  indirectly  around  the  sensor 
end-nucleus  of  the  glosso-pharyngeal  nerve.  The  peripherally  coursing 
branches  enter  the  sheath  of  the  facial  nerve  and  accompany  it  as  far 
as  a  point  about  5  millimeters  above  the  stylo-mastoid  foramen. 

From  its  mode  of  origin,  the  nerve  of  Wrisberg  cannot  be  regarded  as 


THE  ENCEPHALIC  OR  CRANE\L  NERVES 


629 


an  integral  part  of  the  facial  nerve  proper,  but  must  be  considered  as  an 
independent  nerve  composed  of  both  afferent  and  efferent  fibers. 

At  the  beginning  and  in  the  course  of  the  aqueduct  of  Fallopius  the 
facial  trunk  gives  off  the  following  branches:  the  large  superficial  petrosal,  the 
small  superficial  petrosal,  the  stapedius  and  the  chorda  tympani  (Fig.  270). 
I.  The  large  superficial  petrosal  nerve  is  given  off  near  the  geniculate  gan- 
glion. It  then  passes  forward  into  the  spheno-maxillary  fossa  and  be- 
comes associated  with  the  spheno-palatine  or  Meckel's  ganglion.  In  its 
course  it  receives  a  filament  known  as  the  deep  petrosal,  from  the  car- 
otid plexus  of  the  sympathetic.  The  nerve-trunk  formed  by  the  union  of 
these  two  nerves  is  known  as  the 
Vidian  nen^e  and  terminates  as 
stated  above.  The  character  and 
function  of  the  large  petrosal 
nerve  have  been  a  subject  of 
much  discussion.  As  the  out- 
come of  modern  methods  of  in- 
vestigation it  may  be  concluded 
that  it  is  composed  mainly,  if 
not  entirely,  of  fine  medullated 
nerve-fibers  which  are  the  con- 
tinuations of  corresponding  fi- 
bers in  the  nerve  of  Wrisberg 
and  that  their  destination  is 
the  spheno-palatine  ganglion, 
around      the      nerve-cells      of 


Fig.  270.— Chorda  Tympani  Nerve,     i,  2, 
4.  Facial  nerve  passing  through  the  aqueductus 
which    their    terminal    branches   ^^^       5- .  GangUoform  _enlargement. 


6. 


arborize. 

Stimulation  of  the  large  petrosal,  P^^- 


Great  petrosal  nerve.     7.  Spheno-palatine  gan- 

gUon.  8.  Small  petrosal  nerve.     9.  Chorda  tym- 

10,  II,  12,  13.    Various  branches  of  the 

.^,      .    J  1      1      i  •  j_       ia.^iai.     14,  14,  15.  Glosso-pharyngeal  nerve. — 

with  mduced  electric  currents,  (Hirschfeid.)  '^     ■'.  "^ 

gives  rise  to  a  dilatation  of  the 

blood-vessels  of,  and  a  secretion  from  the  mucous  membrane,  of  the 
nose,  soft  palate,  upper  part  of  the  pharynx,  roof  of  the  mouth,  gums, 
and  upper  lip — -the  regions  of  distribution  of  the  post-ganglionic  fibers 
of  cells  of  the  spheno-palatine  ganglion  (see  page  647).  The  nerve 
therefore  contains  both  vaso-dilatator  and  secretor  fibers  which  belong 
to  the  autonomic  system  of  nerves.  As  after  the  administration  of 
nicotine  stimulation  of  this  nerve  is  without  effect,  and  as  stimula- 
tion of  the  spheno-palatine  ganglion  gives  rise  to  the  usual  vaso-dila- 
tator and  secretor  effects  it  may  be  inferred  that  the  ganglion  is  the 
way  station  between  the  pre-ganglionic  fibers  and  the  blood-vessels  and 
glands.  The  deep  petrosal,  which  joins  the  large  petrosal  is  in  all 
probability  a  vaso-constrictor  nerve  coming  from  the  superior  cervical 
ganglion  of  the  sympathetic.  There  is  no  evidence  that  the  large  pe- 
trosal contains  any  fibers  from  the  facial  proper  for  the  innervation  of 
any  striated  muscle  of  the  palate. 
The  small  superficial  petrosal  nerv^e  is  given  off  from  the  facial  at  a 
point  somewhat  external  to  the  large  petrosal  nerve.  In  its  course 
it  is  joined  by  a  small  filament  derived  from  Jacobson's  branch  of  the 
glosso-pharyngeal.     Together  they  pass  into  the  otic  ganglion,  where 


630  TEXT-BOOK  OF  PHYSIOLOGY 

the  fibers  arborize  around  the  nerve-cells  composing  it.  Experiments 
are  lacking  as  to  the  function  of  the  small  petrosal.  The  small  size  of 
its  nerve-fibers  and  their  termination  would  lead  to  the  conjecture 
that  they  are  probably  vaso-dilatator  and  secretor.  Stimulation  of  Jacob- 
son's  nerve  gives  rise  to  a  dilatation  of  the  blood-vessels  of,  and  secretion 
from,  the  mucous  membrane  of  the  cheek-,  lips,  and  gums  and  of  the 
parotid  and  orbit  glands,  the  regions  of  distribution  of  the  post-ganglionic 
fibers  of  the  otic  ganglion.  This  nerve  therefore  contains  both  vaso- 
dilatator and  secretor  fibers  (see  pages  155,  634). 

3.  The  tympanic  nerve  or  stapedius   nerve    is  distributed  directly  to  the 

stapedius  muscle,  and  as  this  muscle  is  of  the  striated  or  skeletal 
variety  it  is  innervated  by  the  facial  proper. 

4.  The  chorda  tympani  nerve  is  given  off  from  the  facial  at  a  point  about 

5  millimeters  above  the  stylo-mastoid  foramen.     It  then  passes  upward 

and  forward  and  enters  the  tympanum  through  the  iter  chordae  posterius, 

crosses  the  tympanic  membrane  between  the  malleus  and  incus,  leaves 

the  tympanum  by  the  iter  chordae  anterius  or  canal  of  Huguier,  and 

finally  joins  the  lingual  branch  of  the  fifth  nerve.     Some  of  its  fibers  can 

be  traced  to  the  mucous  membrane  of  the  dorsum  of  the  tongue,  others 

to  the  submaxillary  and  sublingual  ganglia  with  which  they  become 

associated. 

The  determination  of  the  origin,  course,  and  functions  of  the  chorda 

tympani  nerve  has  given  rise  to  many  investigations  and  discussions,  and  it 

cannot  be  said  that  the  results  thus  far  attained  are  as  satisfactory  as  might 

be  desired. 

If  the  chorda  tympani  nerve  is  divided  there  follows  a  contraction  of 
the  blood-vessels  in  the  neighborhood  of  and  a  diminution  in  the  secretion 
from  the  submaxillary  and  sublingual  glands.  Stimulation  of  the  peripheral 
end  of  the  divided  nerve  gives  rise  to  a  dilatation  of  the  blood-vessels  and  an 
increased  production  and  discharge  of  saliva  from  these  glands  (see  page 
153).  From  these  results  it  is  certain  that  the  chorda  tympani  contains  both 
vaso-dilatator  and  secretor  fibers.  Nicotin  applied  to  the  submaxillary 
and  sublingual  ganglia  abolishes  the  effects  of  stimulation  of  the  chorda 
tympani.  It  does  not  prevent  the  same  effects  when  the  ganglia  themselves 
are  stimulated.  It  is  clear,  therefore,  that  the  vaso-dilatator  and  secretor 
fibers  arborize  around  the  cells  of  the  ganglia  and  are  not  distributed  directly 
to  the  gland  structures.  It  is  highly  probable  that  the  vaso-dilatator  and 
secretor  fibers  in  the  chorda  tympani  are  the  continuations  of  the  efferent 
fibers  found  in  the  pars  intermedia  and  that  they  too  have  their  origin  in  the 
nucleus  salivatorius. 

If  the  nerve  be  divided  as  it  crosses  the  tympanic  cavity  or  before  it  unites 
with  the  lingual  branch  of  the  fifth  nerve,  there  follows  a  loss  of  taste  in  the 
anterior  two-thirds  of  the  tongue  on  the  corresponding  side,  though  the  sensi- 
bility remains  unimpaired.  For  this  and  other  reasons,  the  chorda  tympani 
has  long  been  regarded  as  the  nerve  of  taste  for  this  region.  The  nerve-fibers 
subserving  the  sense  of  taste  are  believed  to  be  the  peripherally  coursing 
fibers  which  have  their  origin  in  the  nerve-cells  of  the  geniculate  ganglion 
and  which  descending  in  the  aqueduct  of  Fallopius  are  continued  as  the 
chorda  tympani.  The  nerve  impulses  developed  in  the  peripheral  termina- 
tions of  this  nerve  by  the  action  of  organic  matter  in  solution  are  transmitted 


THE  ENCEPHALIC  OR  CRANIAL  NERVES  631 

through  the  chorda  tympani,  along  the  facial  nerve  as  far  as  the  geniculate 
ganglion.  The  exact  pathway  for  these  afferent  or  gusta,torY  fibers  beyond  the 
geniculate  ganghon  has  long  been  a  subject  of  much  discussion.  According 
to  some  observers  these  fibers  enter  the  great  petrosal  nerve,  pass  forward  as 
far  as  the  spheno-palatine  ganglion,  then  into  the  superior  maxillary  division 
of  the  trigeminal,  and  so  to  the  brain.  According  to  others,  these  fibers 
pass  into  the  pars  intermedia,  into  the  pons,  where  they  terminate  around 
the  sensor  end-nucleus  of  the  glosso-pharyngeal.  The  evidence  for  and 
against  either  of  these  two  views  is  most  conflicting  and  insufiicient  to  justify 
positive  statements  one  way  or  the  other.  To  the  writer  the  weight  of  evi- 
dence seems  to  favor  the  view  that  the  gustatory  fibers  have  their  origin  in  the 
geniculate  ganglion;  that  they  pass  centrally  through  the  pars  intermedia; 
that  they  are  similar  in  function  to  the  glosso-pharyngeal;  and  that  they  are 
indeed  but  aberrant  branches  of  this  nerve. 

THE  EIGHTH  NERVE.    THE  ACOUSTIC 

The  eighth  cranial  nerve,  the  acoustic,  consists  of  the  centrally  coursing 
axons  of  neurons  which  connect  the  essential  organ  of  hearing  with  sensor 
end-nuclei  in  the  pons  Varolii.  This  nerve  consists  of  two  portions:  viz., 
a  cochlear  or  auditory  and  a  vestibular  or  equilibratory. 

Origin. — The  axons  comprising  the  cochlear  portion  have  their  origin 
in  the  bipolar  nerve-cells  of  the  spiral  ganglion  located  in  the  spiral  canal  near 
the  base  of  the  osseous  lamina  spiralis  (Fig.  271).  From  this  origin  they 
pass  centrally  into  the  central  canal  of  the  modiolus,  at  the  base  of  which 
they  emerge  in  well-defined  bundles  and  enter  the  internal  auditory  meatus. 
Dendritic  processes  from  these  cells  pass  peripherally  to  terminate  on  the 
ciliated  epithelial  cells  of  the  organ  of  Corti. 

The  axons  comprising  the  vestibular  portion  have  their  origin  in  the 
bipolar  nerve-cells  of  the  ganglion  of  Scarpa  located  in  the  internal  auditory 
meatus.  From  this  origin  they  pass  centrally  in  connection  with  the  cochlear 
portion.  Dendritic  processes  from  these  cells  pass  peripherally  into  the 
internal  ear,  where  they  terminate  in  the  epithelial  cells  situated  on  the  inner 
surface  of  the  utricle  and  saccule  and  in  the  ampullae  of  the  semicircular 
canals. 

The  common  trunk  of  the  auditory  nerve,  consisting  of  both  cochlear 
and  vestibular  divisions  after  emerging  from  the  internal  auditory  meatus, 
passes  backward,  inward,  and  downward  as  far  as  the  lateral  aspect  of  the 
pons  where  the  two  divisions  again  separate. 

The  cochlear  nerve,  the  external  root,  passes  to  the  outer  side  of  the 
restiform  body  and  enters  the  ventral  acoustic  nucleus  and  the  lateral  acous- 
tic nucleus,  around  the  cells  of  which  its  end-tufts  arborize.  The  vestib- 
ular nerve,  the  internal  root,  passes  on  the  inner  side  of  the  restiform  body 
to  the  dorsal  portion  of  the  pons,  where,  after  bifurcating,  the  end-tufts  of 
the  axons  arborize  around  the  dorso-internal  or  chief  auditory  nucleus  and 
the  dorso-external  or  Deiters'  nucleus.  Some  of  the  fibers  of  the  vestibu- 
lar branch  descend  through  the  pons  and  medulla  as  far  as  the  cuneate 
nucleus. 

Cortical  Connections. — From  the  nerve-cells  of  the  ventral  and  lateral 
acoustic  nuclei  axons  arise,  some  of  which  cross  the  median  plane  to  enter 


632 


TEXT-BOOK  OF  PHYSIOLOGY 


the  lateral  lemniscus  for  fillet  of  the  opposite  side,  while  others  enter  the 
lemniscus  of  the  same  side.  The  lemniscus  then  passes  upward  to  the  in- 
ternal geniculate  body  around  the  nerve-cells  of  which  its  fibers  terminate. 
From  the  geniculate  body  the  acoustic  radiation  arises  which  then  passes 
upward  and  outward  through  the  posterior  limits  of  the  internal  capsule 
to  terminate  finally  around  the  cells  of  the  super-temporal  convolution. 
From  the  nuclei  around  which  the  vestibular  branch  terminates,  axons 
jQ  arise  which  in  all  probability  pursue  a 

,.'-■'  somewhat  similar  course  and  terminate  in 

the  temporal  lobe. 

Properties. — Stimulation  of  the  coch- 
lear nerve  is  unattended  by  either  motor 
or  sensor  phenomena.  Division  of  the 
nerve  is  followed  by  a  loss  of  the  sense 
of  hearing.  Irritative  pathologic  lesions 
give  rise  to  sensations  of  sound  of  vary- 
ing character  and  intensity.  Degenera- 
tion of  the  nerve  or  destruction  by  tumors, 
etc.,  will  also  be  followed  by  a  loss  of 
the  sense  of  hearing. 

Experimental  lesions  of  the  semicir- 
cular canals  involving  a  destruction  of 
the  physiologic  relations  of  the  vestibular 
nerve  are  followed  by  a  loss  of  the 
coordinating  and  equilibratory  power. 
Disordered  movements,  such  as  rotation 
to  the  right  or  left,  somersaults  back- 
ward and  forward,  follow  destruction  of 
these  canals.  Pathologic  lesions  in  the 
peripheral  distribution  of  the  nerve  are 
attended  in  man  by  disturbances  of  equi- 
librium, e.g.,  vertigo,  or  a  sense  of  sway- 

FiG.  271.— Origin  and  Termination     •         Ditrhino-    nnd  c;ta(To-Printr 
OF   THE.  Acoustic  Nerve,     i.  Cochlea,    mg,  pitcnmg,  ana  Staggermg.^ 
2.  Spiral  ganglion   (Corti).     3.  Cochlear  FuilCtlOnS. —  The     function     of     the 

nerve.    4.  Ventral  acoustic  nucleus.    5.   cochlear  nerveis  toconvey  nerve  impulses 

Lateral    acoustic    nucleus.     6.  Semi-     r  •,  ••      ,       ,1       „  r     ^„      i,"u 

circular  canals.     7.   Ganglion  of  Scarpa.     f^Om   itS   Origm   to   the  ponS,    from  whlch 

8.  Vestibular    nerve.    9.  Dorso-externai  they  are  transmitted  by  the  auditory  tract 
nucleus  (De iters).    10.  Dorso-internal  ^q  ^j^g  acoustic  area  in  the  Cerebral  cortex 

nucleus. — (Alter  Morat  and  Doyon.)  ^  .^  ^  .•  r  i         i 

where  they  evoke  sensations  of  sound  and 
its  different  qualities,  intensity,  pitch,  and  timbre.  The  specific  physiologic 
stimulus  to  the  development  of  these  impulses  is  the  impact  of  atmospheric 
undulations  on  the  tympanic  membrane,  received  and  transmitted  by  the 
chain  of  bones  to  the  structures  of  the  internal  ear— the  organ  of  Corti — with 
which  the  peripheral  terminations  of  the  nerve  are  connected. 

The  function  of  the  vestibular  nerve  is  the  transmission  of  nerve  impulses 
to  the  pons,  whence  they  are  transmitted  to  the  cortex  of  both  the  cerebrum 
and  cerebellum  and  to  other  centers.  The  specific  physiologic  stimulus  is 
supposed  to  be  a  variation  in  pressure  in  the  ampullae  of  the  semicircular 
canals  caused  by  inertia  of  the  endolymph  during  changes  in  the  position  of 
the  head  and  body.     The  impulses  carried  by  the  vestibular  nerve  give  rise 


THE  ENCEPH.\LIC  OR  CRANL\L  NERVES  633 

reflexly  to  certain  adaptive  and  protective  movements  by  which  the  equi- 
librium of  the  body  in  both  dynamic  and  static  conditions  is  maintained. 

THE  NINTH  NERVE.    THE   GLOSSO-PHARYNGEAL 

The  ninth  cranial  ner\^e,  the  glosso-pharyngeal,  consists,  as  shown  by 
both  histologic  and  experimental  methods  of  research,  of  both  afferent  and 
efferent  nerve-fibers,  of  which  the  former,  however,  are  by  far  the  more 
abundant.  Near  its  exit  from  the  cavity  of  the  skull  the  nerve  presents  two 
ganglionic  enlargements  known  as  the  petrosal  and  jugular  ganglia. 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  serve  to  bring  cer- 
tain end-nuclei  in  the  medulla  oblongata  into  anatomic  and  physiologic 
relation  with  portions  of  the  mucous  membrane  of  the  tongue,  pharynx,  and 
middle  ear.  The  afferent  fibers  are  axons  of  the  monaxonic  cells  of  the 
petrosal  and  jugular  ganglia.  The  single  axon  from  each  of  these  cells  soon 
divides  into  two  branches,  one  of  which  passes  centrally,  the  other  peripher- 
ally. The  centrally  directed  branches  collectively  form  the  so-called  roots, 
four  or  five  in  number,  which  enter  the  medulla  between  the  olivary  and 
restiform  bodies.  The  peripherally  directed  branches  collectively  form  the 
two  main  divisions,  from  the  distribution  of  which,  to  the  tongue  and  pharynx, 
the  nerve  takes  its  name. 

Distribution. — The  axons  of  the  centrally  directed  branches  after 
entering  the  medulla  pass  toward  its  dorsal  aspect,  where  they  bifurcate, 
give  off  collateral  branches,  and  terminate  in  fine  end-tufts  in  the  immediate 
neighborhood  of  two  gioups  of  nerve-cells,  the  sensor  end-nuclei.  The  axons 
of  the  peripherally  directed  branches,  after  emerging  from  the  base  of  the 
skull  through  the  jugular  foramen,  pass  forward  and  inward  under  cover  of 
the  stylo-pharyngeal  muscle;  winding  around  this  muscle  they  divide  into 
terminal  branches  which  are  distributed  to  the  mucous  membrane  of  the 
posterior  one-third  of  the  tongue,  pharynx,  soft  palate,  uvula,  and  tonsils. 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  serve  to  bring  the 
nerve-cells  from  which  they  arise  into  connection  with  a  portion  of  the  mus- 
culature of  the  fauces  and  pharynx.  These  nerve-cells  are  located  in  the 
lateral  portion  of  the  forniatio  reticularis  at  some  distance  below  the  floor  of 
the  fourth  ventricle.  They  constitute  the  upper  portion  of  a  collection  of 
the  cells  known  as  the  nucleus  amhiguus. 

Distribution. — From  this  origin  the  efferent  fibers  pass  dorsally  to  near 
sensor  end-nuclei,  then  turn  outward  and  forward  and  finally  emerge  from 
the  medulla  in  intimate  association  with  the  afferent  fibers.  They  are 
ultimately  distributed  to  the  stylo-pharyngeus,  and  to  the  middle  constrictor 
muscle  of  the  pharynx.  In  addition  to  the  foregoing  efferent  fibers  the 
glossopharyngeal  nerve  contains  at  its  emergence  from  the  medulla  both 
vaso-motor  and  secretor  fibers. 

Jacobson's  Nerve. — This  is  a  small  branch  which  leaves  the  glosso- 
pharyngeal at  the  petrous  ganglion.  After  passing  through  a  small  canal  in 
the  base  of  the  skull  it  enters  the  tympanic  cavity,  within  which  it  gives  off 
branches  to  the  great  and  lesser  petrosal  nerves,  to  the  mucous  membrane  of 
the  foramen  ovale,  the  foramen  rotundum,  and  to  the  Eustachian  tube. 

Cortical  Connections. — The  motor  nucleus  is  doubtless  connected  with 
the  general  motor  area  of  the  cortex  through  fibers   descending   in   the 


634  TEXT-BOOK  OF  PHYSIOLOGY 

pyramidal  tract.  The  exact  location  of  the  cortical  area  for  the  pharynx 
is  not  well  determined,  but  is  most  likely  to  be  found  in  the  lower  part  of  the 
general  motor  area  near  the  termination  of  the  Rolandic  fissure.  The  exact 
cortical  connections  of  the  afferent  tract  are  unknown,  but  are  most  likely  to 
be  found  in  the  general  sensor  area. 

Properties. — Stimulation  of  the  glosso-pharyngeal  trunk  with  induced  el- 
ectric currents  calls  forth  evidence  of  pain  and  contraction  of  the  stylo-pharyn- 
geus  and  middle  constrictor  muscles.  Peripheral  stimulation  of  the  termi- 
nals of  the  nerve-fibers  in  the  mucous  membrane  of  the  posterior  third  of  the 
tongue  with  different  kinds  of  organic  matter  in  solution,  develops  nerve 
impulses  which  transmitted  to  the  cortex  evoke  sensations  of  taste.  Division 
of  the  nerve  abolishes  sensibility  in  the  mucous  membrane  to  which  it  is 
distributed,  impairs  the  sense  of  taste  in  the  posterior  third  of  the  tongue, 
and  gives  rise  to  paralysis  of  the  above-mentioned  muscles. 

Stimulation  of  Jacobson's  nerve  is  followed  by  dilatation  of  the  blood- 
vessels of,  and  secretion  from,  the  mucous  membrane  of  the  lower  lip,  cheek, 
and  gums,  and  from  the  parotid  gland.  Division  of  the  nerve  is  followed  by 
the  opposite  results.  The  course  of  the  fibers  which  give  rise  to  these  results 
is  by  way  of  the  lesser  petrosal  to  the  otic  ganglion,  around  the  cells  of  which 
the  fibers  arborize.  From  the  cells  of  this  ganglion  non-medullated  fibers 
pass  to  the  blood-vessels  and  gland  cells.  These  nerve-fibers  are  thus 
members  of  the  autonomic  system  of  nerves. 

Functions. — The  afferent  fibers  of  the  glosso-pharyngeal  transmit 
nerve  impulses  from  the  parts  to  which  they  are  distributed  to  the  cerebral 
cortex,  where  they  evoke  sensations  of  pain  and  sensations  of  taste;  they 
also  assist  in  all  probability  in  the  performance  of  certain  reflexes  connected 
with  deglutition.  The  afferent  fibers  are  therefore  divisable  into  nerves  of 
general  sensibility  and  nerves  of  special  sense.  The  efferent  fibers  transmit 
impulses  to  muscles,  exciting  them  to  activity,  and  to  the  otic  ganglion, 
which  in  turn  dilates  blood-vessels  and  excites  secretion.  The  fibers  excit- 
ing secretion  have  their  origin  in  the  nucleus  salivatorius,  from  which  the 
efferent  autonomic  fibers  in  the  chorda  tympani  nerve  arise. 

THE  TENTH  NERVE.  THE  PNEUMOGASTRIC  OR  VAGUS 

The  tenth  cranial  nerve,  the  pneumogastric  or  vagus,  consists,  as  shown 
by  histologic  methods  of  research,  of  both  afferent  and  efferent  fibers,  in- 
dependent of  those  derived  in  its  course  from  adjoining  motor  or  efferent 
nerves.  Near  the  exit  of  the  nerve  from  the  cavity  of  the  cranium  it  presents 
two  ganglionic  enlargements  known  respectively  as  the  ganglion  of  the  root 
(the  jugular)  and  the  ganglion  of  the  trunk  (the  plexiform). 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  take  their  origin 
in  the  monaxonic  cells  of  the  ganglia  on  the  root  and  trunk.  The  single 
axon  from  each  of  these  cells  soon  divides  into  two  branches,  one  of  which 
passes  centrally,  the  other  peripherally.  The  centrally  directed  branches 
collectively  form  the  so-called  roots,  ten  to  fifteen  in  number,  which  enter  the 
medulla  between  the  restiform  body  and  the  lateral  column.  The  periph- 
erally directed  branches  collectively  form  a  portion  of  the  common  trunk 
of  the  nerve. 

Distribution. — The  axon  of  the  centrally  directed  branches  after  entering 
the  medulla  pass  toward  its  dorsal  aspect,  where  they  bifurcate,  give  collat- 


THE  ENCEPHALIC  OR  CRANIAL  NERVES  635 

erals,  and  terminate  in  fine  end-tufts  in  the  immediate  neighborhood  of  two 
groups  of  nerve-cells,  the  vagal  sensor  end-nuclei. 

The  axons  of  the  peripherally  directed  branches  unite  to  form  a  portion 
of  the  common  trunk,  which,  as  it  descends  the  neck  and  enters  the  thorax  and 
abdomen,  gives  off  a  number  of  branches  which  are  ultimately  distributed  to 
the  mucous  membrane  of  the  esophagus,  larynx,  lungs,  stomach,  and  in- 
testine, and  also  to  the  heart.  The  afferent  fibers  thus  serve  to  bring  into 
anatomic  and  physiologic  relation  the  mucous  membrane  of  these  organs 
and  certain  sensor  end-nuclei  in  the  medulla  oblongata. 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  take  their  origin 
from  nerve-cells  located  in  the  lateral  portion  of  the  formatio  reticularis  at 
some  distance  below  the  floor  of  the  fourth  ventricle.  These  cells  constitute 
the  lower  portion  of  the  nucleus  ambiguus. 

Distribution. — From  their  origin  the  efferent  axons  pass  dorsally  to 
near  the  sensor  end-nuclei,  then  turn  outward  and  forward,  and  finally 
emerge  from  the  medulla  in  close  association  with  the  afferent  branches. 
They  are  ultimately  distributed  to  the  muscles  of  the  lower  two-thirds  of  the 
esophagus;  to  the  muscle-fibers  of  the  stomach  and  perhaps  the  intestines; 
to  the  walls  of  the  gall-bladder  and  to  the  sphincter  of  the  common  bile-duct; 
and  to  the  non-striated  muscle-fibers  of  the  bronchial  tubes,  and  to  the  heart. 
Among  the  efferent  fibers  are  some  which  are  distributed  to  the  gastric 
glands  and  to  the  pancreas.  From  this  distribution  it  is  apparent  that  the 
efferent  fibers  in  the  vagus  are  largely  if  not  entirely  members  of  the  auto- 
nomic system  of  nerves. 

The  efferent  fibers  serve  to  bring  the  nerve-cells  from  which  they  arise 
into  anatomic  and  physiologic  connection  with  a  portion  of  the  musculature 
of  the  alimentary  canal  and  its  diverticulum,  the  lung  as  well  as  the  heart  and 
gastric  glands. 

Communicating  Branches. — At  or  near  the  ganglia  the  vagus  receives 
communicating  branches  from  the  eleventh  nerv^e,  the  spinal  accessory,  the 
facial,  the  hypoglossal,  and  the  anterior  branches  of  the  two  upper  cervical 
nerves.  Owing  to  this  manifold  origin  of  the  efferent  fibers  in  the  trunk  and 
peripheral  branches  of  the  vagus,  it  is,  in  some  instances,  difl&cult,  if  not 
impossible,  to  determine  to  which  of  these  nerves  a  given  muscle  contraction 
is  to  be  referred 

Vagal  Branches. — As  the  vagus  passes  down  the  neck  it  gives  off  the 
following  main  branches: 

1.  The  pharyngeal  nerves,  which,  after  entering  into  the  formation  of  the 

pharyngeal  plexus,  are  distributed  to  the  mucous  membrane  and  to  the 
muscles  of  the  pharynx ;  e.g. ,  superior  and  inferior  constrictors,  the  levator 
palati,  and  the  azygos  uvulae;  according  to  BeevorandHorsley  the  nerves 
for  these  muscles  are  branches  of  the  spinal  accessory. 

2.  The  esophageal  nerves,  which  after  entering  into  the  formation  of  the 

esophageal  plexus,  are  distributed  to  the  mucous  membrane,  and  to  the 
muscles  of  the  lower  two-thirds  of  the  esophagus. 

3.  The  superior  laryngeal  nerve  which,  entering  the  larynx  through  the 

thyro-hyoid  membrane,  is  distributed  to  the  mucous  membrane  lining 
the  interior  of  the  larynx  and  to  the  crico-thyroid  muscle.  From  the 
superior  laryngeal  and  the  main  trunk  small  branches  are  given  off 
which  in  the  rabbit  unite  to  form  a  single  nerve,  the  so-called  depressor 


636  TEXT-BOOK  OF  PHYSIOLOGY 

nerve.  (See  page  325.)  It  is  distributed  to  the  surface  of  the  ventricle 
and  perhaps  to  structures  at  the  root  of  the  aorta.  Though  this 
anatomic  arrangement  is  not  found  in  man,  there  are  many  reasons  for 
beheving  that  analogous  fibers  are  present  in  the  vagus  trunk  of  man  and 
other  animals. 

4.  The  inferior  laryngeal  nerve  which  is  distributed  ultimately  to  all  the 

muscles  of  the  larynx  (except  the  crico-thyroid)  and  to  the  inferior  con- 
strictor of  the  pharynx.  These  motor  fibers  are  derived  from  the  spinal 
accessory. 

5.  The  cardiac  nerves  which,  after  entering  into  the  formation  of  the  cardiac 

plexus,  are  distributed  to  the  heart. 

6.  The  pulmonic  nerves  distributed  to  the  mucous  membrane  of  the  bronchial 

tubes  and  their  ultimate  terminations,  the  lobules  and  air-cells,  as  well 
as  to  their  non-striated  muscle-fibers. 

7.  The  gastric  and  intestinal  nerves,  distributed  to  the  mucous  membrane  and 

muscle  walls  of  the  stomach,  intestines,  and  gall-bladder.  Other  fibers 
in  all  probability  pass  to  the  liver,  spleen,  kidney,  and  suprarenal  bodies. 

Properties  of  the  Pneumogastric  or  Vagus  Nerve  and  its  Various 
Branches. — Faradization  of  the  vagus  nerve  close  to  the  medulla  oblongata 
gives  rise  to  sensations  of  pain  and  to  contraction  of  the  musculature  of  a 
portion  of  the  alimentary  tract,  viz. :  the  esophagus,  stomach,  and  possibly 
of  the  intestine  and  of  the  pulmonary  apparatus,  and  at  the  same  time  causes 
an  inhibition  of  the  heart.  Division  of  the  nerve  is  followed  by  a  loss  of 
sensibiUty  in  the  mucous  membrane  of  the  alimentary  tract  and  of  the  pul- 
monary apparatus,  together  with  a  loss  of  motility  of  the  structures  above 
mentioned,  and  a  loss  of  the  inhibition  of  the  heart. 

Stimulation  of  the  trunk  of  the  nerve  in  different  parts  of  its  course  pro- 
duces a  variety  of  results  dependent  to  some  extent  on  the  presence  of  anas- 
tomosing branches  from  adjoining  nerves. 

The  Pharyngeal  Nerves. — ^Faradization  of  the  pharyngeal  nerves  consisting 
of  both  afferent  and  efferent  fibers,  gives  rise  to  sensations  of  pain,  contraction 
of  the  pharyngeal  muscles,  and  perhaps  to  vomiting.  Division  of  these  nerves 
is  followed  by  a  loss  of  sensibility  in  the  parts  to  which  they  are  distributed  and 
by  paralysis  of  the  muscles  with  a  consequent  impairment  of  deglutition. 

The  Esophageal  Nerves.— Fsirsidiza,tion  of  the  esophageal  nerves,  gives 
rise  to  sensations  of  pain  and  to  contractions  of  the  muscle  coat  of  the  esopha- 
gus. Division  of  these  nerves  is  followed  by  a  loss  of  sensibility  in  the  parts 
to  which  they  are  distributed,  a  partial  paralysis  of  the  muscle  coat  and  an 
impairment  of  deglutition. 

The  Superior  Laryngeal  Nerve. — Faradization  of  the  superior  laryngeal 
nerve  gives  rise  to  sensations  of  pain,  and  to  contraction  of  the  crico-thyroid 
muscle.  Through  reflected  impulses  it  causes  contraction  of  the  muscles 
of  deglutition,  and  of  the  muscles  concerned  in  the  act  of  coughing;  inhibi- 
tion of  the  inspiratory  movement  and  arrest  of  respiration  in  the  condition 
of  expiratory  standstill,  with  perhaps  a  tetanic  contraction  of  the  expiratory 
muscles,  and  contraction  of  the  laryngeal  muscles  with  closure  of  the  glottis. 

Peripheral  stimulation  of  this  nerve — e.g.,  the  contact  of  foreign 
particles — gives  rise  to  a  similar  series  of  phenomena.  Division  of  these 
nerves  is  followed  by  a  loss  of  sensibility  in  the  laryngeal  mucous  mem- 
brane, paralysis  of  the  crico-thyroid  muscle  with  a  consequent  lowering  of 


THE  ENCEPHALIC  OR  CRANIAL  NERVES  637 

the  pitch,  and  a  diminution  in  the  clearness  of  the  voice.  In  consequence 
of  the  loss  of  the  sensibility  there  is  an  inability  to  perceive  the  entrance  of 
foreign  bodies  into  the  larynx. 

The  Depressor  Nerve. — Stimulation  of  the  peripheral  end  of  the  depres- 
sor nerve  is  without  effect;  stimulation  of  the  central  end  retards  and  even 
arrests  the  heart's  pulsations  and  lowers  the  general  blood-pressure.  These 
two  effects,  though  associated,  are  nevertheless  independent  of  each  other.  If 
the  vagus  nerves  be  divided  on  both  sides  between  the  origin  of  the  depres- 
sor and  the  origin  of  the  cardiac  nerves,  and  the  former  stimulated,  there 
will  be  a  fall  of  pressure  without  retardation  of  the  heart.  The  effect  on  the 
heart  is  attributed  to  a  stimulation  of  the  cardio-inhibitor  mechanism  in 
the  medulla  oblongata. 

The  fall  of  general  blood-pressure  was  formerly  attributed  to  a  sudden 
dilatation  of  the  splanchnic  blood-vessels  alone,  in  consequence  of  a  depres- 
sion of  that  portion  of  the  general  vaso-motor  center  which  maintains  through 
the  splanchnic  nerves  a  tonic  contraction  of  their  walls.  It  has  been  satis- 
factorily demonstrated  that  this  is  not  the  sole  cause;  for  after  division  of  the 
splanchnic  nerves,  stimulation  of  the  depressor  causes  a  still  further  fall  of 
from  30  to  40  per  cent,  in  the  general  pressure  (Porter  and  Beyer).  Evi- 
dently, not  any  one,  but  all  portions  of  the  vaso-motor  center  are  subject  to 
the  effects  of  depressor  stimulation. 

The  Inferior  Laryngeal  Nerves. — Faradization  of  the  inferior  laryngeal 
nerves  produces  effects  which  vary  in  accordance  with  the  strength  of  the 
stimulus,  with  different  animals,  and  with  the  same  animal  at  different  periods 
of  life.  In  the  adult  dog  and  in  man,  the  glottis  is  kept  widely  open  for 
respiratory  purposes  by  the  tonic  contraction  of  the  abductor  muscles  (the 
posterior  crico-ary tenoids) ;  for  phonatory  purposes  the  glottis  is  closed  and 
the  vocal  membranes  approximated  by  the  contraction  of  the  adductor  mus- 
cles. It  has  been  shown  that  these  opposed  groups  of  muscles  have  inde- 
pendent nerve-supplies;  that  two  sets  of  fibers  in  the  common  trunk  can  be 
separated  and  stimulated  independently  of  each  other.  Feeble  stimulation  of 
the  common  trunk  produces  a  still  further  abduction  of  the  vocal  cords. 
With  an  increase  in  the  strength  of  the  stimulus,  however,  the  reverse  obtains: 
namely,  adduction  which  increases  until  the  glottis  is  completely  closed. 
Division  of  the  nerves  is  followed  by  paralysis  of  both  the  phonatory  and 
respiratory  muscles,  the  abductors  and  adductors,  with  the  result  of  seriously 
impairing  both  phonation  and  respiration  and  not  infrequently  causing  death. 
The  fibers  of  the  inferior  laryngeal  nerve  are  derived  from  the  eleventh  nerve, 
the  spinal  accessory. 

The  Cardiac  Nerves. — Faradization  of  the  trunk  of  the  vagus  or  of  the 
peripheral  end  of  the  divided  nerv^  gives  rise  to  a  diminution  in  the  frequency 
and  force  of  the  heart's  contractions;  and  if  the  stimulation  be  sufficiently 
powerful,  completely  arrests  it  in  the  phase  of  diastole.  To  these  results  the 
term  inhibition  is  applied.  Division  of  the  vagi  or  of  the  cardiac  branches 
is  followed  by  an  increase  in  the  number  of  contractions  from  loss  of  inhibitor 
influences.  The  inhibitor  fibers  of  the  vagus  are  generally  believed  to  be 
derived  from  the  spinal  accessory,  though  this  has  been  questioned.  Accord- 
ing to  the  recent  investigations  of  Schaternikoff  and  Friedenthal,  they  come 
direct  in  the  vagus,  from  a  nucleus  near  the  vagal  motor  nucleus  in  the  med- 
ulla, the  spinal  accessory  sending  no  branches  to  the  heart.     In  the  frog 


638  TEXT-BOOK  OF  PHYSIOLOGY 

and  other  batrachia  the  vagus  contains  also  accelerator  or  augmentor  fibers 
derived  from  the  sympathetic;  hence  stimulation,  especially  if  feeble,  may 
increase  the  heart's  action. 

The  Pulmonic  Nerves. — The  pulmonic  nerves,  given  off  from  the  trunk 
after  its  entrance  into  the  thorax,  do  not  lend  themselves  readily  to  ex- 
perimentation. Division  of  both  vagi  in  the  neck  above  the  point  of  exit 
of  the  pulmonic  branches  is  followed  by  a  decrease  in  the  frequency  of  the 
respiratory  acts,  with  an  increase  in  their  depth.  At  the  same  time  there  is 
a  loss  of  sensibility  of  the  mucous  membrane  of  the  trachea  and  lungs  and 
a  paralysis  of  non-striated  muscle-fibers. 

Stimulation  of  the  central  end  of  the  divided  vagus  with  weak  induced 
electric  currents  increases  the  frequency,  but  decreases  the  amplitude,  of  the 
respiratory  movements.  This  would  indicate  that  in  the  physiologic  state 
these  nerve-fibers  conduct  afferent  nerve  impulses  that  inhibit  the  inspiratory 
discharge  and  lead  to  an  expiratory  movement  sooner  than  would  otherwise 
be  the  case.  If  the  stimulation  be  increased  in  intensity  the  inspiratory 
movement  gradually  so  exceeds  the  expiratory  that  the  inspiratory  muscles 
pass  into  the  condition  of  tetanus  and  the  chest  walls  come  to  rest  in  the 
condition  of  forced  inspiration. 

Stimulation  of  the  vagus  with  strong  induced  electric  currents  not  infre- 
quently inhibits  the  inspiratory  movement  and  increases  the  expiratory  until 
there  is  a  complete  cessation  of  movement  in  the  condition  of  expiratory 
standstill.  The  effect  thus  produced  is  similar  to,  if  not  identical  with,  that 
produced  by  stimulation  of  the  superior  laryngeal  nerve. 

Faradization  of  the  trunks  of  the  pulmonic  branches  or  stimulation  of 
their  peripheral  terminations  in  the  mucous  membrane  of  the  bronchial 
tubes  or  alveoli  by  the  inhalation  of  chemic  vapors  causes  arrest  of  respira- 
tory movements,  a  fall  of  blood-pressure,  and  a  reflex  inhibition  of  the  heart 
(Brodie). 

The  Gastric  Nerves. — Stimulation  of  the  peripheral  end  of  a  divided  vagus 
nerve  causes  a  distinct  contraction  of  the  right  half  of  the  stomach  and 
secretion  from  the  gastric  glands.  Division  of  the  nerve  abolishes  the  sen- 
sibility of  the  mucous  membrane  of  the  stomach,  impairs  motility,  and  inter- 
feres with  the  secretion  of  the  gastric  juice. 

Similar  experimentation  on  the  trunk  of  the  vagus  has  shown  that  the 
nerve  excites  contraction  of  the  upper  part  of  the  small  intestine  and  of  the 
gall-bladder,  the  secretion  of  the  pancreas,  the  renal  circulation,  the  secretion 
of  urine,  etc. 

Functions. — The  afferent  fibers  transmit  nerve  impulses  from  the  area 
of  their  distribution  to  the  medulla  and  thence  through  cortical  connections 
to  the  sensor  cerebral  areas,  where  they  evoke  sensations.  They  therefore 
endow  all  parts  to  which  they  are  distributed  with  sensibility. 

The  efferent  fibers  transmit  impulses  outward  which  excite  contraction 
of  the  muscle  of  the  lower  two-thirds  of  the  esophagus,  the  stomach,  the 
small  intestine,  and  the  gall-bladder,  and  the  muscles  of  the  bronchial  tubes; 
excite  secretion  from  the  glands  of  the  stomach,  pancreas,  and  kidney,  and 
exert  an  inhibitor  influence  on  the  activity  of  the  heart.  The  efferent  fibers 
belong  to  the  autonomic  system  of  nerves  and  are  not  connected  with  the 
ganglia  of  the  vagus,  but  with  local  peripheral  ganglia. 

The  afferent  fibers  also  assist  in  the_maintenance  of  certain  organic 


THE  ENCEPHALIC  OR  CRANIAL  NERVES 


639 


reflex  actions  which  are  highly  essential  to  the  life  of  the  individual,  e.g., 
respiration,  the  heart-beat,  blood-pressure,  etc.,  all  of  which  have  been  con- 
sidered in  foregoing  pages. 


THE    ELEVENTH    NERVE.    THE    SPINAL    ACCESSORY 

The  eleventh  cranial  nerve,  the  spinal  accessory,  consists  of  peripherally 

coursing  fibers  which  bring  the  nerve-cells 

from  which  they  arise  into  relation  with 

separate  but  functionally  related  muscles, 

such  as  those  entering  into  the  formation 

of  the  larynx.     It  consists  of  two  por- 
tions, the  medullary  or  bulbar  and  the 

spinal. 

Origin. — The  axons  comprising  the 

medullary  portion  arise  from  a  group  of 

nerve-cells  in  the  extreme  lower  part  of 

the  nucleus  amhiguus,  known  as  the  nidus 

laryngei.     From   this   origin   the    axons 

pass  forward  and  outward  to  emerge  from 

the  medulla  just  below  and  in  series  with 

the  roots  of  the  vagus  nerve. 

The  axons  comprising  the  spinal  por- 
tion have  their  origin  in  nerve-cells  in 

the  lateral  margin  of  the  anterior  horn 

of  the  gray  matter  in  the  cervical  portion 

of  the  cord  as  far  down  as  the  fifth  cer- 
vical   vertebra.     From,   this    origin    the 

fibers  pass  to  the  surface  of  the  cord  to 

emerge  between  the  ventral  and  dorsa  ] 

roots  in  from  six  to  eight  filaments,  after 

which  they  unite  from  below  upward  to 

form  a  distinct  nerve.     This  enters  the 

cranial  cavity  through  the  foramen  mag- 
num, where  it  joins  with  the  medullary 

portion  to  form  the  common  trunk,  which 

then  passes  forward  to  emerge  from  the 

cranium  through  the  jugular  foramen. 

(Fig.  272.) 

Distribution. — After  emerging  from 

the  cranial  cavity  the  nerve  soon  sepa- 
rates into  two  branches: 

I.  An  internal  or  anastomotic  branch, 
consisting  chiefly  of  filaments  com- 
ing from  the  medulla  oblongata.  It 
soons  enters  the  trunk  of  the  vagus, 
from  which  fibers  pass  through  the 
pharyngeal  plexus  to  the  superior 
and  inferior  constrictor  muscles  of  the  pharynx,  to  thepalato-pharyngeus, 
to  the  levator  palati  and  azygos  uvulae  muscles  (Beevor  and  Horsley) ;  to 


Fig.  272. — Spinal  Accessory  Nerve. 
I.  Trunk  of  the  facial  nerve.  2,  2. 
Glosso-pharyngeal  nerve.  3,  3.  Pneumo- 
gastric.  4,  4,  4.  Trunk  of  the  spinal  acces- 
sory. 5.  Sublingual  nerve.  6.  Superior 
cervical  ganglion.  7,  7.  Anastomosis  of 
the  first  two  cervical  nerves.  8.  Carotid 
branch  of  the  sympathetic.  9,  10,  11, 
12,  13.  Branches  of  the  glosso-pharyngeal. 
14,  15.  Branches  of  the  facial.  16.  Otic 
ganglion.  17.  Auricular  branch  of  the 
pneumogastric.  18.  Anastomosing  branch 
from  the  spinal  accessory  to  the  pneu- 
mogastric. iQ.  Anastomosis  of  the  first 
pair  of  cervical  nerves  with  the  sublingual. 
20.  Anastomosis  of  the  spinal  accessory 
with  the  second  pair  of  cer\Tcal  nerves.  21. 
Pharyngeal  plexus.  22.  Superior  laryn- 
geal nerve.  23.  External  laryngeal  nerve. 
24.  Middle  cervical  ganglion. — (Hirsch- 
ield.) 


640  TEXT-BOOK  OF  PHYSIOLOGY 

the  muscles  of  the  larynx  through  the  inferior  laryngeal  nerve,  and  to  the 

heart  according  to  some  authorities. 
2.  An  external  branch,  consisting  chiefly  of  the  accessory  fibers  from  the 

spinal  cord. 
It  is  distributed  to  the  sterno-cleido-mastoid  and  trapezius  muscles. 

Cortical  Connections. — The  nucleus  of  origin  of  the  medullary  branch 
at  least,  is  in  relation  with  nerve-cells  in  the  lower  third  of  the  general  cerebral 
motor  area,  the  axons  of  which  descend  in  the  pyramidal  tract. 

Properties. — Faradization  of  the  medullary  portion  of  the  nerve  near  its 
origin  gives  rise  to  contraction  of  the  muscles  to  which  it  is  distributed. 
Destruction  of  the  medullary  root  is  followed  by  impairment  of  deglutition 
from  a  paralysis  of  the  muscles  of  the  pharynx  and  palate  and  a  loss  of  the 
power  of  producing  vocal  sounds  on  account  of  paralysis  of  the  constrictor 
muscles  of  the  larynx.  According  to  some  authorities,  there  is  also  an  ac- 
celeration of  the  heart's  action  from  a  loss  of  inhibitor  influences. 

Stimulation  of  the  external  branch  gives  rise  to  contraction  of  the  sterno- 
cleido-mastoid  and  trapezius  muscles,  though  division  of  the  branch  does 
not  give  rise  to  complete  paralysis,  as  they  are  supplied  with  motor  fibers 
also  from  the  cervical  nerves.  In  consequence  of  division  of  the  external 
branch  animals  experience  extreme  shortness  of  breath  during  exercise,  from 
a  want  of  coordination  of  the  muscles  of  the  fore  limbs  and  the  muscles  of 
respiration. 

Functions. — The  function  of  the  fibers  of  the  spinal  accessory  nerve 
is  the  transmission  of  nerve  impulses  from  the  cells  from  which  they  take 
their  origin  to  the  muscles  to  which  they  are  distributed.  They  therefore 
excite  to  action  some  of  the  muscles  of  deglutition;  the  muscles  which  regu- 
late the  tension  of  the  vocal  bands  during  phonation  and  the  muscles  which 
control  the  respiratory  movements  associated  with  sustained  or  prolonged 
muscle  efforts.  The  fibers  may  also  convey  nerve  impulses  which  exert 
an  inhibitor  influence  on  the  heart. 

THE  TWELFTH  NERVE.    THE  HYPOGLOSSAL 

The  twelfth  cranial  nerve,  the  hypoglossal,  consists  of  peripherally 
coursing  nerve-fibers  which  serve  to  connect  the  nerve-cells  from  which  they 
arise  with  the  musculature  of  the  tongue. 

Origin. — The  axons  composing  the  hypoglossal  nerve  arise  from  a  collec- 
tion of  nerv^e-cells  situated  beneath  the  floor  of  the  fourth  ventricle.  This 
nucleus  is  elongated  and  extends  from  the  medullary  strias  downward  as  far 
as  the  lower  border  of  the  olivary  body.  It  is  located  ventro-laterally  to  the 
spinal  canal.  After  leaving  the  cells  of  the  nucleus  the  axons  pass  forward 
and  outward  toward  the  surface  of  the  medulla,  from  which  they  emerge  in 
ten  or  twelve  small  bundles  or  filaments  in  the  groove  between  the  olivary 
body  and  the  anterior  pyramid.  Beyond  this  point  they  unite  to  form 
a  common  trunk. 

Distribution. — The  common  trunk  thus  formed  passes  out  of  the  cranial 
cavity  through  the  anterior  condyloid  foramen.  In  its  course  it  receives 
filaments  from  the  first  and  second  cervical  nerves,  the  sympathetic  and 
vagus.  It  is  finally  distributed  to  the  intrinsic  muscles  of  the  tongue  and  to 
the   genio-hyo-glossus,    hyo-glossus,    and    stylo-hyoid    muscles.     Branches 


THE  ENCEPH.\LIC  OR  CRANIAL  NERVES  641 

derived  from  the  cervical  plexus  pass  to  muscles  which  elevate  and  depress 
the  hyoid  bone. 

Cortical  Connections. — The  hypoglossal  nerve  nuclei  are  connected 
with  ner\^e-cells  in  the  lower  third  of  the  general  motor  area  around  the  in- 
ferior termination  of  the  fissure  of  Rolando  by  axons  which  descend  in  the 
pyramidal  tract. 

Properties. — Faradization  of  the  nerv^e  gives  rise  to  convulsive  move- 
ments of  the  muscles  to  which  it  is  distributed.  Division  of  the  nerve  is 
followed  by  a  loss  of  motion  and  an  interference  with  deglutition,  mastication, 
and  articulation,  especially  in  the  pronunciation  of  the  consonantal  sounds. 
In  hemiplegia,  complicated  with  paralysis  of  the  tongue  from  injury  to  the 
hypoglossal  tract,  the  opposite  side  of  the  tongue  is  involved  in  the  paralysis. 
On  protrusion  of  the  tongue  the  tip  is  deviated  to  the  paralyzed  side,  due  to 
the  unopposed  action  of  the  muscle  of  the  opposite  side. 

Function. — The  hypoglossal  nerve  transmits  nerve  impulses  from  its 
origin  to  the  intrinsic  and  extrinsic  muscles  of  the  tongue,  exciting  them  to 
activity.  The  coordinate  activity  of  these  muscles  favorably  assists  mastica- 
tion, articulation,  and  deglutition. 


41 


CHAPTER  XXVI 
THE  AUTONOMIC  NERVE  SYSTEM 

Introductory  Statements. — ^The  motor  tissues  of  the  body  may  be 
divided  into  two  groups,  the  one,  comprising  the  striated  skeletal  muscles, 
the  other,  comprising  the  striated  cardiac,  and  the  non-striated  vascular  and 
visceral  muscles,  and  the  epithelium  of  glands.  The  movements  to  which 
they  give  rise  are,  in  the  first  instance,  those  exhibited  mainly  by  the  head, 
upper  and  lower  extremities;  and  in  the  second  instance,  those  exhibited 
by  the  heart  and  blood-vessels;  by  the  walls  of  the  viscera,  e.g.,  stomach,  in- 
testines, bile  and  urinary  bladders,  etc.,  and  those  exhibited  by  glands  (the 
secretion  and  discharge  of  fluids),  e.^.,  salivary,  gastric,  perspiratory,  mam- 
mary, etc.,  as  well  as  the  specific  agents,  e.g.,  adrenalin,  pituitrin,  etc.,  dis- 
charged into  the  blood.  The  activities  which  these  two  groups  of  motor 
tissues  exhibit  are  excited  mainly  by  two  ultimate  and  separate  factors. 

The  Skeletal  Muscles. — ^The  skeletal  muscles  in  the  different  regions  of 
the  body  are  excited  to  action  by  nerve  impulses,  which  come  to  them  directly 
from  special  groups  of  emissive  or  motor  cells  situated  in  the  gray  matter 
beneath  the  aqueduct  of  Sylvius,  beneath  the  floor  of  the  fourth  ventricle, 
and  in  the  ventral  cornua  of  the  gray  matter  of  the  spinal  cord.  The  nerve 
impulses,  discharged  by  these  cells,  are  transmitted  directly  through  but  one 
axon  to  the  muscle.  The  nerve-cells  in  these  different  regions,  though 
irritable,  do  not  possess  spontaneity  of  action,  but  require  for  the  manifesta- 
tion of  their  activity  the  arrival  and  the  stimulating  influence  of  nerve  im- 
pulses. These  may  come  (i)  from  peripheral  regions  of  the  body  through 
afferent  spinal  nerves  in  consequence  of  the  action  of  external  agents,  in 
which  case  the  resulting  movement  is  termed  a  reflex  movement;  or  (2)  from 
the  cerebrum  through  descending  nerve-fibers  in  consequence  of  an  act  of 
volition,  in  which  case  the  resulting  movement  is  termed  a  volitional  or 
voluntary  movement.  In  the  performance  of  daily  work  the  skeletal  muscle 
activities,  though  frequently  caused  by  reflected  nerve  impulses  are,  in  the 
vast  majority  of  instances,  caused  by  nerve  impulses  of  cerebral  (volitional) 
origin.  From  moment  to  moment  these  muscles  are  excited  to  action,  guided 
and  controlled  for  the  most  part  by  this  dominating  and  in  some  respect 
arbitrary  factor.  By  the  constant  exercise  of  the  will,  these  muscles  are 
compelled  to  act  constantly,  and  for  this  reason  it  is  said  that  they  do  not 
possess  independent  activity,  except  in  a  minor  degree. 

The  Striated  Cardiac,  the  Non-striated  Vascular  and  Visceral 
Muscles  and  the  Epithelium  of  Glands. — ^The  striated  cardiac,  the  vas- 
cular and  visceral  muscles,  and  the  epithelium  of  glands  in  all  regions  of  the 
body,  if  not  primarily  excited  to  action,  are  at  least  controlled  and  regulated 
in  their  activities,  by  nerve  impulses  that  come  to  them  indirectly  from  special 
groups  of  emissive  or  motor  nerve-cells  situated  in  (i)  a  small  region  at  the 
extreme  upper  part  of  the  gray  matter  beneath  the  aqueduct  of  Sylvius;  (2) 
in  special  regions  in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle; 

642 


THE  AUTONOMIC  NERVE  SYSTEM  643 

(3)  in  the  ventral  horns  of  the  gray  matter  of  the  spmal  cord  between  and 
including  the  levels  of  origin  of  the  second  thoracic  to  the  second  or  third 
lumbar  nerves;  and  (4)  in  the  gray  matter  of  the  lower  portion  of  the  cord 
between  and  including  the  levels  of  origin  of  the  second  to  the  fourth  sacral 
nerves.  The  nerve  imptilses  discharged  by  these  cells  are  transmitted  indi- 
rectly, not  through  one,  but  through  two  successively  arranged  neurons.  The 
first,  a  fine  white  medullated  fiber,  emerges  from  the  medulla  or  spinal  cord 
in  association  with  the  large  motor  root  fibers  passing  to  the  skeletal  muscles, 
and  after  a  variable  distance  leaves  these  fibers  to  arborize  around  and  to 
become  physiologically  related  to  a  sympathetic  ganglion;  the  second,  a  fine, 
dark  non-medullated  fiber,  emerges  from  one  of  the  cells  of  the  ganglion, 
which,  after  pursuing  a  longer  or  shorter  course,  branches  and  becomes  his- 
tologically and  physiologically  related  to  non-striated  muscle-fibers  and 
epithelium.  The  first  neuron  is  termed  pre-ganglionic,  the  second  post- 
ganglionic,  or  in  accordance  with  long-established  usage,  sympathetic. 

The  nerve-cells  in  the  regions  which  give  origin  to  the  pre-ganglionic 
neurons,  though  irritable,  do  not  possess  spontaneity  of  action,  but  require 
for  the  manifestation  of  their  activities,  the  arrival  and  stimulating  influence 
of  nerve  impulses.  These  may  likewise  come  (i)  from  peripheral  regions 
of  the  body  through  afferent  nerves  in  consequence  of  the  action  of  external 
agents — in  which  case  the  resulting  movement  is  termed  a  reflex  movement; 
or  (2)  from  the  cerebrum  through  descending  nerve-fibers  in  consequence 
of  affective  or  emotional  psychic  states,  in  which  case  the  resulting  movement 
or  modification  of  movement,  is  termed  an  affective  or  an  emotional  move- 
ment. In  the  performance  of  the  functions  of  vascular  and  visceral  organs 
and  glands,  the  non-striated  muscles  and  epithelium  are  in  the  vast  majority 
of  instances  caused  by  reflected  nerve  impulses,  though  frequently  modified 
by  nerve  impulses  of  cerebral  (affective  or  emotional)  origin.^ 

From  the  fact  that  the  nerve-centers  of  the  pre-ganglionic  nerve-fibers  in 
the  cranio-bulbar  and  spinal-cord  regions  are  removed  from  and  not  subject 
to  volitional  control;  and  from  the  further  fact  that  they  are  in  the  vast 
majority  of  instances  stimulated  to  activity  by  nerve  impulses  transmitted 
from  peripheral  regions  (though  modified  from  time  to  time  by  the  ever  vary- 
ing phases  of  affective  or  emotional  psychic  activity)  this  system  of  nerves  is 
regarded  as  in  a  sense  independent,  i.e.,  of  volitional  control,  self -regulative 
or  autonomous  in  its  activity,  and,  therefore,  has  been  designated  the  auto- 
nomic nerve  system  (Langley).  The  tissues  to  which  it  is  distributed  have 
also  been  designated  the  autonomic  tissues. 

In  a  consideration  of  this  subject  it  must  be  borne  in  mind  that  there  are 
not  two  nerve  systems,  but  only  one,  which,  however,  may  be  subdivided 
into  two  portions,  one  of  which  in  its  peripheral  distribution  is  associated 
directly  with  skeletal  muscles  only;  the  other  of  which  is  associated  indirectly, 
through  the  intervention  of  a  ganglion,  with  non-striated  muscle  and  gland 
epithelium.  The  central  origin  of  these  two  portions  is  essentially  the  same, 
and  the  central  cells  are  influenced  in  the  same  way,  though  in  different  de- 
grees, by  peripheral  and  cerebral  activities. 

^  It  is  only  necessary  to  recall  the  well-known  effects  of  psychic  states  on  the  heart,  on  theblood- 
vesseb  and  sweat-glands  of  the  neck  and  face;  the  observations  of  Pavlov  on  the  effects  of  agreeable 
psychic  states  on  the  flow  of  saliva  and  gastric  juice;  the  observations  of  Cannon  of  the  effects 
of  psychic  states  of  an  opposite  character  of  fear,  anger,  etc.,  on  the  gastric  and  intestinal  move- 
ments and  the  secretion  of  the  adrenal  glands. 


644  TEXT-BOOK  OF  PHYSIOLOGY 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  AUTONOMIC  NERVE  SYSTEM 

In  a  consideration  of  the  essential  facts  of  the  physiologic  anatomy  of 
this  system,  it  will  be  found  convenient  to  consider  first  the  sympathetic 
ganglia,  and  the  distribution  of  their  post-ganglionic  fibers. 

The  Sympathetic  Ganglia. — ^The  sympathetic  ganglia  may  for  con- 
venience of  description  be  divided  into  three  groups:  viz.,  the  vertebral  or 
lateral,  the  pre-vertebral  or  collateral,  and  the  peripheral  or  terminal. 

The  vertebral  ganglia  are  arranged  in  the  form  of  chains,  one  on  each 
side  of  the  vertebral  column.  The  number  of  ganglia  in  the  chain  varies  in 
animals  of  different  and  in  animals  of  the  same  species.  In  man  the  number 
varies  from  20  to  22.  Each  chain  may  be  divided  into  a  cervical,  a  thoracic 
a  lumbar,  a  sacral,  and  a  coccygeal  portion.  The  cervical  portion  is  usually 
described  as  consisting  of  three  gangHa — a  superior,  a  middle  and  an  inferior. 
This  statement  is  open  to  question,  however,  as  the  middle  one  is  frequently 
absent  and  the  inferior  is  regarded  by  some  anatomists  as  belonging  to  the 
pre-vertebral  series.  The  thoracic  portion  consists  of  ten  or  twelve  ganglia, 
the  lumbar  and  sacral  portions  of  four  each  and  the  coccygeal  portion  of  one, 
the  so-called  ganglion  impar. 

The  pre-vertebral  ganglia  are  also  united  in  the  form  of  a  chain  situated 
in  the  abdominal  cavity.  The  ganglia  constituting  this  chain  are  known  as 
the  semilunar,  the  renal,  the  superior  and  inferior  mesenteric,  and  hypogas- 
tric, or  pelvic. 

The  peripheral  ganglia  are  in  more  or  less  close  relation  with  tissues  and 
organs  in  different  regions  of  the  body.  Among  the  members  of  this  group 
may  be  mentioned  the  ciliary  or  ophthalmic,  the  spheno-palatine,  the  otic 
and  the  submaxillary  ganglia;  the  ganglia  in  the  walls  of  the  heart,  in  the 
walls  of  the  respiratory  organs,  in  the  walls  of  the  stomach,  intestines  and 
at  the  base  of  the  bladder  (the  pelvic  ganglia). 

Structure  of  the  Ganglia. — Each  ganglion  consists  of  a  capsule  or 
stroma  of  connective  tissue  in  which  are  contained  large  numbers  of  nerve- 
cells,  nerve-fibers,  medullated  and  non-meduUated,  and  blood-vessels.  The 
nerve-cells  give  origin  to  two  or  more  dendrites,  which,  perforating  a  nucle- 
ated capsule  by  which  each  cell  is  surrounded,  branch  and  re-branch  and 
interlace  to  form  a  pericapsular  plexus.  Each  cell  gives  origin  to  an  axon, 
which  as  it  leaves  the  cell  becomes  invested  with  a  sheath  continuous  with 
the  capsule  surrounding  the  cell-body.  It  is,  however,  wanting  in  a  medul- 
lary sheath,  and  hence  the  nerve  presents  a  gray  color.  Such  a  structure, 
in  its  entirety,  is  known  as  a  sympathetic  neuron.  The  axonic  processes  as 
they  emerge  from  the  cells  become  united  and  in  many  instances  form  dis- 
tinct nerve  strands,  but  subsequently  as  they  approach  their  distribution 
divide  and  subdivide,  forming  smaller  and  even  smaller  bundles,  which  pass 
in  different  directions  to  regions  varying  in  position  according  to  the  situa- 
tion of  the  ganglion  from  which  they  come.  These  post-ganglionic  fibers 
are  for  descriptive  purposes  divided  into  two  groups,  viz.:  (i)  the  gray  rami 
communicantes ,  and  (2)  the  rami  viscerates. 

THE  ANATOMIC  RELATIONS  OF  THE  SYMPATHETIC  GANGLIA 
TO  PERIPHERAL  STRUCTURES 

The  Vertebral  Ganglia. — i.  The  grey  rami  communicantes.  The  fibers 
composing  these  branches,  as  the  name  implies,  communicate  with  the 


THE  AUTONOMIC  NERVE  SYSTEM  645 

trunks  of  the  spinal  nerves.  Each  ganghon  of  the  vertebral  chain  gives 
origin  to  one  or  more  of  these  post-ganglionic  branches  which  pass  backward 
and  outward  and  enter  the  sheath  of  the  corresponding  spinal  nerve.  In 
the  cervical  region,  however,  where  the  ganglia  do  not  correspond  in  number 
with  the  cervical  spinal  nerves,  the  ganglia  give  off  two  or  more  gray  rami. 
Thus  in  man  the  superior  cervical  ganglion  sends  branches  to  the  first  four 
cervical  nerves.  The  middle  and  inferior  ganglia  send  a  branch  to  the  fifth 
and  sixth  and  the  seventh  and  eighth  cervical  nerves  respectively.  The  first 
thoracic  ganglion  sends  several  branches  into  the  trunks  of  the  nerves  that 
enter  into  the  formation  of  the  brachial  plexus.  The  ganglia  in  the  thoracic, 
lumbar  and  sacral  regions,  send  at  least  one  gray  ramus,  in  some  instances 
two,  into  the  sheath  of  the  corresponding  thoracic,  lumbar  and  sacral  nerves. 

The  gray  rami  which  thus  enter  the  sheath  of  the  spinal  nerve  trunks,  pass 
in  company  with  their  efferent  motor  fibers,  to  the  periphery,  to  be  finally 
distributed  to  structures  in  the  skin,  viz.:  non-striated  muscles  of  blood- 
vessels, non-striated  muscles  of  hair  follicles,  and  epithelium  of  sweat-glands. 
The  blood-vessels  and  sweat-glands  of  the  skin  of  the  neck  receive  their 
ganglionic  nerve-supply  from  the  superior  and  middle  cervical  ganglia;  those 
of  the  skin  of  the  arm,  from  the  inferior  cervical  and  first  thoracic  ganglia; 
those  for  the  skin  of  the  trunk,  from  the  thoracic  ganglia;  those  for  the  skin 
of  the  hip  and  leg,  from  the  lumbar  and  upper  sacral  ganglia;  those  for  the 
skin  of  the  external  genital  organs,  fr-om  the  lower  sacral  ganglia. 

2,  The  rami  viscerates . — ^The  fibers  composing  these  branches  were  sup- 
posed, as  the  name  implies,  to  pass  directly  to  viscera,  though  it  is  apparent 
from  the  course  they  pursue  that  they  communicate,  if  not  with  spinal,  at 
least  with  cranial  nerves.    ^ 

The  superior  cervical  ganglion  gives  off  from  its  cephalic  extremity  two 
visceral  branches,  which  subsequently  divide  and  subdivide  forming  the 
carotid  and  cavernous  plexuses;  from  these  plexuses  slender  branches  follow 
the  course  of  the  more  superficial  arteries  at  least,  to  their  terminations,  while 
others  pass  into  the  trunks  of  the  trigeminal,  abducent,  and  the  superior  and 
deep  petrosal  branches  of  the  facial  nerve,  to  be  distributed  to  blood-vessels 
and  glands  of  special  regions  of  the  head  and  face.  Still  other  branches 
pass  down  the  neck  and  in  their  course  become  associated  with  correspond- 
ing branches  from  the  middle  and  inferior  cervical  ganglia.  Interlacing  in 
an  intricate  manner  they  assist  in  forming  the  cardiac  plexuses. 

The  middle  cervical  ganglion  gives  off  visceral  branches,  which  in  asso- 
ciation with  branches  from  the  inferior  cervical  ganglion,  pass  to  the  thyroid 
gland  to  be  distributed  to  the  walls  of  the  blood-vessels  and  to  the  gland  cells 
as  well.  Stohr,  Berkley  and  others  have  shown  that  fine  non-medullated 
nerve  filaments  terminate  on  and  between  the  epithelial  cells  lining  the  folli- 
cles, though  the  supply  is  not  very  abundant. 

The  inferior  cervical  ganglion  gives  oft'  visceral  branches  which  pass  down- 
ward and  forward  and  are  ultimately  distributed  to  the  heart-muscle.  With 
these  fibers  there  are  usually  associated,  according  to  the  animal  considered, 
visceral  fibers  which  come  from  the  first  thoracic  or  stellate  ganglion.  These 
fibers  are  known  as  the  sympathetic  cardiac  fibers  and  have  for  the  physiolo- 
gist and  clinician  great  interest  as  they  are  associated  with  the  activities  of 
the  heart.     (See  pages  311  and  319.) 

The  thoracic,  lumbar  and  sacral  ganglia  also  gi\-e  off  visceral  branches 


646  TEXT-BOOK  OF  PHYSIOLOGY 

which  pass  for  the  most  part  to  neighboring  structures,  though  from  the 
lower  lumbar  and  sacral  ganglia  branches  pass  to  viscera  in  the  lower  ab- 
dominal and  pelvic  regions. 

In  accordance  with  the  law  of  distribution  and  relations  of  the  fibers  of 
the  sympathetic  ganglia  to  peripheral  organs,  it  can  be  assumed  that  to  what- 
ever organ  the  visceral  branches  are  distributed  they  too  ultimately  terminate 
in  the  non-striated  muscle-cells  of  the  walls  of  the  blood-vessels  and  the  walls 
of  hollow  viscera,  and  in  some  situations  in  the  epithelium  of  glands  as  well. 

The  Pre-vertebral  Ganglia. — -The  pre-vertebral  ganglia  are  located  in 
the  abdominal  cavity. 

The  semilunar,  the  renal,  and  the  superior  mesenteric  are  situated  in  the 
neighborhood  of  the  celiac  axis  and  on  a  level  with  the  adrenal  bodies.  These 
ganglia  give  off  an  enormous  number  of  visceral  branches  which  interlace  in 
a  very  intricate  manner  forming  what  is  known  as  the  solar  plexus.  Sub- 
divisions of  this  plexus,  taking  their  names  from  the  regions  to  which  they 
are  distributed,  are  known  as  the  gastric,  renal,  adrenal,  splenic,  hepatic  and 
superior  mesenteric.  The  terminals  of  the  fibers  composing  these  plexuses 
are  distributed  to  the  non-striated  muscle-fibers  of  blood-vessels  of  the  stom- 
ach, kidney,  adrenal  body,  liver,  and  small  intestine;  to  the  muscle-walls  of 
the  stomach,  the  small  intestine,  the  walls  of  the  gall-bladder,  as  well  as  the 
sphincter  muscles  surrounding  the  pyloric  and  the  ileo-colic  orifices. 

The  inferior  mesenteric  ganglion  situated  close  to  the  origin  of  the  inferior 
mesenteric  artery  gives  off  visceral  fibers,  which  also  interlace  with  other 
fibers  to  form  the  hypogastric  plexus,  from  which  fibers  pass  to  the  muscle- 
walls  of  the  colon,  bladder,  uterus,  vagina  and  to  the  blood-vessels  of  the 
pelvic  viscera. 

The  Peripheral  Ganglia. — ^The  peripheral  ganglia,  as  previously  stated, 
are  in  more  or  less  close  relation  with  tissues  and  organs  in  different  regions 
of  the  body.  Among  the  members  of  this  group  may  be  mentioned  the 
ciliary  or  ophthalmic,  the  sphenopalatine,  the  otic  and  the  submaxillary 
ganglia;  the  ganglia  in  the  walls  of  the  heart,  in  the  walls  of  the  respiratory 
organs,  in  the  walls  of  the  stomach,  and  intestines  and  the  ganglia  at  the 
base  of  the  bladder  (the  pelvic  ganglia). 

The  ganglia  situated  in  the  head  are  usually  described  in  connection  with 
and  as  constituent  parts  of  the  cranial  nerve  system.  They,  however,  bear 
the  same  relation  to  the  cranial  that  the  vertebral  and  pre-vertebral  ganglia 
bear  to  the  spinal  nerves.  They  consist  of  ganglion  cells  from  which  post- 
ganglionic fibers  pass  to  glands,  blood-vessels,  and  viscera.  Motor  and  sensory 
nerves  pass  through  one  or  more  ganglia,  though  they  have  no  anatomic 
connection  with  them.  In  their  structure,  distribution  and  functions  they 
closely  resemble  the  collateral  ganglia. 

1.  The  ciliary  or  ophthalmic  ganglion  is  situated  in  the  orbital  cavity  posterior 

to  the  eyeball.  It  is  small  in  size,  gray  in  color,  and  consists  of  a  con- 
nective-tissue stroma  containing  nerve-cells.  From  these  cells  post- 
ganglionic fibers  emerge  which,  after  a  short  course  forward,  penetrate 
the  eyeball  and  terminate  in  the  circular  fibers  of  the  iris  and  the  ciliary 
muscle. 

2.  The  spheno-palatine  ganglion  is  situated  in  the  spheno-maxillary  fossa. 

Its  nerve-cells  send  non-meduUated  post-ganglionic  fibers  to  the  blood- 
vessels and  glands  of  the  mucous  membrane  of  the  nasal  and  oral  regions. 


THE  AUTONOMIC  NERVE  SYSTEM  647 

3.  The  otic  ganglion  Is  situated  just  below  the  foramen  ovale  and  internal 

to  the  third  division  of  the  fifth  nerve.  The  cells  give  origin  to  post- 
ganglionic fibers  which  pass  to  the  parotid  gland  by  way  of  the  auriculo- 
temporal division  of  the  fifth  nerve,  and  to  the  blood-vessels  of  the 
mucous  membrane  of  the  lower  Up,  cheek,  and  gums. 
Motor  fibers  from  the  small  or  motor  root  of  the  fifth  nerve  pass 
through  this  ganglion  to  the  tensor  tympani  muscle. 

4.  The  submaxillary  ganglion  is  situated  close  to  the  corresponding  gland. 

The  post-ganglionic  fibers  to  the  blood-vessels  and  gland-cells. 

5.  The  cardiac  ganglia  are  situated  in  different  regions  in  the  walls  of  the 

heart;  their  visceral  branches  are  distributed  directly  to  the  heart  muscle- 
cells. 

6.  The  pelvic  ganglia  are  situated  close  to  the  base  of  the  bladder;  their 

visceral  branches  are  distributed  to  the  blood-vessels  of  the  generative 
organs  and  to  the  muscle-walls  of  the  bladder,  rectum,  etc. 
From  the  distribution  of  the  branches  emerging  from  all  the  different 
groups  of  ganglia  there  is  every  reason  to  believe  that  they  are  in  some  way 
associated  with  vaso-augmentor  and  vaso-inhibitor,  viscero-augmentor  and 
viscero-inhibilor,  secreto-motor  and  secreto-inhibitor,  and  pilo-motor  phenom- 
ena. 

THE  ANATOMIC  RELATION  OF  THE  CENTRAL  NERVE  SYSTEM  TO  THE 

SYMPATHETIC  GANGLIA 

The  central  nerve  system  is  associated  anatomically  and  physiologically 
with  the  sympathetic  gangha  through  the  intermediation  of  fine  meduUated 
nerve-fibers,  the  pre-ganglionic,  which  have  their  origin  in  nerve-cells  situated 
in  four  different  regions,  viz.: 

T.  The  Mid-brain  Region. — The  pre-gangUonic  nerve-fibers  that  leave  the 
brain  in  this  region  arise  from  groups  of  nerve-cells  situated  high  up  in 
the  gray  matter  beneath  the  aqueduct  of  Sylvius  just  where  it  widens 
to  form  the  cavity  of  the  third  ventricle.  From  this  origin  they  enter 
the  trunk  of  the  oculo-motor  nerve  and  in  association  with  it  enter  the 
orbit  cavity.  In  this  situation  these  pre-ganglionic  fibers  leave  the 
oculo-motor  nerve  and  enter  the  ciliary  or  ophthalmic  ganglion  around 
the  nerve-cells  of  which  their  terminal  branches  arborize.  The  gray 
post-ganglionic  fibers  arising  in  the  gray  cells  of  this  ganglion  enter  the 
eyeball  and  are  ultimately  distributed  to  the  sphincter  muscle  of  the 
iris  and  to  the  ciliary  muscle. 
2.  The  Bulbar  Region. — The  pre-ganglionic  fibers  that  leave  the  brain  in 
this  region  arise  from  nerve-cells  situated  in  the  gray  matter  beneath  the 
floor  of  the  fourth  ventricle  a  httle  above  and  below  the  calamus  scrip- 
torius.  These  fibers  leave  this  region  by  three  routes,  viz.:  in  the 
trunks  of  (i)  the  pars  intermedia  or  nerve  of  Wrisberg,  (2)  the  glosso- 
pharyngeal and  (3)  the  vagus. 

The  pre-ganglionic  fibers  that  leave  by  the  nerve  of  Wrisberg  enter 
the  facial  nerve  and  subsequently  pass  by  way  of  the  great  superficial 
petrosal  nerve  to  the  spheno-palatine  ganglion,  and  by  way  of  the  chorda 
tympani  nerve  to  the  submaxillary  ganglion,  around  the  nerve-cells  of 
which  their  terminal  branches  arborize.  The  gray  post-ganglionic  fibers 
which  arise  in  the  cells  of  these  ganglia  are  distributed  to  the  blood- 


648  TEXT-BOOK  OF  PHYSIOLOGY 

vessels  and  glands  of  the  nose  and  mouth  and  to  the  blood-vessels  and 
epithelium  of  the  submaxillary  and  sublingual  glands  respectively. 

The  pre-ganglionic  fibers  that  leave  by  way  of  the  glosso-pharyngeal 
nerve  pass  into  the  tympanic  branch  or  nerve  of  Jacobson  and  ulti- 
mately arborize  around  the  cells  of  the  otic  ganglion.  The  gray  post- 
ganglionic fibers  which  arise  in  the  cells  of  this  ganglion  pass  by  way  of 
the  auriculo-temporal  branch  of  the  trigeminal  nerve  to  the  blood-vessels 
and  epithelium  of  the  parotid  gland. 

The  pre-ganglionic  fibers  that  leave  in  the  trunk  of  the  vagus  nerve 
are  ultimately  distributed  to  the  ganglia  of  the  heart,  stomach,  small 
intestine,  etc.,  around  the  nerve-cells  of  which  their  terminal  branches 
arborize.  The  gray  post-ganglionic  fibers  which  arise  in  these  ganglia 
pass  to  the  heart-fibers,  to  the  non-striated  muscle-fibers  in  the  walls  of 
the  stomach,  intestines,  etc.  These  fibers  contained  in  the  facial,  glosso- 
pharyngeal and  vagus  nerves,  together  with  their  ganglionic  continua- 
tions, have  collectively  been  termed  the  bulbar  autonomic  system. 
Together  with  the  fibers  in  the  oculo-motor  nerve  they  have  been  termed 
the  cranio-bidbar  autmiomic  system. 
3.  The  Mid-spinal  Cord  Region. — The  pre-ganglionic  nerve -fibers  that 
leave  the  spinal  cord  in  this  region  arise  from  groups  of  nerve-cells  situ- 
ated in  the  gray  matter  between  the  levels  of  origin  of  the  second  thoracic 
and  the  second  and  third,  perhaps  the  fourth,  lumbar  nerves.  From  this 
origin  the  fine  pre-ganglionic  fibers  emerge  from  the  cord  in  the  ventral 
roots  of  the  thoracic  and  upper  lumbar  nerves  and  hence  naturally  fall 
into  two  groups,  viz.:  the  thoracic  and  the  lumbar.  Both  groups  of 
nerves  accompany  the  ventral  motor  roots  of  the  spinal  nerves  to  about 
the  point  where  each  nerve  divides  into  an  anterior  and  a  posterior 
branch;  they  then  leave  and  enter  the  vertebral  or  sympathetic  chain 
of  ganglia.  The  branches  of  communication  are  known  as  the  white 
rami  communicantes.  The  nerve -fibers  composing  these  communicating 
branches  all  terminate  around  the  nerve-cells  of  the  ganglia  at  the  same 
and  at  somewhat  different  levels  and  in  different  regions. 

The  thoracic  pre-ganglionic  fibers  in  accordance  with  their  distribu- 
tion may  be  di\dded  into  five  groups,  \\z. : 

{a)  Those  fibers  which  emerge  from  the  spinal  cord  in  each  of  the  tho- 
racic nerves  and  which  terminate  around  the  cells  of  the  ganglia  at 
the  same  level. 
{b)  Those  fibers  which  emerge  from  the  spinal  cord  in  the  thoracic 
nerves  from  the  fourth  to  the  tenth  and  which  after  entering  the 
vertebral  chain  turn  upward  and  finally  terminate  around  the  cells 
of  the  first  thoracic  or  stellate  ganglion, 
(c)  Those  fibers  which  emerge  from  the  cord  in  the  second  and  third 
thoracic  nerves  and  which  after  entering  the  vertebral  chain  turn 
upward  and  terminate  around  the  cells  of  the  inferior  cervical  gan- 
glion. 
{d)  Those  fibers  which  emerge  from  the  cord  in  the  second,  third  and 
perhaps  the  fourth  thoracic  nerves  and  which,  after  entering  the 
vertebral  chain  turn  upward  and  pass  through  the  various  ganglia 
and  cord  and  finally  terminate  around  the  cells  of  the  superior  cer- 
vical ganglion. 


THE  AUTONOMIC  NERVE  SYSTEM  649 

(e)  Those  fibers  which  emerge  from  the  spinal  cord  in  the  thoracic 
nerves  from  the  fifth  to  the  tenth  and  which  after  entering  the  verte- 
bral chain  pass  across  it  and  turn  downward  and  forward  uniting 
at  different  levels  to  form  the  greater  and  lesser  splanchnic  nerves, 
the  terminal  branches  of  which  arborize  around  the  cells  of  the  semi- 
lunar, the  renal,  and  the  superior  mesenteric  ganglia. 
The  lumbar  pre-ganglionic  fibers  in  accordance  with  their  distribu- 
tion may  be  divided  into  three  groups,  ^iz. : 

(a)  Those  fibers  which,  after  pursuing  a  similar  course  to  the  preceding 
termanate  around  the  cells  of  the  ganglia  at  the  same  level. 

(b)  Those  fibers  which  after  entering,  cross  the  vertebral  chain  and 
then  pass  forward  as  the  inferior  splanchnic  nerves  to  terminate 
around  the  cells  of  the  inferior  mesenteric  ganglion. 

(c)  Those  fibers  which  after  entering,  descend  the  vertebral  chain  to 
terminate  around  the  cells  of  the  successive  ganglia  as  far  as  the 
third  sacral.  It  is  probable  that  some  fibers  from,  the  lower  thoracic 
nerves  also  descend  the  lumbar  chain. 

By  reason  of  this  anatomic  distribution,  the  thoracico-lumbar  pre- 
ganglionic fibers  become  related  to  all  the  vertebral  and  the  pre-vertebral 
gangha.  From  these  various  ganglia,  as  stated  on  page  645  post-gan- 
glionic  fibers  arise  which  either  as  gray  rami  communicantes  enter  the 
spinal  nerves  or  as  rami  viscerales  pass  direct  to  their  termination.  In 
either  case  the  nerve-fibers  ultimately  become  histologically  and  physio- 
logically related  to  non-striated  vascular  and  visceral  muscle-fibers  and 
to  epithelium  of  glands.  The  specific  distributions  of  the  post-gan- 
glionic  fibers  are  stated  in  the  paragraphs  relating  to  the  ganglia  them- 
selves. A  connection  is  thus  established  between  the  cells  of  the  spinal 
cord  and  the  motor  tissues  by  means  of  the  pre-  and  post-ganglionic 
nerve-fibers.  These  nerves  include,  therefore,  all  the  vaso-motor  (con- 
strictor) nerves,  the  secreto-motor  nerves  for  the  sweat  glands,  the  thy- 
roid and  the  adrenal  glands,  and  a  large  part  of  the  \iscero-motor  (in- 
hibitor) nerves. 

The  thoracico-lumbar  pre-ganglionic  nerves  together  with  their  post- 
ganglionic continuations  together  constitute  the  thoracico-lumbar  auto- 
nomic nerve  system. 
4.  The  Sacral  Spinal-cord  Region. — The  pre-ganglionic  nerve-fibers  that 
leave  the  spinal  cord  in  this  region  arise  from  groups  of  nerve-cells  situa- 
ted in  the  gray  matter  between  and  including  the  levels  of  origin  of  the 
second,  the  third  and  the  fourth  (?)  sacral  nerves.  From  this  origin  the 
pre-ganglionic  fibers  emerge  from  the  cord  in  association  with  the  large 
motor  fibers  composing  the  ventral  roots  of  these  sacral  nerves  and  pass 
with  them  to  the  interior  of  the  pelvis.  Here  they  leave  the  sacral 
nerves  and  enter  the  pudendal  or  pehic  nerve  (the  nervus  erigens)  and 
finally  terminate  around  the  cells  of  the  pehdc  ganglia.  From  these 
ganglia  post-ganglionic  fibers  arise  which  pass  onward  to  be  distributed 
to  the  non-striated  muscle-fibers  of  pelvic  \'iscera  and  the  blood-vessels 
of  the  external  generative  organs.  These  fibers  contained  in  the  sacral 
nerves  together  with  their  post-ganglionic  continuation  have  collectively 
been  termed  the  sacral  autonomic  system.     It  may  be  regarded  as  a 


650  TEXT-BOOK  OF  PHYSIOLOGY 

special  nerve  system  for  the  anal  end  of  the  gut  and  structures  develop- 
mentally  connected  with  it. 

From  the  distribution  of  the  pre-ganglionic  fibers  and  their  histologic 
connection  with  the  sympathetic  ganglia  and  from  the  distribution  and 
histologic  connection  of  the  branching  fibers  from  the  ganglia  there  is 
every  reason  to  believe  that  the  autonomic  nerve  system  includes  all  the 
vaso-motor,  some  of  the  vaso-inhibitor,  the  viscero-motor  and  the  vis- 
cero-inhibitor,  the  secreto-motor  and  the  secreto-inhibitor  nerves. 
Structure  of  the  Interconnecting  Cords. — The  interconnecting  cords  are 
composed  of  non-medullated  and  medullated  nerve-fibers.     The  former  are 
the  axons  of  cells  found  in  the  ganglia  more  centrally  located;  the  latter  are 
derived  from  the  spinal  nerves,  from  the  fibers  of  which,  however,  they  differ 
in  character,  being  much  smaller  and  finer.     The  fibers  of  the  interconnect- 
ing cords,  as  a  rule,  transmit  nerve  impulses  from  the  more  centrally  to  the 
more  peripherally  located  ganglia,  and  are,  therefore,  termed  rami  efferentes. 
In  the  vertebral  chain  some  of  the  cords  transmit  nerve  impulses  upward, 
others  downward,  others  again  forward,  to  the  pre-vertebral  and  peripheral 
ganglia. 

Among  the  rami  efferentes  or  interconnecting  cords,  there  are  some  which 
possess  special  interest  for  the  physiologist,  viz. : 

1.  The  cervical  which  connects  the  thoracic  ganglia  with  the  superior  cervical 

ganglion.  It  is  composed  mainly  of  pre-ganglionic  medullated  fibers 
derived  originally  from  the  thoracic  spinal  cord. 

2.  The  great  splanchnic  nerve  formed  mainly  by  the  union  of  pre-ganglionic 

fibers  which  cross  the  vertebral  chain  in  the  region  of  the  fifth  to  the 
tenth  thoracic  gangha.  It  connects  the  cells  of  the  spinal  cord  with  the 
cells  of  the  semilunar  ganglion. 

3.  The  stnall  splanchnic  nerve  formed  by  the  union  of  pre-ganglionic  medul- 

lated fibers  that  cross  the  vertebral  chain  in  the  region  of  the  ninth  and 
tenth  thoracic  ganglia.  It  connects  the  cells  in  the  spinal  cord  with 
the  renal  and  perhaps  the  superior  mesenteric  ganglion. 

4.  The  inferior  splanchnic  nerves  formed  by  the  union  of  pre-ganglionic 

medullated  fibers  that  cross  the  upper  lumbar  chain  in  the  region  of  the 
second  and  third  lumbar  ganglia.  They  connect  the  cells  in  the  spinal 
cord  wdth  the  inferior  mesenteric  ganglion. 
Afferent  Sympathetic  Fibers. — With  the  foregoing  groups  of  pre-gan- 
glionic autonomic  fibers  in  the  thoracic  and  lumbar  regions  there  is  associated 
a  certain  number  of  afferent  fibers  which  impart  a  certain  degree  of  sensibiHty 
to  the  organs  of  the  thoracic  and  abdominal  regions.  Though  these  organs  in 
the  physiologic  condition  do  not  appear  to  be  endowed  with  much  sensibihty, 
nevertheless,  in  pathologic  conditions  they  become  exceedingly  sensitive  and 
give  rise  to  extremely  painful  sensations.  The  afferent  nerves  which  mediate 
these  sensations  have  their  origin  in  nerve-cells  located  in  the  ganglia  of  the 
dorsal  roots  of  the  spinal  nerves;  from  this  origin  they  pass  outward  by  way 
of  the  white  rami  into  nerve  trunks  which  pass  to  the  organs  in  the  thoracic 
and  abdominal  cavities,  e.g.,  the  cardiac  plexus  and  the  splanchnic  nerves 
and  the  sacral  nerves  to  the  pelvic  organs.  The  presence  of  afferent  nerves 
in  the  splanchnics  can  be  shown  by  stimulating  the  central  end  or  even  the 
white  rami  with  electric  currents,  a  procedure  that  gives  rise  to  pain  as  well 
as  a  reflex  rise  of  blood-pressure.     The  number  of  afferent  nerve-fibers  in 


THE  AUTONOMIC  NERVE  SYSTEM  651 

any  of  the  these  nerve  trunks  is  quite  small  in  comparison  with  the  efferent 
fibers.  In  pathologic  conditions  of  the  organs  the  sensations  to  which  they 
give  rise  are  frequently  referred  to  areas  of  the  cutaneous  surface  overlying 
the  lesion. 

THE    FUNCTIONS    OF    THE    AUTONOMIC   NERVE    SYSTEM 

The  view  according  to  which  the  sympathetic  ganglia  are  to  be  regarded 
as  independent  organs  endowed  with  functions  of  their  own  and  in  nowise 
directly  dependent  for  their  activities  on  the  central  nerve  system  is  at  the 
present  very  largely  discarded.  Peripheral  structures  cease  to  exhibit 
their  characteristic  functions  after  division  of  the  spinal  nerves  in 
connection  with  their  related  ganglia.  This  does  not  exclude  the  possibihty 
of  the  sympathetic  cell-body,  in  virtue  of  the  interchanges  between  it  and 
the  blood  and  lymph  by  which  it  is  surrounded,  maintaining  its  own  nutri- 
tion and  exerting  possibly  a  favorable  influence  over  the  nutrition  of  periph- 
eral tissues  to  which  its  post-ganglionic  branches  are  distributed. 

From  the  number  and  fineness  of  the  ultimate  terminations  of  the 
branches  given  off  from  the  ganghon  cells  as  well  as  their  extensive  distribu- 
tion in  all  regions  of  the  body,  it  has  been  suggested  that  they  are  distribu- 
tors of  the  nerve  impulses  discharged  in  consequence  of  the  stimulating  influ- 
ence of  nerve  impulses  coming  from  the  spinal  cord,  through  the  pre-gan- 
glionic  fibers. 

From  the  distribution  of  the  post-gangUonic  fibers,  viz.:  to  the  walls  of 
the  blood-vessels,  to  the  walls  of  the  viscera,  and  to  the  epithelium  of  glands 
and  in  some  animals  to  the  muscles  of  the  hair  folHcles,  it  may  be  inferred 
that  the  ganglia  are  associated  with  vaso-motor,  viscero-motor,  secreto-motor 
and  pilo-motor  phenomena;  and  since  the  pre-ganglionic  fibers  are  always 
associated  with  the  gangha,  anatomically  and  physiologically,  it  can  be  said 
that  they  too  are  associated  with  the  same  series  of  phenomena. 

The  functions  of  the  autonomic  nerve  system,  as  determined  from  its 
anatomic  distribution  and  the  results  of  experimental  investigations,  are  to 
augment  or  to  inhibit  the  tonus  of  the  blood-vessels  including  the  heart,  the 
tonus  of  visceral  walls  and  the  activity  of  the  epithelium  of  glands,  and  are, 
therefore,  the  sum  total  of  the  functions  of  the  vaso-motor,  viscero-motor 
and  secreto-motor  nerves,  that  is,  the  nerves  which  collectively  constitute 
this  system.  The  extent  to  which  these  different  modes  of  acti\dty  manifest 
themselves  in  one  or  more  regions  of  the  body  will  depend  to  some  extent 
on  the  strength  of  the  stimulus  and  the  portion  of  the  system  subjected  to 
experiment,  as  will  be  apparent  from  the  following  statements. 

The  Functions  of  the  Mid-brain  Autonomic  Nerves. — The  functions 
of  the  efferent  pre-ganglionic  nerve-fibers  contained  in  the  trunk  of  the  oculo- 
motor nerve,  together  with  their  post-ganglionic  continuations  (the  ciliary 
ganglion  and  its  branches),  are  (a)  To  augment  the  tonus  or  degree  of  con- 
traction of  the  sphincter  pupillce  or  the  sphincter  iridis  in  accordance  with  in- 
creasing intensities  of  light,  and  {b)  to  augment  the  tonus  or  degree  of  con- 
traction of  the  ciliary  muscle  accompanying  efforts  of  accommodation.  The 
result  of  the  contraction  of  the  sphincter  pupillse  is  a  diminution  in  the  diame- 
ter of  the  pupil  and  hence  a  diminution  in  the  amount  of  light  entering  the 
eye  so  that  the  perception  of  the  image  may  be  sharp  and  distinct.  The 
result  of  the  contraction  of  the  ciliary  muscle  is  a  relaxation  of  the  suspensory 


652  TEXT-BOOK  OF  PHYSIOLOGY 

ligament  of  the  lens  whereupon  the  latter  becomes  more  convex  on  its  ante- 
rior surface,  and  hence  adjusted  for  near  \ision.  These  and  similar  facts 
are  shown  by  stimulation  and  division  of  these  nerves  either  pre-  or  post- 
ganglionic with  induced  electric  currents. 

The  Functions  of  the  Bulbar  Autonomic  Nerves. — The  efferent  pre- 
ganglionic nerve-fibers  contained  primarily  in  the  nerve  of  Wrisberg  or  pars 
intermedia,  and  secondarily  in  the  facial  nerve  may  be  divided  into  two 
groups,  viz.:  (a)  those  passing  by  way  of  the  great  petrosal  nerve  to  the  spheno- 
palatine ganglion,  and  {b)  those  passing  by  way  of  the  chorda-tympani  nerve 
to  the  submaxillary  and  sublingual  ganglia.  The  functions  of  the  great 
petrosal  fibers  together  with  their  post-gangHonic  continuations  (the  spheno- 
palatine ganglion  and  its  branches)  are  {a)  To  inhibit  the  contraction  of  the 
blood-vessels  of  the  mucous  membrane  of  the  nose,  soft  palate,  upper  part  of 
the  pharynx,  the  roof  of  the  mouth  and  upper  lip,  and  {b)  to  augment  the 
secretion  of  the  mucous  glands  of  the  corresponding  region.  The  functions 
of  the  chorda-tympani  fibers  and  their  ganglionic  continuations  (the  sub- 
maxillary and  sublingual  ganglia  and  their  branches)  are  (a)  To  inhibit  the 
tonus  of  the  blood-vessels  of  the  submaxillary  and  sublingual  glands  and 
ih)  to  augment  the  activities  of  their  epithelium  and  thus  increase  their 
secretions. 

The  function  of  the  efferent  pre-ganglionic  fibers  contained  in  the  trunk 
of  the  glosso-pharyngeal  nerve  together  with  their  post-ganglionic  continua- 
tions (the  otic  ganglion  and  its  branches)  are  (a)  To  inhibit  the  contraction 
of  the  blood-vessels  of  the  parotid  gland  and  {b)  to  augment  the  activities  of 
its  epithelium  and  thus  increase  its  secretion. 

The  efferent  pre-ganglionic  nerve-fibers  contained  in  the  trunk  of  the 
vagus  nerve  together  with  their  post-ganglionic  continuations  are  as  follows: 
{a)  To  inhibit  the  tonus  and  contractile  power  of  the  heart-muscle;  {b)  to 
inhibit  and  sometimes  augment  the  contraction  of  the  esophagus  and  of  the 
sphincter  cardios;  (c)  to  inhibit  the  tonus  of  the  cardiac  end  of  the  stomach  for 
the  purpose  of  receiving  incoming  amounts  of  food;  (d)  to  augment  the  tonus 
and  the  contractile  power  of  the  gastric  musculature  and  the  pyloric  sphinc- 
ter; (e)  to  augment  and  sometimes  inhibit  the  tonus  of  the  bronchial  muscula- 
ture; (/)  to  inhibit  the  sphincter  muscle  of  the  common  bile  duct  and  to  aug- 
ment the  tonus  and  contractile  power  of  the  visceral  muscles  in  the  walls  of 
the  gall-bladder;  (g)  to  augment  the  tonus  and  contractile  power  of  the  vis- 
ceral muscles  of  the  walls  of  the  small  intestine,  and  (h)  to  augment  and  some- 
times to  inhibit  the  activities  of  the  epithelium  of  the  gastric  and  pancreatic 
glands  and  thus  increase  their  secretions.  These  various  phenomena  result 
when  these  various  efferent  nerves  are  divided  and  their  peripheral  extremi- 
ties are  appropriately  stimulated. 

The  Functions  of  the  Thoracic  and  Liunbar  Autonomic  Nerves. — 
The  efferent  pre-ganglionic  nerve -fibers  that  leave  the  spinal  cord  in  the 
ventral  roots  of  the  thoracic  and  upper  lumbar  nerves  together  with  their 
post-ganglionic  continuations,  have  been  shown  by  experiment  to  be  vaso- 
motor (constrictor),  secreto-motor,  viscero-motor  and  in  some  animals  pilo- 
motor in  functions.  The  origin,  course  and  distribution  of  these  nerve -fibers 
have  been  stated  on  pages  377  and  473.  The  specific  functions  of  these 
fibers  will  depend  on  the  regions  of  the  body  to  which  they  are  distributed 
(See  page  648.) 


THE  AUTONOMIC  NERVE  SYSTEM  653 

The  functions  of  the  thoracic  fibers  that  terminate  in  the  ganglia  of  the 
same  levels  are  (a)  to  augment  the  tonus  and  the  degree  of  the  contraction 
of  the  blood-vessels  of  the  skin  of  the  arm  and  trunk  of  the  body;  and  (b)  to 
augment  the  activities  of  the  epithelium  of  the  sweat-glands  in  the  correspond- 
ing regions.  Stimulation  of  either  the  pre-  or  post-ganglionic  fibers  produces 
the  foregoing  phenomena. 

The  functions  of  the  fibers  that  ascend  the  sympathetic  chain  to  terminate 
in  the  first  thoracic,  the  inferior  cervical  ganglia  are  (a)  to  accelerate  the  rate 
and  the  force  of  the  contraction  of  the  heart-muscle,  and  (b)  perhaps  augment 
the  activities  of  the  epithelium  of  the  thyroid  gland.  The  pre-ganglionic- 
fibers  having  these  important  functions  emerge  from  the  spinal  cord  for  the 
most  part  in  the  ventral  roots  of  the  second  and  third  thoracic  nerves.  After 
leaving  these  nerves  by  v/ay  of  the  rami  communicantes  they  enter  the  thoracic 
chain  and  pass  upward  to  terminate  in  and  around  the  cells  of  the  inferior 
cervical  ganglion  in  man  and  in  the  first  thoracic  ganglion  in  certain  other 
mammals.  The  post-ganglionic  visceral  fibers — the  cardiac  nerves — that 
arise  in  the  cells  of  these  ganglia  pass  downward  and  forward  and  reach  the 
heart  by  way  of  the  cardiac  plexus.  Stimulation  of  these  nerves  causes  the 
same  effects  in  the  heart  as  stimulation  of  the  pre-ganglionic  fibers  in  any 
part  of  their  course.  The  center  of  origin  of  these  accelerator  and  aug- 
mentor  cardiac  fibers  has  not  been  definitely  located  but  there  are  reasons 
for  thinking  it  is  in  the  medulla  oblongata. 

The  functions  of  the  fibers  which  in  their  upward  course  form  the  cervical 
cord  and  finally  terminate  around  the  nerve  cells  of  the  superior  cervical 
ganglion  are: 

(a)  To  augment  the  tonus  and  increase  the  contraction  of  the  blood- 
vessels of  the  skin  and  mucous  membrane  of  dift'erent  portions  of  the  head, 
face  and  neck;  {b)  to  augment  the  contraction  of  the  blood-vessels  of  the  sub- 
maxillary, the  sublingual  and  the  parotid  glands;  and  (c)  to  augment  the  activ- 
ity of  the  epithelium  of  the  sweat  glands  of  the  head,  face  and  neck,  and 
possibly  of  the  salivary  glands  as  well,  (e)  To  augment  the  tonus  and  the 
contractions  of  the  dilatator  pupillce  muscle,  thus  antagonizing  and  overcom- 
ing the  contraction  of  sphincter  pupillce  muscle  and  establishing  a  dilatation 
of  the  pupil. 

The  foregoing  effects  may  be  demonstrated  by  exposing  and  dividing 
the  cervical  cord  and  then  stim.ulating  the  peripheral  end.  The  same  phe- 
nomena follow  stimulation  of  the  post-ganglionic  branches.  That  the 
superior  cervical  ganglion,  like  ganglia  elsewhere  in  the  body,  is  the  cell 
station  between  the  spinal  cord  and  the  peripheral  organs  is  shown  by  the 
fact  discovered  and  applied  by  Langley  that  the  intra-venous  injection  of 
nicotin  or  the  local  application  of  it  to  the  ganglion  itself,  impairs  the  con- 
ductivity of  the  terminals  of  pre-ganglionic  fibers,  after  which  their  stimula- 
tion has  no  effect  on  the  ganglion  cells,  though  the  latter  retain  their  activity, 
as  shown  on  direct  stimulation. 

The  fimctions  of  the  fibers  that  cross  the  sympathetic  chain  and  unite 
to  form  the  greater  and  lesser  splanchnic  nerves  together  with  their  ganglionic 
continuations  (semilunar  ganglion  and  the  fibers  of  the  solar  plexus)  are: 
{a)  To  augment  the  tonus  and  contractile  power  of  the  walls  of  the  blood- 
vessels of  the  stomach  and  intestines  as  far  as  the  descending  colon  (the  so- 
called  splanchnic  vascular  area)  and  the  blood-vessels  of  the  kidney  and 


654  TEXT-BOOK  OF  PHYSIOLOGY 

spleen;  and  (b)  to  inhibit  the  tonus  and  diminish  the  degree  of  contraction 
of  the  muscle  walls  of  the  stomach  and  intestine;  (c)  to  inhibit  the  tonus, 
during  digestion,  of  the  sphincter  muscle  of  the  common  bile  duct;  and  (d) 
to  augment  the  tonus  and  degree  of  contraction  of  the  muscle-walls  of  the 
gall-bladder;  (e)  to  augment  the  tonus  of  the  ileo-colic  sphincter  muscle;  (/) 
to  augment  the  secretion  of  the  adrenal  bodies. 

These  effects  are  readily  observed  if  the  great  splanchnic  nerve  is  first 
divided  and  then  its  peripheral  end  stimulated  with  induced  electric  currents. 
With  the  division  of  the  nerve  the  blood-vessels  of  the  splanchnic  area,  as 
well  as  of  neighboring  organs,  dilate,  attended  by  a  sudden  fall  of  the  general 
blood-pressure;  with  the  beginning  of  the  stimulation  the  blood-vessels  of 
the  corresponding  areas  at  once  contract,  coincidently  with  which  there  is  a 
rapid  rise  in  the  general  blood-pressure.^  The  division  of  the  splanchnics 
is  followed  by  a  rise  in  the  tonus  of  the  muscle-walls  of  the  intestine  (due  to 
the  augmentor  activity  of  the  now  unopposed  vagi  nerves);  stimulation  of 
the  peripheral  end  of  the  splanchnics  is  followed  by  a  marked  inhibition  of 
the  tonus. 

The  functions  of  the  lumbar  fibers  that  terminate  in  the  ganglia  of  the 
same  levels  are:  (a)  To  augment  the  tonus  and  the  degree  of  contraction  of 
the  blood-vessels  of  the  skin  of  the  trunk  to  which  they  are  distributed;  and 
(b)  to  augment  the  activities  of  the  sweat-glands  of  the  corresponding  regions. 

The  functions  of  the  nerve-fibers  that  cross  the  sympathetic  chain  and 
unite  to  form  the  inferior  splanchnic  nerves,  together  with  their  ganglionic 
continuations  (the  inferior  mesenteric  ganglion  and  the  hypogastric  plexus) , 
are :  (a)  To  augment  the  tonus  and  the  contractile  power  of  the  blood-vessels 
of  the  pelvic  viscera;  (b)  to  inhibit  the  tonus  and  diminish  the  contractile 
power  of  the  muscle-walls  of  the  large  intestine;  (c)  to  augment  the  tonus  of 
the  sphincter  muscle  of  the  urinary  bladder,  and  to  inhibit  the  tonus  of  the 
muscle-walls  of  the  bladder  during  the  intervals  of  urination;  {d)  to  augment 
the  tonus  and  contractile  power  of  the  uterus. 

These  various  phenomena  arise  when  either  the  pre-ganglionic  or  the 
post-ganglionic  fibers  in  the  hypogastric  plexus  are  stimulated  with  induced 
electric  currents. 

The  functions  of  the  fibers  which  descend  the  sympathetic  chain  to  ter- 
minate around  the  cells  of  the  lumbar  and  sacral  ganglia  are  (a)  to  augment 
the  tonus  and  the  degree  of  contraction  of  the  blood-vessels  of  the  skin  of  the 
hip  and  leg;  and  (6)  to  augment  the  activities  of  the  epithelium  of  the  sweat- 
glands  of  the  corresponding  parts.  By  reason  of  the  distribution  of  the 
visceral  branches  of  the  sacral  ganglia  a  similar  contraction  of  the  blood- 
vessels is  observed  in  portions  of  the  pelvic  viscera  and  the  external  genitalia. 

The  Functions  of  the  Sacral  Autonomic  Nerves. — The  functions  of  the 
pre-ganglionic  nerve-fibers  that  leave  the  spinal  cord  by  way  of  the  ventral 
roots  of  the  second,  third  and  perhaps  the  fourth  sacral  nerves,  together  with 
their  post-ganglionic  continuations  (the  pelvic  ganglia  and  their  branches) 
are:  {a)  To  inhibit  the  tonus  and  cause  a  dilatation  of  the  blood-vessels  of 
the  generative  organs;  {b)  to  inhibit  the  tonus  of  the  sphincter  muscle  of  the 

1  Occasionally  stimulation  of  the  splanchnic  is  followed  by  a  slight  primary  inhibition  and  dila- 
tation of  the  blood-vessels  which  would  lead  to  the  inference  that  vaso-dilatator  fibers  are  also 
present;  these  fibers,  however,  do  not  belong  properly  to  the  autonomic  system  as  their  center  of 
origin  is  in  the  ganglia  of  the  dorsal  roots  of  the  spinal  nerves.     (See  pages  380.) 


THE  AUTONOMIC  NERVE  SYSTEM  655 

bladder;  (c)  to  augment  the  contraction  of  the  detrusor  muscle  during  urina- 
tion; (d)  to  augment  the  tonus  and  contractile  power  of  the  muscle-walls  of 
the  colon  and  rectum,  and  the  rectal  sphincters. 
Summary. — The  autonomic  system  consists  of: 

1.  The  autonomic  tissues,  viz.:  the  non-striated,  \isceral  and  vascular  muscle 

tissue,  cardiac  striated  muscle  tissue  and  epithelial  tissue. 

2.  The  autonomic  nerves. 

The  autonomic  nerves  consist  of  two  successively  arranged  neurons. 
The  first  has  its  origin  in  cells  of  the  central  nerve  system;  the  second  has 
its  origin  in  cells  of  the  sympathetic  ganglia.  The  former  is  termed  pre- 
ganglionic, the  latter  post-ganglionic.  The  pre-ganglionic  neuron  arborizes 
peripherally  around  the  cells  of  the  sympathetic  ganglia;  the  post-ganglionic 
terminates  by  fine  branchings  around  the  cells  of  the  tissues. 

The  pre-ganglionic  neurons  have  their  origin  in  four  separate  regions  of 
the  central  nerve  system,  viz.:  (i)  The  mid-brain  region;  (2)  the  medulla 
oblongata;  (3)  the  mid-spinal  cord  region  and  (4)  the  sacral  region. 

The  autonomic  system  of  nerves  as  evident  from  their  distribution  com- 
prises : 

1.  The  viscero-augmentor  and  viscero-inhibitor  nerves  for  the  non-striated 

muscles  of  the  eyeball  (iris  and  ciliary  muscle)  of  the  lower  portion  of 
the  esophagus,  of  the  trachea  and  bronchial  tubes,  of  the  stomach,  of 
the  small  and  large  intestine,  the  gall-bladder  and  the  pelvic  organs. 

2.  The  vaso-dilatator  nerves  for  the  non-striated  muscles  of  the  blood-vessels 

of  the  saUvary  glands,  the  mucous  and  serous  glands  of  the  nose,  mouth 
and  pharynx,  and  of  the  genital  organs. 

3.  The  vaso-constrictor  nerves  for  the  non-striated  muscles  of  the  blood-ves- 

sels of  the  skin  of  the  head,  face,  neck,  trunk,  arms,  and  legs,  alimentary 
canal,  liver,  spleen,  kidney,  adrenals  and  genital  organs. 

4.  The  cardio-augmentor  and  car dio -inhibitor  nerves  for  the  heart. 

5.  The  secreto-augmentor  and  secreto-inhibitor  nerves  for  the  epithelial  tissue 

of  the  salivary  glands,  of  the  mucous  and  serous  glands  of  the  mouth, 
nose,  and  pharynx,  of  the  trachea  and  bronchial  tubes,  of  the  gastric 
glands,  of  the  thyroid,  of  the  pancreas,  of  the  adrenals,  of  the  liver,  and 
the  sweat-glands  of  the  skin  of  the  head,  face,  neck,  trunk,  arms,  legs 
and  of  the  glands  in  connection  with  the  genital  organs. 
The  centers  of  origin  of  the  autonomic  nerves  possess  a  certain  degree  of 
tonus,  maintained  in  all  probability  by  changes  of  a  chemic  character,  be- 
tween the  constituents  of  the  nerve  cells  and  the  constituents  of  the  blood  by 
which  they  are  surrounded.     In  consequence  of  this  tonus,  they  maintain  a 
certain  degree  of  activity  in  the  tissues  to  which  the  autonomic  fibers  are  dis- 
tributed.    Given  this  tonus  the  autonomic  centers  may  be  excited  tc  in- 
creased activity:  (i)  By  nerve  impulses  transmitted  by  afferent  nerves  from 
the  surfaces  of  the  body;  and  (2)  by  nerve  impulses  descending  from  the 
cerebrum  in  consequence  of  psychic  states  of  an  affective  or  emotional 
character. 

The  nature  and  strength  of  the  stimulus  applied  to  the  surface  of  the 
body,  and  the  nature  and  intensity  of  the  affective  or  emotional  state  will 
naturally  determine  the  degree  and  extent  of  the  resulting  activity  of  the 
autonomic  tissues. 


656  TEXT-BOOK  OF  PHYSIOLOGY 

Note. — Langley  to  whom  we  are  so  largely  indebted  for  the  present  state  of  our 
knowledge  of  the  anatomy  and  physiologic  relations,  not  only  of  different  portions 
of  this  system,  but  of  the  system  in  its  entirety,  introduced  the  term  "  autonomic," 
on  account  of  the  insufficiency  of  the  older  nomenclature,  e.g.,  "sympathetic," 
"ganglionic,"  "vegetative,"  "organic,"  "visceral,"  etc.,  to  fully  express  its  ana- 
tomic relations  and  physiologic  action.  By  this  term  it  is  implied  that  this  system  is 
independent  in  action;  that  is,  independent  of  volitional  control,  that  its  activity 
is  determined  by  nerve  impulses  coming  from  the  periphery,  though  subject  to 
modifications  by  cerebral  states  of  an  affective  or  emotional  character.  Thus  he 
states  (Journal  of  Physiology,  vol.  23)  "I  propose  to  substitute  the  word  auto- 
nomic "  *  *  *;  "the  autonomic  nervous  system  means  the  nervous  system  of  the 
glands  and  of  the  involuntary  muscle."  "I  propose  the  term  autonomic  nervous 
system  for  the  sympathetic  system  and  the  allied  nerv'ous  systems  of  the  cranial 
and  sarcal  nerves  and  for  the  local  nervous  system  of  the  gut  "  *  *  *.  "I  conclude 
that  there  is  no  fundamental  difference  between  the  pre-ganglionic  fibers  of  the 
body,  whether  they  belong  to  the  cranial,  the  sympathetic  or  the  sacral  autonomic 
systems."  "The  word  autonomic  nervous  system  consists  of  the  sympathetic  sys- 
tem, of  the  cranial  autonomic  system,  and  the  enteric  system,  the  plexuses  of 
Auerbach  and  Meissner." 

Gottlieb  and  Meyer,  partly  on  the  basis  of  the  difference  in  the  physiologic 
action  of  certain  drugs  on  different  portions  of  this  system,  and  partly  on  the  ap- 
parent antagonism  in  the  action  of  the  nerves  which  arise  from  the  bulbar  and 
sacral  region  of  the  spinal  cord,  to  those  which  arise  from  the  thoracic  and  upper 
lumbar  regions,  proposed  a  modification  of  the  terminology.  Thus  they  applied 
to  the  entire  system  the  term  "vegetative,"  but  at  the  same  time  retained  the  term 
autonomic  for  the  fine  meduUated  pre-ganglionic  fibers  in  the  cranio-bulbar  and  in 
the  sacral  nerves,  i.e.,  those  nerv^es  not  in  physiologic  relation  with  the  chain  of 
sympathetic  ganglia,  though  related  to  peripheral  ganglia.  The  pre-gangKonic 
cranio-bulbar  and  sacral  nerve-fibers  together  with  their  post-ganglionic  continua- 
tions innervate  the  anterior  and  posterior  ends  of  the  digestive  tract  and  associated 
structures  respectively,  while  the  pre-ganglionic  thoracico-lumbar  nerve-fibers  and 
their  ganglionic  continuations  innervate  the  sweat-glands  and  blood-vessels  of  the 
head,  face,  extremities  and  the  body-walls  and  the  blood-vessels  and  muscle-walls 
of  the  abdominal  viscera.  Associated  with  these  latter  nerves  are  the  efferent  fibers 
of  the  vagus  which  innervate  the  heart,  the  blood-vessels  and  glands  of  the  viscera 
of  the  thorax  and  part  of  the  abdomen  as  well. 

By  reason  of  the  double  innervation  of  various  organs  of  the  body  and  the  op- 
posite effects  produced  by  stimulation  on  the  one  hand  of  the  autonomic  cranio- 
bulbar  and  sacral  nerve-fibers  and  on  the  other  hand  of  the  pre-  and  post-ganglionic 
fibers  of  the  thoracico-lumbar  nerves,  Gottlieb  and  Meyer  concluded,  that  there  is 
a  fundamental  difference  in  the  functions  of  the  cranio-bulbar  and  sacral  regions 
and  the  thoracico-lumbar  regions  of  the  spinal  cord.  This  antagonism  is  shown 
by  the  effects  observed  in  the  iris  and  in  the  action  of  the  heart  by  alternate  stimula- 
tion of  the  cranio-bulbar  and  thoracic  fibers  which  are  distributed  to  these  struc- 
tures. It  should  be  borne  in  mind,  however,  that  even  though  pre-ganglionic 
fibers  leave  the  spinal  cord  by  way  of  the  thoracic  nerves,  the  nerve-cells  from  which 
they  arise  may  lie  in  other  and  even  distant  regions.  Thus  it  is  well  known  that 
the  vaso-motor  center  which  through  thoracic  nerves  constricts  the  blood-vessels 
of  the  salivary  glands,  the  mucous  glands  of  the  mouth  and  nose  lies  high  up  the 
bulbar  region,  not  far  from  the  vaso-dilatator  center,  which  through  bulbar  fibers 
(in  the  chorda  tympani  and  glosso-pharyngeal)  supplies  and  dilates  the  same  blood- 
vessels of  the  corresponding  structures.  It  is  also  believed  that  the  dilatator  pupillae 
or  iridis  center  which,  through  nerve-fibers  that  leave  the  spinal  cord  through  the 
second  thoracic  nerve  and  then  pass  upward  in  the  sympathetic  chain  to  the  supe- 
rior cervical  ganglion  and  thence  to  the  dilatator  fibers  of  the  iris,  is  also  located 


THE  AUTONOMIC  NERVE  SYSTEM  657 

near  the  group  of  nerve-cells  which  gives  origin  to  the  nerve-fibers  for  the  sphincter 
muscle  of  the  iris.  It  is  also  believed  that  the  accelerator  nerves  for  the  heart 
which  emerge  from  the  spinal  cord  in  the  second  and  third  thoracic  nerves  come 
from  a  center  lying  in  the  bulbar  region  not  far  from  the  center  from  which  the 
inhibitor  fibers  in  the  vagus  come. 

Nevertheless  there  are  other  instances  in  which  this  antagonism  does  exist,  e.g., 
in  the  effects  of  the  efferent  fibers  of  the  vagus  and  the  splanchnic  nerves  on  the 
muscle-wall  of  the  intestine,  the  former  augmenting,  the  latter  inhibiting  the  con- 
traction and  so  in  other  regions.  But  this  does  not  necessitate  the  employment  of 
new  terms.  It  suffices  to  say  that  there  is  a  difference  in  the  action  of  different 
parts  of  the  general  autonomic  system.  It  is  just  as  probable,  however,  that  the 
difference  in  the  effects  observed  following  stimulation  of  the  two  classes  of  nerves 
depends  on  the  character  of  their  peripheral  terminations  rather  than  on  a  differ- 
ence in  the  character  of  the  central  cells. 

Huber,  in  a  recent  review  of  the  morphology  of  the  sympathetic  system,  says 
in  regard  to  the  foregoing  division,  "  such  a  division  of  the  autonomic  nerve  system 
does  not  seem  justifiable  when  viewed  in  the  light  of  a  morphologic  study  of  the 
ganglia  with  their  constituent  neurons,  the  terminations  of  the  neuraxis  of  the 
neurones  in  the  various  tissues,  and  the  connections  of  these  neurones  with  the 
cerebrospinal  axis  by  means  of  the  pre-ganglionic  fibers;  morphologically  con- 
sidered, the  entire  autonomic  system  is  a  unit  and  will  be  treated  as  such.  The 
minor  structural  differences,  more  apparent  than  real,  observed  in  the  neurons  of 
certain  of  the  cranial  autonomic  ganglia  and  in  the  entire  system,  do  not  warrant, 
it  would  seem  to  me,  a  regional  subdivision  of  the  autonomic  system,  when  con- 
sidered from  the  ^•iewpoint  of  structure." 


42 


CHAPTER  XXVII 
PHONATION;  ARTICULATE   SPEECH 

Phonation,  the  emission  of  vocal  sounds,  is  accomplished  by  the  vibration 
of  two  elastic  membranes  which  cross  the  lumen  of  the  larynx  antero- 
posteriorly  and  which  are  thrown  into  vibration  by  a  blast  of  air  from  the 
lungs. 

Articulate  speech  is  a  modification  of  the  vocal  sounds  or  the  voice 
produced  by  the  teeth  and  the  muscles  of  the  lips  and  tongue  and  is  employed 
for  the  expression  of  ideas. 

The  larynx,  the  organ  of  the  voice,  is  situated  in  the  fore  part  of  the  neck, 
occupying  the  space  between  the  hyoid  bone  and  the  upper  extremity  of  the 
trachea.  In  this  situation  it  communicates  with  the  cavity  of  the  pharynx 
above  and  the  cavity  of  the  trachea  below.  From  its  anatomic  relations  and 
its  internal  structure — the  interpolation  of  the  elastic  membranes — the 
larynx  subserves  the  two  widely  different  yet  related  functions,  respiration 
and  phonation. 

THE  ANATOMY  OF  THE  LARYNX 

The  larynx  consists  primarily  of  a  series  of  cartilages  united  one  with 
another  in  such  a  manner  as  to  form  a  more  or  less  rigid  framework,  yet 
possessing  at  its  different  joints,  a  certain  amount  of  motion;  and  secondarily, 
of  muscles  and  nerves  which  conjointly  impart  to  the  cartilages  the  degree 
of  movement  necessary  to  the  performance  of  the  laryngeal  functions.  It 
is  covered  externally  by  fibrous  tissue  and  lined  throughout  by  mucous 
membrane  continuous  with  that  lining  the  pharynx  and  trachea. 

The  larynx  presents  a  superior  or  pharyngeal  and  an  inferior  or  tracheal 
opening.  The  pharyngeal  opening  is  triangular  in  shape,  the  base  being 
directed  forward,  the  apex  backward.  The  plane  of  this  opening  in  the 
living  subject  is  almost  vertical.  The  tracheal  opening  is  circular  in  shape 
and  corresponds  in  size  with  the  upper  ring  of  the  trachea.  Viewed  from 
above,  the  general  cavity  of  the  larynx  is  seen  to  be  partially  subdivided  by 
two  membranous  bands — the  vocal  hands  or  cords — which  run  from  before 
backward  in  a  horizontal  plane.  The  space  between  the  bands,  the  glottis, 
varies  in  size  and  shape  from  moment  to  moment  in  accordance  with  respira- 
tory and  phonatory  necessities.  The  average  width  of  the  glottis,  at  its 
widest  part,  during  quiet  respiration  is  about  13.5  mm.  in  men  and  11.5  mm. 
in  women.  With  the  advent  of  phonation  the  vocal  membranes  are  at  once 
approximated,  and  to  such  an  extent  that  the  glottic  opening  is  reduced  to  a 
mere  slit.     It  is  then  spoken  of  as  the  rima  glottidis,  or  chink  of  the  glottis. 

The  space  above  the  vocal  bands,  the  supra-glottic  or  supra-rimal  space, 
is  triangular  in  shape  and  extends  from  the  pharyngeal  opening  to  the  plane 
of  the  vocal  bands.  The  mucous  membrane  lining  the  walls  of  this  space, 
presents  on  either  side,  just  above  the  vocal  bands,  a  crescentic  fold  which 

658 


PHONATION;  ARTICULATE  SPEECH 


659 


runs  from  before  backward,  and  is  known  as  the  false  vocal  band  or  cord. 
Between  the  true  and  false  bands  there  is  a  cavity  or  space  prolonged. up- 
ward and  outward  for  some  distance,  forming  what  is  known  as  the  ventricle 
of  the  larynx.  The  space  below  the  vocal 
bands,  the  infra-glottic  or  infra-rimal  space, 
is  narrow  above  and  elongated  from  before 
backward,  but  wide  and  circular  below,  cor- 
responding to  the  lumen  of  the  trachea. 
(Fig.  273.) 

The  Laryngeal  Cartilages,  Articula- 
tions, and  Ligaments. — The  cartilages  which 
compose  the  framework  of  the  larynx  are  nine 
in  number,  three  of  which  are  single:  viz.,  the 
cricoid,  the  thyroid,  and  the  epiglottis,  while 
six  occur  in  pairs:  viz.,  the  arytenoids,  the 
cornicula  laryngis,  and  the  cuneiform.  (Figs. 
274  and  275.) 

The  cricoid  cartilage  is  the  foundation 
cartilage,  and  affords  support  to  the  remain- 
ing cartilages  and  the  structures  attached  to 
them.  In  shape  it  resembles  a  signet-ring, 
the  broad  quadrate  portion  of  which  is  directed 
backward,  while  the  narrow  circular  portion 
is  directed  forward.  It  rests  upon  the  upper 
ring  of  the  trachea,  to  which  it  is  firmly  at- 
tached by  fibrous  tissue.  -  The  posterior  upper 
border  of  the  quadrate  portion  presents  on 
either  side  an  oval  convex  facet  for  articula- 
tion with  the  arytenoid  cartilage.  The  long 
axis  of  this  facet  is  directed  downward,  out- 
ward, and  forward. 

The  thyroid,  the  largest  of  the  laryngeal 
cartilages,  is  composed  of  two  flat  quadrilat- 
eral plates,  united  anteriorly  at  an  angle  of 
about  90  degrees.  Each  plate  is  directed 
backward  and  outward  and  terminates  in  a 
free  border,  which  is  prolonged  upward  and 
downward  for  some  distance,  terminating  in 
two  processes,  the  superior  and  inferior  cornua. 
The  upper  border  to  the  thyroid  is  deeply 
notched  in  front.  The  inferior  border  over- 
laps laterally  the  cricoid. 

The  epiglottis  is  a  leaf-shaped  piece  of  cartilage  attached  to  the  thyroid 
at  the  median  notch.  It  is  firmly  united  by  membranes  and  ligaments  to 
the  thyroid  and  arytenoid  cartilages  and  to  the  base  of  the  tongue. 

The  arytenoid  cartilages  are  two  in  number  and  symmetric  in  shape. 
Each  cartilage  is  a  triangular  pyramid,  the  apex  of  which  is  recurved,  and 
directed  backward  and  inward.  The  base  presents  three  angles — an  ante- 
rior, an  external,  and  an  internal.  The  anterior  angle  is  long  and  pointed 
and  projects   forward    in  a  horizontal  plane.     It  serves  for  the  attach- 


FiG.  273. — Longitudinal  Sec- 
tion OF  THE  Human  Larynx, 
Showing  the  Vocal  Bands,  i. 
Ventricle  of  the  larynx.  2.  Supe- 
rior vocal  cord.  3.  Inferior  vocal 
cord.  4.  Arytenoid  cartilage.  5. 
Section  of  the  arytenoid  muscle.  6, 
6.  Inferior  portion  of  the  cavity  of 
the  larynx.  7.  Section  of  the  pos- 
terior portion  of  the  cricoid  carti- 
lage. 8.  Section  of  the  anterior 
portion  of  the  cricoid  cartilage. 
9.  Superior  border  of  the  cricoid 
cartilage.  10.  Section  of  the  thy- 
roid cartilage.  11,  11.  Superior 
portion  of  the  cavity  of  the  larynx. 
12,  13.  Arytenoid  gland.  14,  16. 
Epiglottis.     15,  17.  Adipose  tissue. 

18.  Section  of  the  hyoid  bone.     19, 

19,  20.  Trachea. — (Sappey.) 


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TEXT  BOOK  OF  PHYSIOLOGY 


ment  of  the  vocal  membranes  and  is,  therefore,  termed  the  vocal  process. 
The  external  angle  is  short,  rounded,  and  prominent,  and  serves  for  the 
attachment  of  muscles.  The  internal  angle  affords  a  point  of  insertion  for  a 
ligament.  The  inferior  surface  of  the  arytenoid  is  concave  for  articulation 
with  the  convex  surface  of  the  cricoid  facet.  Its  long  axis,  however,  is 
directed  from  before  backward  and  almost  at  right  angles  to  the  long  axis  of 
the  cricoid  facet. 

The  cornicula  laryngis  and  the  cuneijorm  cartilages  are  small  nodules  of 
yellow  elastic  cartilage  embedded  in  a  fold  of  membrane  which  unites  the 
arytenoid  and  the  epiglottis.  They  are  fragments  of  a  ring  of  cartilage  which 
in  some  animals — e.g.,  ant-eater — extends  between  these  two  cartilages. 


Fig.  274.  Fig.  275. 

Fig.  274. — Laryngeal  Cartilages  and  Ligaments,  Anterior  Surpace.  i.  Hyoid  bone. 
2,  2,  3,  3.  Greater  and  lesser  cornua.  4.  Thyroid  cartilage.  5.  Thyro-hyoid  membrane.  6. 
Thyro-hyoid  ligaments.  7.  Cartilaginous  nodule.  8.  Cricoid  cartilage.  9.  The  crico-thyroid 
membrane.     10.  The  crico-thyroid  Hgaments.     11.  Trachea. — (Sappey.) 

Fig.  275. — Laryngeal  Cartilages  and  Ligaments,  Posterior  Surface,  i,  i.  Thyroid 
cartilage.  2.  Cricoid  cartilage.  3,  3.  Arytenoid  cartilages.  4,  4.  Crico-arytenoid  articulations. 
5,5.  Crico-thyroid  articulations.  6.  Union  of  the  cricoid  cartilage  and  of  the  trachea.  7.  Epiglot- 
tis.    8.  Ligament  uniting  it  to  the  reentering  angle  of  the  thyroid  cartilage. — {Sappey.) 


The  crico-thyroid  articulation  is  formed  by  the  opposition  of  the  tip  of  the 
inferior  cornu  of  the  thyroid  cartilage  and  an  articular  facet  on  the  side  of  the 
cricoid.  The  joint  is  provided  with  a  synovial  membrane  and  enclosed  by  a 
capsular  ligament.  The  movements  permitted  at  this  joint  take  place 
around  a  horizontal  axis  and  consist  of  an  upward  and  downw^ard  movement 
of  both  the  thyroid  and  cricoid,  combined  with  a  sliding  movement  of  the 
latter  upward  and  backward. 

The  crico-arytenoid  articulation  is  formed  by  the  apposition  of  the  articu- 
lating sufaces  of  the  cricoid  and  arytenoid  cartilages.     This  joint  is  provided 


PRONATION;  ARTICULATE  SPEECH  66 1 

with  a  synovial  membrane  and  enclosed  by  a  loose  capsular  ligament  which 
would  permit  of  an  extensive  sliding  of  the  arytenoid  cartilage  downward  and 
outward  were  it  not  prevented  by  the  posterior  crico-arytenoid  ligament, 
which  is  attached,  on  the  one  hand,  to  the  cricoid,  and,  on  the  other,  to  the 
inner  angle  of  the  arytenoid.  The  movements  permitted  at  this  joint  are: 
(i)  Rotation  of  the  arytenoid  around  a  vertical  axis  which  lies  close  to  its 
inner  surface.  (2)  A  sliding  motion  inward  and  forward  with  inward 
rotation  of  the  vocal  process,  or  a  sliding  motion  outward  and  backward 
with  outward  rotation  of  the  vocal  process.  In  either  case  the  process 
describes  an  arc  of  a  circle.  (3)  A  sliding  movement  toward  the  median 
line  in  consequence  of  which  the  inner  surfaces  of  the  arytenoids  are 
brought  almost  in  contact. 

The  crico-thyroid  membrane  is  composed  mainly  of  elastic  tissue.  It 
may  be  divided  into  a  mesial  and  two  lateral  portions.  The  mesial  portion 
is  well  developed,  triangular  in  shape,  and  unites  the  contiguous  borders  of 
the  cricoid  and  thyroid  cartilages.  The  lateral  portion  is  attached  below  to 
the  superior  border  of  the  cricoid.  From  this  attachment  it  passes  upward 
and  inward  under  cover  of  the  thyroid.  As  it  ascends  it  elongates  and  be- 
comes thinner,  and  is  finally  attached  anteriorly  to  the  thyroid  near  the 
median  line,  and  posteriorly  to  the  vocal  process  of  the  arytenoid,  thus  con- 
stituting the  injerior  thyro-arytenoid  ligament.  It  is  covered  internally  by 
mucous  membrane  and  externally  by  the  internal  thyro-arytenoid  muscle. 
The  free  edge  of  this  ligament  forms  the  basis  of  the  true  vocal  band.  A 
superior  thyro-arytenoid  ligament  form-s  the  basis  of  the  false  vocal  band. 

The  thyro-hyoid  membrane,  composed  of  elastic  tissue,  unites  the 
superior  border  of  the  thyroid  to  the  hyoid  bone. 

The  mucous  membrane  lining  the  larynx  is  thin  and  pale.  As  it  passes 
downward  it  is  reflected  over  the  superior  thyro-arytenoid  ligament,  and 
assists  in  the  formation  of  the  false  vocal  band;  it  then  passes  into  and  lines 
the  ventricle,  after  which  it  is  reflected  inward  over  the  superior  border  of  the 
thyro-arytenoid  muscle  and  ligament,  and  assists  in  the  formation  of  the 
true  vocal  band;  it  then  returns  upon  itself  and  passes  downward  over  the 
lateral  portion  of  the  crico  thyroid  membrane  into  the  trachea. 

The  Vocal  Bands. — The  thin,  free,  reduplicated  edge  of  the  mucous 
membrane  constitutes  the  true  vocal  band.  The  surface  of  the  mucous  mem- 
brane is  covered  by  ciliated  epithelium  except  in  the  immediate  neighborhood 
of  the  vocal  bands. 

The  vocal  bands  are  attached  anteriorly  to  the  thyroid  cartilage  near  the 
receding  angle  and  posteriorly  to  the  vocal  processes  of  the  arytenoid  cartil- 
ages. They  vary  in  length  in  the  male  from  20  to  25  mm.  and  in  the  female 
from  15  to  20  mm. 

The  Muscles  of  the  Larynx. — The  muscles  which  have  a  direct  action 
on  the  cartilages  of  the  larynx  and  determine  the  position  of  the  vocal  bands 
both  for  respiratory  and  phonatory  purposes,  and  which  regulate  their 
tension  as  well,  are  nine  in  number  and  take  their  names  from  their  points 
of  origin  and  insertion:  viz.,  two  posterior  crico-arytenoids,  two  lateral  crico- 
arytenoids, two  thyro-arytenoids,  one  arytenoid,  and  two  crico-thyroids 
(Figs.  276  and  277). 

The  posterior  crico-arytenoid  muscle  lies  on  the  posterior  surface  of  the 
quadrate  plate  of  the  cricoid  cartilage,  on  either  side  of  the  median  line, 


662 


TEXT-BOOK  OF  PHYSIOLOGY 


from  which  it  takes  its  origin.  The  fibers  of  the  muscle  pass  upward  and 
outward  and  in  their  course  converge  to  be  inserted  into  the  external  angle  of 
the  arytenoid  cartilage.  The  superior  and  more  horizontally  directed  fibers 
rotate  the  arytenoid  around  its  vertical  axis;  the  inferior  and  obliquely 
directed  fibers  draw  the  cartilage  downward  and  inward.  As  a  result  of  the 
action  of  the  muscle  in  its  entirety,  the  vocal  process  is  turned  upward  and 
outward,  and  as  the  vocal  band  is  carried  with  it  the  glottis  is  widened, 
a  condition  necessary  to  the  free  entrance  of  air  into  the  lungs  (Fig.  278). 
Since  the  contraction  of  the  crico-arytenoid  has  this  result,  it  is  generally 
spoken  of  as  the  abductor  or  respiratory  muscle. 


M 


Fig.  276.  Fig.  277. 

Fig.  276. — ^Posterior  View  OF  THE  Muscles  OF  THE  Larynx,  i.  Posterior  crico-arytenoid 
muscle.  2,  3,  4.  Different  fasciculi  of  the  arytenoid  muscle.  5.  Aryteno-epiglottidean  muscle. — 
{Sappey.) 

Fig.  277. — Lateral  View  of  the  Muscles  of  the  Larynx,  i.  Body  of  the  hyoid  bone. 
2.  Vertical  section  of  the  thyroid  cartilage.  3.  Horizontal  section  of  the  thyroid  cartilage 
turned  downward  to  show  the  deep  attachment  of  the  crico-thyroid  muscle.  4.  Facet  of  articula- 
tion of  the  small  cornu  of  the  thyroid  cartilage  with  the  cricoid  cartilage.  5.  Facet  on  the  cricoid 
cartilage.  6.  Superior  attachment  of  the  crico-thyroid  muscle.  7.  Posterior  crico-arytenoid 
muscle.  8,  10.  Arytenoid  muscle.  9.  Thyro-arytenoid  muscle.  11.  Aryteno-epiglottidean 
muscle.     12.  Middle  thyro-hyoid  ligament.     13.  Lateral  thyro-hyoid  ligament. — (Sappey.) 


The  lateral  crico-arytenoid  muscle  arises  from  the  side  of  the  cricoid 
cartilage.  From  this  point  its  fibers  are  directed  upward  and  backward 
to  be  inserted  into  the  external  process  of  the  arytenoid.  Its  action  is  to 
draw  the  arytenoid  cartilage  forward  and  inward,  thus  approximating 
and  relaxing  the  vocal  band. 

The  thyro-arytenoid  muscle  arises  from  the  inferior  two-thirds  of  the 
inner  surface  of  the  thyroid  cartilage  just  external  to  the  median  line.  From 
this  origin  the  fibers  pass  backward  and  outward,  to  be  inserted  into  the 
anterior  surface  and  external  angle  of  the  arytenoid  cartilage.  The  inner 
portion  of  the  muscle  lies  close  to  and  supports,  if  it  does  not  constitute  a  part 


PHONATION;  ARTICULATE  SPEECH 


663 


of,  the  vocal  band.  The  action  of  the  thyro-arytenoid  muscle  in  conjunction 
with  the  lateral  crico-arytenoid  is  to  rotate  the  arytenoid  cartilage  around  the 
vertical  axis  and  to  draw  the  vocal  process  forward  and  inward,  thus  carry- 
ing the  vocal  cord  toward  the  median  line.  When  the  muscles  of  the  two 
sides  simultaneously  contract,  the  vocal  bands  are  closely  approximated 
and  the  space  between  them,  the  rima  vocalis,  reduced  to  a  mere  slit,  one 
of  the  conditions  essential  to  phonation  (Fig.  279). 

The  arytenoid  muscle  consists  (i)  of  transversely  arranged  fibers  which 
arise  from  and  are  inserted  into  the  outer  surface  of  the  opposite  arytenoid 
cartilages,  and  (2)  of  obliquely  directed  fibers  which  arise  from  the  outer 
angle  of  one  arytenoid  to  be  inserted  into  the  apex  of  the  other.  In  their 
course  they  decussate  in  the  median  line.  The  action  of  this  muscle  is  to 
approximate  the  arytenoid  cartilages  and  thus  obliterate  that  portion  of  the 
glottis  between  the  vocal  processes,  the  rima  respiratoria,  and  so  direct  the 
expiratory  blast  of  air  toward  and  through  the  rima  vocalis. 


Fig.  278. — Glottis  Widely  Opened 
FROM  Simultaneous  Coxtractiox  of 
Both  Crico-arytenoid  Muscles,  b. 
Epiglottis,  rs.  False  vocal  band.  ri. 
True  vocal  band.  ar.  Arytenoid  car- 
tilages, a.  Space  between  the  ar\'tenoids. 
c.  Cuneiform  cartilages,  ir.  Interarytenoid 
fold.  rap.  Ari'epiglottic  fold.  cr.  Car- 
tilage rings. — {Mandl.) 


Fig.  279, — Position  of  the  Vocal 
B.ANDS  Due  to  the  Simultaneous 
Contraction  of  Both  L.^teral  Crico- 
arytenoid Muscles  and  Both  Thy'ro- 
ARYTENOiD  MuscLES.  b.  Epiglottis,  rs. 
False  vocal  band.  ri.  True  vocal  band. 
or.  Space  between  the  arytenoid  cartil- 
ages, the  glottis  respiratoria.  ar.  Ary- 
tenoid cartilages,  c.  Cuneiform  carti- 
lages, rap.  Aryepiglottic  fold.  ir.  In- 
terarytenoid fold. — {Mandl.) 


The  collective  actions  of  the  three  foregoing  muscles  is  to  close  or  con- 
strict the  glottis,  and  for  this  reason  they  are  spoken  of  as  the  adductor  or 
phonatory  muscles. 

The  crico-thyroid  muscle  arises  from  the  side  and  front  of  the  cricoid 
cartilage  and  is  inserted  above  into  the  lower  border  of  the  thyroid  cartilage. 
The  action  of  this  muscle  is  to  draw  up  the  anterior  part  of  the  cricoid  car- 
tilage toward  the  thyroid,  which  remains  stationary,  and  to  swing  the 
quadrate  plate  of  the  cricoid  and  the  arytenoid  cartilages  downward  and 
backward.  This  movement  has  the  result  of  tensing  the  vocal  bands. 
The  cricoid  is  at  the  same  time  drawn  backward  by  the  action  of  the  more 
longitudinally  disposed  fibers. 

Nerves  of  the  Larynj. — The  nerves  which  innervate  the  muscles  of  the 
larynx  and  endow  the  mucous  membrane  with  sensibility  are  derived  from 
the  vagus  trunk.  The  superior  laryngeal  is  for  the  most  part  sensor  and 
distributed  to  the  mucous  membrane,  though  it  contains  motor  fibers  for 
the  crico-thyroid  muscle.  The  inferior  laryngeal  is  purely  motor  and  is 
distributed  to  all  the  muscles  with  the  exception  of  the  crico-thyroid. 


664 


TEXT-BOOK  OF  PHYSIOLOGY 


THE  MECHANISM  OF  PHONATION 

Phonation,  the  production  of  vocal  sounds  in  the  larynx,  is  the  result  of 
the  vibration  of  the  vocal  bands  caused  by  an  expiratory  blast  of  air  from 
the  lungs.  That  a  sound  may  arise  it  is  essential  that  the  glottis  be  approxi- 
mately closed  and  the  vocal  bands  be  made  more  or  less  tense. 

The  closure  of  the  glottis — the  approximation  of  the  vocal  processes  and 
the  vocal  bands — is  accompHshed,  it  will  be  recalled,  by  the  contraction 
of  the  lateral  crico-arytenoid,  the  arytenoid,  and  the  thyro-arytenoid  muscles. 
The  increase  in  tension  is  accomplished  by  the  contraction  of  the  crico-thyroid 
and  the  thyro-arytenoid  muscles,  the  former  by  the  backward  displacement 
of  the  cricoid  and  arytenoid  cartilages,  the  latter  by  converting  the  natural 
concave  edge  of  the  vocal  band  to  a  straight  line.  The  lengthening  and  tens- 
ing of  the  vocal  bands  by  the  crico-thyroid  muscle  is  regarded  by  some  inves- 
igators  as  a  coarse  means,  the  approximation  of  the  free  edges  by  the  thyro- 
arytenoid, as  a  finer  means,  of  adjustment  for  the  producton  of  slight  changes 
in  the  pitch  of  sounds.  The  extent  to  which  the  glottis  is  closed  and  the 
membranes  tensed  will  depend,  however,  on  the  pitch  of  the  sound  to  be 


Fig.  280. — Position  of  the  Vocai. 
Bands  Previous  to  the  Emission  of  a 
Sound,  b.  Epiglottis,  rs.  False  vocal 
band.  ri.  True  vocal  band.  ar.  Ary- 
tenoid cartilages. — {Alandl.) 


Vgp 


Fig.  281. — Position  of  the  Vocai, 
Bands  in  the  Production  of  Notes 
of  Low  Pitch.  /.  Epiglottis,  or.  Glottis. 
ns.  False  vocal  cord.  7ii.  True  vocal  cord. 
ar.  Arytenoid  cartilages. — {Mandl.) 


emitted.  The  appearance  presented  by  the  glottis  just  previous  to  the  emis- 
sion of  a  note  of  medium  pitch,  as  determined  by  laryngologic  examination,  is 
shown  in  Fig.  280.  When  the  foregoing  conditions  in  the  glottis  are  realized, 
the  air  stored  or  collected  in  the  lungs  is  forced  by  the  contraction  of 
the  expiratory  muscles,  through  the  narrow  space  between  the  bands. 
As  a  result  of  the  resistance  offered  by  this  narrow  outlet  and  the 
force  of  the  expiratory  muscles  the  air  within  the  lungs  and  trachea  is 
subjected  to  pressure,  and  as  soon  as  the  pressure  attains  a  certain  level 
the  vocal  bands  are  thrown  into  vibrations,  which  in  turn  impart  to  the 
column  of  air  in  the  upper  air-passages  a  corresponding  series  of  vibrations  by 
which  the  laryngeal  vibrations  are  reinforced.  The  degree  of  pressure  to 
which  the  air  in  the  lungs  and  trachea  is  subjected  was  determined  by  Latour 
to  vary  from  160  mm.  of  water  for  sounds  of  moderate,  to  940  mm.  of  water 
for  sounds  of  highest  intensity.  With  the  escape  of  the  air  or  the  separation 
of  the  vocal  bands  the  vibration  ceases  and  the  sound  dies  away. 

The  Characteristics  of  Vocal  Sounds. — In  common  with  the  sounds 
produced  by  other  music  instruments,  all  vocal  sounds  are  characterized 
by  intensity,  pitch  and  quality,  tone  or  color. 


PRONATION;  ARTICULATE  SPEECH 


665 


Fig.     282. — Glottis    Seen 

WITH  THE  LaRYXGOSCOPE  DUR- 
ING THE  Emission  of  High- 
pitched  Sounds,  i,  2.  Base 
of  the  tongue.  3,  4.  Epiglot- 
tis. 5,  6.  Pharynx.  7.  Ary- 
tenoid cartilages.  8.  Opening 
between  the  true  vocal  cords. 

9.  Arjleno-epiglottidean  folds. 

10.  Cartilage  of  Santorini.  11. 
Cuneiform  cartilage.  12.  Su- 
perior vocal  cords.  13.  In- 
ferior vocal  cords. — (Le  Bon.) 


The  intensity  or  loudness  of  a  sound  depends  on  the  extent  or  amplitude 
of  the  to-and-fro  vibration  or  the  extent  of  the  excursion  of  the  vocal 
band  on  either  side  of  the  position  of  equilibrium  or  rest;  and  this  in  turn 
depends  on  the  force  with  which  the  blast  of  sat  strikes  the  band.  The  more 
forceful  the  blast  of  air,  the  larger,  other  things  , 

being  equal,  will  be  the  primary  vibrations  of 
the  bands,  and  hence  the  secondary  vibrations 
of  the  air  in  the  upper  air-passages. 

The  pitch  of  the  voice  depends  on  the  num- 
ber of  vibrations  in  a  unit  of  time,  a  second. 
This  will  be  conditioned  by  the  length  of  the 
bands  in  vibration  or  the  length  and  width  of 
the  aperture  through  which  the  air  passes  and 
the  degree  of  tension  to  which  the  bands  are  sub- 
jected. In  the  emission  of  sounds  of  highest 
pitch  the  tension  of  the  vocal  bands  and  the 
narrowing  of  the  glottis  attain  their  maximum. 
In  the  emission  of  sounds  of  lowest  pitch  the  re- 
verse conditions  obtain.  In  passing  from  the 
lowest  to  the  highest  pitched  sounds  in  the  range 
of  the  voice  peculiar  to  any  one  individual, 
there  is  a  progressive  increase  in  both  the  ten- 
sion of  the  vocal  bands  and  the  narrowing  of 
the  glottic  aperture.  In  the  production  of  low- 
pitched  notes  of  men,  those  due  to  \dbrations  lying  between  80  and  240  per 
second,  the  tension  is  regulated  by  the  crico-thyroid  muscle;  the  aperture  of 
the  glottis  during  this  time  being  elliptic  in  shape  and  relatively  wide  (Fig. 
283).  In  the  production  of  notes  due  to  vibrations  lying  between  240  and 
512  vibrations  per  second,  the  anterior  fibers  of  the  crico-thyroid  muscle 
relax  and  the  thyroarytenoid  muscle  comes  into  play;  by  its  action  the  vocal 
bands  are  more  closely  approximated  and  the  vocal  aperture  reduced  to  a 
linear  slit.  In  the  high-pitched  notes  emitted  by  soprano  singers  the  vocal 
bands  are  so  closely  applied  to  each  other  that  only  a  very  small  portion  in 
front,  bounding  a  small  oval  aperture,  is  capable  of  vibrating  (Fig.  282). 
The  difference  in  the  pitch  of  the  voice  in  men  and  women  is  due  largely  to 
the  greater  size  and  development  of  the  vocal  bands  in  the  former  than  in  the 
latter. 

The  quality  of  the  voice,  the  timbre  or  tone-color,  depends  on  the  form 
combined  with  the  intensity  and  pitch  of  the  vibration.  As  with  sounds  pro- 
duced by  music  instruments,  the  primary  or  fundamental  vibration  of  the 
vocal  band  is  complicated  by  the  superposition  of  secondary  or  partial  vibra- 
tions (overtones).  The  form  of  the  vibration  will,  therefore,  be  a  resultant  of 
the  blending  of  a  number  of  different  vibrations.  The  quality  of  the  sound 
produced  in  the  larynx  is,  however,  modified  by  the  resonance  of  the  mouth 
and  nasal  cavities;  certain  of  the  overtones  being  reinforced  by  changes  in 
the  shape  of  the  mouth  cavity  more  especially,  thus  giving  to  the  voice  a 
somewhat  different  quality. 

The  Varieties  of  Voice. — The  region  of  the  music  scale,  comprising 
all  vibrations  between  32  and  2048  per  second,  with  which  laryngeal  sounds 
are  in  accord  will  vary  in  the  two  sexes  and  in  different  individuals  of  the 


666  TEXT-BOOK  OF  PHYSIOLOGY 

same  sex.  It  is  customary  to  classify  voices,  especially  those  of  singers,  into 
bass,  baritone,  tenor,  contralto,  mezzo-soprano,  and  soprano,  in  accordance 
with  the  regions  of  the  music  scale  with  which  they  correspond.  Thus  the 
succession  of  notes  characteristic  of  the  bass  voice  vary  in  pitch  from  F,  ia,^, 
to  c',  dOj,  or  from  85  to  256  vibrations  per  second;  those  of  the  baritone 
from  A,  laj,  to  f,  fag,  or  from  106  to  341  vibrations  per  second;  those  of  the 
tenor  from  c',  doj,  to  a',  la^,  or  from  128  to  427  vibrations  per  second;  those 
of  the  contralto  from  e,  mi  2,  to  c",  do^,  or  from  160  to  512  vibrations  per 
second;  those  of  the  mezzo-soprano  from  g,  S0I2,  to  e",  mi^,  or  from  192  to 
640  vibrations  per  second;  those  of  the  soprano  from  b,  si2,  to  g",  sol^,  or 
from  240  to  768  vibrations  per  second.  The  range  of  the  voice  is  thus  seen 
to  embrace  from  one  and  three-quarters  to  two  octaves.  Some  few  individual 
singers  have  far  exceeded  this  range,  but  they  are  exceptional. 

Speech  is  the  expression  of  ideas  by  means  of  articulate  sounds.  These 
sounds  may  be  divided  into  vowel  and  consonant  sounds. 

The  vowel  sounds,  a,  e,  i,  0,  u,  are  laryngeal  tones  modified  by  the 
superposition  and  reinforcement  of  certain  overtones  developed  in  the  mouth 
and  pharynx  by  changes  in  their  shapes.  The  number  of  vibrations  under- 
lying the  production  of  each  vowel  sound  is  a  matter  of  dispute. 

Consonant  sounds  are  produced  by  the  more  or  less  complete  interruption 
of  the  vowel  sounds  during  their  passage  through  the  organs  of  speech. 
These  may  be  divided  into: 

1.  Labials,  p,  h,  m. 

2.  Labio-dentals, /,  v. 

3.  Linguo-dentals,  s,  z. 

4.  Anterior  linguo-palatals,  t,  d,  I,  n,  r,  sh,  zh. 

5.  Posterior  linguo-palatals,  k,  g,  h,  y. 

The  names  of  these  different  groups  of  consonants  indicate  the  region  of 
the  mouth  in  which  they  are  produced  and  the  means  by  which  the  air  blast 
is  interrupted. 

THE  NERVE  MECHANISM  OF  THE  LARYNX 

The  nerve  mechanism  by  which  the  musculature  of  the  larynx  is  excited 
to  action  and  coordinated  so  as  to  subserve  both  respiration  and  phonation 
involves  the  fibers  contained  in  the  superior  and  inferior  laryngeal  nerves 
(both  branches  of  the  vagus)  and  their  related  nerve-centers  in  the  central 
nerve  system. 

For  respiratory  purposes  it  is  essential  that  the  lumen  of  the  glottis  shall 
be  sufficiently  large  to  permit  the  entrance  and  exit  of  air  without  hindrance. 
Laryngoscopic  examination  of  the  larynx  in  the  human  being  shows  that 
during  quiet  respiration  the  vocal  bands  are  widely  separated  and  almost 
stationary,  moving  but  slightly  during  either  inspiration  or  expiration.  At 
this  time,  according  to  the  investigations  of  Semon,  the  area  of  the  glottis  is 
approximately  160  sq.  mm.,  somewhat  less  than  the  area  of  either  the 
supraglottic  or  infraglottic  regions,  which  is  about  200  sq.  mm.  This  con- 
dition of  the  glottis  is  maintained  by  the  steady  continuous  contraction  of  the 
posterior  crico-arytenoid  muscles,  the  abductors  of  the  vocal  bands. 

For  phonatory  purposes  it  is  essential  that  the  respiratory  function  be 
temporarily  suspended  and  the  vocal  bands  closely  approximated.  This  is 
accomplished  by  the  contraction  of  the  remaining  muscles  of  the  larynx, 


PHONATION;  ARTICULATE  SPEECH  667 

with  the  exception  of  the  crico-thyroid,  which  are  collectively  known  as  the 
adductors  of  the  vocal  hands.  During  phonation  the  adductor  muscles  over- 
come the  activity  of  the  abductors.  With  the  cessation  of  phonation  the 
abductors  immediately  restore  the  vocal  bands  to  their  former  respiratory 
position. 

The  activities  of  these  two  antagonistic  groups  of  muscles  are  under  the 
control  of  the  central  nerve  system.  The  only  pathway  for  the  excitator 
nerve  impulses  is  through  the  fibers  of  the  inferior  or  recurrent  laryngeal 
nerve.  The  relation  of  these  nerve-fibers  both  centrally  and  peripherally,  as 
well  as  their  physiologic  action,  has  been  the  subject  of  much  experimenta- 
tion. The  results  have  not  always  been  in  accord,  owing  to  the  choice  of 
animal,  the  use  of  anesthetics,  strength  of  stimulus,  etc. 

As  the  outcome  of  many  investigations  it  is  believed  that  each  muscle 
group  is  innervated  by  its  own  bundle  of  nerve-fibers,  both  of  which  are  con- 
tained in  the  inferior  laryngeal,  though  coming  from  two  separate  centers  in 
the  medulla  oblongata.  Russell  succeeded  in  separating  the  fibers  for  the 
abductors  from  the  fibers  for  the  adductors  in  the  inferior  laryngeal,  and  in 
tracing  them  to  their  terminations.  So  completely  was  this  done  that  it 
became  possible  to  produce  at  will,  through  stimulation,  either  abduction 
or  adduction,  without  contraction  of  the  muscle  of  opposite  function. 

The  laryngeal  respiratory  center  was  located  by  Semon  and  Horsley, 
in  the  cat,  in  the  upper  part  of  the  floor  of  the  fourth  ventricle.  Stimulation 
of  this  area  during  .etherization  was  followed  by  abduction  of  the  vocal 
bands.  The  efferent  fibers-of  this  center  are  believed  by  some  investigators 
to  leave  the  central  nerve  system  in  the  spinal  accessory  nerve,  by  others  in 
the  lower  roots  of  the  vagus. 

From  the  continuous  activity  of  the  abductor  muscle,  and  the  stationary 
position  of  the  vocal  bands,  it  is  probable  that  the  medullary  center  is  in  a 
state  of  continuous  activity  or  tonus,  the  result  probably  of  reflex  influences. 

A  cortical  representation  for  laryngeal  respiratory  movements  has  been 
determined  by  Semon  and  Horsley  in  different  classes  of  animals.  In  the 
cat  especially,  stimulation  of  the  border  of  the  olfactory  sulcus  gives  rise  to 
complete  abduction  of  the  vocal  bands  on  both  sides.  The  representation  is 
therefore  bilateral. 

The  phonatory  center  was  located  by  the  same  investigators  in  the  medulla 
near  the  ala  cinerea  and  the  upper  border  of  the  calamus  scriptorius.  Stimu- 
lation of  this  area  was  invariably  followed  by  bilateral  adduction  of  the  vocal 
bands  and  closure  of  the  glottis. 

A  cortical  representation  for  phonatory  movements  has  also  been  located 
in  the  lower  portion  of  the  pre-central  convolution,  near  the  anterior  border. 
Stimulation  of  this  area  gives  rise  to  marked  adduction  of  both  vocal  bands, 
indicating  that  the  representation  is  therefore  bilateral. 

Faradic  stimulation  of  the  inferior  laryngeal  nerve  during  slight  ether 
anesthetization  gives  rise  to  closure  of  the  glottis;  the  same  stimulation, 
however,  during  deeper  anesthetization  gives  rise  to  opening  or  dilatation  of 
the  glottis,  a  fact  indicating  that  either  the  adductor  muscles  or  their  nerve 
terminals  are  depressed  by  the  action  of  the  ether  before  the  muscles  and 
nerves  of  opposite  function.  The  superior  laryngeal  nerves  contain  motor 
fibers  for  the  crico-thyroid  muscles.  Stimulation  of  the  nerve  gives  rise  to 
contraction  of  the  muscle  and  increased  tension  of  the  vocal  bands.     It  is 


668  TEXT-BOOK  OF  PHYSIOLOGY 

believed  that  these  fibers  are  derived  originally  from  the  efferent  fibers  of  the 
glosso-pharyngeal  nerve.  The  remaining  fibers  of  the  superior  laryngeal 
endow  the  upper  portion  of  the  larynx  with  extreme  sensibility  which  to  a 
certain  extent  protects  the  air-passages  against  the  entrance  of  foreign 
bodies.  Irritation  of  the  terminal  filaments  of  this  nerve  by  particles  of 
food,  solid  or  liquid,  gives  rise  to  marked  reflex  spasm  of  the  adductor 
muscles  and  closure  of  the  glottis,  followed  by  a  strong  expiratory  blast  of  air 
from  the  lungs  by  which  the  offending  particles  are  removed.  Division  of 
this  nerve  on  both  sides  is  followed  by  a  paralysis  of  the  crico-thyroid  muscles, 
a  lowering  of  the  tension  of  the  vocal  bands,  and  a  loss  of  sensibility  of  the 
laryngeal  mucous  membrane. 


CHAPTER  XXVIII 
THE  SENSES  OF  TOUCH,  TASTE  AND  SMELL 

Introductory. — It  is  one  of  the  functions  of  the  nerve  system  to  bring 
the  individual  into  conscious  relation  with  the  external  world.  This  is  ac- 
complished in  part  through  the  intermediation  of  afferent  nerves,  connected 
peripherally  with  highly  specialized  terminal  organs,  and  centrally  with 
specialized  areas  in  the  cerebral  cortex. 

Excitation  of  the  terminal  organs  by  material  changes  in  the  environment 
develops  nerve  impulses  which,  transmitted  to  the  cortical  areas,  evoke 
sensations.  These  sensations,  differing  in  character  from  those  vague  ill- 
defined  sensations — e.g.,  fatigue,  well-being,  discomfort,  etc. — caused  by 
material  changes  occurring  within  the  body,  are  termed  special  sensations — 
e.g.,  touch;  pressure;  pain;  temperature;  taste;  smell;  light  and  its  varying 
qualities,  intensity,  hue,  and  tint;  sound  and  its  varying  qualities,  intensity, 
pitch,  and  timbre. 

The  terminal  organs  which  receive  the  impress  of  the  external  world  are  the 
skin,  tongue,  nose,  eye,  and  ear,  and  collectively  constitute  the  special  sense- 
organs.  The  physiologic  mechanisms  which  underlie  and  develop  these 
special  sensations  are  known  respectively  as  the  tactile,  gustatory,  olfactory, 
optic,  and  auditory.  Each  mechanism  responds  to  but  a  single  form  of 
stimulus  and  to  no  other.  Thus,  the  stimulus  for  the  skin  is  mechanic  pres- 
sure; for  the  tongue,  soluble  organic  and  inorganic  matter;  for  the  nose, 
volatile  or  gaseous  matter;  for  the  eye,  ether  vibrations;  for  the  ear,  atmos- 
pheric undulations.  These  stimuli  alone  are  adequate  to  the  physiologic 
excitation  of  the  different  mechanisms. 

The  factors  involved  in  the  production  of  the  sensations  include  (i)  a 
special  physical  stimulus;  (2)  a  specialized  terminal  organ ;  (3)  an  afferent  nerve 
pathway,  and  (4)  a  specialized  receptive  sensor  cell  in  the  cerebral  cortex. 

Though  the  resulting  sensations  in  each  instance  differ  widely  in  their 
characteristics,  it  is  diflScult  to  present  a  satisfactory  explanation  for  these 
differences.  If  it  be  assumed  that  the  nerve  impulses  which  ascend  the 
different  nerves  of  special  sense  are  alike  in  quality,  then  it  must  be  ad- 
mitted that  the  character  of  the  sensation  is  the  expression  of  a  specialization 
and  organization  of  the  cortical  area.  If,  on  the  other  hand,  specialization 
of  the  cortex  is  denied,  then  there  must  be  admitted  a  specialization  of  the 
peripheral  organ — with  a  resulting  difference  in  quality  or  rapidity  of  the 
nerve  impulses  which  would  impress  or  excite  the  non-specialized  cortex  in 
such  a  way  as  to  call  forth  the  characteristic  sensation.  It  is  possible,  how- 
ever, that  neither  supposition  is  wholly  correct,  and  that  the  character  of  the 
sensation  depends  on  the  construction  and  adaptation  of  the  entire  sense 
apparatus  to  the  character  of  the  stimulus. 

Whatever  the  conditions  for  their  origin  and  whatever  their  character- 
istics, sensations  in  themselves  do  not  constitute  knowledge;  they  are  but 
elementary  states  of  consciousness,  raw  materials  out  of  which  the  mind 
elaborates  conceptions  and  forms  judgments  as  to  the  character  of  any  given 
object  in  comparison  with  former  experiences. 

669 


670  TEXT-BOOK  OF  PHYSIOLOGY 

THE  SENSE  OF  TOUCH 

The  physiologic  mechanism  involved  in  the  sense  of  touch  includes  the 
skin  and  the  mucous  membrane  lining  the  mouth,  the  afferent  nerves,  their 
cortical  connections,  and  nerve-cells  in  the  cortex  of  the  parietal  lobe. 

Peripheral  excitation  of  this  mechanism  develops  nerve  impulses  which, 
transmitted  to  the  cortex,  evoke  sensations  of  touch  and  temperature.  To 
the  skin,  therefore,  is  ascribed  a  touch  sense  and  a  temperature  sense. 
Of  the  touch  sensations  two  kinds  may  be  distinguished:  viz.,  pressure 
sensations  and  place  sensations.  With  the  contact  of  an  external  body 
there  arises  the  perception  not  only  of  the  pressure,  but  also  the  perception 
of  the  place  or  locality  of  the  contact.  In  accordance  with  this,  it  is 
customary  to  attribute  to  the  skin  a  pressure  sense  and  a  location  sense. 

The  specific  physiologic  stimuli  to  the  terminal  organs  in  the  skin  and 
oral  mucous  membrane  are  mechanic  pressure  and  thermic  vibrations. 

The  Skin. — The  skin,  which  constitutes  the  basis  for  the  sense  of  touch, 
covers  and  closely  invests  the  entire  body.  It  varies  in  thickness  and  deli- 
cacy in  different  regions,  though  its  structure  is  everywhere  essentially  the 
same.  As  the  physiologic  anatomy  of  the  skin  has  elsewhere  been  detailed 
(page  472),  it  is  only  necessary  to  state  here  that  it  is  divided  into  a  deep 
and  a  superficial  layer.  The  former,  known  as  the  derma,  consists  of  an 
inner  layer  of  rather  loose  connective  tissue  and  an  outer  layer  of  condensed 
connective  tissue.  The  latter,  known  as  the  epidermis,  consists  of  an  inner 
layer  of  pigment  cells  and  a  thick  outer  layer  of  epithelial  cells.  The 
derma  is  characterized  by  the  presence  of  elevations  (papillae)  which  are 
everywhere  extremely  abundant.  Throughout  the  derma  ramify  blood- 
vessels and  nerves. 

The  Peripheral  or  Terminal  Organs. — Between  the  contact  surface 
and  the  afferent  nerves  specialized  structures  are  found  which  serve  as  inter- 
mediatos  between  the  stimulus,  on  the  one  hand,  and  the  afferent  nerves, 
on  the  other  hand.  By  virtue  of  their  structure  they  are  far  more  irritable 
than  the  nerve-fibers  and  hence  respond  more  quickly  to  the  physiologic 
stimulus  than  the  nerve-fiber  itself.  To  these  specialized  organs,  found  not 
only  in  the  skin  but  in  other  sense-organs  as  well,  the  term  peripheral  or 
terminal  organ  is  given.  It  is  these  structures  that  are  primarily  excited 
to  activity  by  the  physiologic  stimulus,  and  that  in  turn  arouse  the  nerve 
to  activity.  Peripheral  organs  are  to  be  regarded  as  special  modes  of  termi- 
nation of  afferent  nerves  adapted  for  the  impress  of  a  specific  stimulus. 
The  peripheral  organs  of  afferent  nerves  found  in  the  skin  and  oral  mucous 
membrane  present  a  variety  of  forms,  some  of  which  are  as  follows: 

1.  Free  Endings. — These  are  pointed  or  club-shaped  processes,  the  ultimate 

terminations  of  afferent  nerve-fibrils,  found  in  and  among  epidermic  cells. 

2.  Tactile  Cells. — These  are  oval  nucleated  bodies  found  in  the  deeper 

layers  of  the  epidermis.  They  rest  upon  or  are  embraced  by  a  crescentic- 
shaped  body,  the  tactile  meniscus,  which  in  turn  is  directly  connected 
with  the  nerve-fibril  and  probably  a  modification  of  it. 

3.  The  Corpuscles  of  Meissner  and  Wagner. — In  the  papillae  of  the  derma, 

especially  in  the  palm  of  the  hand  and  in  the  finger-tips,  are  found 
elliptical  bodies  consisting  of  a  connective-tissue  capsule  containing  a 
number  of  tactile  discs  with  which  the  nerve-fibrils  are  connected.     If 


THE  SENSE  OF  TOUCH  671 

the  afferent  nerve  is  traced  to  the  capsule,  it  is  found  to  lose  both  its 
neurilemma  and  its  medulla,  after  which  the  naked  fibril  penetrates  the 
capsule,  breaks  up  into  a  number  of  branches,  and  after  pursuing  a 
more  or  less  spiral  course  becomes  connected  with  the  tactile  discs. 

4.  Hair  Wreaths. — Just  below  the  openings  of  the  sebaceous  glands  the 

hair-follicles  are  surrounded  by  naked  axis-cylinder  fibrils  in  the  form 
of  a  wreath,  which  in  all  probability  terminate  in  the  cells  of  the  external 
root-sheath.  These,  too,  are  to  be  regarded  as  part  of  the  touch 
apparatus. 

5.  Corpuscles  of  Vater  or  Pacini. — These  are  oval-shaped  structures  found 

along  the  ner\^es  distributed  to  the  palms  of  the  hands  and  the  soles 
of  the  feet,  on  the  nerves  distributed  to  the  external  genital  organs,  to 
joints  and  other  structures.  They  consist  of  a  thick  capsule  of  lamel- 
lated  connective  tissue  in  the  interior  of  which  is  a  bulb  resembling 
granular  protoplasm.  The  axis-cylinder  of  the  nerve-fiber  enters  the 
capsule  and  becom.es  connected  with  the  bulb. 

Other  forms  of  peripheral  organs  are  found  in  special  regions  of  the  skin 
as  well  as  in  different  animals. 

Touch  Sense. — The  area,  stimulation  of  which  evokes  sensations  of 
touch  is  coextensive  with  the  skin  and  that  limited  portion  of  the  mucous 
membrane  lining  the  mouth.  Careful  stimulation  of  the  skin  by  means  of  a 
fine  stiff  bristle  has  revealed  the  fact,  however,  that  the  touch  area  is  not 
continuous,  but  discrete,  presenting  itself  under  the  form  of  small  areas  or 
spots,  separated  by  relatively  large  areas  insensitive  to  the  same  agent. 
Stimulation  of  these  spots  always  calls  forth  a  sensation  of  touch.  For  this 
reason  they  are  known  as  ''touch  spots."  The  number  of  such  spots  in  any 
given  area  of  skin  varies  considerably.  Thus,  in  the  skin  of  the  calf  fifteen 
such  spots  have  been  counted  in  a  square  centimeter.  In  the  palm  of  the  hand 
from  forty  to  fifty  have  been  counted  in  an  area  of  the  same  extent.  They  are 
also  especially  abundant  in  the  immediate  neighborhood  of  the  hair-follicles. 

The  peripheral  end-organ  associated  with  the  touch  spots  in  the  neigh- 
borhood of  a  hair-follicle  is  in  all  probability  the  wreath  of  nerve-fibrils 
surrounding  the  follicle.  In  regions  devoid  of  hairs  the  end-organ  is  the 
Meissner  corpuscle,  for  in  the  palmar  surface  of  the  distal  phalanx  of  the 
index-finger,  where  the  touch  sense  is  quite  acute,  about  20  corpuscles  are 
present  in  each  square  millimeter  of  surface.  The  specific  stimulus  neces- 
sary to  evoke  the  sensation  of  touch  is  a  deformation  of  the  skin;  and  the 
greater  this  is,  within  physiologic  limits,  the  more  pronounced  is  the  sensation. 

Pressure  Sense. — The  contact  of  an  external  body  is  attended  by  a 
certain  amount  of  pressure,  which,  however,  must  attain  a  certain  degree 
before  the  sensation  can  be  evoked.  This  is  known  as  the  threshold  value, 
or  the  degree  of  liminal  intensity.  Since  the  sensations  are  the  result  of 
pressure,  they  are  termed  pressure  sensations,  and  their  intensity  may  be 
expressed  in  terms  of  pressure. 

The  sensitivity  of  the  skin  as  determined  by  the  pressure  sense  varies  in 
different  regions  of  the  body  and  in  accordance  with  the  size  of  the  area 
pressed.  Thus,  the  liminal  intensity  of  a  stimulus  for  an  area  of  nine  square 
millimeters  for  the  skin  of  the  forehead  is  0.002  gram;  for  the  flexor  aspect 
of  the  forearm,  0.003  g^s-^n;  and  for  the  hips,  thigh,  and  abdomen,  0.005 
gram;  for  the  palmar  surface  of  the  finger,  0.019  gram;  for  the  heel,  i  gram. 


672  TEXT-BOOK  OF  PHYSIOLOGY 

The  delicacy  of  the  sense  of  touch  is  measured  by  the  shght  increase  or 
decrease  in  the  intensity  of  the  stimulus  that  will  produce  an  appreciable 
change  in  the  intensity  of  the  sensation.  Not  all  changes  in  the  stimulus, 
however,  are  attended  by  a  change  in  the  sensation.  It  has  been  deter- 
mined that  the  latter  will  change  only  when  the  former  changes  in  a  definite 
ratio,  which  for  the  volar  surface  of  the  third  phalanx  of  the  index-finger  is  as 
29  is  to  30.  Thus,  other  things  being  equal,  a  sensation  caused  by  a  given 
weight  will  only  change  with  moderate  stimulation  when  one-thirtieth  of  the 
weight  is  either  added  or  subtracted.  The  ratio  of  change,  however,  varies 
in  different  regions  of  the  body:  thus,  for  the  back  of  the  hand  the  ratio 
varies  from  one-tenth  to  one-twentieth;  for  the  tongue,  one-thirtieth  to  one- 
fortieth.  The  difference  of  stimulus  necessary  to  evoke  a  sensation  is 
known  as  the  threshold  difference.  It  seems  to  be  a  law  not  only  for  the 
skin,  but  for  other  senses  as  well,  that  a  change  in  the  intensity  of  a  sensa- 
tion, to  an  appreciable  extent,  will  occur  only  when  the  objective  stimulus 
changes  in  a  definite  ratio.  This  ratio,  however,  will  vary  not  only  in  dif- 
ferent regions  of  the  skin,  in  different  individuals,  but  with  the  sense-organ 
investigated. 

Place  Sense. — The  sensation  evoked  by  stimulation  of  the  skin  is  always, 
under  normal  conditions,  referred  to  the  place  stimulated.  This  holds  true 
not  only  for  two  or  more  points  near  or  widely  separated  on  the  same  side, 
but  also  for  corresponding  points  on  opposite  sides  of  the  body,  even  when 
the  stimuli  have  the  same  intensity  and  are  simultaneously  applied.  The 
cause  for  this  localizing  power  is  to  be  found  in  a  difference  in  the  quality  of 
the  sensation  related  in  some  way  to  the  part  stimulated.  Each  cutaneous 
area  is  supposed  to  give  to  the  tactile  sensation  a  quality  or  local  sign  by 
virtue  of  which  the  mind  is  enabled  to  localize  the  point  of  contact. 

Each  cutaneous  area  which  has  a  local  sign  of  its  own  is  known  as  a 
sensor  circle,  for  the  reason  that  the  mind  does  not  refer  the  sensation  to  a 
point,  but  to  an  area  more  or  less  circular  in  outline.  The  skin  may,  there- 
fore, be  regarded  as  composed  of  myriads  of  such  circles  varying  in  size  in 
different  regions  of  the  body. 

The  delicacy  of  the  localizing  power  in  any  part  of  the  skin  is  determined 
by  testing  the  power  which  the  part  possesses  of  distinguishing  the  sensa- 
tions produced  by  the  contact  of  the  points  of  a  pair  of  compasses  placed 
close  together.  The  distance  to  which  the  points  must  be  separated  in 
order  to  evoke  two  separate  recognizable  sensations  is  a  measure  of  the 
diameter  of  the  sensor  circle.  Within  this  circle  the  two  sensations  become 
fused  into  one  sensation.  The  discriminative  sensibility  of  different  regions 
as  determined  by  compass  points  is  shown  in  the  following  table;  the 
numbers  represent  the  distances  at  which  two  sensations  are  recognized: 

mm. 
Tip  of  tongue i .  i 

Palmar  surface  of  third  phalanx  of  index-finger 2.2 

Red  surface  of  lips 4.5 

Palmar  surface  of  first  phalanx  of  finger 5.5 

Tip  of  nose 6.8 

Palm  of  hand 8.9 

Lower  part  of  forehead 22.6 

Dorsum  of  hand 31  -6 

Dorsum  of  foot 40 .6 

Middle  of  the  back 67 .7 


THE  SENSE  OF  TOUCH 


673 


The  discriminative  sensibility  of  any  portion  of  the  body  is  a  function  of 
its  mobility.  This  is  shown  by  the  fact  that  it  increases  rapidly  from  the 
shoulders  to  the  fingers  and  from  the  hips  to  the  toes. 

The  Temperature  Sense. — The  sensations  of  heat  and  cold  which  are 
experienced  from  time  to  time  are  caused  by  changes  in  the  temperature  of 
the  skin  produced  in  a  variety  of  ways.  As  these  sensations  are  specifically 
different  from  those  of  touch,  as  well  as  different  from  each  other,  it  is  highly 
probable  that  for  each  sensation  there  are  special  nerve-endings  distributed 
throughout  the  skin.  Investigations  have  shown  that  all  over  the  skin  there 
are  innumerable  spots  of  varying  size  which  if  stimulated  evoke  sensations  of 
heat  or  cold.  Such  points  are  termed  heat  and  cold  spots.  Each  responds 
to  but  one  kind  of  stimulus.  A  warm  object  applied  to  a  heat  spot  will 
.  evoke  a  sensation  of  warmth.  It  will  have  no  effect  on  the  cold  spot.  The 
reverse  is  also  true.  Between  the  cold  and  heat  spots  there  are  areas  that 
are  neutral,  insensitive  to  either  heat  or  cold.     The  cold  spots  are  more 


Fig.  283. — Cold  and  Hot  Spots  from  the  Anterior  Surface  of  the  Forearm,  a.  Cold 
spots,  b.  Hot  spots.  The  dark  parts  are  the  most  sensitive,  the  hatched  the  medium,  the  dotted 
the  feebly,  and  the  vacant  spaces  the  non-sensitive. — {Landois  and  Stirling.) 

numerous  than  the  heat  spots  in  almost  all  regions  of  the  body.     (See  Fig. 

283.) 

The  sensitivity  of  the  skin  to  temperature  changes  is  very  acute,  as  shown 
by  the  fact  that  even  o.o5°C.  is  readily  appreciable.  This  holds  true, 
however,  only  when  the  temperature  of  the  object  lies  between  27°  and  33°C. 
This  capability  varies  in  different  regions  of  the  skin,  and  depends  on  the 
number  of  heat  and  cold  spots  present,  the  thickness  of  the  epidermis,  the 
thermal  conductivity  of  the  object  touching  it,  and  the  extent  to  which  it  is 
habitually  exposed  or  protected. 

The  physiologic  stimulus  to  the  thermic  end-organs  is  the  passage  of 
heat  through  the  skin  from  the  interior  of  the  body  to  the  surrounding  air. 
If  the  radiation  is  continuous  and  uniform,  the  end-organs  soon  adapt  them- 
selves to  the  temperature  of  the  surrounding  air  and  the  sensation  of  heat, 
under  physiologic  conditions,  is  not  evoked.  If  there  is  a  sudden  rise  in  the 
43 


674  TEXT-BOOK  OF  PHYSIOLOGY 

external  temperature  caused  by  natural  or  artificial  means,  which  diminishes 
the  radiation,  the  temperature  of  the  skin  will  at  once  rise,  the  end-organs 
will  be  stimulated,  and  a  sensation  of  warmth  developed.  If,  on  the  other 
hand,  there  is  a  sudden  fall  in  temperature  and  an  increased  radiation,  the 
temperature  of  the  skin  will  fall,  the  end-organs  will  be  stimulated,  and  a 
sensation  of  cold  developed.  Experiment  also  teaches  that  the  intensity  of 
a  warm  or  cold  sensation  will  depend  on  the  existing  temperature  of  the  skin, 
and  not  upon  the  absolute  temperature  of  the  object.  Thus,  water  at  2o°C. 
will  evoke  a  sensation  of  heat  or  cold  respectively  according  as  the  skin 
has  previously  been  cooled  below  or  warmed  above  this  temperature. 

The  Muscle  Sense. — As  a  result  of  the  activities  of  the  musculature  of 
the  body  or  even  of  its  individual  parts,  there  arises  in  consciousness  a 
series  of  sensations  which  are  termed  muscle  sensations.  These  sensations 
give  rise  to  the  perception — 

1.  Of  the  direction  and  duration  of  both  passive  (due  to  external  causes) 

and  active  movements  (due  to  internal,  volitional  efforts)  which  take 
place  without  hindrance; 

2.  Of  the  resistance  offered  to  movements  by  external  bodies;  and 

3.  Of  the  posture  of  the  body  or  of  its  individual  parts. 

As  to  the  seat  of  the  physiologic  processes  which  precede  and  underlie 
the  development  of  the  sensations  two  views,  at  least,  may  be  advanced,  viz. : 

1.  That  the  processes  are  central  in  origin  and  partake  of  the  nature  of  a 

discharge  of  nerve  impulses  from  the  nerve-cells  through  the  motor 
nerves  to  the  muscles,  the  entire  process  being  accompanied^ by  sensation. 
This  is  known  as  the  innervation  theory. 

2.  That  the  processes  are  peripheral  in  origin,  initiated  by  stimulation  of 

specialized  end-organs  in  the  muscles  and  tendons  which  are  connected 
through  the  intermediation  of  afferent  nerves  with  nerve-cells  in  the 
cerebral  cortex. 


^•"  c.  pre.  ' 

Fig.  284. — A  Neuro-muscle  Spindle  of  a  Cat.  (Ruffini.)  c.  Capsule,  pr.  e.  Primary 
ending,  s.  e.  Secondary  ending,  pi.  e.  Plate  ending.  (All  these  are  probably  sensor  in  function.) 
— {Starling's  "Physiology.") 

The  physiologic  mechanism  subserving  the  muscle  sense,  according  to  the 
second  theory,  now  held  by  many  physiologists,  thus  involves  peripheral  end- 
organs,  afferent  nerves,  their  cortical  connections  and  nerve-cells  in  the 
cerebral  cortex  at  or  near  the  junction  of  the  superior  and  inferior  parietal 
convolutions. 

The  End-organs. — These  are  small  fusiform  structures  found  in  and 
among  the  muscle  bundles  of  all  the  muscles  of  the  body  with  the  exception 
of  the  diaphragm  and  eye  muscles.  In  the  muscles  of  the  arm  and  in  the 
small  muscles  of  the  hand  they  are  especially  abundant.  From  their  shape 
they  are  known  as  muscle  spindles.     They  vary  in' length  from  2  to  12  mm. 


THE  SENSE  OF  TOUCH  675 

and  in  breadth  from  0.15  to  0.4  mm.  Each  spindle  (Fig.  284)  consists  of  a 
connective-tissue  capsule  containing  from  two  to  ten  longitudinally  arranged 
striated  muscle-fibers  of  fine  diameter.  In  the  middle  or  equatorial  region  of 
these  intra-fiisal  fibers  there  is  frequently  found  a  quantity  of  non-striated 
protoplasmic  matter.  The  spindle  is  supplied  with  both  sensor  and  motor 
nerves.  The  sensor  fiber  loses  its  external  investments  as  it  approaches  the 
capsule.  The  naked  axis-cylinder  then  penetrates  the  capsule,  and  after 
dividing  several  times  terminates  in  a  ribbon-like  or  spiral  manner  around  the 
intra-fusal  muscle-fiber.  This  ending  was  described  by  and  is  known  as 
Ruffini's  "  annulo-spiral  ribbon."  The  motor  nerve  also  penetrates  the 
capsule  and  terminates  in  the  polar  extremities  of  the  intra-fusal  fiber. 
Sensor  end-organs  supposed  to  be  connected  with  the  muscle  sense  are  also 
found  in  the  tendons  of  muscles. 

Afferent  Nerves. — That  muscles  are  abundantly  supplied  with  afferent 
nerves  has  been  proved  by  different  methods  of  investigation.  With  histo- 
logic methods  Sherrington  has  traced  afferent  fibers  from  the  muscle  spindles 
directly  into  the  spinal  nerve  ganglia.  The  contractions  of  muscles  from 
electric  stimulation  as  well  as  the  contractions  known  as  muscle  cramp, 
due  to  unknown  agents,  give  rise  to  sensations  of  pain,  a  fact  which  in- 
dicates the  presence  in  muscles  of  afferent  or  sensor  nerves. 

Cortical  Area. — Pathologic  findings  have  shown  that  an  impairment 
or  a  loss  of  the  muscle  sense  is  associated  with  destructive  lesions  of  perhaps 
the  super-  and  sub-parietal  convolutions  (Figs.  249  and  252).  In  a  case  re- 
ported by  Starr  the  removal  of  a  small  tumor  in  the  pia  mater  situated 
over  the  junction  of  the  superior  and  inferior  parietal  lobules  was  followed 
by  a  loss  of  the  muscle  sense  and  marked  ataxia  in  the  right  hand  for  a 
period  of  six  weeks,  after  which  recovery  took  place.  These  symptoms 
were  attributed  to  injury  of  the  cortex  from  unavoidable  surgical  procedures. 

The  muscle  sensations,  as  stated  in  foregoing  paragraphs,  form  the 
basis  of  the  perception  not  only  of  the  direction  and  the  duration  of  a  body 
movement  and  the  resistance  experienced,  but  also  of  the  position  and  the 
tension  of  the  muscle  groups.  The  latter  fact  more  especially  makes  it 
possible  for  the  mind  to  direct  the  muscles  and  to  graduate  the  energy 
necessary  to  the  accomplishment  of  a  definite  purpose. 

Active  Touch. — Active  touch  or  the  application  of  the  fingers  to  the 
surfaces  of  external  objects  implies  the  cooperation  of  the  skin  and  the 
muscles.  The  sensations  which  are  evoked  are  combinations  of  contact 
and  muscle  sensations.  The  union  of  these  sensations  forms  the  basis  of 
the  perception  of  hardness,  softness,  smoothness,  and  roughness  of  bodies. 

THE  SENSE  OF  TASTE 

The  physiologic  mechanism  involved  in  the  sense  of  taste  includes  the 
tongue,  the  gustatory  nerves,  their  cortical  connections  and  nerve-cells  in  the 
gray  matter  of  the  sub-collateral  convolution.  The  peripheral  excitation  of 
this  apparatus  gives  rise  to  nerve  impulses  which  transmitted  to  the  brain 
evoke  the  sensations  of  taste.  The  specific  physiologic  stimulus  is  matter, 
organic  and  inorganic,  in  a  state  of  solution. 

The  nerves  of  taste  contained  in  the  trunks  of  the  chorda  tympani  (page 
630)  and  of  the  glosso-pharyngeal  (page  634)  after  entering  the  medulla 
oblongata  terminate  around  certain  sensor  end  nuclei,  the  exact  location  of 


676 


TEXT-BOOK  OF  PHYSIOLOGY 


which  is  uncertain.  From  these  nuclei  axons  arise  which  in  all  probability 
cross  the  median  plane  and  ascend  to  the  sub-collateral  convolution.  The 
exact  course  of  this  gustatory  tract  is,  however,  obscure. 

The  Tongue. — The  tongue  consists  of  both  intrinsic  and  extrinsic  mus- 
cles, in  virtue  of  which  it  is  susceptible  of  a  change  both  in  shape  and  in 
position.  The  movements  of  the  tongue,  though  not  essential  to  taste,  are 
made  use  of  in  the  finer  discrimination  of  tastes. 

The  tongue  is  covered  over  by  mucous  membrane  continuous  with  that 
lining  the  oral  cavity.  The  dorsum  of  the  tongue  presents  a  series  of  papillae 
richly  supplied  with  blood-vessels  and  nerves.  Of  these  there  are  three 
varieties,  the  filiform,  the  fungiform,  and  the  circumvallate. 

1.  The  filiform  papillae,  the  most  numerous,  cover  the  anterior  two-thirds  of 

the  tongue;  they  are  conical  or  filiform  in  shape  and  covered  with 
horny    epithelium    which    is   often   prolonged    into    filamentous  tufts. 

2.  The  Jungijorm  papillce,  found  chiefly  at  the  tip  and  sides  of  the  tongue. 

are  less  numerous  but  larger  than  the  preceding  and  of  a  deep  red 
color. 

3.  The  circumvallate  papillce,   from   eight  to  ten  in  number,  are  situated^ 

at  the  base  of  the  tongue  arranged  in  the  form  of  the  letter  V.     They 
consist  of  a  central  projection  surrounded  by  a  wall  or  circumvallation 
from  which  they  take  their  name. 
The  Peripheral  End-organs.    The  Tastebuds. — Embedded  in  the 
epithelium  covering  the  mucous  membrane  not  only  of  the  tongue  but  of 
the  palate  and  posterior  surface  of  the  epiglottis  are  small 
ovoid  bodies  which  from  their  relation  to  the  gustatory 
nerves  are  regarded  as  their  peripheral  end-organs  and 
known  as  taste-buds  or  taste-beakers.     Each  bud  is  ovoid 
in  shape  (Fig.  285).     Its  base  rests  on  the  tunica  propria; 
its  apex  comes  up  to  the  epithelium,  where  it  presents 
a   narrow    funnel-shaped  opening,  the  taste-pore.     The 
wall  of  the  bud  is  composed  of  elongated  curved  epithe- 
lium.     The    interior    contains   narrow   spindle-shaped 
neuro-epithelial  cells  provided  at  their  outer  extremity  with 
stiff  hair-like  filaments  which  project  into  the  taste-pore. 
The  neuro-epithelial  cells  are  in  physiologic  relation 
with  the  nerves  of  taste.     The  terminal  branches,  after 
entering  the  bud  at  its  base,  develop  fine  tufts  which 
come  into  contact  with  the  cells.     That  the  taste-buds 
are  connected  with  the  nerves  of  taste  is  rendered  prob- 
able from  the  fact  of  their  degeneration  after  division  of 
the  nerves. 

The  Taste  Area. — The  taste  area,  though  confined 
for  the  most  part  to  the  tongue,  extends  in  different  in- 
dividuals to  the  mucous  membrane  of  the  hard  palate, 
to  the  anterior  surface  of  the  soft  palate,  to  the  uvula, 
the  anterior  and  posterior  half  arches,  the  tonsils,  the 
posterior  wall  of  the  pharynx,  and  the  epiglottis. 

The  Taste  Sensations. — The  sensations  which  arise  in  consequence  of 
impressions  made  by  different  substances  on  the  peripheral  apparatus  of 
this  area  are  in  so  many  instances  combinations  of  taste,  touch,  temperature, 


Fig.  285. — Taste- 
bud  FROM  Circum- 
vallate Papilla  of 
A  Child.  The  oval 
structure  is  limited  to 
the  epithelium  (e) 
lining  the  furrow, 
encroaching  slightly 
upon  the  adjacent 
connective  tissue  (/); 
o,  taste-pore  through 
which  the  taste-cells 
communicate  with 
the  mucors  surface. 
— (After   Piersol.) 


THE  SENSE  OF  SMELL  677 

and  smell  that  they  are  extremely  difficult  of  classification.  Nevertheless 
six  primary  tastes  can  be  recognized:  bitter,  sweet,  acid  or  sour,  salt  or  saline, 
alkaline  and  metallic.  Though  the  contact  of  any  bitter,  sweet,  acid,  salt, 
etc.,  substance  with  any  part  of  the  tongue  will,  if  the  substance  be  present 
in  sufficient  quantity  or  concentration,  develop  a  corresponding  sensation, 
some  regions  of  the  tongue  are  more  sensitive  and  responsive  than  others. 
Thus,  the  posterior  portion  is  more  sensitive  to  bitter  substances  than  the 
anterior;  the  reverse  is  true  for  sweet  substances  and  perhaps  for  acids  and 
salines. 

The  intensity  of  the  resulting  sensation  in  any  given  instance  will  depend 
on  the  degree  of  concentration  of  the  substance,  while  its  massiveness  will 
depend  on  the  area  affected. 

THE  SENSE  OF  SMELL 

The  physiologic  mechanism  involved  in  the  sense  of  smell  includes  the 
nasal  fossae,  the  olfactory  nerves,  the  olfactory  tracts,  and  nerve-cells  in  those 
areas  of  the  cortex  known  as  the  uncinate  convolution  and  anterior  part  of 
the  gyrus  fornicatus.  Peripheral  stimulation  of  this  mechanism  develops 
nerve  impulses  which,  transmitted  to  the  cortex,  evoke  the  sensations  of  odor. 
The  specific  physiologic  stimulus  is  matter  in  the  gaseous  or  vaporous  state. 

The  Nasal  Fossae. — The  nasal  fossae  are  irregularly  shaped  cavities 
separated  by  a  vertical  septum  formed  by  the  perpendicular  plate  of  the 
ethmoid  bone,  the  vomer,  and  the  triangular  cartilage.  The  outer  wall 
presents  three  recesses  separated  by  the  projection  inward  of  the  turbinated 
bones.  Each  fossa  opens  anteriorly  and  posteriorly  by  the  anterior  and 
posterior  nares,  the  latter  communicating  with  the  pharynx.  Both  fossae  are 
lined  throughout  by  mucous  membrane.  The  upper  part  of  the  fossa  is 
known  as  the  olfactory,  the  lower  portion  as  the  respiratory  region.  In  the 
former,  the  mucous  membrane  over  the  septum  and  superior  turbinated 
bone  is  somewhat  thicker  than  elsewhere  and  covered  with  a  neuro-epithelium 
which  constitutes 

The  Peripheral  End-organ. — This  consists  of  a  basement  membrane 
supporting  two  kinds  of  cells,  the  olfactory  and  the  sustentacular.  The 
olfactory  cells  are  bipolar  nerve-cells,  the  center  of  which  contains  a  large 
spheric  nucleus.  The  peripheral  pole  is  cylindric  or  conic  in  shape  and 
provided  at  its  extremity  with  several  hair-like  processes.  The  central 
pole  becomes  the  axon  process  and  passes  directly  to  the  olfactory  bulb. 

The  sustentacular  cells  are  epithelial  in  character  and,  as  their  name 
implies,  support  or  sustain  the  olfactory  cells. 

For  the  appreciation  of  odorous  particles  the  air  must  be  drawn  through 
the  nasal  fossae  with  a  certain  degree  of  velocity.  If  the  particles  are  widely 
diffused  in  the  air,  they  must  be  drawn  not  only  more  quickly  but  more 
forcibly  into  contact  with  the  olfactory  hairs,  as  in  the  act  of  sniffing,  the 
result  of  short  energetic  inspirations.  To  many  substances  the  olfactory 
apparatus  is  extremely  sensitive.  Thus,  it  has  been  shown  that  a  particle 
of  mercaptan  the  actual  weight  of  which  was  calculated  to  be  jyoToo^ro^ 
of  a  milligram  gives  rise  to  a  distinct  sensation. 

The  Olfactory  Sensations. — The  sensations  which  arise  in  consequence 
of  the  excitation  of  the  olfactory  apparatus  are  very  numerous  and  their 


678  TEXT-BOOK  OF  PHYSIOLOGY 

classification  is  extremely  difficult.  For  this  reason  it  is  customary  to  divide 
them  into  two  groups:  viz.,  agreeable  and  disagreeable,  in  accordance  with 
the  feelings  they  excite  in  the  individual.  As  the  olfactory  sensations  give 
rise  to  feelings  rather  than  ideas,  this  sense  plays  in  man  a  subordinate  part 
in  the  acquisition  of  knowledge.  In  lower  animals  this  sense  is  employed 
for  the  purpose  of  discovering  and  securing  food,  for  detecting  enemies  and 
friends,  and  for  sexual  purposes.  In  land  animals  the  entire  olfactory  appa- 
ratus is  well  developed  and  the  sense  keen;  in  some  aquatic  animals,  as  the 
dolphin,  whale,  and  seal,  the  apparatus  is  poorly  developed  and  the  sense 
dull. 


CHAPTER  XXIX 
THE  SENSE  OF  SIGHT 

The  physiologic  mechanism  involved  in  the  sense  of  sight  includes  the 
eyeball,  the  optic  nerve,  the  optic  tracts,  the  thalamo-occipital  tract  or  the 
optic  radiation,  and  nerve-cells  in  the  cuneus  and  adjacent  gray  matter. 
Peripheral  stimulation  of  this  mechanism  develops  nerve  impulses  which 
transmitted  to  the  cortex  evoke  (i)  the  sensation  of  light  and  its  different 
quaHties — colors;  (2)  the  perception  of  light  and  color  under  the  form  of 
pictures  of  external  objects;  and  (3)  in  connection  with  the  ocular  muscles, 
the  production  of  muscle  sensations  by  which  the  size,  distance,  and  direc- 
tion of  objects  may  be  judged. 

The  specific  physiologic  stimulus  to  the  terminal  end-organ,  the  retina, 
is  the  impact  of  ether  vibrations.  In  general,  it  may  be  said  that,  at  least 
for  the  same  color,  the  intensity  of  the  objective  vibration  determines  the 
intensity  of  the  sensation. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EYEBALL 

The  eyeball  is  situated  at  the  fore  part  of  the  orbit  cavity,  and  in  such  a 
position  as  to  permit  of  an  extensive  range  of  vision.  It  is  loosely  held  in 
position  by  a  fibrous  membrane,  the  capsule  of  Tenon,  which  is  attached, 
on  the  one  hand,  to  the  eyeball  itself,  and,  on  the  other,  to  the  walls  of  the 
orbit  cavity.  Thus  suspended,  the  eyeball  is  susceptible  of  being  turned  in 
any  direction  by  the  contraction  of  the  muscles  attached  to  it. 

The  ball  is  spheroid  in  shape,  measuring  about  24  millimeters  in  its 
antero-posterior  diameter  and  a  little  less  in  its  transverse  and  vertical 
diameters.  When  viewed  in  profile,  it  is  seen  to  consist  of  the  segments  of 
two  spheres,  of  which  the  posterior  is  the  larger,  occupying  five-sixths,  and 
the  anterior  is  the  smaller,  occupying  one-sixth  of  the  ball. 

If  a  horizontal  section  of  the  eyeball  is  made  it  will  be  seen  to  consist 
of  several  concentrically  arranged  membranes  enclosing  various  refracting 
media  essential  to  vision.     (See  Fig.  286.) 

The  membranes,  enumerating  them  from  without  inward,  are  as  follows: 
the  sclera  and  cornea,  the  chorioid  and  iris,  and  the  retina.  The  refracting 
media  are  the  aqueous  humor,  the  crystalline  lens,  and  the  vitreous  humor. 

The  Sclera  and  Cornea. — The  sclera  is  the  thick  opaque  membrane 
covering  the  posterior  five-sixths  of  the  ball.  It  is  composed  of  layers  of 
connective  tissue  which  are  arranged  transversely  and  longitudinally.  It 
is  pierced  posteriorly  by  the  optic  nerve  about  3  or  4  millimeters  internal  to 
the  optic  axis.  By  virtue  of  its  firmness  and  density  the  sclera  gives  form  to 
the  eyeball,  protects  delicate  structures  enclosed  by  it,  and  serves  for  the 
attachment  of  the  muscles  by  which  the  ball  is  moved.  The  cornea  is  the 
transparent  membrane  forming  the  anterior  one-sixth  of  the  ball  It  is  nearly 
circular  in  shape,  measuring  in  its  horizontal  meridian  12  mm.,  in  its  vertical 
meridian  11  mm.     The  curvature  is,  therefore,  sharper  in  the  latter  than  in 

679 


68o 


TEXT-BOOK  OF  PHYSOLOGY 


the  former.  The  radius  of  curvature  of  the  anterior  surface  at  that  central 
portion  ordinarily  used  in  vision  is  7.829  mm.;  that  of  the  posterior  surface 
about  6  mm. 

The  substance  of  the  cornea  is  made  up  of  thin  layers  of  delicate  trans- 
parent fibrils  of  connective  tissue  continuous  with  those  found  in  the  sclera. 
Lymph-spaces  are  present  throughout  the  cornea,  in  which  are  to  be  found 
lymph-corpuscles.  The  anterior  surface  of  the  cornea  is  covered  with  several 
layers  of  nucleated  epithelium  supported  by  a  structureless  membrane,  the 
anterior  elastic  lamina.  The  posterior  surface  also  is  covered  by  a  layer  of 
epithelium  supported  by  a  similar  membrane,  the  posterior  elastic  lamina  or 
the  membrane  of  Descemet,  which  at  its  periphery  becomes  continuous 


Visual  axis        Optic  axis 


Suspejtsori/  iiff. 


Sclera--- 
(^tic  ?terve 


""  Be  a  no. 
C/iorioid 


Tnacula  luiea 


Fig.  286. — The  Right  Eye  viewed  in  Horizontal  in  Section. — (After  Toldl.) 

with  the  iris.  At  the  junction  of  the  cornea  and  sclera  there  is  a  circular 
groove,  known  as  the  canal  of  Schlemm. 

The  posterior  elastic  lamina,  near  the  margin  of  the  cornea,  breaks  up 
into  fibers  to  form  a  network  structure,  the  intervals  between  the  fibers  of 
which  are  known  as  the  spaces  of  Fontana.  These  spaces  are  in  communi- 
cation with  the  canal  of  Schlemm. 

The  Chorioid,  Iris,  Ciliary  Muscle,  and  Ciliary  Processes.  The 
Chorioid. — The  chorioid  is  the  dark  brown  membrane  which  extends  forward 
nearly  to  the  cornea,  where  it  terminates  in  a  series  of  folds,  the  ciliary 
processes.  Posteriorly,  it  is  pierced  by  the  optic  nerve.  It  is  composed 
largely  of  blood-vessels,  arteries,  capillaries,  and  veins,  supported  by  con- 
nective tissue..-  Externally  it  is  loosely  connected  to  the  sclera;  internally  it 
is  lined  by  a  layer  of  hexagonal  cells  containing  black  pigment  which 


THE  SENSE  OF  SIGHT 


68i 


though  usually  described  as  a  part  of  the  chorioid,  are  now  known  to  belong, 
embryologically  and  physiologically,  to  the  retina.  Lying  within  the  outer 
layer  of  arteries  and  veins  there  is  a  thick  layer  of  small  arterioles  and 
capillaries,  known  as  the  chorio-capillaris.  The  chorioid  with  its  con- 
tained blood-vessels  bears  an  important  relation  to  the  nutrition  and  function 
of  the  eye.  It  provides  a  free  supply  of  lymph  and  presents  a  uniform  tem- 
perature to  the  retina  in  contact  with  it. 

The  Iris. — The  iris  is  the  circular,  variously  colored  membrane  in  the 
anterior  part  of  the  eye  just  behind  the  cornea.  It  presents  a  little  to  the 
nasal  side  of  the  center  a  circular  opening,  the  pupil.  The  outer  or  circum- 
ferential border  is  united  by  connective  tissue  to  the  cornea,  sclera,  and  ciliary 
muscle;  the  inner  border  forms  the  boundary  of  the  pupil.  The  iris  consists 
of  a  framework  of  connective  tissue  supporting  blood-vessels,  muscle-fibers, 
and  pigmented  connective-tissue  cells.  The  anterior  surface  is  covered  by  a 
layer  of  cells  continuous  with  those  covering  the  posterior  surface  of  the  cor- 
nea. The  posterior  surface  is  formed  by  a  thin  structureless  membrane  sup- 
porting a  layer  of  pigment  cells  continuous  with  those  lining  the  chorioid. 
The  color  which  the  iris  presents  in  different  individuals  depends  on  the  rela- 


Fig.  287. — Section  through  the  Cilwry  Region  of  the  Human  Eye.  a.  Radiating 
bundles  of  the  ciliary  muscle,  b.  Deeper  bundles,  c.  Circular  network,  d.  Annular  muscle  of 
Miiller.  e.  Tendon  of  ciliary  muscle.  /.  Muscle-fibers  on  posterior  side  of  the  iris.  g.  Muscles 
on  the  ciliary  border  of  the  same.     h.  Ligamentum  pectinatum. — {After  Iwanoff.) 

tive  amount  of  pigment  in  the  connective-tissue  corpuscles.  In  blue  eyes  the 
pigment  is  wanting.  In  gray,  brown,  and  black  eyes  the  pigment  is  present 
in  progressively  increasing  amounts.  The  blood-vessels  are  connected  with 
those  of  the  chorioid  coat. 

The  muscle-fibers  are  of  the  non-striated  variety  and  arranged  in  two  sets, 
one  circularly,  the  other  radially,  disposed. 

The  circular  fibers  are  found  close  to  the  pupil  near  the  posterior  surface 
of  the  iris.  Contraction  of  this  band  of  fibers  diminishes,  relaxation  increases, 
the  size  of  the  pupil.  This  muscle  is  known  as  the  sphincter  pupillcB  or 
sphincter  iridis. 

The  radial  fibers  form  a  more  or  less  continuous  layer  in  the  posterior 
part  of  the  iris,  extending  from  the  margin  of  the  pupil,  where  they  blend 
with  the  circular  fibers,  to  the  outer  border.  Contraction  of  the  fibers  in- 
creases the  size  of  the  pupil.     The  muscle  is  known  as  the  dilatator  pupillcs. 

The  nerves  exciting  the  sphincter  pupiUce  to  action  are  the  ciliary  nerves, 
axons  of  nerve-cells  located  in  the  ciliary  or  ophthalmic  ganglion.  Stimula- 
tion of  these  fibers  gives  rise  to  contraction  of  the  sphincter  and  diminution  in 
the  size  of  the  pupil.     The  nerves  exciting  the  dilatator  pupillos  to  action  are 


682 


TEXT-BOOK  OF  PHYSIOLOGY 


I.  Pigment-layer  (not  shown). 


2.  Layer  of  rods  and  cones. 


Outer  nuclear  layer. 


axons  of  nerve-cells  located  in  the  superior  cervical  ganglion.  They  reach 
the  iris  by  way  of  the  cervical  sympathetic,  the  ophthalmic  division  of  the 
fifth,  and  the  long  ciliary  nerve.  Stimulation  of  these  nerves  is  followed  by 
contraction  of  the  dilatator  and  an  increase  in  the  size  of  the  pupil.  Both 
the  ciliary  and  superior  cervical  ganglia  are  in  relation  with  pre-ganglionic 
fibers  coming  from  the  central  nerve  system  (see  page  653), 

The  Ciliary  Muscle. — The  ciliary  muscle  is  a  gray  circular  band  about  two 
millimeters  in  width,  consisting  of  non-striated  muscle-fibers.  The  majority 
of  these  fibers  pursue  a  radial  or  meridional  direction.  Taking  their  origin 
from  the  junction  of  the  sclera,  cornea,  and  iris,  they  pass  backward  to  be  in- 
serted into  the  chorioid  coat  opposite  the  ciliary  processes.  The  inner  por- 
tion of  the  muscle  is  interrupted  by  bundles  of  fibers  which  pursue  a  circular 
direction  (Fig.  287).  They  collectively  constitute  the  annular  or  ring 
muscle  of  Muller.  The  ciliary  muscle  in  common  with  the  circular  fibers 
of  the  iris  receives  its  nerve  supply  direct  from  the  nerve-cells  in  the  ciliary 

ganglion.  Contraction  of  the 
ciliary  muscle  tenses  the  cho- 
rioid coat,  and  for  this  reason 
it  is  frequently  termed  the  ten- 

3.  External  limiting  membrane.     SOr  cJlOrioideCB. 

The  Retina. — The  retina 
is  the  internal  coat  of  the  eye, 
extending  forward  almost  to 
the  ciliary  processes,  where  it 
terminates  in  an  indented  bor- 
der, known  as  the  or  a  serrata. 
In  the  living  condition  it  is 
clear,  transparent  and  pink 
in  color.  After  death  it  be- 
comes opaque.  The  retina  is 
abundantly  supplied  with 
blood-vessels,  derived  from 
the  arteria  centralis  retina,  a 
branch  of  the  ophthalmic, 
which  pierces  the  optic  nerve 
near  the  sclera,  runs  forward 
in  its  center,  to  the  retina,  in 
which  its  terminal  branches  are  distributed.  The  veins  arising  from  the 
capillary  plexus  leave  the  retina  by  the  same  route. 

In  the  posterior  portion  of  the  retina,  at  a  point  corresponding  with  the 
axis  of  vision,  there  is  a  small  oval  area  about  2  mm.  in  its  transverse  and 
about  0.8  mm.  in  its  vertical  diameter.  From  the  fact  that  it  presents  a 
yellow  appearance,  it  is  known  as  the  macula  lutea.  This  area  presents  in 
its  center  a  depression  with  sloping  sides,  known  as  the  fovea  centralis. 
About  3.5  mm.  to  the  nasal  side  of  the  macula  is  the  point  of  entrance  of  the 
optic  nerve. 

The  retina  is  remarkably  complex  in  structure,  presenting  an  appearance, 
when  viewed  microscopically,  something  like  that  represented  in  Fig.  288, 
indicating  that  it  is  composed  of  different  cellular  elements  arranged  in 
layers.     These  have  been  named,  from  behind  forward,  as  follows: 


Fig. 


*    5. 


Outer  molecular  layer. 
Inner  nuclear  layer. 

Inner  molecular  layer. 

Layer  of  ganglion  cells. 
Layer  of  nerve-fibers. 


1. — Vertical  Section  of  Human  Retina. 
— {S  diaper.) 


THE  SENSE  OF  SIGHT 


683 


1.  The  layer  of  pigment  cells. 

2.  The  layer  of  rods  and  cones,  or  Jacobson's  layer. 

3.  The  external  limiting  membrane. 

4.  The  outer  nuclear  or  granular  layer. 

5.  The  outer  molecular  or  reticular  layer. 

6.  The  inner  nuclear  or  granular  layer. 

7.  The  inner  molecular  or  reticular  layer. 

8.  The  layer  of  ganglion  cells. 

9.  The  layer  of  nerv^e-fibers. 

Modern  histologic  methods  of  research  have  made  it  possible  to  reduce  the 
.  retina,  exclusive  of  the  pigment  cells,  to  three  successive  layers  of  nerve-cells, 

supported  by  a  highly  developed  neuroglia,  forming  what  has  been  termed  the 
-  fibers  of  Miiller.     These  nerve-cells  are  as  follows: 

1.  The  visual  cells. 

2.  The  bipolar  cells. 

3.  The  ganglion  cells. 

The  relation  of  these  nerve-cells  one  to  another  and  to  the  supporting  neuro- 
glia tissue  and  the  manner  in  which  they  unite  to  form  the  above-mentioned 
layers  are  schem.atically  shown  in  Fig.  289. 

The  pigment-layer  is  composed  of 
hexagonal  cells.  Though  formerly 
described  as  forming  a  part  (the  inner 
layer)  of  the  chorioid,  these  cells  belong 
embryologically  to  the  retina.  From 
their  retinal  surface  delicate  pigmented 
processes  extend  into  and  between 
the  rods  and  cones.  On  exposure  to 
light  these  procesess  elongate  and 
push  themselves  between  the  rods. 
In  the  dark  they  retract  and  withdraw 
into  the  cell-body. 

The  visual  cells  which  form  the 
layer  of  rods  and  cones  are  of  two  varie- 
ties, the  rod  shaped  and  the  cone 
shaped. 

The  rod-shaped  visual  cell  con- 
sists of  a  straight  elongated  cylinder 
extending  through  the  entire  thickness 
of  Jacobson's  membrane  and  a  fine 
fiber  containing  a  nucleus,  which, 
after  piercing  the  •  external  limiting 
membrane,  passes  into  the  outer  mo- 
lecular layer,  where  it  terminates  in  a 
spheric  enlargement.  The  outer  por- 
tion of  the  rod  is  clear  and  homo- 
geneous, though  containing  a  pigment 
known  as  visual  purple  or  rhodopsin; 
the  inner  portion  of  the  rod  is  slightly  granular. 

The  cone-shaped  visual  cells  also  consist  of  two  portions,  a  conic  portion 
situated  in  Jacobson's  membrane  between  the  rods,  and  a  fine  fiber,  contain- 


FiG.  289. — Cross-section  of  the  Retina 
FROM  A  Maximal.  A.  Layer  of  rods  and 
cones.  B.  Visual  cells  (outer  granules). 
C.  Outer  molecular  layer.  E.  Bipolar  cells 
(inner  granules).  F.  Inner  molecular  laver. 
G.  Ganglion  cells.  H.  Layer  of  nerve- 
fibers,  a.  Rods.  b.  Cones.  e.  Bipolar 
rod.  f.  Bipolar  cone.  r.  Lower  ramifica- 
tion of  a  bipolar  rod.  f.  Lower  ramification 
of  a  bipolar  cone,  g,  h,  i,  j,  k.  Ganglion 
cells  in  various  stages,  branching  from  F. 
X,  z.  Bipolar  contact  of  rods  and  cones,  t. 
Muller's  supporting  fibers.  S.  Centrifugal 
nerve-fibers. — {After  Ramon  y  Cajal.) 


684 


TEXT-BOOK  OF  PHYSIOLOGY 


ing  a  nucleus,  which,  after  piercing  the  external  limiting  membrane,  passes 
into  the  outer  molecular  layer,  where  it  terminates  in  a  fine  tuft.  The 
inner  portion  of  the  cone  is  thicker  than  the  rod  and  rests  on  the  limiting 
membrane;  the  outer  portion  tapers  to  a  fine  point  and  is  known  as  the 
cone  style.  The  cones,  as  a  rule,  are  shorter  than  the  rods.  The  proportion 
of  rods  to  cones  varies  in  different  parts  of  the  retina,  though  there  are  on 
the  average  about  fourteen  rods  to  one  cone.  In  the  macula  the  rods  are 
entirely  absent,  cones  alone  being  present. 

The  layer  of  visual  cells  together  with  the  neuroglia  constitutes  the  first 
of  the  three  layers  of  the  retina  proper.  The  external  limiting  membrane  is 
formed  by  the  bending  of  the  ends  of  neuroglia  cells. 

The  bipolar  cells  consist  of  a  central  portion,  found  in  the  inner  nuclear 
layer,  from  which  are  given  off  two  processes  which  pass  in  opposite  direc- 
tions, one  toward  the  visual  cells,  the  other  toward  the  ganglion  cells.     The 


Fig.  2QO. — Horizontal  Section  through  the  Macula  and  Fovea  of  a  Man  Sixty  Years 
Old.  The  section  is  not  through  the  exact  center  of  the  fovea,  for  there  are  only  cone  visual  cells 
and  no  remnants  of  the  confluence  of  the  inner  granule  and  ganglion  cell  layers  are  present,  i. 
Cones.  2.  External  hmiting  membrane.  3.  Outer  nuclear  layer.  4.  Henle's  fiber  layer.  5. 
Outer  molecular  or  reticular  layer.  6.  Inner  nuclear  layer.  7.  Inner  molecular  or  reticular 
layer.     8.  Layer  of  ganglion  cells.     9.  Nerve-fiber  layer. — {After  Schaper,St6hr's  "Histology") 

former  terminate  in  tufts  which  arborize  around  the  tufts  and  spheric  en- 
largements of  the  visual  cells,  and  assist  in  the  formation  of  the  outer  molec- 
ular layer;  the  latter  terminate  in  similar  tufts  in  the  inner  molecular 
layer. 

The  ganglion  cells  are  arranged  in  a  single  layer,  as  a  rule.  They 
are  large  and  nucleated.  From  the  inner  side  of  each  cell  there  is  given 
off  a  single  axon  which  passes  toward  the  center  of  the  retina  (forming  the 
nerve-fiber  layer),  where  it  enters  and  assists  in  forming  the  optic  nerve. 
From  the  outer  side  of  the  ganglion  cell  dendrites  pass  into  and  assist  in 
forming  the  inner  molecular  layer.  These  dendrites  come  into  physiologic 
relation  with  those  of  the  inner  processes  of  the  bipolar  cells. 

Horizontally  disposed  nerve-cells  are  also  present  in  the  outer  molecular 
layer  in  relation  with  the  visual  cells.  Spongioblasts  or  amacrine  cells  are 
also  present  at  the  border  of  and  in  the  inner  molecular  layer. 


THE  SENSE  OF  SIGHT  685 

From  the  relation  of  the  ganglion  cells,  in  which  the  optic  nerve-fibers 
take  their  origin,  to  the  visual  cells  and  the  bipolar  cells,  the  former  may 
be  regarded  as  the  terminal  visual  organ,  the  intermediary  between  the 
ether  vibrations  and  the  ganglion  cell.  The  visual  cells  are  directed  toward 
the  chorioid,  away  from  the  entering  light,  dipping  into  the  pigment  cells. 
They,  with  the  pigment -layer,  are  the  elements  by  which  the  ether  vibrations 
are  transformed  into  nerve  energy. 

In  the  fovea  most  of  the  retinal  elements  are  wanting  or  are  reduced  in 
thickness.  The  cones  alone  are  present.  The  cone-fibers  with  their  nuclei 
are  directed  obliquely  upward  and  outward  along  the  slope  of  the  fovea, 
to  end  in  tufts  which  come  into  physiologic  relation  with  the  dendrites  of 
the  ganglion  cells,  which  at  the  top  of  the  fovea  are  generally  increased  in 
number  (Fig.  290). 

It  is  estimated  that  the  optic  nerve  contains  about  500,000  nerve-fibers, 
and  that  for  each  fiber  there  are  about  7  cones,  100  rods,  and  7  pigment 
cells.  In  accordance  with  this  estimate  there  would  be  about  3,500,000 
cones,  50,000,000  rods,  and  3,500,000  pigment  cells.  The  distance  be- 
tween the  centers  of  two  adjacent  cones  in  the  fovea  is  4  micromillimeters. 

The  Refracting  Media. — The  refracting  media  enclosed  by  the  fore- 
going membranes  are  the  aqueous  humor,  the  lens  and  the  vitreous  humor. 

The  Aqueous  Humor. — The  aqueous  humor  is  small  in  amount  in  compari- 
son with  the  \itreous  and  is  found  in  the  space  bounded  by  the  cornea,  the 
ciliary  body,  the  suspensory  ligament,  and  the  lens.  The  projection  of  the 
iris  into  this  space  partially  divides  it  into  an  anterior  and  a  posterior  portion 
or  chamber.  The  aqueous  humor  is  a  clear,  watery,  alkaline  fluid  derived 
from  or  secreted  by  the  capillary  blood-vessels  of  the  ciliary  body.  From 
this  origin  it  passes  through  the  pupil  into  the  anterior  chamber.  It  serves 
to  keep  the  cornea  tense  and  smooth.  The  ocular  tension  depends  partly 
on  the  presence  of  this  fluid  in  the  eyeball.  There  is  every  reason  for  believ- 
ing that  there  is  a  constant  stream  of  fluid  from  the  blood-vessels  into  the 
eye  and  from  the  eye  through  the  spaces  of  Fontana  at  the  base  of  the  iris 
into  the  canal  of  Schlemm,  and  so  into  the  blood.  Any  interference  with  the 
exit  of  this  fluid  rapidly  increases  the  intra-ocular  tension.  Inasmuch  as 
the  aqueous  humor  has  the  same  refracting  power  as  the  cornea  the  two  may 
be  regarded  as  a  single  body. 

The  Lens. — The  lens  is  the  transparent  biconvex  body  situated  just  behind 
the  iris,  in  the  concavity  of  the  vitreous.  The  thickness  of  the  lens  is  3.6 
mm.,  the  diameter  about  9  mm.  It  consists  of  a  transparent  capsule  contain- 
ing elongated  hexagonal  fibers  which,  having  their  origin  near  the  anterior 
central  portion  of  the  lens,  pass  out  toward  the  margin,  where  they  bend 
around  to  terminate  in  a  triradiate  figure  on  the  opposite  side.  Chemically 
the  lens  consists  of  water,  a  globulin  body  (crystallin) ,  and  salts. 

The  lens  is  held  in  position  by  the  suspensory  ligament,  formed  in  part 
by  the. hyaloid  membrane  and  in  part  by  fibers  derived  from  the  ciliary  pro- 
cesses. The  former  becomes  attached  to  the  posterior  surface,  the  latter  to 
the  anterior  surface  of  the  lens  near  the  equator.  The  space  between  the 
two  layers  of  the  ligament  is  the  canal  of  Petit.  The  anterior  surface  of  the 
ligament  presents  a  series  of  plications  conforming  to  corresponding  plica- 
tions on  the  surface  of  the  ciliary  processes. 

The  Vitreous  Humor. — The  vitreous  humor  is  the  largest  of  the  refracting 


686  TEXT-BOOK  OF  PHYSIOLOGY 

media  and  occupies  by  far  the  largest  portion  of  the  interior  of  the  eyeball. 
From  its  ])osition  it  gives  support  to  the  retina.  Anteriorly  it  ])resents  a  con- 
cavity, in  which  the  crystalline  lens  is  lodged.  The  vitreous  humor  consists 
of  water  (97  per  cent.),  organic  matter  and  salts,  enclosed  in  a  transparent 
membrane,  the  tunica  hyaloidea.  The  mass  of  the  vitreous  humor  is  pene- 
trated by  a  species  of  connective  tissue. 

These  bodies,  the  cornea  and  ac[ueous  humor,  the  lens  and  the  vitreous 
humor,  together  form  a  refracting  system  by  which  jmrallel  rays  of  light, 
which  enter  the  eye  when  it  is  in  repose,  are  converged  and  brought  to  a  focus 
on  the  retina. 

The  relations  of  the  parts  entering  into  the  structure  of  the  eye  are  shown 
in  Fig.  286. 

THE  PHYSIOLOGY  OF  VISION 

The  Retinal  Image. — The  general  function  of  the  eye  is  the  formation 
of  images  of  external  objects  on  the  free  ends  of  the  percipient  elements  of 
the  retina,  the  rods  and  cones.  The  existence  of  an  image  on  the  retina 
can  be  readily  seen  in  the  excised  eye  of  an  albino  rabbit,  when  placed 
between  a  lighted  candle  and  the  eye  of  an  observer.  Its  presence  in  the 
human  eye  can  be  demonstrated  with  the  ophthalmoscope.  It  is  this  image, 
composed  of  focal  points  of  luminous  rays,  that  stimulate  the  rods  and 
cones,  which  is  the  basis  of  our  sight  perceptions,  and  out  of  which  the  mind 
constructs  space  relations  of  external  objects.  In  only  two  essential  re- 
spects as  far  as  space  relations  go,  does  the  image  on  the  retina  differ  from 
the  appearance  of  the  object,  aside  from  the  fact  that  the  object  has  usually 
three,  the  image  only  two,  dimensions — viz.,  in  size  and  position.  What- 
ever the  distance,  the  image  is  generally  smaller  than  the  object;  it  is  also 
reversed,  the  upper  part  of  the  object  becoming  the  lower  part  of  the  image, 
and  the  right  side  of  the  object  the  left  side  of  the  image. 

The  Dioptric  or  Refracting  Apparatus. — The  formation  of  an  image  is 
made  possible  by  the  introduction  of  a  complex  refracting  apparatus  consist- 
ing of  the  cornea,  aqueous  humor,  lens,  and  vitreous  humor.  Without  these 
agencies  the  ether  vibrations  would  give  rise  only  to  a  sensation  of  diffused 
luminosity.  Rays  of  light  emanating  from  any  one  point — that  is,  homo- 
centric  rays — arriving  at  the  eye  must  traverse  successively  the  different  re- 
fracting media.  In  their  passage  from  one  to  the  other,  they  undergo  at  the 
surfaces  changes  in  direction  before  they  are  finally  converged  to  a  focal  point. 
In  order  to  follow  mathematically  the  rays  in  all  their  deviations  through  the 
media,  to  determine  their  focal  points  and  to  construct  an  image,  a  knowl- 
edge of  the  form  of  the  refracting  surfaces,  the  refractive  indices  of  the  dif- 
ferent media,  and  the  distance  of  the  surfaces  from  one  another  must 
be  known. 

The  following  constants  are  now  accepted:  The  radius  of  curvature 
of  that  portion  of  each  refracting  surface  used  for  distinct  vision  is  for  the 
cornea  7.829  mm.,  for  the  anterior  and  posterior  surfaces  of  the  lens  10  and 
6  mm.,  respectively.  The  indices  of  refraction  of  the  different  media  are 
as  follows:  cornea  and  aqueous  humor,  1.3365;  lens,  1.4371;  vitreous  body, 
1.3365.  The  distance  from  the  vertex  of  the  cornea  to  the  lens  is  3.6  mm.; 
the  thickness  of  the  lens,  3.6  mm.;  the  distance  from  the  posterior  surface 
of  the  lens  to  the  retina,  15  mm.  As  the  two  surfaces  of  the  cornea  are  prac- 


THE  SENSE  OF  SIGHT  687 

tically  parallel,  and  as  the  index  of  refraction  of  the  aqueous  humor  is  the 
same  as  that  of  the  cornea,  they  may  be  regarded  as  but  one  medium.  The 
refracting  surfaces  may,  therefore,  be  reduced  to  the  anterior  surface  of  the 
cornea,  the  anterior  surface  of  the  lens,  and  the  posterior  surface  of  the 
lens.^ 

Parallel  rays  of  light  entering  the  eye  pass  from  air,  with  an  index  of  re- 
fraction of  1.00025,  into  the  cornea,  with  an  index  of  refraction  of  1.3365. 
In  passing  from  the  rarer  into  the  denser  medium  they  undergo  refraction 
in  accordance  with  the  laws  of  optics  and  are  rendered  somewhat  convergent. 
The  extent  of  this  first  refraction 
and  convergence  is  sufficiently  |^ 
great  to  bring  parallel  rays,  if  con- 
tinued, to  a  focus  about  10  mm. 
behind  the  retina.  This  would  be 
the  condition  in  aphakia  whether 
the  lens  is  congenitally  absent  or 
has  been  removed  by  surgical  pro- 
cedures.    Perfect  vision,  however, 

reniiirpc;    tVtat    tlip  ronvfrfrpnrp  nf        ^^^-     291.— Refraction     of     Homocentric 
requires    tnat    tne  convergence  01  ^^^^s  and  the  Formation  of  an  Image. 

the  light  must  be  great  enough  to 

bring  the  focal  point,  the  image,  on  the  retina.  This  is  accomplished  by 
the  introduction  of  an  additional  refracting  body,  the  lens.  On  entering 
the  lens  the  rays  are  for  the  same  reason — i.e.,  the  passage  from  a  rarer  into 
a  denser  medium — again  refracted  and  converged,  and  if  continued  would 
come  to  a  focus  about  6.5  mm.  behind  the  retina.  On  passing  from  the  lens 
into  the  vitreous — i.e.,  from  a  denser  into  a  rarer  medium — the  rays  are 
once  more  converged  and  to  an  extent  sufficient  to  focalize  them  on  the 
retina  (Fig.  291). 

While  it  is  thus  possible  to  follow  the  rays  geometrically  through  these 
media  by  means  of  the  above-mentioned  factors,  the  procedure  is  attended 
with  many  difficulties.  Moreover,  as  the  relations  all  change  when  rays 
enter  the  eye  from  objects  situated  progressively  nearer  the  eye,  a  separate 
calculation  is  necessitated  for  each  distance  for  the  determination  of  the 
size  of  the  image. 

A  method  by  which  these  difficulties  are  much  reduced  was  suggested 
by  Gauss  and  developed  by  Listing.  It  was  demonstrated  by  Gauss  that 
in  every  complicated  system  of  refracting  media  separated  by  centered 
spheric  surfaces  there  may  be  assumed  certain  ideal  or  cardinal  points,  to 
which  the  system,  may  be  reduced,  and  which,  if  their  relative  position  and 
properties  be  known,  permit  of  the  determination,  either  by  calculation 
or  geometric  construction,  of  the  path  of  the  refracted  ray,  and  the  position 
and  size  of  the  image  in  the  last  medium,  if  those  of  the  object  in  the  first 
medium  be  known. 

Every  dioptric  system  can  be  replaced,  as  Gauss  showed,  by  a  single 
system  composed  of  six  cardinal  points  and  six  planes  perpendicular  to  the 

'  Strictly  speaking,  the  posterior  surface  of  the  cornea  is  not  parallel  to  the  anterior  surface, 
and  the  index  of  refraction  of  the  cornea  is  a  trifle  greater  than  that  of  the  aqueous  humor,  viz.: 
1.377.  But  as  the  increase  in  the  corneal  refraction  due  to  the  higher  index  is  almost  exactly 
counteracted  by  a  decrease  in  refraction  due  to  the  higher  curvature  of  the  posterior  corneal 
surface,  the  usual  assumptions  furnish  quite  accurate  results. 


688 


TEXT-BOOK  OF  PHYSIOLOGY 


common  axis — e.g.,  two  focal  points,  two  principle  points,  two  nodal  points, 
two  focal  planes,  two  principal  planes,  and  two  nodal  planes. 

Properties  of  the  Cardinal  Points. — The  first  focal  point,  jp,,  in  Fig. 
292,  has  the  property  that  every  ray  which  before  refraction  passes  through 
it,  after  refraction  is  parallel  to  the  axis. 

The  second  focal  point,  F^,  has  the  property  that  every  ray  which  before 
refraction  is  parallel  to  the  axis,  passes  after  refraction  through  it. 

The  second  principal  point,  H^,  is  the  image  of  the  first,  H^;  that  is, 
rays  in  the  first  medium  which  go  through  the  first  principal  point  pass  after 


^ 


jr, 


firA    1^ 


^ 


J5 


Fig.  292. — Diagram  showing  the  Positiox  and  Relation  of  the  Cardinal  Polnts. 

the  last  refraction  through  the  second.  Planes  at  right  angles  to  the  axis  at 
these  points  are  principal  planes.  The  second  principal  plane  is  the  image 
of  the  first.  Every  point  in  the  first  principal  plane  has  its  image  after 
refraction  at  a  corresponding  point  in  the  second  principal  plane  at  the  same 
distance  from  the  axis  and  on  the  same  side. 

The  second  nodal  point,  N.^,  is  the  image  of  the  first,  N^:  a  ray  which  in 
the  first  medium  is  directed  to  the  first  nodal  point  passes  after  refraction 
through  the  second  nodal  point,  and  the  direction  of  the  rays  before  and 
after  refraction  are  paralled  to  each  other.  In  Fig.  292  let  A  B  represent 
the  axis.     The  distance  of  the  first  focal  point,  F^,  from  the  first  principal 


Ji/  //^ 


Fig.  293. — Diagram  to  Find  the  Image  nsr  Last  Medium  of  a  Luminous  Point  in 

THE  First. 


plane,  H ^,  is  the  anterior  focal  distance.  The  distance  of  the  second  focal 
point,  F^,  from  the  second  principal  plane,  H^,  is  the  posterior  focal  distance. 
The  distance  of  the  first  nodal  point,  N ^,  from  the  first  focal  point,  F^,  is  equal 
to  the  posterior  focal  distance  H.^  F^.  The  distance  of  the  second  nodal 
point,  A'^2'  from  the  second  focal  point,  F^,  is  equal  to  the  anterior  focal 
distance,  H^  F^.  It  is  evident,  therefore,  that  the  distance  of  the  corre- 
sponding principal  and  nodal  points  from  each  other  is  equal  to  the  differ- 
ences between  the  two  focal  distances.  Also  the  distance  of  the  two 
principal  points  from  each  other  is  equal  to  the  distance  of  the  two  nodal 
points  from  each  other.     Finally,  the  focal  distances  are  proportional  to  the 


THE  SENSE  OF  SIGHT 


689 


refractive  indices  of  the  first  and  last  media.     Planes  passing  through  the 
focal  points  vertically  to  the  axis  are  known  as  focal  planes. 

From  these  properties  of  the  cardinal  points  the  position  of  an  image  in 
the  last  medium  of  a  luminous  point  in  the  first  may  be  determined,  and  the 
course  of  a  refracted  ray  in  the  last  medium  be  constructed  if  its  direction  in 
the  first  be  given  according  to  the  following  rules : 

1.  To  find  the  image  in  the  last  medium  of  a  luminous  point  in  the  first :     Let 

A  (Fig.  293)  be  this  given  point.  Draw  A  B  parallel  to  the  axis  until  it 
meets  the  second  principal  plane  in  B ;  then  B  F^  will  be  this  ray  after 
refraction.  Draw  a  second  ray  from  A  to  the  first  nodal  point;  then 
draw  another  ray,  D  E,  from  the  second  nodal  point  parallel  to  A  C. 
This  will  be  the  refracted  ray  in  the  last  medium.  Where  the  two  re- 
fracted rays,  BF^^  and  D  E,  intersect,  the  image  of  A  will  be  A^.^ 

2.  To  find  the  refracted  ray  in  the  last  medium  of  a  given  ray  in  the  first 

medium:  Let  A  B  (Fig.  294)  be  the  given  ray.     Continue  this  ray  until 


///  ^, 


Fig.  294. — Diagram  to  Find,  the  Refracted  Ray  in  the  Last  Medium  of  a  Given 
Ray  in  the  First  Medium. 


it  meets  the  first  principal  plane  in  C.     Draw  C  D  parallel  to  the  axis. 
Now  assume  any  point,  such  as  E,  in  the  given  ray,  and  find  its  image 
El  by  the  Rule  i.     Then  D  E^  becomes  the  course  of  the  refracted 
ray. 
The  Schematic  Eye. — Accepting  the  system  of  cardinal  points,  Listing, 
Donders',  and  v.  Helmholtz  have  constructed  "schematic"  eyes  to  be  sub- 
stituted for  the  refracting  system  of  the  natural  eye. 

For  this  purpose  it  is  necessary  to  make  use  of  the  various  estimates  of 
the  indices  of  refraction  of  the  different  media,  of  the  radii  of  curvatures 
of  the  different  refracting  surfaces,  and  of  the  distances  separating  them,  to 
deduce  an  average  eye  as  a  basis  for  calculation.  The  most  widely  accepted 
attempt  is  that  of  v.  Helmholtz.  The  data  he  assumed  are  as  follows :  The 
refractive  index  of  air  =  i;  of  the  cornea  and  aqueous  humor,  1.3365;  of  the 
lens,  1.437 1 ;  of  the  vitreous  humor,  1.3365;  the  radius  of  curvature  of  the 
cornea,  7.829  mm. ;  of  the  anterior  surface  of  the  lens,  10  mm. ;  of  the  posterioi 
surface,  6  mm.;  the  distance  from  the  apex  of  the  cornea  to  the  anterior 
surface  of  the  lens,  3.6  mm.;  thickness  of  lens,  3.6  mm.  From  the  above- 
mentioned  data  V.  Helmholtz  calculated  the  position  of  the  cardinal  points 
for  the  eye  as  follows  (see  Fig.  295) :     The  first  focal  point  is  situated  13.745 

'  If  the  point  A  is  infinitely  far  from  the  eye,  all  the  rays  striking  the  eye  will  be  parallel  to 
each  other.  The  nodal  ray  must  therefore  be  drawn,  and  the  point  where  this  nodal  ray  meets 
the  second  focal  plane  will  be  the  image  ol  A,  oi  A^  where  all  rays  parallel  to  the  nodal  ray  will 
meet. 

44 


690 


TEXT-BOOK  OF  PHYSIOLOGY 


mm.  before  the  anterior  surface  of  the  cornea;  the  second  focal  point  is 
situated  15.619  mm.  behind  the  posterior  surface  of  the  lens;  the  first 
principal  point,  1.753  '^^^  behind  the  cornea;  the  second  principal  point, 
2.106  mm.  behind  the  cornea;  the  first  and  second  nodal  points,  6.968  and 
7.321  mm.  behind  the  apex  of  the  cornea,  respectively.  The  anterior  focal 
distance  of  this  schematic  eye,  the  distance  between  F^  and  H^,  there- 
fore amounts  to  15.498  mm.,  and  the  posterior  focal  distance,  H^  to  F^,  to 
20.713  mm. 

When  the  eye,  however,  is  accommodated  for  near  vision,  the  relations 
of  the  cardinal  points  are  changed  and  will  be  as  follows,  if  the  point  accom- 
modated for  lies  152  mm.  from  the  cornea:  Anterior  focal  distance,  13.990 
mm.;  posterior  focal  distance,  18.689  mm.;  distance  from  cornea  of  the  first 


Fig.  295. — Diagram  showing  the  Position  of  the  Cardinal  Points  in  the  "Schematic 
Eye."  The  continuous  lines  in  the  upper  half  of  the  figure  show  their  position  in  the  passive 
emmetropic  eye.  The  dotted  lines  indicate  the  change  in  their  position  in  an  eye  accommodated 
for  the  object  A  at  the  distance  a  from  the  cornea,  or  152  mm.  The  lower  half  of  the  figure  shows 
the  formation  of  a  distinct  image  on  the  retina  of  an  eye  accommodated  for  the  object  A  at  the 
distance  a  from  the  cornea. 


and  second  principal  points,  1.858  and  2.257  m^-  respectively;  distance  of 
the  posterior  focus,  20.955  ^^^-  from  cornea.  Given  this  schematic  eye  in  the 
accommodated  state,  the  course  of  the  rays  and  the  determination  of  the 
position  of  an  image  in  the  last  medium  of  a  luminous  point  in  the  first  can 
easily  be  determined  by  the  rules  already  given. 

The  Reduced  Eye. — As  suggested  by  Listing,  this  schematic  eye  may 
be  yet  further  simplified  or  reduced  to  a  single  refracting  surface  bounded 
anteriorly  by  air  and  posteriorly  only  by  aqueous  or  vitreous  humor.  Without 
introducing  any  noticeable  error  in  the  determination  of  the  size  of  the  retinal 
image,  the  anterior  principal  and  the  anterior  nodal  points  may  be  disre- 
garded, owing  to  the  minuteness  of  the  distances  (0.39  mm.)  separating  the 
two  systems  of  points.     There  is  thus  obtained  one  principal  point  and  one 


THE  SENSE  OF  SIGHT 


691 


nodal  point,  which  latter  becomes  the  center  of  curvature  of  the  single  re- 
fracting surface.  The  dimensions  of  this  "reduced"  eye  are  as  follows  (see 
Fig.  296).  From  the  anterior  surface  of  the  cornea,  corresponding  to  the 
principal  plane  H,  to  the  nodal  point  N,  5.215  mm.,  from  the  anterior  focal 
point  jPj,  to  the  principal  plane  H,  i.e.,  the  anterior  focal  distance/',  15.498 
mm.;  from  the  principal  plane  H  to  the  posterior  focal  point  F^,  i.e.,  the 
posterior  focal  distance/",  20.713  mm.;  the  index  of  refraction  is  1.3365. 
There  is  thus  substituted  for  the  natural  eye  a  single  refracting  surface  with 
a  radius  of  curvature,  r,  of  5.125  mm.  In  such  an  eye  luminous  rays  emanat- 
ing from  the  anterior  focal  point  are  parallel  to  the  axis  after  refraction  in 
the  interior  of  the  eye.  Also  rays  parallel  to  the  axis  before  refraction  unite 
at  the  posterior  focal  point. 

By  means  of  this  reduced  eye  the  construction  of  the  refracted  ray,  the 
various  calculations  as  to  the  size  of  the  image,  the  size  of  diffusion  circles, 
etc.,  are  greatly  facilitated:  e.g., 

In  Fig.  297  let  A  B  represent  an  object.  From  A  a  pencil  of  rays  falls 
on  the  single  refracting  surface.  One  of  the  rays,  the  nodal  ray,  falling  on 
the  surface  perpendicularly,  passes  unrefracted  through  the  single  nodal  point, 


Fig.  296. — The  Reduced  Eye. 


Fig.  297, — The  Formation  of  an  Image  in  the 
Reduced  Eye. 


N,  to  the  posterior  focal  plane.  The  remaining  rays,  partially  represented 
in  the  figure,  falling  on  this  surface  under  varying  degrees  of  incidence, 
undergo  corresponding  degrees  of  refraction,  by  which  they  form  a  converg- 
ing cone. of  rays  which  unite  at  a  point  situated  on  the  nodal  ray.  These 
two  points,  A,  a,  are  known  as  conjugate  foci.  The  same  holds  true  for 
a  pencil  of  rays  emanating  from  B  or  any  other  point  of  the  object. 

The  Size  of  the  Retinal  Image. — The  size  of  the  retinal  image,  I  (in 
Fig.  297  a  b),  may  now  be  easily  calculated,  when  the  size  of  the  object,  O 
(in  Fig.  297  AB),  and  its  distance,  D,  from  the  refracting  surface  with  radius 
of  curvature,  r,  are  known,  by  the  following  formula: 


O  :  I=D  +  r:f"-r. 

For,  as  the  triangles  A  N  B  and  a  N  b  are  similar,  we  have 

ABXNg         ,    ,       ,        ,      0{f"-r\ 
s.   „„j  therefore /  =  — ^•^—      '' 


A  B:  ab  =f  N:N  g,  or  ab  = 


fN 


and 


D+r 


Independent  of  the  foregoing  method,  the  size  of  the  retinal  image  may  be 
calculated  if  it  is  remembered  that  the  eye,  like  any  optic  system,  has  a  point 
of  such  a  quality  that  a  ray  of  light  which  before  entering  the  eye  was  directed 
toward  it,  after  refraction  continues  as  if  it  came  from  this  point.  In  other 
words,  there  is  in  the  eye  a  point  which  allows  a  ray  of  light  to  pass  unre- 
fracted as  would  a  pinhole   instead  of  a  lens.     This   point,    termed  the 


692 


TEXT-BOOK  OF  PHYSIOLOGY 


Fig.  298. — Drawing  Designed  to  show 
HOW  THE  Visual  Angle  and  Size  of  Retinal 
Image  Varies  with  the  Distance  of  an 
Object  of  Given  Size.  For  the  distant  position 
of  A-B  the  visual  angle  is  a;  for  the  near 
position  (dotted  lines)  /3. — (From  Stewart.') 


nodal  point  of  the  eye,  determines  the  size  of  the  image;  for  if  a  Hne  be 
drawn  from  both  the  upper  and  lower  ends  of  an  object  through  this  nodal 
point,  it  is  clear  that  the  images  of  the  respective  points  must  lie  on  these 
two  rays  where  they  intersect  the  retina.  The  distance  of  this  nodal  point 
from  the  retina  is  15.498  mm.  It  is  clear,  therefore,  that  the  size  of  the 
object  is  to  the  size  of  the  image,  as  the  distance  of  the  object  from  the 

nodal  point  is  to  the  distance  of  the 
nodal  point  from  the  retina;  or,  in 
other  words,  to  find  the  size  of  the 
retinal  image :  multiply  the  diameter 
of  the  object  by  15.5  mm.  and  divide 
by  the  distance  of  the  object  from 
the  eye. 

The  Visual  Angle. — The  visual 
angle  is  defined  as  the  angle  formed 
by  the  intersection  of  two  lines 
drawn  from  the  extremities  of  an 
object  to  the  nodal  point  of  the  eye. 
Beyond  the  nodal  point,  however,  the  lines  again  diverge  and  form  an  in- 
verted or  reversed  image  of  the  object  on  the  retina.  The  size  of  the 
visual  angle  increases  with  the  nearness  and  decreases  with  the  remote- 
ness of  the  object;  the  retinal  image  correspondingly  increases  and  de- 
creases in  size.  These  facts  will  become  apparent  from  an  examination  of 
Fig.  298.  As  the  size  of  the  retinal  image  diminishes  when  the  visual  angle 
diminishes  either  as  a  result  of  the  removal  of  a  given  object  from  the  eye,  or 
of  a  diminution  of  the  size  of  the  object,  there  comes  a  limit  in  the  size  of  the 
visual  angle,  beyond  which  it  is  impossible  to  see  the  two  end  points  {A  and  B) 
of  the  object  separately.  When  this  limit  is  reached  the  size  of  the  angle  ex- 
pressed in  degrees  of  the  circle,  may  be  determined  if  the  distance  between 
the  two  points  and  their  distance  from  the  eye  be  known.  Thus  it  has  been 
experimentally  determined  that  at  a  distance  of  5  meters,  the  smallest  object  or 
the  smallest  interval  between  two  points  which  permits  the  eye  to  distinguish 
them  as  such,  is  about  1.454  mm.  Lines  drawn  from  the  extremities  of 
such  an  object  or  interval,  to  the  nodal  point,  subtend  an  angle  of  60  seconds.^ 
Beyond  this  the  two  points  are  indistinguishable.  In  other  words  the 
emmetropic  eye  possesses  the  power  of  distinguishing  the  correspondingly 

*  The  size  of  the  visual  angle,  under  which  an  object  of  this  size  and  situated  at  a  distance 
of  5  meters  is  distinctly  seen,  can  be  determined  from  the  following  Fig.  316,  in  which  A  B  represents 
the  size  of  the  object  i  .454  mm. ; 
N,  the  nodal  point;  CN,  the 
line  which  bisects  the  object, 
represents  the  distance  of  the 
object  from  the  nodal  point; 
a,  the  visual  angle  subtended 
and  whose  value  it  is  desired 
to  know,  and  b  one-half  of  the 
angle  a.  By  trigonometry  the 
size  of  the  angle  a  can  be  de- 
termined in  the  following  way: 
one-half  the  size  of  the  object 
A  B,  is  divided  by  its  distance 
from  the  nodal  point;  the  quo- 
tient is  the  tangent  of  half  the  angle. 


Fig.  209. — Figure  showing  the  Method  of  Obtaining 
THE  Visual  Angle  Expressed  in  Degrees  or  Fraction 
OF  a  Degree  of  an  arc. 


Thus  0.727 -^  5000  =0.0001454.  By  reference  to  tables  of 
natural  tangents,  it  will  be  found  that  the  angle  or  fraction  of  the  circle  corresponding  to  this 
tangent  is  30  seconds,  and  that  therefore  the  whole  angle  is  60  seconds. 


THE  SENSE  OF  SIGHT  693 

small  interval  between  the  two  images  on  the  retina  of  the  two  objective 
points.  The  size  of  the  image  or  the  interval  between  the  two  retinal  points, 
determined  from  the  foregoing  factors  by  the  formulae  on  page  692  is  0.004 
mm.,  which  would  correspond  to  a  visual  angle  of  60  seconds.  If  the 
retinal  distance  is  less  than  this  the  two  sensations  fuse  into  one.  The  reason 
assigned  for  this  is,  that  the  distance  between  the  centers  of  two  adjoining 
cones  in  the  macula  is  0.004  iiini'  With  a  visual  angle  not  less  than  60 
seconds,  the  two  foci  fall  on  separate  cones.  With  a  smaller  visual  angle 
the  two  foci  fall  on,  and  excite  but  a  single  cone  and  hence  there  arises  the 
sensation  of  but  a  single  point.  The  acuteness  of  vision,  therefore,  of  the 
emmetropic  eye  depends  on  its  power  of  distinguishing  the  smallest  retinal 
image  or  the  smallest  interval  between  two  cones  on  the  retina,  correspond- 
ing to  a  visual  angle  of  60  seconds. 

In  ophthalmic  practice  it  is  customary  in  testing  the  acuteness  of  vision 
to  employ  test  letters  of  specific  sizes  for  specific  distances.  The  letters  are 
so  proportioned  that  when  they  are  placed  at  the  specified  distances,  the 
extremities  of  the  letters  subtend  an  angle  of  5  minutes.  The  letters  have 
been  constructed  on  the  following  basis:  Since  to  an  angle  of  60  seconds 
there  corresponds  an  object  of  1.454  mm.  at  the  distance  of  5  meters  as 
shown  before  and  as  the  object  decreases  in  proportion  to  the  distance 


M   T 


L 


Fig.  300. — Standard  Letters,  for  Testing  the  Acuity  of  Vision. 

(for  the  same  visual  angle)  it  is  evident  that  the  object  would  have  to  be  one- 
fifth  of  1.454  mm.  or  0.2908  mm.  in  order  to  subtend  an  angle  of  60  seconds 
at  one  meter.  From  this  the  size  for  any  other  distance  in  meters  is  found 
simply  by  multiplying  0.2908  mm.  by  the  distance.  The  standard  letters 
are  so  constructed  that  each  is  inscribed  within  a  square,  the  sides  of  which 
at  a  specific  distance,  5  meters,  subtend  an  angle  of  5  minutes  and  which  is 
again  subdivided  into  25  small  squares  each  side  of  which  subtends  an  angle 
of  I  minute.  These  partial  little  squares  correspond  to  the  details  of  the 
letter  while  the  whole  letter  of  course,  embraces  an  angle  of  5  minutes  both 
as  to  height  and  to  breadth  (Fig.  300).  The  letter  that  could  be  distinctly 
seen  at  a  distance  of  5  meters,  would  have,  therefore,  a  vertical  and  a  hori- 
zontal dimension  of  5  times  1.454  mm.  or  7.27  mm.  (Fig.  300  A),  and  at  to 
meters  corresponding  dimension  of  14.54  mm.,  etc.     (Fig.  300  B.) 

If  with  the  accommodation  suspended,  the  emmetropic  eye  could 
clearly  distinguish  at  a  distance  of  5  meters  a  letter  7.27  mm.  in  size  whiclj 
would,  therefore,  subtend  an  angle  of  5  minutes,  then  the  acuity  of  th^ 
vision  would  be  normal  and  could  be  expressed  as  follows:  V=|-  or  V  =  Ti 
If  on  the  contrary  at  this  distance  the  smallest  letter  that  could  be  clearly 
seen  is  one  that  would  subtend  an  angle  of  5  minutes  at  a  distance  of  10  meters 
then  the  visual  acuity  would  be  only  one-half  the  normal  and  could  be 
expressed  as  follows :  V  =3^  or  V  =  ^,  etc.     The  acuity  of  vision  is  expressed. 


694 


TEXT-BOOK  OF  PHYSIOLOGY 


.../' 


Fig.  301. — The  Refraction  of  Parallel  and 
Divergent  Rays  in  the  Emmetropic  Eye  in  the 
Passive  .and  in  the  Active  or  Accommodated 
Condition. 


therefore,  by  a  fraction  the  numerator  of  which  is  the  distance  at  which  the 
test  is  made  and  whose  denominator  is  the  distance  at  which  the  smallest 
letters  distinguished  by  the  patient  subtend  an  angle  of  5  minutes,  or  in 

other  words  the  distance  at 
which  the  patient  reads  di- 
vided by  the  distance  at  which 
he  ought  to  read  the  smallest 
letters  seen  by  him  on  the 
chart. 

Accommodation.  —  Ac- 
commodation may  be  defined 
as  the  power  which  the  eye 
possesses  of  adjusting  itself  to 
vision  at  different  distances; 
or  in  other  words,  the  power 
of  focusing  rays  of  light  on  the 
retina,  which  come  from 
different  distances  at  different 
times.  That  such  a  power 
is  a  necessity  is  apparent  from 
the  fact  that  it  cannot  focus 
rays  coming  from  a  distant 
and  a  near  object  at  the  same  time.  Thus,  if  an  object  is  held  before 
one  eye  at  a  distance  of  22  centimeters,  for  example,  and  the  vision 
is  directed  to  a  distant  object  it  is  evident  that  the  near  object  is  indistinctly 
seen;  but  if  the  vision  is  then  directed  to  the  near  object,  it  in  turn  becomes 
clear  and  distinct,  while  the  distant  object  becomes  blurred  and  indistinct. 
It  is  evident,  therefore,  that  rays  of  light  coming  from  a  distant  and  a  near 
object  cannot  be  simultaneously,  but  only  alternately,  focused  on  the  retina. 
The  observer  at  the  same  time  becomes  conscious,  as  the  vision  is  directed 
from  the  distant  to  the  near  object,  of  a  change  in  the  eye  itself,  a  change 
that  involves  time  and  effort.  The  reasons  for  these  facts  will  become 
apparent  from  a  consideration  of  the  following  facts: 

In  a  normal  or  emmetropic  eye,  parallel  rays  of  light  (Fig.  301,  a,  h) 
after  passing  through  the  optic  media  are  converged  and  brought  to  a 
focus  on  the  retina,  /.  Rays,  however,  which  come  from  a  luminous 
point  situated  near  the  eye,  P,  and  are  therefore  divergent,  passing  through 
the  optic  media  at  the  same  time,  are  intercepted  by  the  retina  before  they 
are  focused,  and  give  rise  to  the  formation  of  diffusion-circles  and  indis- 
tinctness of  vision.  The  reverse  is  also  true.  When  the  eye  is  adjusted 
for  the  refraction  and  focusing  of  divergent  rays  (Fig.  301,  P)  parallel  rays 
will  be  brought  to  a  focus  before  reaching  the  retina,  and,  again  diverging, 
will  form  diffusion-circles.  It  is  evident,  therefore,  that  it  is  impossible 
to  focus  simultaneously  both  parallel  and  divergent  rays,  and  to  see  dis- 
tinctly at  the  same  time,  two  objects  which  are  situated  at  different  distances. 
The  eye  must  be  alternately  adjusted  first  to  one  object  and  then  to  another. 
To  this  adjustment  the  term  accommodation  has  been  given. 

|The  following  table  of  Listing  shows  the  size  of  the  diffusion-circles 
formed  of  objects  situated  at  different  distances  when  the  accommodative 
power  is  suspended  in  an  emmetropic  eye: 


o.o 

mm. 

0.0    mm. 

0.005 

mm. 

o.ooii  mm. 

0.012 

mm. 

0.0027  mm. 

0.025 

mm. 

0.0050  mm. 

0.050 

mm. 

0.0112  mm. 

O.IOO 

mm. 

0.0222  mm. 

0.20 

mm. 

0.0443  ™™- 

0.40 

0.80 

mm. 
mm. 

0.0825  mm. 
0.1616  mm. 

1.60 

mm. 

0.3122  mm. 

3.20 

3-42 

mm. 
mm. 

0.5768  mm. 
0.6484  mm. 

THE  SENSE  OF  SIGHT  695 

Distance  of  the  Focal 
Distance  of  Ltiminous  Point.  Point  behind  the  Posterior  Diameter  of  the  Diffusion-circle. 

Surface  of  the  Retina. 
00 

65  m. 
25  m. 
12    m. 

6    m. 

3    m. 

1 .  500  m. 

0.750  m. 

0.375  m. 

0.188  m. 

0.094  m. 

0.088  m. 

From  the  foregoing  table  it  is  evident  that  between  infinity  and  65  meters, 
the  dififusion-circles  are  so  sUght  that  no  perceptible  accommodative  effort 
is  required  to  eHminate  them.  From  65  meters  to  6  meters  the  diffusion- 
circles  gradually  become  larger,  though  they  are  yet  so  faint  as  to  require 
for  their  correction  an  accommodative  effort  which  is  scarcely  measurable. 
From  6  meters  up  to  6  centimeters,  however,  a  progressive  increase  in  accom- 
modative power  is  demanded  for  distinct  vision. 

The  normal  eye  when  adjusted  for  distant  vision  is  in  a  passive  condition, 
and  hence  vision  of  distant  objects  is  unattended  with  fatigue.  In  the  act 
of  adjustment,  however,  for  near  vision  the  eye  passes  into  an  active  state, 
the  result  of  a  muscle  effort,  the  energy  of  which  is  proportional  to  the  near- 
ness of  the  object  toward  which  the  eye  is  directed. 

Mechanism  of  Accommodation. — Inasmuch  as  neither  the  corneal 
curvature  nor  the  shape  of  the  eyeball  undergoes  any  change  during  accom- 
modation, the  necessary  change,  whatever  it  may  be,  is  ^^^^ 
to  be  sought  for  in  the  lens.     As  to  the  character  of  the        ^^|^^^ 
changes  in  this  body,  two  views  are  held,  based  largely  on       hVH^^ 
the  fact  and  its  interpretation,  that  images  of  a  luminous       BillllJ^P 
point  reflected  from  the  anterior  surface  of  the  cornea  and       ^m^^^^^ 
the  anterior  and  posterior  surfaces  of  the  lens,  change         ^^^^T 
their  relative  positions  during  accommodation.  ^     i,    c 

Thus,  if  in  a  darkened  room  a  lighted  candle  be  placed 
in  front  of  and  to  the  side  of  an  individual  whose  eye  is    .j-^jc  images  in  the 
directed  to  a  distant  object,  an  observer  placed  in  the     Eye.      a.    Upright 
same  relative  position  as  the  candle  will  observe  three     i^i^ge  of  reflection, 

,       ^  1  r  r    1  1      from  the  cornea,     o. 

images  m  the  eye,  one  at  the  surface  of  the  cornea  and  Upright  image  from 
two  at  the  pupillary  margin  (Fig.  302).  Of  the  two  latter,  the  anterior  siirface 
one  is  quite  large  and  situated  apparently  in  front  of  the  °erted  image,  '^"from 
third,  which  is  faint,  small,  and  inverted.  The  middle  the  posterior  surface 
image  is  reflected  from  the  anterior  surface  of  the  lens,  9^  *^^  lens.— {Heim- 
the  last  from  the  posterior  surface.  These  images  of  re- 
flection are  known  as  catoptric  images.  If  now  the  individual  be  directed  to 
fix  the  gaze  on  a  near  object,  the  second  image  changes  its  position,  ad- 
vances toward  the  corneal  image  and  at  the  same  time  becomes  smaller, 
a  change  which,  in  accordance  with  the  laws  of  optics,  could  only  be  due  to 
an  increase  in  the  convexity  of  the  anterior  surface  of  the  lens.  A  slight 
displacement  of  the  third  image  sometimes  observed  indicates  a  possible 
increase  in  the  convexity  of  the  posterior  surface  of  the  lens. 

According  to  Helmholtz,  during  accommodation  the  entire  anterior  sur- 


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face  of  the  lens  becomes  more  convex,  while  at  the  same  time  it  slightly  ad- 
vances, possibly  as  much  as  0.4  mm.  in  extreme  efforts.  This  change  is 
represented  in  Fig.  303.  According  to  Tscherning,  the  increase  in  con- 
vexity of  the  anterior  surface  is  confined  to  the  central  portion,  the  re- 
mainder of  the  surface  becoming  somewhat  flattened.  There  is,  moreover, 
no  evidence  that  there  is  any  advance  of  the  surface  or  any  increase  in  the 
thickness  of  the  lens.  A  series  of  new  and  ingenious  experiments  lend' 
support  to  Tscherning's  view,  though  of  late  Hess  has  brought  forward  defi- 
nite experimental  evidence  in  favor  of  the  view  of  Helmholtz.  The  radius 
of  curv^ature  in  either  case  approximates  6  mm.'  in  extreme  efforts  of  ac- 
commodation. The  increase  in  convexity  naturally  increases  the  refracting 
power. 

Whicheverview  is  accepted,  the  nearer  the  object — that  is,  the  greater  the 
degree  of  divergence  of  the  light  rays — the  more  pronounced  must  be  the 
increase  in  convexity  in  order  that  they  may  be  sufficiently  converged  and 
focalized  on  the  retinal  surface.  Changes  in  the  convexity  of  the  lens, 
either  of  increase  or  decrease,  are  attended  by  changes  in  the  distinctness  of 
images.  Coincidently  with  the  lens  change,  the  pupillary  margin  advances 
and  the  pupil  itself  becomes  smaller.     By  this  means  an  indistinctness  of  the 


Xpi/^elium. 


•^or^^nuznj  Miemirelnm 


Fig. 303. 


ccssuj  CUiar^ 


-The  Left  Half  Represents  the  Eye  in  a  State  of  Rest. 
Half  in  State  of  Accommodation, 


The  Right 


image  is  prevented  by  cutting  off  the  rays  which  would  give  rise,  owing  to 
the  angle  at  which  they  fall  on  the  surface,  to  diffusion-circles,  from  spheric 
aberration. 

The  Function  of  the  Ciliary  Muscle. — Though  it  is  generally  admit- 
ted that  the  increase  in  the  convexity  of  the  lens  is  caused  by  the  contrac- 
tion of  the  ciUary  muscle,  the  exact  manner  in  which  this  is  accomplished 
is  not  clearly  understood.  According  to  Helmholtz,  when  the  eye  is  in 
repose  and  directed  to  a  distant  object  the  lens  is  somewhat  flattened  from 
a  traction  exerted  by  the  suspensory  ligament.  When  the  eye  is  directed 
to  a  near  object,  the  ciliary  muscle  contracts,  thereby  relaxing  the  ligament, 
as  a  result  of  which  the  lens,  by  virtue  of  an  inherent  elasticity,  bulges  for- 
ward and  becomes  more  convex.  In  consequence  of  this  latter  fact  the 
refracting  power  is  proportionally  increased.  In  extreme  efforts  of  accom- 
modation it  is  believed  by  some  observers  that  the  circularly  arranged 
fibers,  the  so-called  annular  muscle,  contract  and  exert  a  pressure  on  the 
periphery  of  the  lens  and  thus  aid  other  mechanisms  in  relaxing  the  liga- 
ment and  in  increasing  the  convexity.  This  view  appears  to  be  supported 
by  the  fact  that  in  hypermetropia,  where  a  constant  effort  is  required  to 
obtain  a  distinct  image  of  even  distant  objects,  the  annular  muscle  becomes 


THE  SENSE  OF  SIGHT  697 

very  much  hypertrophied,  thus  reinforcing  the  meridional  fibers.  In 
myopia,  on  the  contrary,  where  the  accommodative  effort  is  at  a  mini- 
mum, the  entire  muscles  possesses  less  than  its  average  size  and 
development. 

According  to  Tscherning,  a  different  explanation  of  the  action  of  the 
ciliary  muscle  must  be  given.  Thus,  when  it  contracts,  the  antero-internal 
angle,  that  portion  in  close  relation  with  the  suspensory  ligament,  recedes 
and  exerts  on  the  ligament  a  pressure  which  in  turn  exerts  a  traction  on  the 
peripheral  portions  of  the  anterior  surface  of  the  lens,  which  produces  the 
deformation  observed.  At  the  same  time  the  postero-external  portion  of 
the  muscle  exerts  traction  on  the  chorioid,  thus  sustaining  the  vitreous 
and  indirectly  the  lens. 

The  reason  for  the  flattening  of  the  periphery  of  the  lens  from  zonular 
compression  and  the  sharpening  of  the  central  convexity  is  to  be  found  in 
the  fact  that  the  convexity  of  the  more  solid  central  portion,  the  nucleus, 
is  greater  than  that  of  the  lens  itself.  Hence  it  is  easily  understood  why  a 
zonular  traction  would  give  rise  to  peripheral  flattening. 

There  is,  however,  one  point  which  seems  difficult  to  harmonize  with 
Tscherning's  view;  that  is,  the  fact  that  during  accommodation  the  lens 
appears  to  be  slightly  tremulous,  thus  showing  relaxation,  and  not  increased 
tension,  of  the  suspensory  ligament. 

Range  of  Accomniodation. — It  has  been  stated  that  rays  of  light 
coming  from  a  luminous  point  situated  at  any  distance  beyond  65  meters 
are  so  nearly  parallel  that  no  accommodative  effort  is  required  for  their 
focalization.  So  long  as  the  luminous  point  remains  between  infinity  and 
65  meters,  the  eye,  directed  toward  it,  remains  completely  relaxed.  The 
point  at  which  the  object  can  be  distinctly  seen  without  accommodation 
is  termed  the  far  point  or  the  punctum  remotum.  This  for  the  normal  eye 
is  at  a  distance  of  65  meters  or  beyond.^  If  the  luminous  point  gradually 
approaches  the  eye  from  a  point  65  meters  distant,  the  accommodative 
power  comes  into  play  and  gradually  increases  until  it  attains  its  maximum. 
The  nearest  point  up  to  which  the  eye  is  able  to  form  distinct  images  of 
objects  is  called  its  near  point  or  punctum  proximum.  This  near  point  in  a 
healthy  boy  of  twelve  years  will  lie  at  a  point  situated  7  cm.  from  the  eye, 
while  the  same  point  lies  20  cm.  distant  in  a  man  of  forty  years.  Of  objects 
which  lie  nearer  than  the  punctum  proximum  the  eye  cannot  form  distinct 
images.  The  distance  between  the  punctum  remotum  and  the  punctum  proxi- 
mum is  termed  the  range  of  accommodation. 

Force  of  Accommodation. — The  increase  in  curvature  of  the  lens 
necessary  to  focalize  rays  when  the  eye  is  directed  from  the  far  to  the  near 
point  necessitates  the  expenditure  of  energy  on  the  part  of  the  ciliary  muscle. 
The  force  expended  in  the  act  of  accommodation  may  be  measured  by 
a  lens,  the  refracting  power  of  which  is  such  as  to  enable  it  to  produce  the 
same  result — that  is,  to  give  the  diverging  rays  coming  from  the  near  point, 
e.g.,  20  cm.,  a  parallel  direction.  A  lens,  therefore,  which  has  a  focal  dis- 
tance of  20  cm.  would  be  a  measure  of  the  force  expended ;  for  such  a  lens 
placed  in  front  of  the  crystalline  lens,  when  in  a  state  of  repose,  would, 
with  the  assistance  of  the  latter,  bring  diverging  rays  coming  from  the  near 

*  In  practical  ophthalmic  work  a  point  six  meters  distant  is  taken  as  the  far  point  for  the 
reason  that  the  rays  at  this  distance  are  practically  parallel. 


698 


TEXT-BOOK  OF  PHYSIOLOGY 


point  to  a  focus  on  the  retina.  A  lens  of  this  character  is  said  to  have  a 
refracting  power  of  5  dioptrics.^ 

The  refracting  media  of  the  human  eye  in  repose  have  collectively  a 
refracting  power  of  about  64  dioptries,  the  reciprocal  of  its  anterior  focal 
distance.  The  refracting  power  of  the  corneal  surface  alone  is  equivalent  to 
42  dioptries.  The  crystalline  lens  by  reason  of  its  relations  and  situation 
in  the  optic  media  has  a  refracting  power  of  about  20  dioptries. 

The  capability  of  the  lens  to  increase  its  refraction  during  accommodative 
efforts  beyond  the  20  dioptries  varies  considerably  at  different  periods  of  life. 
From  youth  to  old  age,  the  elasticity  of  the  lens  steadily  declines,  and  the 
range  of  accommodation  diminishes  from  the  recession  of  the  near  point. 

The  following  table  shows  the  decrease  in  the  accommodative  power  with 
increasing  years,  the  sjiortening  of  the  range  of  accommodation  and  the 
distance  to  which  the  near  point  has  receded  from  the  eye. 


Age,  Years 

Range  of  Accom- 
modation, Dioptries 

Near  point, 
cm. 

Age,  Years 

Range  of  Accom- 
modation, Dioptries 

Near  point, 
cm. 

10 

20 

30 
40 

1                             1 

140                         70          ,            45 

50 

10. 0                       10. 0                     60 
7.0                       14.0                     70 
4.5                       22.0          1 

3-5 

2-5 

1 .0 
Zero 

28.0 

40.0 

100. 0 

Zero 

Convergence  of  the  Eyes  during  Accommodation. — When  the  eyes 
are  at  complete  rest  and  directed  to  some  far  distant  object  the  visual  lines 
are  parallel  and  the  optic  axes  are  directed  outward  at  an  angle  of  about 
5  degrees,  which  is  known  as  the  angle  alpha.  If,  however,  it  is  desired  to 
see  distinctly  with  both  eyes,  any  object  within  the  range  of  accommodation, 
the  eyeballs  must  be  converged  toward  the  median  line,  the  object  being  to 
enable  the  rays  of  light  emanating  from  a  point  to  fall  directly  into  the  foveas 
so  that  the  two  retinal  images  shall  give  rise  to  but  a  single  impression  and 
thus  prevent  double  vision  or  diplopia  which  would  otherwise  result.  Con- 
vergence of  the  eyeballs,  therefore,  increases  as  the  visual  axes  are  directed 
toward  objects  which  are  placed  progressively  nearer  the  eyeballs.  This  is 
accomplished  by  the  conjoint  and  harmonious  action  of  the  internal  recti 
muscles  balanced  by  the  action  of  their  physiologic  antagonists — the  external 
recti  muscles.  Convergence  of  the  eyeballs  is  always  accompanied  by  in- 
crease in  the  accommodation,  the  two  being  necessary  for  distinct  binocular 
vision. 

Functions  of  the  Iris. — For  purposes  of  distinct  vision  it  is  essen- 
tial that  the  quantity  of  light  entering  the  interior  of  the  eye  shall  be  so 

^  Since  lenses  of  the  same  curvature  made  from  different  materials  have  different  refracting 
powers,  it  becomes  necessary  to  have,  for  purposes  of  comparison,  some  unit  of  measurement. 
The  unit  now  accepted  is  the  refracting  power  of  a  glass  lens  which  is  sufficient  to  focalize  parallel 
rays  at  a  distance  of  loo  cm.  or  i  meter.  This  amount  of  refracting  power  is  termed  a  dioptry. 
Lenses  which  would  focahze  parallel  rays  at  a  distance  of  50,  20,  or  10  cm.  are  said  to  have  a  re- 
fractive power  of  2,  5,  10  dioptries  respectively,  obtained  by  dividing  100  cm.  by  the  focal  distance. 
The  refracting  power  of  a  biconcave  lens  is  determined  by  prolonging  backward  in  the  direction 
the  parallel  rays  have  come,  the  rays  which  have  been  rendered  divergent  by  the  lens,  and  using  a 
corresponding  negative  figure.  Thus  a  lens  which  diverges  parallel  rays  in  such  a  way  as  to  make 
them  appear  to  radiate  from  a  point  20  centimeters  behind  itself  is  said  to  have  a  refractive  power  of 
minus  5  dioptries. 


THE  SENSE  OF  SIGHT  699 

adjusted  that  the  formation  and  subsequent  perception  of  the  image  shall  be 
sharp  and  distinct.  This  is  accompHshed  by  the  iris,  the  circular  fibers 
of  which  respectively  contract  and  relax  with  increasing  and  decreasing  in- 
tensities of  the  light.  The  size  of  the  pupil,  therefore,  through  which  the 
light  passes,  will  vary  from  moment  to  moment  and  in  accordance  with 
variation  in  the  light  intensity.  The  quantity  of  light  necessary  to  distinct 
vision  is  thus  regulated. 

In  the  total  absence  of  light  the  sphincter  pupillse  muscle  is  relaxed  and 
the  pupil  widely  dilated.  With  the  appearance  of  light  and  an  increase 
in  its  intensity  the  muscle  again  contracts  and  the  pupil  progressively  narrows. 
With  a  given  intensity  in  the  light,  the  sphincter  contraction  is  greater  when 
the  light  falls  directly  upon  the  fovea.  Contraction  of  this  muscle  is  an 
associated  movement  in  the  convergence  of  the  eyes  during  accommodation 
and  in  consensus  with  the  other  eye. 

In  addition  to  this  function  of  the  iris,  it  constitutes,  by  virtue  of  the 
sphincter  muscle  contraction,  an  important  corrective  apparatus.  Being 
non-transparent,  it  serves  as  a  diaphragm  intercepting  those  rays  which 
would  otherwise  pass  through  the  peripheral  portions  of  the  lens  and  by 
spheric  aberration  give  rise  to  indistinctness  of  the  image.  The  movements 
of  the  iris  by  which  the  size  of  the  pupil  is  determined  are  caused  by  the  con- 
tractions and  relaxations  of  the  sphincter  pupiUcE  and  dilatator  pupillcB 
muscles.  The  contraction  of  the  sphincter  is  entirely  reflex  and  involves 
those  structures  necessary  to  the  performance  of  any  reflex  act,  viz. :  a  recep- 
tive surface,  the  retina;  afferent  nerves,  the  pupillary  fibers  of  the  optic  nerve; 
a  central  emissive  center  situated  in  the  gray  matter  beneath  the  aqueduct 
of  Sylvius;  and  efferent  nerves,  the  motor  oculi  and  the  cihary  nerves.  The 
stimulus  requisite  to  the  excitation  of  this  mechanism  is  the  impact  of  light 
waves  or  ether  vibrations  on  the  rods  and  cones.  According  to  the  intensity 
of  these  vibrations  will  be  the  resulting  contraction  of  the  muscle.  The 
contraction  of  the  dilatator  pupillas  muscle  is  determined  by  the  activity 
of  a  continuously  active  nerve-center  in  the  medulla  oblongata  which  trans- 
mits its  nerve  impulses  through  the  spinal  cord,  along  the  first  and  second 
dorsal  nerves  to  the  superior  cervical  ganglion,  and  thence  to  the  iris  by 
way  of  the  fifth  nerve.  (See  Fig.  268,  page  620.)  These  two  muscles 
appear  to  bear  an  antagonistic  relation  to  each  other,  for  section  of  the  motor 
oculi  is  followed  by  relaxation  of  the  sphincter  muscle  and  dilatation  of  the 
pupil.  Stimulation  of  the  sympathetic  is  followed  by  a  more  pronounced 
dilatation.  The  size  of  the  pupil  is  the  resultant  of  a  balancing  of  these 
two  forces. 

OPTIC  DEFECTS 

Presbyopia. — ^Presbyopia  may  be  defined  as  a  condition  of  the  normal 
eye  in  which  the  accommodation  has  become  so  reduced  by  age  that  reading  has 
become  impossible  at  ordinary  distances.  As  age  advances  the  lens  gradu- 
ally loses  its  elasticity  and  hence  its  power  to  increase  in  convexity  and  thick- 
ness to  the  same  extent  as  in  earlier  life,  in  response  to  efforts  of  accommo- 
dation. The  refractive  power  is,  thereby,  lessened  and  the  eye  is  no  longer 
able  to  see  distinctly  at  the  normal  reading  distances,  viz. :  22  to  28  cm.  Rays 
of  light  emanating  from  a  luminous  point  at  the  normal  reading  distances  are 
less  and  less  converged  on  the  retina  and  hence  the  diffusion  circles  increase 


700  TEXT-BOOK  OF  PHYSIOLOGY 

in  size.  The  near  point,  the  point  from  which  divergent  rays  can  be  focal- 
ized, therefore  advances  toward  the  far  point,  or  recedes  from  the  individual. 
The  range  of  accommodation  is,  thereby,  diminished.  At  forty  years  the  near 
point  is  about  22  cm.,  indicating  an  increase  in  refracting  power  on  the  part 
of  the  lens  of  4.5  dioptrics;  at  forty-five  years  the  near  point  has  receded  to 
28  cm.  This  indicates  that  the  lens  at  this  period  has  only  a  refracting 
power  of  approximately  3.5  dioptrics,  showing  therefore  a  loss  in  the  five 
years  of  i  dioptry;  at  fifty  years  the  near  point  recedes  to  40  cm.,  and  at  sixty 
to  100  cm.,  indicating  a  loss  in  refracting  power  on  the  part  of  the  lens  of 
2  and  3.5  dioptrics  respectively.  Convex  lenses  placed  before  the  eyes  hav- 
ing a  refracting  power  of  i,  2,  and  3.5  dioptrics  would  in  the  three  instances 
return  the  near  point  to  its  normal  position.  At  the  age  of  seventy  the  lens 
is  incapable  of  any  increase  during  an  accommodative  effort.  A  lens  of  4.5 
dioptrics  would  therefore  be  required  by  such  a  man  to  return  the  near  point 
to  its  normal  position,  22  cm.  from  the  eye. 

Myopia. — Myopia  may  be  defined  as  a  condition  of  the  eye  characterized 
by  an  increase  in  the  antero-posterior  diameter  or  by  a  hypernormal  refrac- 
ting power  of  the  lens.  The  former  is  the  usual  condition.  In  either  case 
parallel  rays  of  light  which  enter  the  eye  are  brought  to  a  focus  in  front  of 


Fig.  304. — Myopia.    Parallel  rays  Fig.  305. — Correction  of  Myopia  by 
focus  at  F,  cross  and  form  diffusion-  a  Concave  Lens. 

circles;  divergent  rays  from  A  focus 
on  the  retina. 

the  retina,  after  which  they  diverge  and  give  rise  to  diffusion  circles  and  indis- 
tinctness of  vision.  Divergent  rays,  however,  which  enter  the  eye  are  focal- 
ized as  usual  on  the  retina  even  in  its  new  position.  The  distant  point,  the 
punctum  remotum  is  always  at  a  finite  distance,  but  approaches  the  eye  as 
the  myopia  increases.  The  near  point  is  usually  much  nearer  the  eye  than 
20  cm.     For  this  reason  the  condition  is  termed  near  sight  (Fig.  304). 

The  increase  in  the  length  of  the  antero-posterior  diameter  may  range 
from  a  fraction  of  a  millimeter  up  to  3.8  mm.  With  an  increase  of  0.16  mm. 
the  far  point  is  but  200  cm.  distant;  and  with  an  increase  of  3.8  mm.  it  is  but 
10  cm.  distant.  Inasmuch  as  only  divergent  rays  can  be  focalized  by  the 
myopic  eye  normal  vision  can  be  established  by  placing  before  the  eyes  a  bi- 
concave lens  with  a  diverging  power  in  the  first  instance  of  0.5  dioptry  and 
the  second  of  10  dioptrics  (Fig.  305). 

Hypermetropia. — Hypermetropia  may  be  defined  as  a  condition  of  the 
eye  characterized  by  decrease  of  the  normal  antero-posterior  diameter  or 
by  a  subnormal  refracting  power  of  the  lens.  The  former  is  the  usual  con- 
dition. In  either  case  parallel  rays  of  light  which  enter  the  eye  are,  therefore, 
not  brought  to  a  focus  when  the  accommodation  is  suspended.  Falling  on 
the  retina  previous  to  focalization,  they  give  rise  to  diffusion-circles  and  indis- 
tinctness of  the  image.     As  no  object  can  be  seen  distinctly  no  matter  how 


THE  SENSE  OF  SIGHT  701 

remote,  there  is  no  positive  far  point.  The  near  point  is  abnormally  distant 
— sometimes  as  far  as  200  cm.  For  this  reason  the  condition  is  termed /ar 
sight.  A  hypermetropic  eye  without  accommodative  effort  can  focalize  only 
converging  rays  on  the  retina.  If  rays  of  light  were  to  come  from  the  retina 
of  such  an  eye,  they  would,  on  emerging,  take  a  divergent  direction,  as  shown 
in  Fig.  306,  dotted  line  C  and  D.  If  these  same  rays  were  to  be  prolonged 
backward,  they  would  meet  at  the  point  K,  which  is  the  punctum  remotmn; 
and  as  it  is  behind  the  eye,  it  is  termed  negative.  Since  rays  coming  from 
the  retina  take  a  divergent  direction  on  emerging  from  the  eye,  it  is  evident 


Fig.  306. — The  Hypermetropic  Eye.  Parallel  rays  {A,  B)  can  be  focused  only  at  a  point 
behind  the  eye,  as  at/;  rays  of  light  coming  from  the  retina  take,  on  emerging  from  the  eye,  a 
divergent  direction,  C,  D.     K.  The  negative  punctum  remotum. 

that  only  converging  rays  can  be  focalized  by  a  passive  hypermetropic  eye. 
The  hypermetropic  person  attempts,  and  partially  succeeds,  in  focalizing  the 
rays  by  increasing  the  convexity  of  the  lens  through  an  increased  accommoda- 
tive effort  which  often  gives  rise  to  accommodation  fatigue  and  headache. 
As  there  are  no  convergent  rays  in  nature,  it  is  necessary  for  distinct  vision 
that  all  rays,  parallel  and  divergent,  shall  be  given  a  convergent  direction 
before  entering  the  eye.  The  decrease  in  the  length  of  the  antero-posterior 
diameter  may  range  from  a  fraction  of  a  millimeter  up  to  2.78  mm.  Normal 
vision  may  be  established  by  placing  before  the  hypermetropic  eye  convex 


Fig.  307. — Hypermetropu.    Par-  Fig.  308. — Correction  of  Hyper- 

ALLEL  Rays  Focused  behind  the  metropu  by  a  Convex  Lens. 

Retina. 

lenses  with  a  converging  power,  in  the  first  instance,  of  0.5  dioptry  and,  in  the 
second  instance,  of  10  di^^p tries  (Figs.  307  and  308). 

Astigmatism. — Astigmatism  may  be  defined  as  a  condition  of  the  eye 
characterized  by  an  inequality  of  curvature  of  its  refracting  surfaces  in  con- 
sequence of  which  not  all  of  a  homocentric  bundle  of  rays  are  brought  to 
the  same  focus.  The  inequality  may  be  either  in  the  cornea  or  lens,  or 
both,  though  usually  in  the  cornea. 

In  the  normal  cornea  the  radius  of  curv-ature  in  the  vertical  meridian 
is  a  tritle  shorter,  7.6  mm.,  than  that  of  the  horizontal,  7.8  mm.,  and  hence 
its  focal  distance  is  slightly  shorter.     The  difference,  however,  in  the  focal 


702 


TEXT-BOOK  OF  PHYSIOLOGY 


distances  is  so  slight  that  the  error  in  the  formation  of  the  image  is  scarcely 
noticeable.  A  transverse  section  of  a  cone  of  light  coming  from  the  cornea  is 
practically  a  circle.  If,  however,  the  vertical  curvature  exceeds  the  normal 
to  any  marked  extent,  the  rays  passing  in  the  vertical  plane  will  be  more 
sharply  refracted  and  brought  to  a  focus  much  sooner  than  the  rays  passing 
through  the  horizontal  plane.  The  result  will  be  that  the  cone  of  light  will 
be  no  longer  circular,  but  more  or  less  elliptic.  The  variations  of  the  shape 
of  this  cone  are  shown  in  Fig.  309,  which  represents  the  appearance  pre- 
sented on  cross-section  both  before  and  after  focalization  of  each  set  of  rays. 
Though  the  vertical  plane  has  usually  the  sharper  curvature,  it  not  infre- 
quently happens  as  illustrated  in  this  figure,  that  the  reverse  is  true.  For 
the  reason  that  the  rays  from  one  point  do  not  all  come  to  the  same  focus 
or  point,  the  condition  is  termed  astigmatism. 

Spheric  Aberration. — When  the  rays  of  light  which  emanate  from  a 
point  fall  upon  a  spheric  lens,  they  do  not  after  passing  through  it  reunite 
at  one  point  because  of  the  fact  that  the  more  peripheral  rays  have  a 
shorter  focus  than  the  central  rays.  To  this  condition  the  term  spheric 
aberration  is  given.  Spheric  aberration  can  be  demonstrated  in  the  human 
eye.     That  this  condition  is  present  to  but  a  slight  extent  in  the  normal 


Fig.  309. — Refraction  by  an  Astigmatic  Surface. — {Hansel!  and  Sweet.) 

eye  is  due  to  the  presence  of  the  iris,  which  intercepts  those  rays  which  would 
otherwise  pass  through  the  marginal  portions  of  the  refracting  media. 
In  widely  dilated  eyes  the  spheric  aberration  of  the  peripheral  parts  may 
amount  to  as  much  as  4  or  5  dioptrics. 

Chromatic  Aberration. —  When  a  beam  of  light  is  made  to  pass 
through  a  prism,  it  is  decomposed  into  the  primary  colors  owing  to  a  difference 
in  the  refrangibility  of  the  rays.  In  passing  through  the  refracting  media  of 
the  eye  the  different  rays  composing  white  light  also  undergo  unequal  refrac- 
tion and  those  rays  which  give  rise  to  one  color  are  brought  to  a  focus  at  a 
point  somewhat  different  from  those  which  give  rise  to  other  colors.  If  the 
eye  is  accommodated  for  one  set  of  rays,  it  is  not  for  another,  and  the  result  is  a 
fringe  of  colors  around  the  image.  This  defect  in  the  normal  eye  is  so  slight 
that  the  mind  fails  to  take  cognizance  of  it.  That  the  eye  is  incapable  simul- 
taneously of  focalizing  rays  of  widely  different  refrangiblity,  as  those  which 
give  rise  to  the  blue  and  red  colors,  is  shown  by  the  following  experiment: 
The  eye  being  directed  to  a  luminous  point,  a  plate  of  cobalt-glass  is 
placed  between  the  light  and  the  observer  close  to  the  eye.  This  substance 
has  the  property  of  intercepting  all  rays  but  the  red  and  the  blue  and  hence 
these    alone  will    be  seen.     The   center  of    the  image  produced  will   be 


THE  SENSE  OF  SIGHT 


703 


red  and  clearly  defined,  the  periphery  blue  and  ill-defined.  The  reason 
for  this  is  clear.  The  eye  more  readily  accommodates  itself  for  the 
red  rays,  and  hence  their  focal  point  is  distinct.  The  blue  rays,  having  a 
higher  degree  of  refrangibility,  come  to  a  focus,  cross  and  diverge,  and  give 
rise  to  diffusion-circles.  If  a  biconcave  glass  be  placed  before  the  cobalt, 
the  blue  rays  can  be  focalized  on  the  retina,  while  the  red  will  fall  on  the  retina 
without  focalization.  The  image  will  now  be  blue  and  distinct  in  the  center, 
the  periphery  red  and  ill-defined.  With  the  removal  of  the  minus  glass  the 
reverse  condition  again  obtains. 

Imperfect  Centering. — From  a  purely  physical  point  of  view,  the  eye 
is  not  a  perfect  optic  instrument.  In  addition  to  the  defects  noticed  in  the 
foregoing  paragraphs,  there  is  yet  another,  viz.:  an  imperfect  centering  of 
the  refracting  surfaces.  In  first-class  optic  instruments  the  lenses  are 
centered — that  is,  their  optic  centers  are  situated  on  the  same  axis.  In 
viewing  an  object  through  such  a  system  the  visual  line  corresponds  with 
the  axis  of  the  lens  system.     This  is  not  the  case  with  the  refracting  system 


2emporaZ  Si^te 


/^Tasai  iSuie. 

Fig.  310. — Diagram  showing  the  Corneal  Axis  D  D,  the  Optic  Axis  O  A,  the  Visual 
Axis  V  L,  and  the  Line  of  Fixation  V  C;  also  the  Three  Angles,  a,  /?,  u. 


of  the  eye.  A  line  passing  through  the  center  of  the  cornea  and  the  center 
of  the  eye,  the  optic  axis  (O  A  in  Fig.  310),  does  not  pass  exactly  through 
the  center  of  the  lens  and  does  not  fall  into  the  point  for  most  distinct  vision, 
the  fovea.  This  has  lead  to  the  recognition  of  other  lines  the  relations  of 
which  must  be  kept  in  mind  in  all  optic  discussions,  viz. : 

1.  The  visual  axis  or  visual  line  (F  L),  the  line  connecting  the  point  viewed, 

the  nodal  point,  and  the  fovea  centralis. 

2.  The  line  of  fixation  or  line  oj  regard  {V  C),  the  line  connecting  the  point 

viewed  with  the  center  of  rotation,  the  latter  being  situated  6  mm.  behind 
the  nodal  point  of  the  eye  and  9  mm.  before  the  retina.  The  relation 
of  these  lines  and  certain  angles  connected  with  them  are  shown  in  Fig. 
327.  The  angle  included  between  the  line  D  D  (the  major  axis  of  the 
corneal  ellipse)  and  the  visual  line  is  the  angle  alpha,  amounting  on  the 
average  to  5°.  The  angle  incuded  between  the  optic  axis  and  the  line  of 
fixation  or  regard  is  the  angle  gamma,  while  the  angle  between  the  optic 
axis  and  the  line  of  vision  is  the  angle  beta.  In  emmetropia  the  angle  alpha 
is  about  5°.     In  hypermetropia  it  Is  greater,  amounting  to  7°  or  8°, 


704  TEXT-BOOK  OF  PHYSIOLOGY 

giving  to  the  eye  an  appearance  of  divergence.  In  myopia  it  is  much 
smaller — 2° — or  in  extreme  cases  may  be  abolished,  the  line  of  vision 
corresponding  with  the  optic  axis  or  even  passing  beyond  it.  The 
angle  gamma  is  of  value  in  determining  the  actual  deviation  of  the  eye 
in  squint. 

Functions  of  the  Retina. — Of  all  the  layers  of  the  retina,  the  rods  and 
cones  appear  to  be  the  most  essential  to  vision.  It  is  only  this  layer  that  is 
capable  of  receiving  the  light  stimulus  and  of  transforming  it  into  some 
specific  form  of  energy,  which  in  turn  arouses  in  the  fibers  of  the  optic  nerve 
the  characteristic  nerve  impulses.  A  ray  of  light  entering  the  eye  passes 
entirely  through  the  various  layers  of  the  retina,  and  is  arrested  only  upon 
reaching  the  pigmentary  epithelium  in  which  the  rods  and  cones  are  embedded. 
As  to  the  manner  in  which  the  objective  stimuli — light  and  color,  so  called — 
are  transformed  into  nerve  impulses,  but  little  is  known.  It  is  probable 
that  the  ether  vibrations  are  transformed  into  heat,  which  excites  the  rods 
and  cones.  These,  acting  as  highly  specialized  end-organs  of  the  optic 
nerve,  start  the  impulses  on  their  way  to  the  brain,  where  the  seeing  process 
takes  place.  As  to  the  relative  function  of  the  rods  and  cones,  it  has  been 
suggested,  from  the  study  of  the  facts  of  comparative  anatomy,  that  the  rods 
are  impressed  only  by  differences  in  the  intensity  of  light,  while  the  cones,  in 


Fig.  311. — Diagram  for  Observing  the  Situation  of  the  Blind  Spot. — 

(Helmholtz.) 

addition,  are  impressed  by  qualitative  differences  in  color.  The  nerve- 
fibers  themselves  are  insensible  to  the  impact  of  the  ether  vibrations,  and 
require  for  their  excitation  some  intermediate  form  of  energy.  That  this  is 
the  case  was  shown  by  Bonders,  who  reflected  a  beam  of  light  on  the  optic 
nerve  at  its  entrance  without  the  individual  experiencing  any  sensation  of 
light.  This  region,  occupied  only  by  the  optic-nerve  fibers  and  devoid  of  any 
special  retinal  elements,  is  therefore  an  insensitive  or  blind  spot.  The 
diameter  of  this  spot  is  about  1.5  mm.,  and  occupies  in  the  field  of  vision  a 
space  of  about  6°.  It  is  situated  about  3.5  mm.  to  the  nasal  side  of  the 
visual  axis.  Its  existence  can  be  demonstrated  by  the  familiar  experiment 
of  Mariotte,  which  consists  in  placing  before  the  eye  two  objects  having  the 
relation  to  each  other  shown  in  Fig.  311.  With  the  left  eye  closed  and  the 
right  eye  directed  to  the  cross,  both  objects  may  be  visible.  But  by  moving 
the  figure  away  from  or  toward  the  eye,  there  will  be  found  a  distance, 
about  30  cm.,  when  the  circle  will  be  invisible.  This  occurs  when  the 
image  falls  on  the  optic  nerve  at  its  entrance.  The  experiment  of  Purkinje 
as  described  in  the  following  paragraph  demonstrates  also  the  fact  that  the 
sensitive  portion  of  the  retina  is  to  be  found  only  in  the  layer  of  rods  and 
cones. 

It  is  well  known  that  the  blood-vessels  of  the  retina  are  situated  in  its 
innermost  layers  a  short  distance  behind  the  optic  nerve-fibers.      Owing  to 


THE  SENSE  OF  SIGHT 


705 


this  anatomic  arrangement,  a  portion  of  the  light  coming  through  the  pupil 
will  be  intercepted  by  the  vessels  and  a  shadow  projected  on  the  layer  of 
rods  and  cones.  Ordinarily,  these  shadows  are  not  perceived,  for  the  reason 
that  the  shaded  parts  are  more  sensitive,  so  that  the  small  amount  of  light 
passing  through  the  vessels  produces  as  strong  an  impression  on  this  part 
as  does  the  full  amount  of  light  on  the  unshaded  parts  of  the  retina,  and 
perhaps  because  the  mind  has  learned  to  disregard  them.  But  if  light  be 
made  to  enter  the  eye  obliquely,  the  position  of  the  shadows  will  be  changed, 
when  at  once  they  become  apparent.  This  can  be  shown  in  the  following 
way:  If  in  a  darkened  room  a  lighted  candle  be  held  several  inches  to  the 
side  and  to  the  front  of  the  eye,  and  then  moved  up  and  down,  there  will 
be  perceived,  apparently  in  the  field  of  vision,  an  arborescent  figure  corres- 
ponding to  the  retinal  blood-vessels.  This  is  due  to  the  falling  of  the 
shadows  on  unusual  portions  of  the  layer  of  rods  and  cones. 


Fig.  312. — Section  of  the  Retina  of  a  Frog.     A.  In  darkness.     B.  In  light. 
(After  Van  Genderen  Start,  from  Tscherning's  "Physiologic  Optics.^') 


Excitability  of  the  Retina. — The  retina  is  not  equally  excitable  in  all  parts 
of  its  extent.  The  maximum  degree  of  sensibility  is  found  in  the  macula 
lutea,  and  especially  in  its  central  portion,  the  fovea.  In  this  region  the 
layers  of  the  retina  almost  entirely  disappear,  the  layers  of  rods  and  cones 
alone  remaining,  and  in  the  fovea  only  the  cones  are  present.  That  this 
area  is  the  point  of  most  distinct  vision  is  shown  by  the  observation  that 
when  the  eye  is  directed  to  any  given  point  of  light,  its  image  always  falls 
in  the  fovea.  Any  pathologic  change  in  the  fovea  is  attended  by  marked 
indistinctness  of  vision.  The  sensibility  of  the  retina  gradually  but  irregu- 
larly diminishes  from  the  macula  toward  the  periphery.  This  diminution 
in  insensibility  holds  true  for  monochromatic  as  well  as  white  light. 

As  stated  above,  the  nature  of  the  molecular  processes  which  take  place  in 
the  retinal  tissue,  caused  on  one  hand  by  the  light  vibrations,  and  on  the  other 
hand  developing  nerve  impulses,  is  entirely  unknown.  The  discovery  of 
the  visual  purple  in  the  outer  segment  of  the  rods  gave  promise  of  some 

45 


7o6  TEXT-BOOK  OF  PHYSIOLOGY 

explanation  of  the  process,  especially  when  it  was  shown  to  undergo 
changes  when  exposed  to  the  action  of  light.  But  as  the  pigment  is  wanting 
in  the  cones,  and  especially  in  the  fovea,  it  cannot  be  considered  essential 
to  distinct  vision,  although  that  it  plays  some  important  r61e  in  the  visual 
process  is  highly  probable.  It  was  observed  by  Van  Genderen  Stort, 
that  when  an  animal  is  kept  in  darkness  some  time  before  death,  the  cones 
are  long  and  filiform;  but  if  the  animal  has  been  exposed  to  light,  they  are 
short  and  swollen.  It  was  discovered  by  Boll  that  if  an  animal  is  kept  in 
darkness  an  hour  or  two  before  death  the  pigment  is  massed  at  the  ends  of 
the  rods  and  cones,  but  after  exposure  to  light  it  becomes  displaced  and 
extends  over  and  between  the  rods  almost  to  the  external  limiting  mem- 
brane.    These  conditions  are  represented  in  Fig.  312. 

The  Eye  a  Living  Camera. — In  its  construction,  in  the  arrangement 
of  its  various  parts,  and  in  their  mode  of  action  the  eye  may  be  compared 
to  a  camera  ohscura.  Though  the  comparison  may  not  be  absolutely  exact, 
yet  in  a  general  way  it  is  true  that  there  are  many  striking  points  of  simi- 
larity between  them;  e.g.,  the  sclera  and  chorioid  may  be  compared  to  the 
walls  of  the  camera;  the  combined  refracting  media  to  the  component  glasses 
of  the  lens,  the  action  of  which  results  in  the  focusing  of  the  light  rays;  the 
retina  to  the  sensitive  plate  receiving  the  image  formed 
at  the  focal  point;  the  iris  to  the  diaphragm  for  the 
regulation  of  the  amount  of  light  to  be  admitted,  and 
for  the  partial  exclusion  of  those  marginal  rays  which 
give  rise  to  spheric  aberration;  the  ciliary  muscle  to  the 
adjusting  screw,  by  means  of  which  the  image  is  brought 
to  a  focus  on  the  sensitive  plate,  notwithstanding  the 
Fig.  313. — Retina  op  varying  distances  of  the  object  from  the  lens.  The 
OF  ^A^^WiNDow^°Fo^R  P^'^sence  of  the  visual  purple  in  the  rods  of  the  retina 
Meters  Distant  a.  capable  of  being  altered  by  light  makes  the  compari- 
Yellow    spot,    b,  b.    son  still  more  striking. 

^•—-'^-      -'     Medullate('' 

-{Kiihne.)  ,      ,  .  . 

or  an  optogram  of  an  external  object  m  a  manner 
similar  to  that  by  which  an  image  is  fixed  on  the  sensitive  plate  of  a 
camera.  An  animal  is  kept  in  the  dark  for  about  ten  minutes  in 
order  to  permit  the  retinal  pigment  to  be  completely  regenerated. 
The  animal,  with  the  eyes  covered,  is  then  brought  into  a  room  with 
a  single  window.  While  the  head  is  steadily  directed  to  the  window, 
the  eye  is  exposed  for  several  minutes.  The  eyes  are  again  covered,  the 
animal  killed,  and  the  eyes  removed  by  the  light  of  a  sodium  flame.  The 
retina  is  then  placed  in  a  4  per  cent,  solution  of  alum.  In  a  short  time  the 
image  of  the  window,  the  optogram,  will  be  fixed  (Fig.  313).  That  portion 
of  the  image  corresponding  to  the  window  lights  will  be  quite  bleached  in 
appearance  from  the  action  of  the  light  on  the  pigment,  while  that  corre- 
sponding to  window  bars  will  have  the  usual  color  of  the  retina.  Dur- 
ing life  the  regeneration  of  the  visual  purple  must  take  place  with  extreme 
rapidity  if  a  similar  change  takes  place  with  the  formation  of  each  image. 
The  visual  purple  is  believed  to  be  derived  from  a  pigment  secreted  by  the 
layer  of  pigment  cells. 

Binocular  Vision. — Though  two  images  are   formed,   one  on   each 
retina,  when  the  eyes  are  directed  to  a  given  object,  there  results  but  one 


nfr::^befs.4lS.f  ^uhne  even  succeeded  in  obtaining  a  fixed  image 


THE  SENSE  OF  SIGHT  707 

sensation.     If  the  direction  of  either  visual  axis  be  changed  by  pressure  on 

the  eyeball,  there  arise  two  sensations,  and  the  object  appears  to  be  doubled. 

The  reason  assigned  for  this,  in  the  first  instance,  is  that  the  two  images 

fall  into  the  foveae,  on  two  corresponding  points;  while  in  the  second  instance 

they  fall  on  non-corresponding  points.     It  would  appear,  therefore,  that  for 

the  purpose  of  seeing  an  object  singly  when  the  eyes  are  directed  toward 

it,  the  rays  emanating  frcm  it  must  fall  on  corresponding  parts  of  the  retina. 

As  all  portions  of  the  retina  are  sensitive 

to  light,  though  in  varying  degrees,  it  is 

not  essential  that  the  images  always  fall 

in  the  fovese.     The  parts  of  the  retinae 

which    correspond    physiologically    are 

shown   in  Fig.  314.     In  this  figure  the 

retinal  area  is  divided   into   quadrants   ^  ^ 

,  ,.     ,         ,   ,       .        ,    1  !•  f  Fig. -jiA. — Corresponding  Areas  OF  THE 

by  vertical  and  horizontal  lines  of  sepa-  Retina. 

ration,  as  they  are  termed.     If  one  retina 

is  placed  in  front  of  or  over  the  other,  it  will  be  found  that  the  quadrants 
bearing  similar  letters  cover  each  other.  So  long  as  the  rays  of  light, 
entering  the  eye,  fall  on  corresponding  areas  the  sensation  of  but  one 
object  arises.  If,  however,  they  fall  on  non-corresponding  areas,  two  sen- 
sations arise.  Normal  binocular  \ision  enlarges  very  considerably  the  area 
of  the  visual  field,  permits  of  a  better  estimation  of  the  size  and  distance 
of  objects,  enables  the  mind  to  form  more  readily  a  perception  of  depth, 
and  increases  the  intensity  of  sensations. 

The  Horopter. — When  the  eyes  are  in  a  so-called  secondary  position 
— that  is,  in  a  position  in  which  the  visual  axes  are  converged  and  directed 
to  a  point  in  front  of  and  in  the  middle  plane  of  the  body — it  will  be  found 
on  examination  that  rays  of  light  from  a  number  of  other  objects  enter  the 
eye,  pass  through  the  nodal  point,  and  fall  on  corresponding  parts  of  the 
two  retinas  and  give  rise  to  but  single  images.  All  such  points  lie,  for  the 
horizontal  line  of  separation,  on  a  line  termed  the  horopter.  The  form  of 
this  line  is  that  of  a  circle  which  passes  through  the  fixation  point  and  the 
two  nodal  points.  Any  object  on  the  horopter  will  give  rise  to  but  a  single 
image.  This  is  shown  in  Fig.  315,  in  which  the  objects  I,  II,  III  project 
their  rays  into  both  eyes  and  upon  corresponding  areas. 

In  addition  to  the  horopter  for  the  horizontal  line  of  separation,  there 
is  also  an  horopter  for  the  vertical  line  of  separation.  At  a  distance  of  two 
meters  the  vertical  horopter  is  a  plane.  Within  this  distance  it  is  concave  to 
the  face;  beyond  this  distance  it  is  convex. 

An  object  which  lies  either  in  front  of  or  behind  the  fixation  point  will 
project  its  rays  on  parts  of  the  retinae  which  do  not  correspond,  and  hence 
give  rise  to  double  images.  This  is  evident  from  examination  of  Fig.  316. 
While  the  eyes  are  directed  to  figure  2,  of  which  there  is  but  a  single  image, 
the  objects  B  and  A  give  rise  to  double  images,  for  reasons  already 
given.  If  the  eyes  are  now  directed  to  B,  double  images  will  be  formed  of 
2  and  A. 

At  all  times,  therefore,  double  images  are  formed  on  the  retinae  the 
existence  of  which  is  scarcely  noticed  unless  the  attention  is  directed  to  them. 
This  is  due  to  the  fact  that  many  of  the  images  fall  on  the  peripheral,  less 
sensitive  parts  of  the  retinae.    At  the  same  time,  from  a  want  of  accommo- 


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dation  and  the  formation  of  diffusion-circles,  they  are  indistinct.  For  these 
reasons  they  are  readily  neglected. 

In  the  primary  position  of  the  eyes — that  is,  a  position  in  which  the 
visual  axes  are  parallel — the  horopter  is  a  plane  at  infinity.  In  the  tertiary 
positions  the  horopter  is  a  curve  of  complex  form. 

Movements  of  the  Eyeball. — The  almost  spheric  eyeball  lies  in  the 
correspondingly  shaped  cavity  of  the  orbit,  like  a  ball  placed  in  a  socket, 
and  is  capable  of  being  rotated  to  a  considerable  extent  by  the  six  muscles 
which  are  attached  to  it.  These  muscles  are  the  superior  and  inferior  recti, 
the  external  and  internal  recti,  and  the  superior  and  inferior  obliqui.  The 
four  recti  muscles  arise  from  the  apex  of  the  orbit  cavity,  from  which  point 
they  pass  forward  to  be  inserted  into  the  sclera  about  7  to  8  mm.  from  the 
corneal  border.  The  superior  oblique  muscle  having  a  similar  origin  passes 
forward  to  the  upper  and  inner  angle  of  the  orbit  cavity,  at  which  point  its 


B 


Fig.  311;. — Horopter  for  the 
Secondary  Position,  with  Con- 
vergence OF  the  Visual  Axes. 
— {Landois.) 


Fig.  316. — Scheme  of  Identical  and 
Non-identical  Points  of  the  Retina. — 
{Landois.) 


tendon  passes  through  a  cartilaginous  pulley,  after  which  it  is  reflected  back- 
ward to  be  inserted  into  the  superior  surface  of  the  sclera  about  16  mm. 
behind  the  corneal  border.  The  inferior  oblique  muscle  arises  from  the  inner 
and  inferior  angle  of  the  orbit  cavity.  It  then  passes  outward,  upward,  and 
backward,  to  be  inserted  into  the  upper,  posterior,  and  temporal  portion  of 
the  sclera  about  4  or  5  mm.  from  the  optic  nerve  entrance. 

The  movements  of  each  eye  are  referred  to  three  fixed  lines  or  axes, 
which  have  their  origin  at  the  point  of  rotation  of  the  eyeball,  this  point 
lying  about  1.7  mm.  behind  the  center  of  the  globe.  If  the  eye  looks  straight 
forward  in  the  horizontal  plane  (the  head  being  erect),  the  line  joining  the 
center  of  rotation  with  the  object  looked  at  is  the  line  of  fixation  or  line  of 
regard.  Around  this  antero-posterior  axis  the  eye  may  be  regarded  as  per- 
forming its  circular  rotation  or  torsion.  At  right  angles  to  this  line,  and 
joining  the  centers  of  rotation  of  both  eyes,  is  the  horizontal  or  transverse 


THE  SENSE  OF  SIGHT  709 

axis,  around  which  the  movements  of  elevation  (up  to  34  degrees)  and  de- 
pression (down  to  57  degrees)  take  place.  At  right  angles  to  both  of  these 
lines  there  is  the  vertical  axis,  around  which  the  movements  of  adduction 
(toward  the  nose  up  to  45  degrees)  and  abduction  (toward  the  temple  up 
to  42  degrees)  occur.  The  six  muscles  may  be  divided  into  three  pairs,  each 
of  which  has  a  common  axis  around  which  it  tends  to  move  the  eyeball. 
But  only  the  common  axis  of  the  internal  and  external  recti  coincides  with 
one  of  three  axes  before  mentioned — namely,  with  the  vertical  axis — thus 
moving  the  ball  only  inwardly  or  outwardly — respectively.  The  other  two 
pairs,  however,  have  their  own  axes  of  action,  and  their  movements  of  the 
ball  must  be,  therefore,  analyzed  with  regard  to  all  the  three  axes,  each 
of  these  four  muscles  producing  rotation,  elevation,  and  depression,  and 
abduction  or  adduction.  The  superior  and  inferior  recti  muscles,  form- 
ing one  pair,  move  the  eye  around  a  horizontal  axis  which  intersects  the 
median  plane  of  the  body  in  front  of  the  eyes  at  an  angle  of  63  degrees, 
and  the  superior  and  inferior  obhque  muscles  forming  the  third  pair  rotate 
the  globe  around  a  horizontal  axis  which  cuts  the  median  plane  of  the  body 
behind  the  eyes  at  an  angle  of  39  degrees.  Thus  it  is  that  each  muscle 
moves  the  eye  as  follows,  the  movement  for  practical  purposes  being  referred 
to  the  cornea:  The  rectus  externus  draws  the  cornea  simply  to  the  temporal 
side,  the  rectus  Internus  simply  to  the  nose;  the  superior  rectus  displaces  the 
cornea  upward,  slightly  inward,  and  turns  the  upper  part  toward  the  nose 
(medial  torsion);  the  inferior  rectus  moves  the  cornea  downward,  slightly 
inward,  and  twists  the  upper  part  away  from  the  nose  (lateral  torsion); 
the  superior  oblique  displaces  the  cornea  downward,  slightly  outward,  and 
produces  medial  torsion;  while  the  inferior  oblique  moves  the  cornea  upward, 
slightly  outward,  and  produces  lateral  torsion.  These  facts  show  that  for 
certain  movements  of  the  eye  at  least  three  muscles  are  necessary  (see 
following  table): 

Inward Rectus  internus.  Inward  and                   f  Rectus  internus. 

Outward Rectus  externus.  downward \  Rectus  inferior. 

Uinnard                     I  ^^^^us  superior.  I  Obliquus  superior. 

^             1  Obliquus  inferior.  Outward  and                j  Rectus  externus. 

n^.»«„   *j  J  Rectus  inferior.  upward \  Rectus  superior. 

Uowtvward .  ObUquus  superior.  ObUquus  inferior. 

Inward  and               [  Rectus  internus.  Outward  and                |  Rectus  externus. 

upward •<  Rectus  superior.  downward \  Rectus  inferior. 

[  Obliquus  inferior.  [  Obliquus  superior. 

If  both  eyes  have  their  line  of  vision  in  the  horizontal  plane  parallel 
with  each  other  and  with  the  median  plane  of  the  body,  they  are  said  to  be 
in  the  primary  position.  All  other  positions  are  called  secondary  and  tertiary. 
Both  eyes  always  move  simultaneously,  which  is  called  the  associated  move- 
ment  oj  the  eyes.  There  are  three  forms  of  associated  movements:  (i)  move- 
ment of  both  eyes  in  the  same  direction;  (2)  movements  of  convergence  by 
which  the  visual  lines  are  converged  on  a  point  in  the  middle  line  of  the  body; 
(3)  movements  of  divergence,  by  which  the  eyes  are  brought  back  from 
convergence  to  parallelism,  or  even  to  divergence,  as  in  certain  stereoscopic 
exercises.  A  combination  of  (i)  and  (2)  or  of  (i)  and  (3)  takes  place  for 
certain  positions  of  the  object  looked  at. 

Color-perception. — A  beam  of  sunlight  passed  through  a  glass  prism 
is  decomposed  into  a  series  of  colors — red,  orange,  yellow,  green,  blue,  and 


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TEXT-BOOK  OF  PHYSIOLOGY 


violet — the  so-called  spectral  colors,  so  well  exemplified  in  the  rainbow. 
The  spectral  colors  are  termed  simple  colors,  because  they  cannot  be  any 
further  decomposed  by  a  prism.  Objectively,  the  spectral  colors  consist  of 
very  rapid  transverse  electro-magnetic  vibrations  of  the  ether,  from  about 
400  millions  of  milHons  per  second  for  red  to  about  760  millions  of  millions 
for  violet,  but  subjectively  they  are  sensations  caused  by  the  impact  of  the 
ether-waves  on  the  percipient  layer  of  the  retina. 

It  is  possible  to  mix  or  blend  these  spectral  color-sensations  in  the  eye 
by  stimulating  the  same  area  of  the  retina  by  different  spectral  colors,  either 
at  the  same  time  or  in  rapid  succession.  The  following  table  shows  the 
results  of  such  experiments  as  performed  by  v.  Helmholtz  (Dk.  =dark; 
Wh.=  whitish): 


Violet 


Indigo 


Cyan-blue 


Bluish-green 


Green 


Greenish- 
yeUow 


Yellow 


Red. 

Orange. 

Yellow. 

Gr.-yellow. 

Green. 

Bluish-green. 

Cyan-blue. 


Purple. 
Dk.-rose. 
Wh.-rose. 
White. 


Wh.-rose. 
White. 
Wh. -green 


Dk.-rose. 

Wh.-rose. 

White. 

Wh. -green. 
White-blue.    Water-blue.    Bl. -green. 

Water-blue.    Water-blue 

Indigo.  I 


:  White. 
Wh. -yellow. 
Wh. -yellow. 


Wh.- green.  Green. 


Wh. -yellow. 

Yellow. 

Gr.-yellow. 


Gold-yellow. 
Yellow. 


Orange. 


These  are  the  mixed  colors.  But  it  is  to  be  observed  that  only  two  new  color- 
sensations  can  be  produced,  white  and  purple,  the  remaining  mixed  colors 
already  finding  their  equivalent  in  the  spectrum.  White  and  purple, 
therefore,  are  color-sensations  which  have  no  objective  equivalent  in  a 
simple  number  of  ether-vibrations  like  the  spectral  colors. 

Two  spectral  colors  which  by  their  mixture  produce  the  sensation  of 
white  are  called  complementary  colors.  Such  are  red  and  green-blue,  golden 
yellow  and  blue,  green  and  violet.  The  mixture  of  all  the  spectral  colors 
produces  white  again.  This  is  the  result  of  adding  two  or  more  color- 
sensations.  Different  results  are  obtained,  however,  by  adding  color  pig- 
ments. Yellow  and  blue,  for  example,  produce  in  the  eye  white,  but  on  the 
painter's  palette  green.  The  colors  of  nature  are  usually  mixtures  of  simple 
colors,  as  can  be  shown  by  spectroscopic  analysis  or  by  a  synthesis  of  spectral 
colors. 

In  all  color-sensations  we  must  distinguish  three  primary  qualities:  (i) 
hue;  (2)  purity  or  tint;  (3)  brightness  or  luminosity.  The  first  quality  gives 
the  main  name  to  the  color — e.g.,  red  or  blue — this  depending  on  the  spectral 
color  or  the  mixture  of  two  spectral  colors  with  which  it  can  be  matched. 
The  second  quality,  the  tint,  depends  on  the  admixture  of  white  with  the 
ground  color;  and  the  third  quality,  brightness,  depends  on  the  objective 
intensity  of  the  light  and  the  subjective  sensitiveness  of  the  retina.  Color- 
perception  thus  far  refers  only  to  the  most  sensitive  part  of  the  retina.  At 
the  more  peripheral  parts  of  the  retina  the  colors  are  seen  somewhat  differ- 
ently, as  is  shown  by  the  following  table  giving  the  limits  up  to  which  the 
colors  are  recognized: 

White.  Blue.  Red.  Green. 

Externally 90°  80°  65°  50° 

Internally 60°  55°  50°  40° 

Superiorly 45°  40°  35°  30° 

Inferioriy 70°  60°  45°  35° 


THE  SENSE  OF  SIGHT  711 

Theories  of  Color-perception.— T^e  theory  of  v.  HelmhoUz,  originated 
by  Thomas  Young  (1807),  assumes  in  its  latest  form  the  existence  in  the 
human  retina  of  three  different  kinds  of  end-organs,  each  of  which  is  loaded 
with  its  own  photo-chemical  substance  capable  of  being  decomposed  by  a 
certain  color,  and  thus  exciting  the  fiber  of  the  optic  nerve. 

In  the  first  group  these  end-organs  are  loaded  with  a  red-sensitive  sub- 
stance, which  is  afi'ected  mainly  by  the  red  part  of  the  spectrum;  the  second 
group  has  its  end-organs  provided  with  a  green-sensitive  substance,  which 
is  mainly  excited  by  the  green  color;  while  the  third  group  is  provided  with 
a  blue-sensitive  substance,  this  latter  being  mainly  affected  and  decomposed 
by  the  blue-violet  portion  of  the  spectrum.  All  these  three  different  end- 
organs  are  present  in  every  part  of  the  most  sensitive  area  of  the  retina,  and 
are  connected  by  separate  nerve-fibers  with  special  parts  of  the  brain,  in  the 
cells  of  which  each  calls  up  its  separate  sensation  of  red  or  green  or  blue. 

Out  of  these  three  primary  color-sensations  all  other  color-sensations 
arise.  If  a  light  mainly  excites  the  red-  or  green-  or  blue-sensitive  substance 
of  a  retinal  area,  we  term  it  red,  green,  or  blue,  respectively.  But  if  two  of 
these  photo-chemical  substances  are  stimulated  simultaneously,  quite  differ- 
ent sensations  arise.  Thus  simultaneous  stimulation  of  the  red  and  green 
substances  gives  rise  to  the  sensation  of  yellow,  that  of  red  and  blue  to  the 
sensation  of  purple,  and  that  of  blue  and  green  to  the  sensation  of  blue-green. 
Simultaneous  stimulation  of  all  three  substances  of  a  certain  area  produces 
the  sensation  of  white.  According  to  this  theory,  complementary  colors 
are  any  two  which  together  excite  all  three  substances.  Color-blindness 
is  explained  by  this  theory,  on  the  assumption  that  two  of  the  photo-chemical 
substances  have  become  similar  or  equal  in  composition  to  each  other. 

The  theory  of  Hering,  brought  forward  in  1874,  has  the  underlying 
assumption  that  the  process  of  restitution  in  a  nerve-element  is  capable  of 
exciting  a  sensation.  This  theory  asserts  that  there  are  three  visual  sub- 
stances in  the  retina — a  white-black,  a  red-green,  and  a  yellow-blue  visual 
substance.  A  destructive  process  in  the  white-black  substance,  such  as  is 
induced  not  only  by  white  light,  but  also  by  any  other  simple  or  mixed  color, 
produces  the  sensation  of  white,  while  the  process  of  restitution  or  assimila- 
tion in  this  substance  produces  the  sensation  of  black.  Similarly,  red  light 
produces  dissimilation  or  decomposition  in  the  red-green  substance,  and 
this,  again,  the  sensation  of  red.  Green  light,  however,  favors  the  process 
of  restitution  or  assimilation  in  the  red-green  substances,  and  thus  gives  rise 
to  the  sensation  of  green.  In  the  same  way  the  sensation  of  yellow  has  its 
cause  in  the  decomposition  of  yellow-blue  substance  Induced  by  yellow  light, 
while  the  sensation  of  blue  is  produced  by  an  assimilative  process  in  the 
same  substance.  Simultaneous  processes  of  dissimilation  and  assimila- 
tion in  the  same  visual  substance  antagonize  each  other,  and  consequently 
produce  no  color-sensation  by  means  of  this  substance,  but  only  the 
sensation  of  white,  by  reason  of  decomposition,  by  both  colors,  in  the 
white-black  substance.  Thus,  yellow  and  blue,  impinging  on  the  same 
retinal  area,  have  no  effect  on  the  yellow-blue  substance,  because  they  are 
antagonistic  In  their  action  on  this  substance,  but  produce  only  the  sensation 
of  white,  as  both  yellow  and  blue  decompose  the  white-black  material. 
Color-blindness  is  explained  by  the  assumption  of  the  absence  of  either  the 
red-green  or  the  yellow-blue  visual  substance  in  the  retina. 


712  TEXT-BOOK  OF  PHYSIOLOGY 

Accessory  Structures. — The  eyeball  is  protected  anteriorly  by  the 
eyelids  and  their  associated  structures,  the  Meibomian  glands,  the  lachrymal 
glands,  and  tears. 

The  eyelids  consist  of  a  central  framework  of  connective  tissue  support- 
ing muscle-tissue  (the  orbicularis  palpebrarum  muscle)  and  glands,  and 
covered  externally  by  skin  and  internally  by  a  modified  skin,  the  conjunctiva. 
The  free  border  of  each  lid  is  strengthened  by  a  semilunar  plate  of  dense 
fibrous  tissue,  the  tarsus.  The  cutaneous  edge  of  the  lid  is  bordered  with 
short  stiff  hairs.  At  the  inner  extremity  each  eyelid  presents  a  small  opening, 
the  punctum  lacrimale,  the  beginning  of  the  lachrymal  duct.  The  two  ducts 
after  uniting  open  into  the  nasal  duct. 

The  Meibomian  glands  are  modified  sebaceous  glands  imbedded  in  the 
posterior  portion  of  the  lids.  Their  ducts  open  on  the  free  border  of  the  lid. 
These  glands  secrete  an  oleaginous  material  resembling  sebaceous  matter, 
which  accumulates  along  the  margin  of  the  lid  and  prevents  the  tears  from 
flowing  down  the  cheek. 

The  lachrymal  gland  is  situated  at  the  upper  and  outer  part  of  the  orbit 
cavity.  It  consists  of  a  series  of  compound  tubules  lined  by  epithelium. 
The  secretion  (the  tears)  is  conducted  from  the  gland  to  the  outer  part  of  the 
conjunctiva  by  seven  or  eight  ducts.  The  lachrymal  secretion  consists  of 
water  and  inorganic  salts.  It  is  distributed  over  the  corneal  surface  during 
the  act  of  winking,  thus  keeping  it  moist  and  free  from  foreign  particles. 
It  eventually  passes  into  the  lachrymal  ducts  and  then  into  the  nose.  The 
lachrymal  glands  receive  secretory  fibers  by  way  of  the  fifth  nerve  and  the 
cervical  sympathetic.  The  secretion  can  be  excited  reflexly  from  stimulation 
of  sensor  nerves  as  well  as  by  emotional  states. 


CHAPTER  XXX 
THE  SENSE  OF  HEARING 

The  physiologic  mechanism  involved  in  the  sense  of  hearing  includes  the 
ear,  the  acoustic  nerve,  the  acoustic  tract  (the  lateral  fillet  or  lemniscus),  the 
acoustic  radiation,  and  nerve-cells  in  the  cortex  of  the  temporal  lobe. 

Peripheral  stimulation  of  this  mechanism  develops  nerve  impulses  which, 
transmitted  to  the  cerebral  cortex,  evoke  the  sensation  of  sound  and  its  vary- 
ing qualities — intensity,  pitch,  and  timbre. 

The  specific  physiologic  stimulus  to  the  terminal  organ,  the  organ  of 
Corti,  is  the  impact  of  atmospheric  pulsations  of  varying  energy  and  rapidity. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EAR 

The  ear,  the  organ  of  hearing,  is  lodged  within  the  petrous  portion  of 
the  temporal  bone.  It  may,  for  convenience  of  description,  be  divided  into 
three   portions:  viz.,   the  external,   the  middle,   and  the  internal  portion 

(Fig.  317)- 

The  External  Ear. — The  external  ear  consists  of  the  pinna  or  auricle 

and  the  external  auditory  canal. 

The  Pinna. — The  pinna  is  composed  of  "a  thin  layer  of  cartilage  which 
presents  a  series  of  elevations  and  depressions.  It  is  attached  by  fibrous  tis- 
sue to  the  outer  edge  of  the  auditory  canal  and  covered  by  a  layer  of  skin  con- 
tinuous with  that  covering  adjacent  structures.  The  general  shape  of  the 
pinna  is  concave.  Its  anterior  surface  presents,  a  little  below  the  center,  a 
deep  depression — the  concha. 

The  External  Auditory  Canal. — The  external  auditor}'  canal  extends  from 
the  concha  inward  for  a  distance  of  from  25  to  30  mm.  It  is  directed  at  first 
upward,  forward,  inward,  and  then  somewhat  downward  to  its  termination. 
It  is  composed  partly  of  bone  and  partly  of  cartilage  and  fined  by  a  reflection 
of  the  skin  covering  the  pinna.  At  the  external  portion  of  the  canal  the  skin 
contains  a  number  of  tubular  glands,  the  ceruminoiis  glands,  which  resemble 
in  their  conformation  the  perspiratory  glands.  They  secrete  cerumen  or  ear- 
wax. 

The  Middle  Ear. — The  middle  ear,  or  tympanum,  is  an  irregularly 
shaped  cavity  hollowed  cut  of  the  temporal  bone  and  situated  between  the 
external  auditory  canal  and  the  internal  ear.  It  is  narrow  from  side  to  side, 
though  wider  above  than  below.  It  is  relatively  long  in  its  antero-posterior 
and  vertical  diameters.  The  upper  portion  is  known  as  the  attic.  The 
middle  ear  is  in  communication  posteriorly  with  the  mastoid  cells,  anteriorly 
uith  the  pharynx  through  the  Eustachian  tube. 

The  Eustachian  Tube. — The  passageway  between  the  tympanic  cavity  and 
the  naso-pharynx  is  known  from  its  discoverer  as  the  Eustachian  tube.  It 
is  composed  internally  of  bone,  externally  of  cartilage,  and  is  lined  by  mucous 

713 


714 


TEXT-BOOK  OF  PHYSIOLOGY 


membrane  covered  with  ciliated  epithelium.  Near  the  middle  of  its  course 
the  tube  is  contracted,  though  expanded  at  either  extremity  (Fig.  317).  It 
measures  about  40  mm.  in  length.  Its  general  direction  from  the  pharyn- 
geal orifice  is  outward,  backward,  and  upward  at  an  angle  of  about  45 
degrees. 

The  middle  ear  cavity  is  separated  from  the  external  ear  by  a  membrane 
— the  membrana  tympani — and  from  the  internal  ear  by  an  osseo- 
membranous  partition  which  forms  a  common  wall  for  both  cavities. 
The  interior  of  the  cavity  is  crossed  from  side  to  side  by  a  chain  of 
bones  and  lined  by  a  mucous  membrane  continuous  with  that  lining  the 
pharynx. 

The  Membrani  Tympani. — The  membrana  tympani,  is  a  thin,  translucent 
nearly  circular  membrane,  measuring  about  10  mm.  in  diameter,  placed  at 


Fig.  317. — The  Ear.  i.  Pinna,  or  auricle.  2.  Concha.  3.  External  auditory  canal. 
4.  Membrana  tympani.  5.  Incus.  6.  Malleus.  7.  Manubrium  mallei.  8.  Tensor  tympani. 
9.  Tympanic  cavity.  10.  Eustachian  tube.  11.  Superior  semicircular  canal.  12.  Posterior  semi- 
circular canal.  13.  External  semicircular  canal.  14.  Cochlea.  15.  Internal  auditory  canal. 
16.  Facial  nerve.  17.  Large  petrosal  nerve.  18.  Vestibular  branch  of  auditory  nerve.  19. 
Cochlear  branch. — (Sappey.) 


the  inner  termination  of  the  external  auditory  canal.  It  is  inclosed  in  a  ring 
of  bone  which  in  the  fetal  condition  can  be  easily  removed,  but  in  the  adult 
condition  cannot  be  removed,  owing  to  its  consolidation  with  the  surrounding 
bone.  This  membrane  consists  primarily  of  a  layer  of  fibrous  tissue  which 
is  covered  externally  by  a  thin  layer  of  skin  continuous  with  that  lining  the 
auditory  canal,  and  internally  by  a  thin  mucous  membrane.  The  tympanic 
membrane  is  placed  obliquely  at  the  bottom  of  the  auditory  canal,  inclining 
from  above  and  behind  downward  and  forward  at  an  angle  of  about  forty- 
five  degrees.  The  external  surface  of  this  membrane  presents  a  funnel- 
shaped  depression,  the  sides  of  which  are  slightly  convex. 

The  Ear-bones. — Running  across  the  tympanic  cavity  and  forming  an 
irregular  line  of  joined  levers  is  a  chain  of  bones,  which  articulate  one  with 


THE  SENSE  OF  HEARING 


715 


another  at  their  extremities.  These  bones  are  known  as  the  malleus,  incus, 
and  stapes.  The  form  and  arrangement  of  these  bones  are  shown  in  Figs. 
318,  319- 

The  malleus,  or  hammer  bone,  consists  of  a  head,  neck,  and  handle, 
of  which  the  latter  is  attached  to  the  inner  surface  of  the  membrana  tympani. 
The  incus  or  anvil  bone  presents  a  concave  articular  surface  which  receives 
the  head  of  the  malleus.  The  stapes,  or  stirrup  bone,  articulates  externally 
with  the  long  process  of  the  incus,  and  internally,  by  its  oval  base,  with  the 
edges  of  an  oval  opening,  the  foramen  ovale.  The  entire  chain  is  partially 
supported  by  a  ligament  attached  to  the  shrot  process  of  the  incus  and 
to  the  walls  of  the  tympanic  cavity. 

The  Tensor  Tympani  Muscle. — This  is  a  delicate  muscle,  about  15  mm. 
in  length,  situated  in  a  narrow  groove  just  above  the  Eustachian  tube  (Fig. 

320).  It  arises  from  the  car- 
tilaginous portion  of  the  Eusta- 
chian tube  and  the  adjacent 
portion  of  the  sphenoid  bone. 
From  this  origin  it  passes  nearly 
horizontally  backward  to  the 
tympanic    cavity;  just  opposite 


Fig.  318. — ^Tympanic  Membrane  and  the  Audi- 
tory Ossicles  (Left)  seen  prom  within,  ie., 
FROM  the  Tympanic  Cavity.  M.  Manubrium 
or  handle  of  the  malleus.  T.  Insertion  of  the 
tensor  tympani,  h.  Head.  IF.  Long  process  of 
the  malleus,  a.  Jncus,  with  the  short  (K)  and  the 
long  (/)  process.  5.  Plate  of  the  stapes.  Ax, 
Ax,  is  the  common  axis  of  rotation  of  the  auditory 
ossicles.  5'.  The  pinion-wheel  arrangement  be- 
tween the  malleus  and  incus. — (Landois.) 


Fig.  319. — Audi- 
tory Ossicles,  i. 
Head  of  malleus.  2. 
Processus  brevis.  3. 
Processus  gracilis. 
4.  Manubrium.  5. 
Long  process  of  in- 
cus. 6.  Articulation 
between  incus  and 
stapes.  7.  Stapes. 
— (Sappey.) 


the  foramen  ovale  its  tendon  bends  at  a  right  angle  over  the  processus 
cochleariformis  and  then  passes  outward  across  the  tympanic  cavity  to  be 
inserted  into  the  handle  of  the  malleus  near  the  neck. 

The  Stapedius  Muscle. — The  stapedius  muscle  emerges  from  the  cavity  of 
a  pyramid  of  bone  which  projects  from  the  posterior  wall  of  the  tympanum. 
Its  tendon  passes  forward  to  be  inserted  into  the  neck  of  the  stapes  bone  near 
its  point  of  articulation  with  the  incus. 

The  Internal  Ear. — The  internal  ear,  or  labyrinth,  is  located  within 
the  petrous  portion  of  the  temporal  bone.  It  consists  of  an  osseous  and  a 
membranous  portion  of  which  the  latter  is  contained  within  the  former. 

The  osseous  labyrinth  is  subdivided  into  vestibule,  semicircular 
canals,  and  cochlea. 

The  Vestibule. — The  vestibule  is  a  small,  triangular-shaped  cavity  between 


7i6 


TEXT-BOOK  OF  PHYSIOLOGY 


the  semicircular  canals  and  the  cochlea.  It  is  separated  from  the  cavity  of 
the  middle  ear  by  an  osseous  partition  which  presents  near  its  center  an  oval 
opening,  the  foramen  ovale.  (See  Fig.  322.)  In  the  living  condition  this 
opening  is  closed  by  the  base  of  the  stapes  bone,  which  is  held  in  position  by 

an  annular  Hgament.  The  inner  wall  presents  a 
number  of  openings  for  the  passage  of  nerve- 
fibers. 

The  Semicircular  Canals. — The  semicircular 
canals  are  three  in  number,  and  named  from  their 
position,  the  superior  vertical,  the  posterior  verti- 
cal and  the  horizontal.  These  canals  are  at 
right  angles  one  to  the  other  and  open  by  five 
orifices  into  the  vestibule,  one  of  the  orifices,  how- 
ever, being  common  to  two  of  the  canals.  Each 
canal  near  the  vestibular  orifice  is  enlarged  to 
almost  twice  the  size  of  the  rest  of  the  canal  form- 
ing what  has  been  termed  the  ampulla.  The 
superior  canal  is  placed  transversely  to  the  long 
axis  of  petrous  portion  of  the  temporal  bone;  the 
posterior  canal  is  placed  almost  parallel  to  the  posterior  surface  of  the  petrous 
portion  of  the  temporal  bone.  The  lateral  or  horizontal  canal  is  placed 
horizontally  to  the  vertical  canals.  Their  relation  to  one  another  on  opposite 
sides  of  the  median  plane  of  the  body  is  shown  in  Fig.  321.  From  the  ana- 
tomic relations  of  these  canals,  it  is  apparent  that  the  horizontal  or  lateral 
canals  lie  in  one  and  the  same  plane;  that  the  two  superior  vertical  canals, 
as  well  as  the  two  posterior  vertical  canals,  lie  in  planes  which  are  at  right 
angles  to  each  other,  and  that  the  superior  vertical  canal  of  the  one  side, 


Fig.  320. — M,  The  Tensor 
Tympani  Muscle — the  Eus- 
tachian Tube  (Left). — 
(Landois.) 


Fig.  321. — DIGRAMS  of  Semicircular  Ganals  to  Show  their  Position  in  Three  Planes 
AT  Right  Angles  to  One  Another.  It  will  be  seen  that  the  two  horizontal  canals  lie  in  the 
same  p\ane{H),  and  that  the  superior  vertical  of  one  side  {SV)  is  in  the  same  plane  as  the 
posterior  vertical  (FV)  of  the  other  side. — (from  Ewald.) 

lies  in  a  plane  parallel  to  the  plane  in  which  the  posterior  vertical  canal  of 
the  opposite  side  lies. 

The  Cochlea. — The  cochlea,  the  anterior  portion  of  the  labyrinth,  is  a 
gradually  tapering  canal,  about  35  mm.  in  length,  wound  spirally  two  and  a 
half  times  around  a  central  bony  axis,  the  modiolus.  The  cavity  of  the  coch- 
lea is  partially  subdivided  into  two  ca^dties  by  a  thin  spiral  plate  of  bone 
which  projects  from  the  inner  wall,  known  as  the  lamina  ossea  spiralis.  In 
the  natural  condition  this  partition  is  completed  by  a  connective-tissue  mem- 
brane, so  that  the  two  passages  are  completely  separated  from  each  other. 


THE  SENSE  OF  HEARING 


717 


The  upper  passage  or  scala  is  in  free  communication  with  the  vestibule, 
and  is  known  as  the  scala  vestibiili;  the  lower  passage  or  scala  in  the  dead  con- 
dition communicates  with  the  tympanum  by  means  of  a  round  opening,  the 
foramen  rotundum,  and  is,  therefore,  known  as  the  scala  tympani.     In  the  liv- 


FiG.  322. — Bony  Cochlea,  i. 
Ampulla  of  superior  semicircular 
canal.  2.  Horizontal  canal.  3. 
Junction  of  superior  and  posterior 
semicircular  canals.  4.  The  pos- 
terior semicircular  canal.  5.  Fora- 
men rotunduro.  6.  Foramen  ovale. 
7.  Cochlea. 


Fig.  323. — I.  Utricle.  2. 
Saccule.  3.  \"estibular  end  of 
cochlea.  4.  Canalis  reuniens. 
5.  Membranous  cochlea.  6. 
Membranous  semicircular 

canals. 


B>C^ 


i::j^^ 


ing  condition  this  opening  is  completely  closed  by  a  membrane,  a  second 
membrana  tympani.  Both  the  scalas  vestibuli  and  tympani  communicate 
at  the  apex  of  the  cochlea  by  a  small  opening,  the  helicotrema.  The 
modiolus,  the  central  bony  axis,  is  perforated  from  base  to  apex  by 
a  canal  for  the  passage  of  the  auditory  nerve-fibers;  lateral  canals,  diverg- 
ing from  the  central  canal,  pass  through 
the  osseous  lamina  spiralis  and  transmit  a^-SWA-'-- 

fibers  of  the  auditory  nerve.     The  interior  ^:';fl^-^^^^'W^i^-0 

of  the  bony  labyrinth  is  lined  by  periosteum 
covered  by  epithelium  and  in  communication 
with  lymph-spaces  at  the  base  of  the  skull 
by  means  of  the  aqueduct  of  the  vestibule. 

The  membranous  labyrinth,  lying 
within  the  osseous  labyrinth,  corresponds 
with  it  in  form,  though  it  is  smaller  in  size. 
It  may  be  subdivided  into  vestibule,  semi- 
circular canals,  and  cochlea  (Fig.  323). 

The  Vestibular  Portion. — The  vestibular 
portion  consists  of  two  small  sacs,  the  utricle 
and  the  saccule,  which  communicate  with 
each  other  by  means  of  the  two  branches  of 
a  duct  passing  through  the  aqueduct  of  the 
vestibule — the  ductus  endolyniphaticus. 

The  Semicircular  Canals. — The  semicir- 
cular canals,  three  in  number,  communicate 
with  the  utricle  in  the  same  manner  as  the  bony  canals  communicate  with 
the  vestibule.  The  saccule  communicates  with  the  membranous  cochlea 
by  a  short  canal,  the  canalis  reuniens.  The  walls  of  the  utricle,  saccule,  and 
semicircular  canals  are  composed  of  connective  tissue  lined  by  epithelium. 
At  the  points  of  entrance  of  the  auditory  nerve,  the  macules  acusticce,  in  all 


Fig.  324. —  Sectiox  of  W.\ll  of 
Utricle  of  the  Internal  Ear, 
through  macular  region,  from 
rabbit,  showing  otoliths  {o),  em- 
bedded within  granular  substance 
{g).  h.  Ciliated  cells  wdth  proc- 
esses, {p),  extending  between 
sustentacular  elements  {s).  m. 
Basement  membrane,  n.  Nerve- 
fibers  within  fibrous  tissue  (/) 
passing  toward  hair-cells  and 
becoming  non-medullated  at  base- 
ment-membrane.— {After  Piersol.) 


7i8 


TEXT-BOOK  OF  PHYSIOLOGY 


three  structures,  the  epithelium  undergoes  a  marked  change  in  appearance 
and  structure.  It  becomes  columnar  in  shape  and  provided  with  stiff  hair- 
like processes  or  threads,  which  project  into  the  cavity.  In  the  saccule  and 
utricle  the  hair-like  processes  are  covered  by  a  layer  of  small  crystals  of 
calcium  carbonate  held  together  by  a  gelatinous  material.  The  crystals  are 
known  as  otoliths  (Fig.  324). 

The  Vestibular  Nerve. — The  fibers  of  the  vestibular  nerve,  arising  from 
the  cells  of  the  ganglion  of  Scarpa  in  the  internal  auditory  meatus,  send  their 
peripherally  directed  branches  through  the  foramina  in  the  inner  wall  of  the 
vestibule,  through  the  walls  of  the  utricle  and  semicircular  canals  near  the 
ampulla.  As  the  fibers  approach  the  maculae  acusticse  they  subdivide  into 
delicate  fibrillae,  which  ultimately  become  histologically  and  physiologically 
related  to  the  neuroepithelium.  From  the  relation  of  the  nerve-fibers  to 
the  epithelium,  the  latter  must  be  regarded  as  the  highly  specialized  terminal 
organ  of  the  vestibular  portion  of  the  auditory  nerve. 

The  Cochlea. — The  cochlea  is  a  closed  membranous  tube  situated  between 
the  osseous  lamina  spiralis  and  the  outer  bony  wall.  A  transverse  section  of 
the  entire  cochlea  shows  the  relation  of  the  osseous  and  membranous  portions 
(Fig.  325).  The  cochlear  tube  is  triangular  in  shape.  The  base  is  attached 
to  the  bony  wall,  the  apex  to  the  edge  of  osseous  lamina  spiralis.  One  side  of 
the  tube  forms  in  part  the  membrane  of  Reissner,  the  other  side  forms  in  part 
the  basilar  membrane.  The  sides  of  the  cochlea  toward  the  scala  vestibuli 
and  scala  tympani  are  covered  with  epithelium.  The  triangular  cavity  of 
the  cochlear  tube  is  known  as  the  scala  media.     The  inner  surface  of  the 

cochlear  tube  is  lined  by  epithelium, 
which  becomes  extraordinarily  modi- 
fied and  specialized  along  the  surface 
of  the  basilar  membrane,  to  constitute 
what  is  known  as 

The  Organ  of  Corti. — In  Fig.  325 
this  organ  is  represented  as  it  appears 
on  cross-section  of  the  cochlea.  It  con  - 
sists  primarily  of  an  arch  composed 
of  two  modified  epithelial  cells  known 
as  the  rods  or  pillars  of  Corti,  which 
rest  below  on  the  basilar  membrane, 
but  meet  and  interlock  above;  it  con- 
sists secondarily  of  a  series  of  colum- 
nar epithelial  cells  provided  with  hair- 
like  processes  which  rest  upon  and  are 
supported  by  the  rods  both  on  the  inner  and  outer  aspects  of  the  arch. 
The  space  beneath  the  arch  is  known  as  the  tunnel.  The  inner  hair  cells 
are  not  nearly  so  numerous  as  the  outer  hair  cells.  The  epithelial  cells  ex- 
ternal to  the  outer  and  inner  hair  cells  are  supporting  or  sustentacular  in 
character. 

The  organ  of  Corti  extends  the  entire  length  of  the  cochlea.  The  num- 
ber of  rods  which,  standing  side  by  side,  form  the  inner  limb  of  the  arch  is 
estimated  at  5600;  the  number  which  form  the  outer  limb  is  estimated  at 
3850.  The  outer  rods  are  broader  than  the  inner  and  at  some  places  articu- 
late with  two  or  three  inner  rods.     The  upper  edges  of  the  rods  are  flattened, 


Fig 


325. — A    Transverse  Section  of 
Turn  of  the  Cochlea. 


THE  SENSE  OF  HEARING  .     719 

elongated,  and  project  outward,  forming  a  reticulated  membrane  through 
the  meshes  of  which  the  hair-like  processes  of  the  cells  project. 

From  the  connective-tissue  thickening  on  the  upper  surface  of  the  os- 
seous lamina  spiralis  there  extends  outward  over  the  organ  of  Corti  a  thin 
membrane,  the  membrana  tectoria.  The  common  cavity  between  the  walls 
of  the  osseous  and  membranous  labyrinth  in  the  vestibule,  the  semi- 
circular canals,  in  the  scala  vestibuli  and  scala  tympani  of  the  cochlea,  is 
filled  with  a  clear  fluid — the  perilymph;  the  common  cavity  within  the  walls 
of  the  entire  membranous  labyrinth  is  also  filled  with  a  similar  fluid — the 
endolymph.  The  hair-like  processes  of  the  epithelial  cells  covering  the 
maculae  acusticae  and  the  rods  of  Corti  are  consequently  bathed  by  endo- 
lymph. Both  fluids  are  in  relation  with  the  subarachnoid  lymph-spaces  at 
the  base  of  the  brain,  the  perilymph  through  the  aqueduct  of  the  vestibule, 
the  endolymph  through  the  endolymphatic  duct. 

The  Cochlear  Nerve. — The  libers  of  the  cochlear  nerve,  arising  from  the 
ganglion  cells  of  the  spiral  ganglion  situated  in  the  osseous  lamina  spiralis 
near  the  modiolus,  send  their  peripheral  branches  to  the  saccule  and  to  the 
organ  of  Corti.  As  they  approach  this  structure  they  lose  their  medullary 
sheath  and  become  naked  axis-cylinders.  The  fibers  then  divide  into  two 
parts,  of  which  one  passes  to  the  inner  hair  cells;  the  other  passes  between 
the  inner  rods  and  crosses  the  tunnel  between  the  outer  rods  to  the  outer  hair 
cells.  The  exact  method  of  termination  of  these  fibers  in  the  hair  cells  is 
unknown. 

From  the  relation  of  the  nerve- fibers  to  the  organ  of  Corti  the  latter  must 
be  regarded  as  the  highly  specialized  terminal  organ  of  the  cochlear  division 
of  the  auditory  nerve. 

THE  PHYvSIOLOGY  OF  HEARING 

The  general  function  of  the  ear  is  the  reception  of  atmospheric  vibrations 
and  the  transmission  of  the  effects  they  produce  to  the  percipient  elements — 
the  hair  cells — of  the  organ  of  Corti.  The  vibratory  excitation  of  these  end- 
organs  thus  caused,  is  the  basis  of  auditory  perceptions.  The  atmospheric 
vibrations  are  collected  by  the  pinna  and  concha,  conveyed  by  the  auditory 
canal  to  the  tympanic  membrane,  transmitted  by  the  chain  of  bones  to  the 
labyrinth  to  pass  successively  through  the  perilymph,  the  membranous  walls, 
and  the  endolymph,  to  the  hair  cells.  The  nerve  impulses  generated  by  these 
vibrations  are  then  transmitted  by  the  cochlear  (acoustic)  nerve  to  the  acous- 
tic centers  of  the  cerebrum,  where  the  sensations  of  sound  are  evoked.  In 
this  general  process  each  one  of  the  individual  structures  composing  the  ear 
in  its  entirety,  has  a  special  function.  In  order  to  fully  appreciate  these 
functions  the  characteristic  features  of  atmospheric  vibrations,  viz. :  intensity 
pitch  and  quality  must  be  kept  in  mJnd. 

Atmospheric  Vibrations. — The  vibrations  of  the  atmosphere,  the  ob- 
jective causes  of  the  sensations  of  sound,  are  imparted  to  the  atmosphere  by, 
the  vibrations  of  elastic  bodies  such  as  tuning-forks,  rods,  strings,  membranes, 
etc.  These  produce  in  the  air  a  to-and-fro  movement  of  its  particles,  re- 
sulting in  a  succession  of  alternate  condensations  and  rarefactions  which  are 
propagated  in  all  directions.  The  impact  of  a  rhythmic  succession  of  such 
condensations  on  the  ear  gives  rise  to  musical  sounds;  the  impact  of  an 
arrhythmic  or  irregular  succession  gives  rise  to  noises. 


720  TEXT-BOOK  OF  PHYSIOLOGY 

If  a  writing  point  attached  to  a  tuning-fork  in  vibration  be  placed  in  con- 
tact with  a  traveling  recording  surface,  each  vibration  will  be  recorded  in  the 
form  of  a  wave.  For  this  reason  atmospheric  vibrations  are  generally 
spoken  of  as  sound-waves.  A  line  drawn  horizontally  through  such  a  curve 
indicates  the  position  of  rest  of  the  fork;  the  extent  of  the  curve  on  each  side 
of  this  line  indicates  the  excursion  of  the  fork  or  the  amplitude  of  its 
movement. 

The  sensations  of  sounds  which  physiologically  result  from  the  stimula- 
tion of  the  auditory  apparatus  are  characterized  by  loudness,  pitch  and  qual- 
ity or  timbre  and  are  the  result  of  the  intensity  or  vigor,  frequency,  and  form 
of  the  atmospheric  vibrations. 

The  intensity  or  loudness  of  a  sound  depends  on  the  amplitude  of  the 
vibration  which  causes  it.  The  greater  the  ampHtude  or  swing  of  the  vibrat- 
ing body,  the  greater  is  the  energy  with  which  it  strikes  the  ear. 

The  pitch  of  a  sound  depends  on  the  number  of  \ibrations  which  strike 
the  ear  in  a  unit  of  time — a  second.  The  greater  the  number,  the  higher  the 
pitch.  Thus  while  the  pitch  of  the  sound  caused  by  the  note  C,  on  the  first 
leger  line  below  the  G  clef,  of  the  music  scale,  corresponds  to  256  vibrations, 
the  pitch  of  the  sound  caused  by  the  note  C  an  octave  above,  corresponds  to 
512  vibrations.  The  lowest  rate  of  vibration  which  can  produce  a  distinct 
sound  varies  in  different  individuals  from  14  to  18;  the  highest  rate  varies 
from  35,000  to  40,000  per  second.  Between  these  two  extremes  lies  the  range 
of  audibility,  which  embraces  about  1 1  octaves.  Vibrations  less  than  14  per 
second  cannot  be  perceived  as  a  continuous  sound;  vibrations  beyond 
40,000  also  fail  to  be  so  perceived.  In  the  ascent  of  the  music  scale  from  the- 
lowest  to  the  highest  regions  there  is  a  gradual  increase  in  the  vibration 
frequency. 

The  quality  of  a  sound  depends  on  the/orw  of  the  vibration.  It  is  this 
feature  which  gives  rise  to  those  differences  in  sensations  which  permit  one  to 
distinguish  one  instrument  from  another  when  both  are  emitting  the  same 
note.  The  form  of  the  sound-wave  in  any  given  instance  is  the  resultant  of  a 
combination  of  a  fundamental  vibration  and  certain  secondary  vibrations  of 
subdivisions  of  the  vibrating  body.  These  secondary  vibrations  give  rise 
to  what  is  known  as  overtones.  By  their  union  with  and  modification  of  the 
fundamental  vibration  there  is  produced  a  special  form  of  vibration  which 
gives  rise  not  to  a  simple  but  a  composite  sensation.  It  is  for  this  reason  that 
the  same  note  of  the  piano,  the  violin,  and  the  human  voice  varies  in  quality. 

The  Function  of  the  Pinna  and  External  Auditory  Canal. — In  those 
animals  possessing  movable  ears  the  pinna  plays  an  important  part  in  the 
collection  of  sound-waves.  In  man  the  pinna  plays  but  a  subordinate  part 
in  this  process.  Nevertheless  an  individual  with  defective  hearing  may  have 
the  perception  of  sound  increased  by  placing  the  pinna  at  an  angle  of  90 
degrees  to  the  side  of  the  head  or  by  placing  the  hand  behind  it.  The  external 
auditory  canal  transmits  the  sonorous  vibrations  to  the  tympanic  membrane. 
From  the  obliquity  of  this  canal  it  has  been  supposed  that  the  vibrations, 
after  passing  the  concha,  undergo  a  series  of  reflections  on  their  way  to  the 
tympanic  membrane,  which,  owing  to  its  inclination,  would  be  struck  by 
them  in  a  much  more  effective  manner. 

The  Function  of  the  T3mipanic  Membrane. — The  function  of  the 
tympanic  membrane  is  the  reception  of  the  atmospheric  vibrations  which 


THE  SENSE  OF  HEARING  721 

are  transmitted  to  it.  This  it  does  by  vibrating  in  unison  with  them.  The 
vibrations  which  the  membrane  exhibits  correspond  in  ampHtude,  in  fre- 
quency, and  in  form  to  those  of  the  atmosphere.  That  this  membrane 
actually  reproduces  all  vibrations  within  the  range  of  audibility  has  been 
experimentally  demonstrated.  The  membrane,  not  being  fixed  as  far  as  its 
tension  is  concerned,  does  not  possess  a  fixed  fundamental  note,  like  a  station- 
ary fixed  membrane,  and  is.  therefore,  just  as  well  adapted  for  the  reception  of 
one  set  of  vibrations  as  another.  This  is  made  possible  by  variations  in  its 
tension  in  accordance  with  the  pitch  or  frequency  of  the  atmospheric  vibra- 
tions. In  the  absence  of  vibration  the  membrane  is  in  a  condition  of  re- 
laxation; with  the  advent  of  sound-waves  possessing  a  gradual  increase  of 
pitch,  as  in  the  ascent  of  the  music  scale,  the  tension  of  the  membrane  in- 
creases until  its  maximum  is  reached  at  the  upper  limit  of  the  range  of 
audibility.  By  this  change  in  tension  certain  tones  become  perceptible  and 
distinct,  while  others  become  imperceptible  and  indistinct. 

The  Function  of  the  Tensor  Tympani  Muscle. — The  function  of  this 
muscle  is,  as  its  name  indicates,  to  change  and  to  fix  the  tension  of  the  tym- 
panic membrane,  so  that  it  can  most  readily  vibrate  in  unison  with  vibrations 
of  varying  degrees  of  rapidity.  The  tendon  of  this  muscle  playing  around 
the  processus  cochleariformis  is  attached  almost  at  a  right  angle  to  the  handle 
of  the  malleus.  Hence  as  the  muscle  contracts  it  exerts  its  traction  from  the 
process  and  draws  the  handle  of  the  malleus  inward,  thus  increasing  the 
convexity  of  the  tympanic  membrane  and  at  the  same  time  its  tension. 
With  the  relaxation  of  the  muscle  the  handle  of  the  malleus  passes  outward, 
and  the  convexity  and  tension  diminish. 

In  the  ascent  of  the  music  scale,  each  note  corresponding  to  an  increase 
in  vibration  frequency  requires  for  its  perception  an  increase  in  tension  and 
an  increase  in  the  force  of  the  contraction  of  the  tensor  muscle.  In  the 
descent  of  the  music  scale  the  reverse  conditions  obtain.  The  contraction  of 
the  muscle  is  of  the  nature  of  a  single  twitch,  and  of  just  sufificient  force  and 
duration  to  tense  the  membrane  for  a  given  rate  of  vibration. 

The  contraction  of  the  muscle  is  excited  reflexly.  The  afferent  path  is 
through  fibers  of  the  trigeminal  nerve  distributed  to  the  tympanic  mem- 
brane; the  efferent  path  is  through  fibers  in  the  small  root  of  the  trigeminal. 
The  stimulus  is  sudden  pressure  on  the  tympanic  membrane.  The  more 
frequently  and  forcibly  the  stimulus  is  applied,  the  greater  is  the  muscle 
response.  The  tensor  tympani  muscle  may  therefore  be  regarded  as  an 
accommodative  apparatus  by  which  the  tympanic  membrane  is  adjusted 
for  the  reception  of  vibrations  of  varying  degrees  of  frequency. 

The  Function  of  the  Chain  of  Bones. — The  function  of  the  chain  of 
bones  is  to  transmit  the  effects  of  the  atmospheric  vibrations  to  the  fluid  of  the 
labyrinth.  The  manner  4n  which  this  is  accomplished  becomes  evident 
from  the  relation  which  the  bones  of  this  chain  bear  to  one  another  and  to  the 
tympanic  membrane  on  the  one  hand  and  to  the  fluid  of  the  labyrinth  on 
the  other. 

When  pressure  is  made  on  the  outer  surface  of  the  tympanic  membrane 
it  is  at  once  pushed  inward,  carrying  with  it  the  "handle  of  the  malleus,  the 
head  at  the  same  time  rotating  outward  around  an  axis  corresponding  to  its 
ligamentous  attachments.  As  the  handle  moves  inward  a  small  ledge  of  bone 
just  below  the  malleo-incudal  joint  locks  with,  and  hence  pushes  inward, 
46 


722  TEXT-BOOK  OF  PHYSIOLOGY 

the  long  process  of  the  incus.  Since  this  process  is  united  at  almost  a 
right  angle  to  the  stapes  bone,  the  latter  is  forced  toward  and  into  the 
foramen  ovale,  thus  producing  a  pressure  on  the  perilymph.  With  the 
cessation  of  the  pressure  the  elastic  forces  of  the  membrane  and  of  the  liga- 
ments return  the  handle  of  the  malleus  to  its  former  position;  by  the  unlock- 
ing of  the  malleo-incudal  joint  the  entire  chain  also  returns  to  its  former  posi- 
tion without  exerting  undue  traction  on  the  basal  attachment  of  the  stapes. 

As  the  long  process  of  the  incus  is  shorter  than  the  handle  of  the  malleus, 
and  as  the  movement  between  them  takes  place  around  an  axis  from  be  fore 
backward,  it  follows  that  the  excursion  of  the  incus  and  stapes  will  be  less 
than  that  of  the  malleus,  while  the  force  will  be  greater.  Hence  as  the 
vibrations  are  transferred  from  the  tympanic  membrane  of  large  area  to  the 
base  of  the  stapes  of  small  area  (20  to  1.5),  they  lose  in  amplitude  but  in- 
crease is  force.  Their  pressure  on  the  perilymph  is  therefore  13.3  times 
greater  than  on  the  membrana  tympani.  In  addition  to  its  function  as  a 
transmitter  of  vibrations,  the  chain  of  bones  serves  as  a  point  of  attachment 
for  muscles  which  regulate  the  tension  of  the  tympanic  membrane  and 
the  pressure  on  the  labyrinth. 

The  Function  of  the  Stapedius  Muscle. — The  function  of  the  stapedius 
muscle  is  a  subject  of  much  discussion.  According  to  Henle,  its  function 
is  so  to  adjust  the  stapes  bone  that  it  will  be  prevented  from  exerting  an  undue 
pressure  on  the  perilymph  during  the  inward  excursions  of  the  incus  process. 
According  to  Toynbee,  its  function  is  to  press  the  posterior  part  of  the  stapes 
inward,  make  it  a  fixed  point,  and  place  the  anterior  part  in  such  a  position 
that  it  will  vibrate  freely  and  accurately. 

The  Function  of  the  Eustachian  Tube. — In  order  that  the  tympanic 
membrane  may  vibrate  freely  it  is  essential  that  the  air  pressure  on  both 
sides  shall  be  equal  at  all  times.  This  is  made  possible  by  the  Eustachian 
tube.  Were  it  not  for  this  passageway,  with  each  inward  swing  of  the  mem- 
brane the  air  in  the  tympanic  cavity  would  be  condensed  and  its  pressure 
raised,  in  consequence  of  which  the  movement  of  the  membrane  would  be 
retarded;  with  each  outward  swing,  the  air  would  be  rarefied  and  its  pressure 
lowered  below  that  of  the  atmosphere,  and  in  consequence  the  movement 
outward  would  be  retarded;  the  maximum  response,  therefore,  of  the  mem- 
brane to  a  given  vibration  could  not  be  attained  and  the  resulting  sound 
would  be  mulHed  and  indistinct.  But  as  with  each  vibration  of  the 
membrane  the  air  can  pass  into  and  out  of  the  tympanum  through  this 
partially  closed  tube,  inequalities  of  pressure  are  prevented  and  a  free 
vibration  permitted. 

The  impairment  in  the  acuteness  of  hearing  which  is  caused  by  either 
a  rise  or  fall  of  pressure  in  the  middle  ear  can  be  shown — 

1.  By  closing  the  mouth  and  nose  and  then  forcing  air  from  the  lungs 

through  the  Eustachian  tube  into  the  tympanum,  thus  increasing  the 
pressure. 

2.  By  closing  the  mouth  and  nose  and  then  making  an  effort  of  deglutition. 

As  this  act  is  attended  by  an  opening  of  the  pharyngeal  end  of  the 
Eustachian  tube,  the  air  in  the  tympanum  is  partly  withdrawn  and  the 
pressure  lowered.  In  each  instance  hearing  is  impaired.  After 
either  experiment  the  normal  condition  is  restored  by  swallowing  with 
the  nasal  passages  open. 


THE  SENSE  OF  HEARING         '  723 

The  Functions  of  the  Internal  Ear. — From  the  anatomic  arrange- 
ment of  the  structures  of  the  internal  ear  it  is  evident  that  if  the  vibrations 
of  the  stapes  bone  are  to  reach  the  peripheral  organs — the  hair  cells — of 
both  the  vestibular  and  cochlear  nerves,  they  must  traverse  successively 
the  perilymph,  the  membranous  walls,  and  the  endolymph.  As  the  perilymph 
is  incompressible,  the  inward  movement  of  the  stapes  would  be  prevented 
were  it  not  for  the  elastic  character  of  the  membrane  closing  the  foramen 
rotundum.  The  pressure  wave  occasioned  by  each  inward  movement  of  the 
stapes  is  transmitted  through  the  scala  vestibuli,  the  helicotrema,  and  the 
scala  tympani,  to  this  membrane,  which  by  virtue  of  its  elasticity  is  pressed 
into  the  tympanic  cavity.  With  the  outward  movement  of  the  stapes, 
equilibrium  is  at  once  restored. 

The  Functions  of  the  Cochlea. — The  cochlea  is  the  portion  of  the  in- 
fernal ear  which  is  concerned  in  the  perception  of  tones.  The  arrangement 
of  the  histologic  elements  of  the  organ  of  Corti  indicates  that  they  in  some 
way  respond  to  the  vibrations  of  varying  frequency  and  form,  and  through 
the  development  of  nerve  impulses,  evoke  the  sensations  of  pitch  and  quality. 
The  manner  in  which  this  is  accomplished  is  largely  a  matter  of  speculation. 
While  many  theories  have  been  offered  in  explanation  of  the  power  to  distin- 
guish the  pitch  and  the  quality  or  timbre  of  a  tone,  most  physiologists  prefer 
that  of  Helmholtz,  who  regarded  the  transverse  fibers  of  the  basilar  mem- 
brane as  the  elements  immediately  concerned,  and  compared  them,  both  in 
their  arrangement  and  power  of  sympathetic  vibration,  with  the  strings  of  a 
piano.  He  said :  "  If  we  could  so  connect  every  string  of  a  piano  with  a  nerve- 
fiber  that  the  nerve-fiber  would  be  excited  as  often  as  the  string  vibrated, 
then,  as  is  actually  the  case  in  the  ear,  every  musical  note  which  affected  the 
instrument  would  excite  a  series  of  sensations  exactly  corresponding  to  the 
pendulum-like  vibrations  into  which  the  original  movements  of  the  air  can 
be  resolved;  and  thus  the  existence  of  each  individual  overtone  would  be 
exactly  perceived,  as  is  actually  the  case  with  the  ear.  The  perception  of 
tones  of  different  pitch  would,  under  these  circumstances,  depend  upon 
different  nerve-fibers,  and  hence  would  occur  quite  independently  of  each 
other.  Microscopic  investigation  shows  that  there  are  somewhat  similar 
structures  in  the  ear.  The  free  ends  of  all  the  nerve-fibers  are  connected 
with  small  elastic  particles  which  we  must  assume  are  set  into  sympathetic 
vibration  by  sound-waves"  (Stirling). 

The  mechanism  might  be  regarded,  therefore,  somewhat  as  follows: 
The  sound-waves  received  by  the  membrana  tympani  and  transmitted  by  the 
chain  of  bones  to  the  fenestra  ovalis  produce  variable  pressures  in  the 
fluids  of  the  internal  ear;  these  pressures  vary  in  intensity,  in  number, 
and  in  quahty,  and  correspond  with  the  intensity,  pitch,  and  quality  of  the 
tones.  If,  therefore,  a  compound  wave  of  pressure  be  communicated  by 
the  base  of  the  stapes,  it  will  be  resolved  into  its  constituents  by  the  different 
transverse  fibers  of  the  basilar  membrane,  each  picking  out  its  peculiar 
portion  of  the  wave  and  thus  stimulating  corresponding  nerve  filaments. 
Thus  difi'erent  nerve  impulses  are  transmitted  to  the  brain,  where  they  are 
fused  in  such  a  manner  as  to  give  rise  to  a  sensation  of  a  particular 
quahty,  but  still  so  imperfectly  fused  that  each  constituent,  by  a  strong 
effort  of  attention,  may  be  still  recognized.  The  transverse  fibers  of  the 
basilar  membrane  vary  in  length  from  0.04155  mm.  at  the  base  of  the 


724  TEXT-BOOK  OF  PHYSIOLOGY 

cochlea  to  0.495  ^^-  ^^  the  apex,  and,  according  to  Retzius,  are  about  24,000 
in  number.  As  the  human  ear  usually  cannot  distinguish  more  than  11,000 
tones,  it  is  evident  that  there  is  a  sufficient  anatomic  basis  for  this  theory. 

The  Functions  of  the  Semicircular  Canals. — From  the  pronounced 
disturbances  of  equilibrium  and  progression  which  follow  injury  or  destruc- 
tion of  one  or  more  of  the  semicircular  canals  or  of  the  vestibular  branch  of 
the  acoustic  nerve  (phenomena  which  have  been  alluded  to  in  connection 
with  a  consideration  of  the  functions  of  the  cerebellum  (see  page  608),  it  is 
apparent  that  they  are  among  the  peripheral  sense-organs,  the  physiologic 
action  of  which  is  the  development  of  nerve  impulses,  which  when  trans- 
mitted to  the  brain  assist  the  equilibratory  mechanism  to  maintain  the  equi- 
librium of  the  body,  both  in  the  standing  position  and  in  the  various  modes 
of  progression.  The  character  of  the  stimulus,  however,  and  the  manner  in 
which  it  acts  on  the  specialized  portion  of  the  sense-organs  (the  hair  cells)  is 
not  entirely  clear. 

In  any  explanation  it  must  be  recalled  (i)  that  the  membranous  canals 
are  in  direct  connection  with  the  utriculus,  and  that  both  are  filled  with 
endolymph;  and  (2)  that  these  canals  act  in  pairs  at  least,  in  the  varying 
positions  or  movements  of  the  head,  e.g.,  the  two  horizontal  or  lateral  canals 
act  together  when  the. head  is  rotated  around  a  vertical  axis;  the  two  superior 
and  the  two  posterior  verticals  act  together  when  the  head  is  rotated  around 
a  horizontal  axis;  the  superior  and  posterior  vertical  of  one  side  act  together 
with  the  corresponding  canals  of  the  opposite  side  when  the  head  is  rotated 
around  an  antero-posterior  or  sagittal  axis;  the  superior  vertical  of  one  side 
acts  with  the  posterior  vertical  of  the  opposite  side  when  the  head  is  rotated 
around  an  oblique  axis. 

Goltz  was  the  first  to  present  the  theory  that  the  stimulus  is  to  be  found 
in  the  pressure  of  the  endolymph.  He  stated  that  in  any  given  position  of 
the  head,  the  endolymph  would  gravitate  to  the  most  dependent  portion  of 
the  canals  and  thus  press  on  the  nerve  endings  in  one  or  more  of  the  ampullae, 
and  thus  develop  nerve  impulses  which,  when  transmitted  to  the  brain, 
would  evoke  more  or  less  conscious  sensations.  These  sensations  would 
indicate  the  position  of  the  head,  and  lead  to  the  necessary  effort  to  control 
the  movement,  to  maintain  equilibrium  and  to  restore  the  head  to  the  normal 
position.  If  this  mechanism  is  injured  this  knowledge  and  this  control  is 
interfered  with  and  a  want  of  equilibrium  is  developed. 

Later  Breuer  propounded  the  view  that  the  stimulus  was  to  be  found  in 
a  movement  of  the  endolymph  in  one  or  more  canals  when  the  head  was 
rotated,  a  movement  which  varied  in  rapidity  in  proportion  to  the  rate  of 
head  rotation.  Mach  subsequently  presented  the  view  that  the  stimulus  was  to 
be  found  in  a  variation  of  pressure,  positive  or  negative  in  one  or  more  of 
the  ampullae  rather  than  in  an  actual  movement  of  the  fluid.  Crum  Brown 
made  the  further  suggestion  that  in  addition  to  the  movement  or  pressure 
of  the  endolymph  there  was  a  movement  of  the  perilymph  as  well  which 
involved  also  the  walls  of  the  canal.  This  general  theory  is  usually  desig- 
nated as  the  dynamic  in  contradistinction  to  the  theory  of  Goltz  which  is 
designated  as  the  static.  In  the  dynamic  theory,  the  movement  of  the  fluid 
in  the  canal  or  canals,  the  plane  of  which  corresponds  to  the  plane  in  which 
the  head  is  rotated,  though  in  the  opposite  direction,  is  over  the  hair  cells  at 
the  ampullae  and  thus  acts  as  a  stimulus.     Thus,  if  the  head  is  rotated  to  the 


THE  SENSE  OF  HEARING  725 

right  around  a  vertical  axis  and  in  a  horizontal  plane  the  endolymph  in  the 
right  half  of  each  horizontal  or  lateral  canal  will  flow  toward  the  ampulla, 
and  in  the  left  half  away  from  the  ampulla.  If  the  head  is  rotated  to  the 
left  the  reverse  movements  would  arise. 

If  the  head  is  rotated  backward  around  a  horizontal  axis  the  endolymph 
will  flow  in  both  posterior  vertical  canals  in  the  opposite  direction,  i.e.,  away 
from  the  ampullae  and  if  the  head  is  rotated  around  the  same  axis  forward, 
the  reverse  movement  takes  place.  If  the  head  is  rotated  around  an  oblique 
axis,  the  endolymph  will  flow  in  the  superior  vertical  canal  of  one  side  away 
from  the  ampulla  and  in  the  posterior  vertical  canal  of  the  other  side  toward 
the  ampulla.  Thus  whatever  the  plane  of  rotation  of  the  head  may  be, 
there  is  always  a  flow  of  endolymph  in  the  opposite  direction,  which 
movement,  or  variation  in  pressure,  becomes  the  effective  stimulus.  The  nerve 
impulses  developed  by  this  stimulation  when  transmitted  to  the  brain  evoke 
more  or  less  conscious  sensations,  which  inform  us  as  to  the  direction  and  the 
extent  of  movement  of  the  head  and  calls  forth  the  action  of  the  mechanism 
by  which  the  movement  is  regulated  and  controlled  and  the  equilibrium  of 
the  body  maintained. 


CHAPTER  XXXI 
REPRODUCTION 


Reproduction  is  the  process  by  which  a  new  individual  is  initiated  and 
developed  and  the  species  to  which  it  belongs  is  preserved.  Reproduc- 
tion is  the  result  of  the  union  and  subsequent  development  of  germ-  and 
sperm-cells.     These  cells  are  produced  and  their  union  accomplished  by  the 

,  cooperation  of  the  reproduc- 


/'^f^,* 


'^m^ 


^■h- 


'  OS. 


tive  organs  characteristic  of 
the  two  sexes. 

Embryology  is  a  depart- 
ment of  anatomic  science 
which  has  for  its  object  the 
investigation  of  the  successive 
stages  that  the  new  being 
passes  through  during  its 
gradual]  development  prior  to 
birth. 

THE  REPRODUCTIVE  OR- 
GANS OF  THE  FEMALE 

The  reproductive  organs  of 
the  female  comprise  the  ova- 
ries, Fallopian  tubes,  uterus, 
and  vagina. 

The  Ovaries. — The  ova- 
ries are  two  small,  flattened 
bodies,  measuring  about  40 
mm.  in  length  and  20  in 
breadth.  They  are  situated 
in  the  cavity  of  the  pelvis,  one 
on  either  side,  and  embedded 
in  a  fold  of  the  peritoneum, 
known  as  the  broad  ligament. 
A  section  of  the  ovary  shows 
that  it  consists  externally  of  a 
thin,  firm,  connective-tissue 
membrane  and  internally  of  a  fine  connective-tissue  stroma,  supporting 
blood-vessels,  non-striated  muscle-fibers  and  nerves,  and  containing  in  its 
meshes  a  very  large  number  of  spheric  sacs  named  after  their  discoverer,  de 
Graaf ,  the  Graafian  sacs  or  follicles.  These  follicles  are  very  numerous  and 
are  present  in  all  portions  of  the  ovary,  though  they  are  most  abundant  to- 
ward its  peripheral  portions.  It  is  estimated  that  each  human  ovary  con- 
tains from  20,000  to  40,000  follicles.     The  follicles  vary  considerably  in  size; 

726 


Fig.  :?26. — Section  of  Cortex  of  Cat's  Ovary, 
Exhibiting  Large  Graafian  Follicles,  a.  Pe- 
ripharal  zone  of  condensed  stroma,  b.  Groups  of  im- 
niiiture  lollicies.  c.  Theca  of  follicle,  d.  Membrana 
granulosa,  e.  Discus  proligerus.  /.  Zona  pellucida. 
g.  Vilellus.  h.  Germinal  vesicle,  i.  Germinal  spot. 
k.  Cavity  of  liquor  folliculi. — {After  Piersol.) 


REPRODUCTION 


727 


while  many  are  visible  to  the  unaided  eye,  others  require  for  their  detection 
high  powers  of  the  microscope.  Although  the  follicles  are  present  in  the 
ovary  at  the  time  of  birth,  it  is  not  until  the  period  of  puberty  that  they  as- 
sume functional  activity. 

From  this  time  on  to  the  catamenial  period  there  is  a  constant  growth 
and  development  of  these  follicles.  Each  follicle  consists  of  an  external 
investment  of  fibrous  tissue  and  blood-vessels,  and  an  internal  investment  of 
cells,  the  memhrana  granulosa.  At  the  lower  portion  of  this  membrane  there 
is  an  accumulation  of  cells,  the  proligerous  disc  (Fig.  326).  The  cavity  of 
the  follicle  contains  a.  slightly  yellowish,  alkaline,  albuminous  fluid,  a 
transudate  in  all  probability  from  the  blood-vessels.  The  Graafian  follicle 
is  of  especial  interest,  for  it  is  in  this  structure,  and  more  especially  in  the 
proligerous  disc,  that  the  true  germ-cell  or  ovum  is  developed. 


Fig.  327.  —  Ovum  of  a  Covv.  i.  Zona 
pellucida.  2.  Cytoplasm,  vitellus.  3.  Nu- 
cleus, germinal  vesicle.  4.  Nucleolus,  germ- 
inal spot.  5.  Corona  radiata.  The  radial 
striation  of  the  zona  pellucida  cannot  be 
seen. — (Stohr.) 


Fig.  328. — Frontal  Sec- 
tion of  the  Uterus.  I.  Cav- 
ity of  the  body.  2,  3.  Lateral 
walls.  4,4.  Comua.  5.  Os 
internum.  6.  Cavity  of  the 
cervix.  7.  Arbor  vitae  of  the 
cer^-ix.  8.  Os  externum.  9. 
Vagina. — (Sappey.) 


The  ovixm  is  a  spheric  body  measuring  about  0.3  mm.  in  diameter.  It 
consists  of  a  mass  of  living,  protoplasmic  material,  cytoplasm,  a  nucleus  or 
germinal  vesicle,  'and  a  nucleolus  or  germinal  spot.  The  cytoplasm  presents 
toward  its  central  portion  a  quantity  of  granular  material,  partly  fatty  in 
character,  the  deuioplasm  or  vitellus.  The  peripheral  portion  of  the  cyto- 
plasm is  surrounded  by  a  clear  thick  membrane,  the  zona  pellucida,  external 
to  which  is  a  layer  of  radially  placed  columnar  epithelium,  the  corona  radiata 

(Fig.  327)- 

The  nucleus  consists  of  a  nuclear  membrane  enclosing  contents.  The 
latter  consist  of  an  amorphous  material  in  which  is  embedded  a  network, 
some  of  the  threads  of  which  have  a  strong  afl&nity  for  certain  staining 
materials,  and  hence  are  known  as  chromatin,  in  the  meshes  of  which  lies  a 
material  that  stains  less  deeply  and  known  as  achromaiin. 


728  TEXT-BOOK  OF  PHYSIOLOGY 

The  Fallopian  Tubes. — The  Fallopian  tubes  are  about  12  centimeters 
in  length  and  extend  from  the  upper  angles  of  the  uterus  to  the  ovaries. 
Each  tube  is  somewhat  trumpet-shaped,  the  narrow  portion  being  close  to 
the  uterus,  the  wide  portion  close  to  the  ovary.  The  outer  extremity  of  the 
tube  is  expanded  and  subdivided,  and  presents  a  series  of  processes  termed 
fimbriae,  one  of  which  is  attached  to  the  ovary.  The  tube  consists  of  three 
coats — an  external  or  fibro-serous;  a  middle  or  muscle,  the  fibers  of  which 
are  arranged  longitudinally  and  circularly;  and  an  internal  or  mucous,  which 
is  folded  longitudinally.  The  surface  of  the  mucous  coat  is  covered  with  a 
layer  of  ciliated  epithelial  cells,  the  direction  of  motion  of  which  is  toward 
the  uterus. 

The  Uterus. — The  uterus  is  pyriform  in  shape  and  divided  into  a  body 
and  neck.  It  measures,  before  the  first  pregnancy,  about  7  cm.  in  length, 
5  cm.  in  breadth  and  2^  cm.  in  thickness.  A  frontal  section  of  the  uterus 
shows  a  central  cavity  which  in  the  body  is  triangular  in  shape,  in  the  neck 
oval  or  fusiform  (Fig.  328).  At  the  upper  angles  of  the  uterus  the  cavity  is 
continuous  with  the  cavity  of  each  Fallopian  tube.  At  the  junction  of  the 
body  and  the  neck,  the  cavity  presents  a  constriction,  the  internal  os.  The 
constriction  at  the  end  of  the  neck  is  known  as  the  external  os.  The  walls  of 
the  uterus  are  extremely  thick  and  composed  of  non-striated  muscle-fibers 
arranged  in  a  very  complicated  manner.  The  interior  of  the  uterus  is  lined 
by  mucous  membrane  covered  with  cylindric  ciliated  epithelial  cells,  the 
motion  of  which  is  toward  the  external  os.  Tubular  glands  are  found  in 
large  numbers  in  the  mucous  membrane  lining  the  cavity,  while  racemose 
glands  are  found  in  the  mucous  membrane  lining  the  neck.  Owing  to  the 
flattening  of  the  uterus  from  before  backward  the  walls  are  almost  in  contact 
and  the  cavity  almost  obliterated. 

The  Vagina. — The  vagina  is  a  musculo-membranous  canal,  from  12  to 
18  cm.  in  length,  situated  between  the  rectum  and  bladder.  It  extends 
from  the  surface  of  the  body  to  the  brim  of  the  pelvis,  and  embraces  at  its 
upper  extremity  the  neck  of  the  uterus. 

Ovulation.— After  the  establishment  of  puberty  a  Graafian  follicle 
develops  and  ripens  or  matures  periodically,  usually  every  twenty-eight  days. 
During  the  time  of  maturation  the  follicle  increases  in  size,  from  an  augmen- 
tation of  its  fluid  contents,  and  approaches  the  surface  of  the  ovary,  where  it 
forms  a  projection  varying  from  6  to  12  mm.  in  size.  When  maturation  is 
complete  the  vesicle  ruptures,  and  the  ovum  and  liquid  contents  are  discharged. 
The  ovum,  by  a  mechanism  not  fully  understood,  is  received  by  the  fimbriated 
extremity  of  the  Fallopian  tube  and  enters  its  cavity.  The  ovum  is  then 
transferred  through  the  tube  by  the  peristaltic  contraction  of  its  muscle- 
fibers  and  by  the  action  of  the  cilia  of  its  lining  epithelium.  The  time 
occupied  in  the  transference  of  the  ovum  from  the  ovary  to  the  interior  of  the 
uterus  has  been  estimated  to  be  from  four  to  ten  days. 

Either  at  the  time  of,  or  very  shortly  after,  its  discharge  from  the  follicle, 
the  ovum,  and  more  especially  the  nucleus,  undergoes  a  series  of  histologic 
changes  which  eventuates  in  an  extrusion  of  a  portion  of  the  chromatin 
material.  The  extruded  portions  are  known  as  the  polar  bodies.  The  non- 
extruded  portion  of  the  chromatin  material  is  known  as  the  female  pronu- 
cleus or  germ  nucleus.  The  chromosomes  are  reduced  to  one-half  the  somatic 
number.     The  succession  of  changes  which  the  nucleus  undergoes  is  termed 


REPRODUCTION  729 

maturation.  As  the  nucleus  is  regarded  as  the  part  of  the  ovum  which 
transmits  parental  characteristics  it  is  assumed  that  the  extrusion  of  a  por- 
tion of  the  nuclear  material  is  a  means  by  which  an  excess  of  inherited 
substance  is  prevented. 

Menstruation. — Menstruation  is  a  periodic  discharge  of  blood  and 
mucus  from  the  surface  of  the  mucous  membrane  of  the  uterus,  and  occurs 
about  every  twenty-eight  days.  The  duration  of  the  menstrual  period  ex- 
tends over  four  or  five  days  and  the  amount  of  blood  discharged  varies 
from  180  c.c.  to  200  c.c.  Menstruation  is  usually  an  accompaniment  of 
ovulation,  though  the  latter  process  may  take  place  independently  of  the 
former.  It  is  characterized  by  both  local  and  systemic  changes.  The 
local  changes  are  most  marked  in  the  uterus,  the  mucous  membrane  of  which 
increases  in  thickness  from  a  proliferation  of  the  connective  tissue  and  a 
hyperemic  condition  of  the  blood-vessels.  Subsequently  to  these  changes 
the  epithelial  surface,  as  well  as  the  more  superficial  portions  of  the  connec- 
tive tissue,  undergo  degeneration  and  exfoliation,  after  which  the  finer 
blood-vessels  rupture  and  permit  of  an  escape  of  blood  into  the  uterine 
cavity.  At  the  end  of  the  menstrual  period  regenerative  changes  set  in 
which  continue  until  the  normal  condition  of  the  mucous  membrane  is 
reestablished. 

The  Corpus  Luteum. — With  the  rupture  of  the  Graafian  follicle  there  is 
an  effusion  of  blood  into  the  follicular  cavity  which  soon  coagulates,  loses  its 
color  and  assumes  the  characteristics  of  iibrin.  The  walls  of  the  follicle, 
which  have  become  thickened  from  the  deposition  of  a  reddish-yellow  glutin- 
ous substance,  now  become  convoluted  and  undergo  a  still  further  hypertrophy, 
until  they  encroach  upon  and  almost  obliterate  the  follicular  cavity.  In  a 
few  weeks  the  mass  loses  its  red  color  and  becomes  decidedly  yellow,  when  it 
is  known  as  the  corpus  liiieum.  With  the  continuance  of  reparative  changes 
this  body  gradually  disappears  until  at  the  end  of  two  months  nothing 
remains  but  a  small  cicatrix  on  the  surface  of  the  ovary.  Such  are  the 
changes  in  the  follicle  if  the  ovum  has  not  been  impregnated. 

The  corpus  luteum,  after  impregnation  has  taken  place,  undergoes  a 
much  slower  development,  becomes  larger,  and  continues  during  the  entire 
period  of  gestation.  The  difference  between  the  corpus  luteum  of  the  un- 
impregnated  and  pregnant  condition  is  expressed  in  the  following  table  by 
Dal  ton: 

Corpus  Luteum  of  Menstruation.  Corpus  Luteum  of  Pregnancy. 

At  the  end  of  three    Three-quarters  of  an  inch  in  diameter;  central  clot  reddish;  convoluted 
weeks.  wall  pale. 

One  month Smaller;  convoluted  wall  bright  !  Larger;  convoluted  wall  bright  yellow; 

yellow;  clot  still  reddish.  clot  still  reddish. 

Two  months Reduced  to  the  condition  of  an     Seven-eighths  of  an  inch  in  diameter;  con- 
insignificant  cicatrix.  voluted  wall  bright  yellow;  clot  perfectly 

decolorized. 

Four  months ,  Absent  or  unnoticeable I  Seven-eighths  of  an  inch  in  diameter;  clot 

1    pale  and  fibrinous;  convoluted  wall  dull 
I    yellow. 
Still  as  large  as  at  the  end  of  second 
month;  clot  fibrinous;  convoluted  wall 
paler. 
Half  an  inch  in  diameter;  central  clot  con- 
verted into  a  radiating  cicatrix;  external 
wall  tolerably  thick  and  convoluted,  but 
without  any  bright  yellow  color. 


Six  months Absent. 

~ ! 
i 

Nine  months 1  Absent. 


730 


TEXT-BOOK  OF  PHYSIOLOGY 


THE  REPRODUCTIVE  ORGANS  OF  THE  MALE 

The  reproductive  organs  of  the  male  comprise  the  testicles,  vasa  deferen- 
tia,  vesiculae  seminales,  and  penis. 

The  Testicles. — The  testicles  are  oblong  glands,  about  40  mm.  in 
length,  30  mm.  in  breadth  and  20  mm.  in  thickness,  and  contained  within 
the  cavity  of  the  scrotum.  A  section  of  the  testicle  (Fig.  329)  reveals  the 
presence  externally  of  a  dense  fibrous  membrane,  the  tunica  albuginea,  and 
internally  a  connective-tissue  framework  consisting  mainly  of  septa,  which 
enter  the  organ  on  its  posterior  aspect  at  the  mediastinum  testis,  passing  in- 
ward in  a  diverging  manner.  The  spaces  between  the  septa  are  occupied 
by  the  true  gland  substance,  the  seminiferous  tubules. 


Fig.  329. — Diagram  of  a  Ver- 
TiC-\L  Section  through  a  Tes- 
ticle. I.  Mediastinum  testis.  2, 
2.  Trabeculae.  3.  One  of  the 
lobules.  4,  4.  Vasa  recta.  5. 
Globus  major  of  the  epidid}Tnis. 
6.  Globus  minor.  7.  Vas  def- 
erens.— {H olden.) 


Fig.  330. — Vas  Deferens,  Vesicxjl^ 
Seminales,  and  Ejaculatory  Ducts. 
a.  Vas  deferens,  b.  Seminal  vesicle. 
c.  Ejaculator}'  duct.  d.  Termination  of 
the  ejaculatory  duct.  e.  Opening  of  the 
prostatic  utricle.  /,  g.  Veru  montanum. 
h,  I.  Prostate. — (Liegeois.) 


The  seminiferous  tubules  are  very  numerous,  the  estimate  as  to  their 
number  varying  from  800  to  1000.  When  unraveled  they  measure  from  30 
to  40  cm.  in  length  and  0.3  mm.  in  diameter.  At  their  peripheral  extremities 
the  tubules  are  very  much  convoluted,  but  as  they  pass  toward  the  mediasti- 
num testis,  the  convolutions  disappear,  and  after  uniting  with  one  another 
terminate  in  from  twenty  to  thirty  straight  tubes,  of  small  diameter, 
the  vasa  recta,  which  pass  through  the  mediastinum  and  form  the  rete  testis. 
At  the  upper  part  of  the  mediastinum  the  tubules  unite  to  form  from  nine 
to  thirty  small  ducts,  the  vasa  efferentia,  which  soon  become  very  much  con- 
voluted. After  a  short  course  they  unite  to  form  a  single  tortuous  tube, 
about  7  meters  in  length  and  0.4  mm.  in  diameter,  which  descends  behind 
the  testicle  to  its  lower  border.  This  tube  is  known  as  the  epididymis. 
The  seminal  tubule  consists  of  a  basement  membrane  lined  by  granular 
nucleated  epithelium. 


REPRODUCTION  731 

The  vas  deferens,  the  excretory  duct  of  the  testicle,  is  about  60  cm.  in 
length  and  from  2  to  3  mm.  in  diameter,  and  extends  upward  from  the 
epididymis  to  the  inguinal  canal,  through  which  it  passes  into  the  abdominal 
cavity  and  then  to  the  under  surface  of  the  base  of  the  bladder,  where  it 
unites  with  the  duct  of  the  vesicula  seminalis  to  form  the  ejaculatory  duct. 

The  vesiculae  seminales  are  two  lobulated  pyriform  bodies,  about  40 
mm.  in  length,  situated  on  the  under  surface  of  the  bladder.  Each  vesicula 
seminalis  consists  of  an  external  fibrous  coat,  a  middle,  muscular  coat,  and  an 
internal  mucous  coat.  '  The  mucous  coat  contains  a  number  of  small 
tubular   albumin-producing   glands   which   secrete   a   characteristic   fluid. 

The  ejacidatory  duct,  formed  by  the  union  of  the  vas  deferens  and  the 
duct  of  the  vesicula  seminalis,  opens  into  the  prostatic  portion  of  the  urethra 
(Fig.  330). 

The  prostate  gland  is  a  musculo-glandular  mass  surrounding  the 
posterior  extremity  of  the  urethra.  It  contains  a  large  number  of  tubules, 
more  or  less  branched  and  convoluted,  and  lined  by  columnar  epithelium, 
They  secrete  a  fluid  which  is  poured  into  the  urethra  at  the  time  of  the  ejacu- 
lation   of    semen  and    impart  motility   to   the    spermatozoa   or   spermia. 

The  penis  consists  of  three  parts:  the  corpus  spongiosum  below,  through 
which  passes  the  urethra,  and  the  two  corpora  cavernosa,  one  on  either  side 
and  above.  The  corpus  spongiosum  terminates  anteriorly  in  a  conic- 
shaped  structure,  the  glans  penis;  the  corpora  cavernosa  consist  externally 
of  a  fibrous  investment  and  internally  of  a  fibrous  investment  and  internally 
of  erectile  tissue.  These  bodies  are  abundantly  supplied  with  blood,  which 
after  entering  their  substance  by  the  arteries,  passes  into  sinuses  or  reser- 
voirs, from  which  it  is  carried  away  by  veins.  These  vessels  pass  to  the 
dorsum  of  the  penis  and  unite  to  form  a  large  vein  by  which  the  blood  is  re- 
turned to  the  general  circulation.  By  virtue  of  the  erectile  tissue  in  the  cor- 
pora cavernosa  the  penis  becomes  erect  and  rigid  when  the  blood  supply 
is  increased.  This  takes  place  in  response  to  peripheral  stimulation  or 
emotional  states,  or  both  combined.  When  these  conditions  are  established 
nerve  impulses  pass  outward  through  nerves,  the  nervi  erigentes,  which  have 
their  origin  in  the  lumbar  segment  of  the  spinal  cord,  and  bring  about  an 
active  dilatation  of  the  arteries  and  a  relaxation  of  the  non-striated  muscle- . 
fibers  in  the  corpora  cavernosa.  (See  page  380.)  With  these  events  there 
is  a  rapid  influx  of  blood  and  a  distention  and  an  erection  of  the  organ. 
This  condition  is  furthered  and  maintained  by  a  partial  compression  of  the 
dorsal  vein  by  the  fibrous  capsule. 

Semen. — The  semen  is  a  complex  fluid  composed  of  the  'secretions  of 
the  testicles,  the  vesiculas  seminales,  the  prostatic  tubules,  and  urethral  or 
Cowper's  glands.  It  is  grayish-white  in  color,  mucilaginous  in  consistency, 
characteristic  in  odoTf-  and  somewhat  heavier  than  water.  In  response  to 
appropriate  stimulation  the  muscle-fibers  in  the  walls  of  the  vasa  deferentia, 
vesiculae  seminales,  and  prostatic  tubules  contract  and  discharge  their  con- 
tents into  the  urethra,  from  which  they  are  forcibly  ejected  by  the -rhythmic 
contraction  of  the  ejaculatory  muscles,  the  ischio-  and  bulbo-cavernosi.  The 
amount  of  semen  discharged  at  each  ejaculation  varies  from  i  to  5  c.c. 

Spermatozoa. — The  spermatozoa  or  spermia  are  peculiar  morphologic 
elements  which  arise  within  the  seminiferous  tubules  as  a  result  of  com- 
plex histologic  changes  in  the  lining  epithelium.     An  adult  spermatozoon 


732 


TEXT-BOOK  OF  PHYSIOLOGY 


consists  of  a  conoid  slightly  flattened  head,  from  the  posterior  part  of 
which  there  projects  a  short  straight  rod,  provided  with  a  long  filamentous 
tail  or  cilium  and  an  end-piece  (Fig.  331).  The  head  contains  a  nucleus 
of  chromatin  material.  The  total  length  of  a  spermatozoon  varies  from 
50  to  80  micro-millimeters.  The  characteristic  physiologic  feature  of 
spermatozoa  is  incessant  locomotion  when  in  a  suitable  medium.  So  long 
as  they  are  confined  to  the  vas  deferens  they  are  quiescent,  but  with  their 
advent  into  the  vesicula  seminalis  and  dissemination  in  its  contained  fluid, 
they  become  extremely  active  and  move  around  with  con- 
siderable rapidity.  The  power  of  locomotion  depends  on 
the  possession  of  the  tail  which,  by  lashing  the  sur- 
rounding fluid  now  in  this  and  now  in  that  direction, 
propels  the  head  from  place  to  place.  The  vitality  of 
spermatozoa  is  such  as  to  enable  them  to  retain  their  phy- 
siologic activities  in  the  uterus  for  more  than  eight  days. 
The  development  of  spermatozoa  from  testicular  cells 
as  observed  in  lower  animals  indicates  that  each  cell  gives 
rise  to  four  embryonic  forms — spermatids  — which  subse- 
quently develop  into  adult  spermatozoa.  In  this  process 
the  primary  nuclear  chromatin  undergoes  a  division,  so 
that  each  spermatozoon  receives  but  a  fractional  amount 
representing  one-half  the  number  of  somatic  chromo- 
somes. The  changes  by  which  this  condition  is  brought 
about  are  comparable  to  the  changes  exhibited  by  the 
ovum,  and  have  for  their  result  a  reduction  in  the  quan- 
tity of  hereditary  substance  to  be  transmitted. 

Fecundation. — Fecundation  is  the  union  of  the 
spermatozoon  (the  sperm-cell)  with  the  ovum  (the  germ- 
cell)  and  takes  place  in  the  great  majority  of  instances  in 
the  Fallopian  tube.  After  the  introduction  of  the  sper- 
matozoa into  the  vagina  during  the  act  of  copulation, 
they  soon  begin  to  pass  upward,  into  and  through,  the 
uterine  cavity  and  out  into  the  Fallopian  tube,  where 
they  accumulate  in  large  numbers  and  retain  their  vi- 
tality for  some  days.  The  migration  is  effected  by  the 
propelling  power  of  the  filamentous  tail  and  by  the  action  of  the  cilia  of  the 
uterus  and  tubes. 

From  observations  made  on  the  behavior  of  the  spermatozoa  toward 
the  ovum  in  lower  animals,  and  on  the  manner  in  which  their  union  is 
effected,  the  inference  may  be  drawn  that  a  similar  procedure  takes  place  in 
mammals.  In  lower  animals  the  spermatozoa  on  approaching  an  ovum 
take  on  increased  activity,  swimming  around  it  in  all  directions  and  appar- 
ently seeking  a  point  of  entrance.  In  fish  and  molluscs  the  zona  pellucida 
presents  a  distinct  opening,  the  micropyle,  through  which  the  spermatozoon 
passes.  Inasmuch  as  the  mammalian  ovum  is  devoid  of  such  an  opening, 
the  mechanism  of  entrance  of  the  spermatozoon  is  not  clearly  understood. 
Notwithstanding  their  enormous  numbers  it  is  generally  believed  that  but  a 
single  spermatozoon  effects  an  entrance  into  the  ovum.  With  the  accom- 
plishment of  this,  however,  the  spermatozoon  loses  its  mobility,  after  which 
the  tail  disappears. 


Fig.  33 1. — Htjman 
Spermatozo5n.  I . 
Front  view,  2,  side 
view,  of  the  head. 
h.  Head.  m.  mid- 
dle piece.  /.  Tail. 
e.  Terminal  fila- 
ment.— {After  Ret- 
zitis.) 


REPRODUCTION  733 

The  germ  nucleus  proceeds  to  the  middle  of  the  ovum  where  it  is  followed 
by  head  and  middle-piece  of  the  spermium;  the  middle-piece  forms  a  central 
spindle  while  the  germ  nucleus  and  head  of  the  spermium  each  resolves 
itself  into  one-half  the  number  of  chromatic  loops  of  a  somatic  cell.  In  this 
condition  the  fertilized  ovum  represents  a  parent  cell,  that  possesses  the 
physiologic  activities  and  characters  of  both  ancestral  cells.  From  this 
parent  cell  the  offspring  develops  through  successive  division,  multiplication 
and  differentiation  of  the  resulting  cells.  The  chromatic  material  of  the 
germ  nucleus  and  head  .of  the  spermium  represent  the  transmitters  of  in- 
herited characters. 

The  Fixation  of  the  Ovum. — The  ovum,  after  fertilization  in  the 
oviduct,  continues  to  divide  and  pass  slowly  to  the  uterus  (8  to  10  days) 
where  it  is  retained  until  the  end  of  gestation.  A  menstrual  mucosa  having 
developed  the  ovum  lodges  on  a  smooth  thick  area  and  gradually  sinks 
beneath  the  surface.  During  the  passage  down  the  oviduct  the  zona  pellu- 
cida  has  become  attenuated  and  has  been  finally  replaced  by  a  thick  layer 
of  ameboid  and  phagocytic  cells  called  the  trophoderm.  Upon  lodgment 
of  the  ovum  these  cells  destroy  the  underlying  mucosa  and  produce  a  cavity 
into  which  the  ovum  sinks.  As  the  ovum  increases  in  size  the  mucosa 
gradually  covers  it;  that  portion  of  the  mucosa  toward  the  uterine  cavity  is 
called  the  decidua  capsularis  {d.  reflexa) ,  that  beneath  the  ovum  the  decidua 
basilar  is  {placental  d.),  while  the  remainder  constitutes  the  decidua  parietalis 
(d.  vera).  As  development  proceeds  the  decidua  basilaris  becomes  greater, 
ultimately  developing  into  the  placenta. 

Segmentation  of  the  Ovum. — Immediately  after  fertilization  the  ovum 
divides  and  redivides,  within  the  diminishing  zona  pellucida,  forming  an 
irregular  mass  called  the  morula.  The  peripheral  cells  form  a  layer,  the 
trophoderm,  beneath  the  attenuated  zona  pellucida  ultimately  replacing 
that  structure.  The  remaining  cells  of  the  morula  differentiate  into  three 
masses — ectodermal,  entodermal  and  mesodermal;  the  central  cells  of  these 
masses  liquefy  and  disappear  forming  thus  the  ectodermal  or  amniotic 
cavity,  limited  by  the  ectoderm;  the  entodermal  cavity,  limited  by  the  ento- 
derm; and  the  mesodermal  or  celomic  cavity,  limited  by  the  extra-embryonic 
mesoderm.  Meanwhile  cells  in  various  parts  of  the  thickened  trophoderm 
have  disappeared  leaving  this  layer  in  the  form  of  delicate  trophodermal 
villi,  the  future  chorionic  and  placental  villi. 

The  Embryonic  Shield. — The  floor  of  the  amniotic  cavity,  consisting 
of  ectoderm  and  entoderm,  constitutes  the  embryonic  shield  or  disc.  As 
the  shield  increases  in  size  a  median  longitudinal  thickening  is  seen 
occupying  the  caudal  half  of  the  area.  This  is  the  primitive  streak,  a  tem- 
porary structure  that  is  soon  overshadowed  by  changes  in  the  areas  just  in 
front  of  it.  Here  is  formed  a  median,  longitudinal,  grooved  ridge  of  ecto- 
derm, that  develops  rapidly  in  length.  This  is  the  neural  groove  2in6.  folds. 
The  dorsal  lips  of  the  groove  approach  each  other  in  the  mid-line  and  fuse, 
separating  from  the  original  ectoderm  which  closes  over  the  ectodermal 
tube.  This  ectodermal  tube  is  the  neural  tube  from  which  the  nerve  system 
is  developed. 

In  the  immediate  vicinity  of  the  head  end  of  the  primitive  streak  is  seen 
a  darkened  area,  Hensen's  node  that  represents  the  beginning  invagination 
of  the  ectoderm  in  the  formation  of  the  embrvonic  mesoderm  and  notochord 


734 


TEXT-BOOK  OF  PHYSIOLOGY 


to  be  considered  later.  That  portion  of  the  embryonic  shield  that  gives 
rise  to  the  embryo  itself  becomes  distinctly  outlined  laterally  and  in  the  head 
and  tail  regions  of  the  neural  groove.  Just  external  to  this  area,  the  embry- 
onic area  proper,  is  a  transparent  area,  the  areapellucida,  beyond  which  is  the 
area  opaca  in  which  the  first  blood-vessels  appear. 

Mesoderm  and  Notochord. — So  far  in  the  embryonic  area  only  ecto- 
derm and  entoderm  exist.  Hensen's  node  at  the  head  end  of  the  primitive 
streak  represents  an  invagination  (gastrulation)  of  ectoderm  between  ecto- 
derm and  entoderm.  This  invagination  elongates  headward  in  the  embry- 
onic area  constituting  a  tube  of  ectodermal  cells,  the  chordal  canal.  Later 
the  ventral  wall  of  the  canal  and  the  adjacent  entoderm  disappear  so  that 
the  chordal  ectoderm  temporarily  forms  the  dorsal  median  boundary  of  the 
entodermal  cavity.  By  this  process  a  communication  is  established  between 
the  entodermal  cavity  and  neural  groove,  called  the  neur enteric  canal.  The 
chordal  ectoderm  separates  from  the  entoderm  and  then  forms  a  solid  cord 
of  cells,  the  notochord,  between  entoderm  and  neural  groove,  the  neurenteric 
canal,  however,  persisting  for  some  time.  In  the  meantime  other  ecto- 
dermal cells  in  the  region  of  the  chordal  invagination  spread  between  ecto- 
derm and  entoderm  and  form  the  anlage  of  the  mesoderm.  These  cells  by 
rapid  proliferation  soon  separate  ectoderm  and  entoderm  and  join  the 
extra-embryonic  mesoderm.  The  separation  of  ectoderm  and  entoderm  is 
complete  except  in  the  regions  of  the  bucco-pharyngeal  and  cloacal  membranes. 
Upon  each  side  of  the  neural  groove  the  mesoderm  becomes  transversely 
grooved  on  its  ectodermal  surface,  forming  a  number  of  successive  block- 
like masses  called  primitive  somites  or  segments.  Of  these  there  are  thirty- 
eight  of  the  trunk  and  possibly  four 
for  the  head  region.  Each  segment 
consists  of  three  parts — the  sclero- 
tome, the  myotome  and  the  dermatome. 
Lateral  to  the  somite  is  a  thickened 
mass  of  mesoderm,  the  intermediate 
cell-mass,  that  laterally  splits  into  two 
layers,  the  outer  accompanies  the  ecto- 
derm forming  the  somatopleure  which 
gives  rise  to  the  body-wall,  the  inner 
joins  the  entoderm  forming  the 
splanchnopleure  from  which  the  gut- 
tract,  vitelline  duct  and  yolk-sac  are 
derived. 

Fetal  Membranes. — As  the  primi- 
tive streak  and  neural  groove  are  form- 
ing, the  extra-embryonic  mesoderm 
that  lies  beneath  the  trophoderm  invades  the  trophodermal  villi  forming  thus 
the  chorion  with  its  villi.  Gradually  the  mesoderm  of  the  roof  of  the  am- 
niotic cavity  splits  into  two  layers,  the  upper  constituting  chorionic  meso- 
derm while  the  under  one  attached  to  the  ectoderm  of  the  amniotic  forms 
with  the  latter  the  amnion.  In  the  chick  and  some  mammals,  the  amnion 
is  derived  from  the  somatopleure  in  the  folding  ofi  of  the  body.  In  amniotes 
the  amniotic  cavity  is  at  first  small  but  rapidly  increases  in  size.  It  con- 
tains a  clear,  transparent  liquid,  the  amniotic  fluid,  which  amounts  to  about 


Fig.  332. — Human  Embryo  .\nd  its  En- 
velopes AT  THE  End  of  the  Third  Month, 
— (Dalton.) 


REPRODUCTION 


735 


one  liter  at  term;  it  serves  to  protect  the  fetus  during  gestation  and  at 
parturition  it  dilates  the  os  cervicis,  and  flushes  the  birth  canal.  This  liquid 
is  derived  mainly  from  the  blood  as  it  contains  albumin,  sugar,  fat  and  in- 
organic salts.  Traces  of  urea  indicate  that  some  of  its  constituents  are  de- 
rived from  the  embryo  itself. 

The  caudal  end  of  the  embryonic  area  is  left  connected  with  the  chorion 
by  a  heavy  band  of  mesoderm  termed  the  belly-stalk,  to  which  the  caudal 
part  of  the  amnion  is  attached.  The  entoderm  is  invaginated  into  the 
belly-stalk  for  a  short  distance  constituting  the  allantois  of  higher  forms. 
In  oviparous  forms  the  allantois  grows  out  between  the  closing  somatopleuric 
folds  that  form  the  body- wall  and  constitutes  a  free  sac  upon  which  vessels 
(allantoic  arteries  and  vehis)  develop  from  the  embryo.  This  sac  then 
spreads  beneath  the  white  shell  membrane  forming  the  organ  of  nutrition 
and  respiration  of  these  forms  during  the  last  half  of  their  incubation  periods. 
In  mammals  the  extra-embryonic  portion  of  the  allantois  is  of  little 
importance  (Fig.  332). 

Placenta  Formation. — The  chorionic  villi  increase  rapidly  in  size  and 
number  and  usually  surround  the  whole  fetal  sac,  giving  it  a  peculiar  shaggy 


Amnion,   '^'^•mmmmms 
Chorion. 


d 

Compact 

.£ 

layer. 

0 

u 

c; 

ci  ' 

:j 

.      1 

TJ 

Cavernous  / 

0 
0 

layer.       (, 

Muscularis. 


•  Chorionic  villi. 

Intervillous  spaces. 
Floating  villus. 

Attached  villi. 
Vein. 


^IG.  373. — Di.\GR.A.M  OF  Human  Placexta  at  the  Close  of  Pregn.anxy. — (Schdper.) 


appearance.  Blood-vessels  now  proceed  from  the  embryo  along  the  belly- 
stalk  (not  the  allantois  in  higher  forms  as  formerly  stated).  These,  the 
umbilical  arteries  and  veins,  pass  to  the  chorionic  villi  and  send  branches  to 
those  of  the  placental  area;  these  vascularized  villi  constitute  the  chorion 
frondosum,  while  the  avascular  villi  form  the  chorimi  leve.  The  villi  of  the 
latter  disappear  during  the  second  month,  leaving  the  chorionic  membrane 
smooth.  The  villi  of  the  chorion  frondosum  now  penetrate  the  uterine  glands 
of  the  decidua  basilaris,  which  by  this  time  have  been  denuded  of  epithelium, 
and  have  gained  connection  with  the  blood-vessels  of  the  mucosa;  in  this 
manner  these  uterine  glands  have  become  converted  into  blood  sinuses.  The 
chorionic  villi  either  attach  themselves  to  the  tunica  propria  of  the  mucosa 


736  TEXT-BOOK  OF  PHYSIOLOGY 

( fixed  villi)  or  remain  free  {floating  villi) .  At  the  edge  of  the  placental  area 
very  few  villi  develop,  leaving  a  circular  channel  called  the  marginal  sinus. 
This  attachment  of  villi  becomes  marked  from  the  third  month  on  and 
this  is  considered  the  beginning  of  placentation.  From  this  time  on  to  term 
there  is  merely  an  increase  in  number  of  villi  and  vessels  with  thus  a  cor- 
responding increase  in  the  size  of  the  placenta.    (Fig,  2)?)Z') 

The  Placenta. — Of  all  the  embryonic  structures  the  placenta  is  the 
most  important.  It  is  formed  by  the  end  of  the  third  month,  after  which  it 
gradually  increases  in  size  up  to  the  end  of  the  eighth  month,  by  which 
time  it  is  fully  developed.  It  then  measures  from  i8  to  24  cm.  in  diameter 
and  weighs  from  400  to  600  grams.  It  is  most  frequently  attached  to  the 
upper  and  back  part  of  the  uterine  wall.  Though  exceedingly  complex 
in  structure  it  consists  essentially  of  two  portions,  a  fetal  and  a  maternal. 

The  fetal  portion  consists  primarily  of  those  villi  on  the  chorion  in  relation 
with  the  decidua  basilaris.  These  structures  gradually  increase  in  size  and 
number,  and  receive  the  ultimate  branches  of  the  umbilical  arteries.  The 
maternal  portion  consists  primarily  of  the  decidua  basilaris.  As  gestation 
advances  the  placental  villi  rapidly  increase  in  size  and  number,  and  re- 
ceive the  branches  of  the  umbilical  arteries.  At  the  same  time  the  decidua 
basilaris  becomes  hypertrophied  and  vascular.  With  the  continued  growth 
and  development  of  these  two  structures  they  gradually  fuse  together  and 
finally  become  inseparable.  In  accordance  with  the  needs  of  the  embryo, 
the  decidua  basilaris  and  its  contained  blood-vessels  undergo  certain  histo- 
logic changes  which  result  in  the  formation  of  large  cavities,  sinuses,  or 
lakes,  into  which  the  blood  of  the  uterine  vessels  is  emptied.  As  the  pla- 
centa develops,  the  structures  separating  the  blood  of  the  mother  from  that 
of  the  child  gradually  become  modified  until  they  are  represented  by  a  thin 
cellular  or  homogeneous  membrane.  The  conditions  now  are  such  as  to 
permit  of  a  free  exchange  of  material  between  the  mother  and  child.  Whether 
by  osmosis  or  by  an  act  of  secretion,  the  nutritive  materials  of  the  maternal 
blood  pass  through  the  intervening  membrane  into  the  fetal  blood  on  the 
one  hand,  while  waste  products  pass  in  the  reverse  direction  into  the  maternal 
blood  on  the  other  hand.  Inasmuch  as  oxygen  is  absorbed  and  carbon 
dioxid  exhaled  by  the  same  structures,  the  placenta  is  to  be  regarded  as 
both  an  absorptive  and  a  respiratory  organ.  So  long  as  these  exchanges  are 
permitted  to  take  place  in  a  normal  manner  the  nutrition  of  the  embryo  is 
secured. 

The  Nutritive  Supply  of  the  Embryo. — The  growth  and  development 
of  the  embryo  from  the  period  of  fertilization  to  the  period  of  birth  require 
a  continuous  and  ever  increasing  supply  of  food  materials.  This  is  derived 
from  several  sources  and  requires  for  its  utilization,  the  development,  in 
different  classes  of  animals,  of  specialized  forms  of  the  circulatory  apparatus, 
the  relative  importance  of  which  varies  in  accordance  with  the  source  of 
the  food  supply.  These  are  know^n  as  the  vitelline,  the  allantoic,  and  the 
placental  circulations.  All  these  forms  are  present  at  successive  stages  in 
the  development  of  the  human  embryo  but  only  the  last  is  of  major 
importance. 

As  the  ovum  passes  down  the  oviduct  it  imbibes  its  nutritive  material 
from  the  mucosa.     When  it  lodges  itself  in  the  uterus  it  probably  receives 


REPRODUCTION 


737 


additional  material  in  the  same  way.  The  period  during  which  it  does  so 
is,  however,  very  limited. 

The  Vitelline  Circulation. — The  vitelline  circulation,  which  in  oviparous 
animals,  e.g.,  the  chick,  is  of  primary  importance  because  of  the  large 
amount  of  food  stored  in  the  vitellus  or  yolk,  is  in  mammals  of  relatively 
slight  importance  because  of  the  limited  supply  of  food  in  the  vitellus.  It 
is  nevertheless  present  in  early  stages. 

The  Allantoic  Circulation  which  in  oviparous  animals  is  also  of  primary 
importance  in  the  latter  half  of  the  incubation  period  both  as  an  absorption 


Fig.  334. — The  Fetal  Circulation,  ao.  Aorta,  a.pu.  Pulmonary  artery,  au.  Umbilica 
artery,  da.  Ductus  arteriosus,  dv.  Ductus  venosus.  mt.  Intestine,  vci  and  vcs.  Inferior  and 
superior  venae  cavae.  vh.  Hepatic  vein.  vp.  Venaportae.  v.  pu.  Pulmonary  vein.  vu.  Umbilical 
vein. — (From  Kallmann.) 

and  respiratory  apparatus  is  also  present  in  mammals  to  a  slight  extent, 
but  it  is  merely  a  transition  stage  in  the  development  of  placental  circulation. 
The  Placental  Circulation. — The  development  of  the  fetal  or  placental 
circulatory  apparatus  by  which  the  fetus  obtains  its  food  supply  and  neces- 
sary oxygen  and  frees  itself  from  carbon  dioxid  has  been  alluded  to  in  a 
foregoing  paragraph  relating  to  the  formation  of  the  placenta.  After  the 
blood-vessels  of  the  embryo,  the  umbilical  arteries  and  vein  have  come  into 
histologic  and  physiologic  relations  with  the  uterine  blood-vessels,  the 
nutritive  materials  and  the  oxygen  are  derived  entirely  from  the  maternal 

47 


738  TEXT-BOOK  OF  PHYSIOLOGY 

blood-stream  which  at  the  same  time  receives  carbon  dioxid  and  perhaps 
other  waste  products  from  the  fetal  blood-stream.  The  placenta  thus 
serves  as  a  digestion  and  respiratory  organ.  The  blood  having  undergone 
these  changes  now  leaves  the  placenta  and  returns  to  the  fetus  by  the  um- 
bilical vein.  This  blood  is  relatively  rich  in  nutritive  material  and  of  a 
scarlet  red  color  by  reason  of  the  presence  of  an  increased  amount  of  oxygen. 
As  it  passes  into  the  abdominal  cavity  a  portion,  about  one-half,  of  the  blood 
is  directed  by  the  ductus  venosus  into  the  inferior  vena  cava,  while  the  re- 
mainder is  emptied  into  the  portal  vein,  by  which  it  is  distributed  to  the 
liver  and  from  which  it  emerges  by  the  hepatic  veins  and  is  poured  into  the 
inferior  vena  cava.  The  blood  in  the  vena  cava  is  thus  a  mixture  of  venous 
blood  from  the  lower  extremities  and  liver,  and  oxygenated  blood  from  the 
placenta.  After  its  discharge  into  the  right  auricle  the  blood  is  directed  by 
a  fold  of  the  lining  membrane,  the  Eustachian  valve,  through  an  opening  in 
the  interauricular  septum,  the  foramen  ovale,  into  the  left  auricle.  It  then 
flows  through  the  auriculo-ventricular  opening  into  the  left  ventricle,  thence 
into  the  aorta,  and  by  its  branches  is  distributed  to  all  parts  of  the  body. 

The  blood  from  the  head  and  upper  extremities  is  emptied  by  the  superior 
vena  cava  into  the  right  auricle,  but  as  it  passes  in  front  of  the  Eustachian 
valve,  it  flows  directly  into  the  right  ventricle  and  then  into  the  pulmonary 
artery.  On  account  of  the  unexpanded  condition  of  the  lungs  and  the 
almost  impervious  condition  of  the  pulmonary  capillaries,  but  a  small 
portion  of  the  blood  passes  through  them,  while  the  larger  portion  by  far 
passes  into  the  aorta  directly  through  a  duct,  the  ductus  arteriosus,  which 
enters  at  a  point  below  the  origin  of  the  left  carotid  and  subclavian  arteries. 
A  comparison  of  the  blood  distributed  to  the  head  and  upper  extremities, 
with  that  distributed  to  the  lower  extremities,  will  show  a  larger  percentage 
of  nutritive  material  and  oxygen  in  the  former  than  in  the  latter,  a  fact 
which  has  been  offered  as  an  explanation  of  the  more  rapid  growth  of  the 
liver  and  upper  half  of  the  body.  As  the  blood  passes  through  the  aorta, 
a  portion  is  directed  from  the  main  current  by  the  hypogastric  and  umbilical 
arteries  to  the  placenta,  where  it  loses  carbon  dioxid  and  gains  oxygen,  and 
changes  in  color  from  a  bluish  red  to  a  scarlet  red. 

Parturition. — At  the  end  of  gestation — approximately  280  days  from 
the  time  of  conception — a  series  of  changes  occur  in  the  uterine  structures 
which  lead  to  an  expulsion  of  the  child,  the  placenta,  and  decidua  vera.  To 
this  process  in  its  entirety  the  term  parturition  is  given.  At  this  time,  from 
causes  not  clearly  defined,  the  uterine  walls  begin  to  exhibit  throughout  their 
extent  a  series  of  slight  contractions  which  are  somewhat  peristaltic  in  char- 
acter; these  contractions,  which  gradually  increase  in  frequency  and  vigor, 
bring  about  a  dilatation  of  the  internal  os  and  a  descent  of  the  membranes 
into  the  cervical  canal.  The  pressure  exerted  by  these  membranes  during 
the  time  of  the  contraction  materially  assists  in  the  relaxation  of  the 
circular  fibers  and  a  dilatation  of  the  external  os.  When  the  dilatation  has 
so  far  advanced  that  the  diameter  of  the  external  os  attains  a  measure  of 
7  or  8  cm.,  the  tension  of  the  membranes  becomes  sufficiently  great  to  lead 
to  their  rupture  and  to  a  partial  escape  of  the  amniotic  fluid.  With  this 
event,  the  presenting  part  of  the  child,  usually  the  head,  descends  into  the 
vagina.  After  a  short  period  of  rest  the  uterine  contractions  return  and 
rapidly  increase  in  vigor  and  duration.     As  a  result  of  the  pressure  thus 


REPRODUCTION  739 

exerted  from  all  sides  on  the  body  of  the  child,  the  head  gradually  descends 
still  further  into  the  vagina  and  finally  emerges  through  the  vulva  to  be 
followed  in  a  short  time  by  expulsion  of  the  trunk  and  limbs,  and  a  discharge 
of  the  remaining  amniotic  fluid.  With  the  expulsion  of  the  child  the  uterine 
contractions  cease  for  a  period  of  ten  or  fifteen  minutes,  when  they  again 
recur,  with  the  result  of  detaching  the  placenta  and  expelling  it  into  the 
vagina.  It  is  then  removed  by  the  cooperative  action  of  the  abdominal 
and  perineal  muscles.  The  hemorrhage  which  would  naturally  occur  with 
the  detachment  of  the  placenta  and  the  laceration  of  the  maternal  vessels 
is  prevented  by  the  firm  continuous  contraction  of  the  uterine  walls,  by 
which  the  vessels  are  compressed  and  permanently  closed. 

The  Establishment  of  Inspiration  and  the  Adult  Circulation. — 
After  the  birth  of  the  child  and  the  detachment  of  the  placenta,  there  speedily 
occurs  a  decrease  in  the  quantity  of  oxygen  and  an  increase  in  the  quantity 
of  carbon  dioxid  in  the  blood,  a  condition  which  causes  a  discharge  of  nerve 
energy  from  the  inspiratory  center,  a  contraction  of  the  inspiratory  muscles, 
an  expansion  of  the  thorax,  and  an  inflow  of  air  into  the  lungs. 

In  addition  it  is  very  probable  that  the  stimulation  of  the  inspiratory 
center  is  also  occasioned  by  the  arrival  of  nerve  impulses  from  the  skin, 
developed  by  the  cooling  of  the  skin  due  to  the  vaporization  of  the  amniotic 
fluid. 

In  the  later  months  of  intrauterine  life  the  vascular  apparatus  undergoes 
certain  anatomic  changes  which  favor  the  transition  from  the  placental  to 
the  adult  circulation.  Thus  the  ductus  venosiis  contracts,  and  sends  a 
a  larger  volume  of  blood  into  and  through  the  liver;  the  Eustachian  valve 
diminishes  in  size  and  at  the  time  of  birth  has  almost  disappeared;  a  mem- 
branous fold  grows  upward  and  backward  from  the  edge  of  the  foramen 
ovale  on  the  left  side;  the  ductus  arteriosus  also  contracts.  With  the  first  in- 
spiration and  the  expansion  of  the  lungs,  the  blood  which  enters  the  pul- 
monary artery  passes  through  the  pulmonary  capillaries  in  large  volume  and 
is  returned  by  the  pulmonary  veins  to  the  left  auricle.  The  entrance  of 
the  blood  into  this  cavity  presses  the  membranous  fold  against  the  margins 
of  the  foramen  ovale  and  thus  prevents  the  further  flow  of  blood  from  the  right 
auricle.  The  blood  entering  the  right  auricle  by  the  inferior  vena  cava  now 
flows  into  the  right  ventricle,  which  is  favored  by  the  small  size  of  the  Eu- 
stachian valve.  The  foramen  ovale  is  permanently  closed  at  the  end  of  a 
week  or  ten  days;  the  ductus  arteriosus  at  the  end  of  four  days.  The  um- 
bilical vein  and  ductus  venosus,  at  the  end  of  four  or  five  days,  have  also 
become  almost  impervious  from  the  contraction  of  their  walls.  The 
proximal  ends  of  the  hypogastric  arteries  remain  open  and  carry  blood  to  the 
walls  of  the  bladder.  The  distal  ends  of  the  arteries  are  converted  into  im- 
pervious cords. 

Lactation. — As  pregnancy  advances  the  mammary  glands  increase  in 
size,  partly  from  a  deposition  of  fat  and  connective  tissue  and  partly  from 
a  multiplication  of  the  secreting  acini.  The  lining  epithelial  cells  at  the  same 
time  increase  in  size,  and  toward  the  end  of  pregnancy  begin  to  exhibit 
functional  activity.  At  the  time  of  birth,  or  within  a  day  or  so  after  birth, 
the  acini  are  filled  with  a  fluid  which  in  its  qualitative  composition  resem- 
bles milk  and  is  known  as  colostrum.  It  is  distinguished  from  milk  more 
especially  in  the  fact  that  it  contains  in  large  quantity  a  proteid  which  coag- 


740  TEXT-BOOK  OF  PHYSIOLOGY 

ulates  on  boiling,  and  certain  inorganic  salts  which  have  a  laxative  effect 
on  the  new-born  child.  Normal  lactation  and  the  phenomena  which 
accompany  it  are  fully  established  by  the  end  of  the  second  or  third  day. 

The  composition  of  milk  and  the  mechanism  of  its  production  have  been 
stated  in  the  chapter  on  Secretion. 

Physiologic  Activities  of  the  Embryo. — During  intrauterine  life  the 
evolution  of  structure  is  accompanied  by  an  evolution  of  function.  The 
relatively  simple  and  uniform  metabolism  of  the  undifferentiated  blastoder- 
mic membranes  gradually  increases  in  complexity  and  variety,  as  the  individ- 
ual tissues  and  organs  make  their  appearance  and  assume  even  a  slight 
degree  of  functional  activity.  As  to  the  periods  at  which  different  organs 
begin  to  functionate,  but  little  is  positively  known. 

The  primitive  heart,  in  all  probability,  begins  to  pulsate  very  early,  as  in 
an  embryo  from  fifteen  to  eighteen  days  old  and  measuring  but  2  mm.  in 
length,  Coste  found  the  amnion,  the  allantois,  the  omphalo-mesenteric 
vessels,  and  the  two  primitive  aortae  developed.  In  the  earlier  weeks,  all 
products  of  metabolism  are  doubtless  eliminated  by  the  placental  structures; 
but  as  metabolism  increases  in  complexity  the  liver  and  kidney  assume  ex- 
cretory activity.  Thus,  at  the  end  of  the  third  month  the  intestine  contains 
a  dark,  greenish,  viscid  material — meconium — composed  of  bile  pigments, 
bile  salts,  and  desquamated  epithelium;  the  amniotic  fluid,  as  well  as  the 
fluid  within  the  bladder,  contains  urea  at  the  end  of  the  sixth  month,  in- 
dicating the  establishment  of  both  hepatic  and  renal  activity.  Contractions 
of  the  skeletal  muscles  of  the  limbs  begin  about  the  fifth  month,  from  which 
it  may  be  inferred  that  the  mechanism  for  muscle  activity,  viz.:  muscles, 
efferent  nerves,  and  spinal  centers,  has  become  anatomically  developed  and 
associated,  and  capable  of  coordinate  activity.  These  contractions  are,  in  all 
probability,  automatic  or  autochthonic  in  character  due  to  stimuli  arising 
within  the  spinal  centers.  The  remaining  organs  remain  more  or  less 
inactive. 

After  birth,  with  the  first  inspiration  and  the  introduction  of  food  into  the 
alimentary  canal,  the  physiologic  mechanisms  which  subserve  general 
metaboHsm  begin  to  functionate  and  in  the  course  of  a  week  are  fully  estab- 
lished. At  this  time  the  cardiac  pulsation  averages  about  135  a  minute;  the 
respiratory  movements  vary  from  30  to  35  a  minute,  and  are  diaphragmatic 
in  type;  the  urine,  which  was  at  first  scanty,  is  now  abundant  and  propor- 
tional to  the  food  consumed;  the  digestive  glands  are  elaborating  their  re- 
spective enzymes,  digestion  proceeding  as  in  the  adult.  The  hepatic  secre- 
tion is  active  and  the  lower  bowel  is  emptied  of  its  contents;  the  coordinate 
activites  of  the  nerve-,  muscle-,  and  gland-mechanisms  are  entirely  reflex  in 
character.  Psychic  activities  are  in  abeyance  by  reason  of  the  incomplete 
development  of  the  cerebral  mechanisms. 


APPENDIX 


PHYSIOLOGIC  APPARATUS 

The  study  of  the  physical  and  physiologic  properties  of  muscles  and 
nerves  necessitates  the  employment  of  some  stimulus  which,  when  applied  to 
either  tissue,  will  call  forth  a  contraction  of  the  muscle,  or  the  development  of 
a  nerve  impulse  in  the  nerve.  The  most  convenient  stimulus  is  electricity, 
for  the  reason  that,  with  appropriate  apparatus,  its  intensity  and  duration 
can  be  graduated  with  the  utmost  nicety.  Moreover,  it  does  not  destroy 
the  tissues,  as  do  many  chemic,  physical,  and  mechanic  stimuli. 

It  is,  therefore,  necessary  that  the  student  should  have  a  practical 
acquaintance  with  those  appliances  by  means  of  which  electricity  is 
generated,  applied  and  controlled. 

The  electric  cell  is  an  apparatus  composed  of  different  elements,  which, 
by  virtue  of  chemic  actions  taking  place  among  them,  generate  and  conduct 
electricity.  In  its  simplest  form  an  electric  cell  consists  of  two  metals — 
zinc  and  copper,  or  carbon,  or  platinum,  etc.,  immersed  in  an  exciting  fluid, 
usually  dilute  sulphuric  acid  (Fig.  335). 

The  zinc  element  is  the  one  acted  on  chemically  by  the  sulphuric  acid, 
and  at  the  expense  of  which  the  electricity  is  maintained.  It  is  known  as  the 
generating  element.     The  copper  is  the  collecting  and  conducting  element. 

With  the  immersion  of  these  elements  in  a  solution  of  H2SO4  a  chemic 
action  at  once  takes  place  between  the  zinc  and  the  acid,  with  the  formation 
of  zinc  sulphate  and  the  liberation  of  hydrogen,  as  expressed  in  the  following 
formula: 

Zn  +  HjSO.^ZnSO,  +  H^. 

The  zinc  sulphate  passes  into  the  solution,  while  the  hydrogen  accu- 
mulates on  the  surface  of  the  copper  element. 

As  all  chemic  action  is  accompanied  by  the  development  of  electricity, 
it  can  be  shown  by  appropriate  means  that  this  is  the  case  at  the  surface  of 
the  zinc.  Such  a  combination  is  the  means  of  establishing  a  dijfference  oj 
potential  between  two  points;  the  point  of  highest  potential  being  in  the  acid  at 
the  surface  of  the  zinc,  the  point  of  lowest  potential  being  in  the  interior  of  the 
zinc.  So  long  as  theelements  remain  unconnected  there  is  no  movement  of 
electricity,  no  current. 

If  the  ends  of  the  elements  projecting  beyond  the  fluid  are  connected  by 
a  copper  wire,  a  pathway  or  circuit  is  established,  and  a  movement  of  the 
electricity  takes  place.  As  electricity  flows  from  the  point  of  high  to  the 
point  of  low  potential,  it  follows  that  inside  the  cell  the  current  flows  in  the 
acid  from  the  zinc  to  the  copper,  and  outside  the  cell  from  the  copper  to  the 
zinc.  Such  a  current  is  termed  a  continuous,  a  galvanic  or  a  voltaic  current. 
Inasmuch  as  there  is  a  progressive  fall  in  potential  between  the  highest  and 

741 


742 


TEXT-BOOK  OF  PHYSIOLOGY 


lowest  points,  it  follows  that  any  two  points  in  the  circuit  will  exhibit  a 
similar  difference  of  potential.  For  this  reason  the  projecting  end  of  the 
copper  element  is  at  a  higher  potential  than  the  projecting  end  of  the  zinc 
element.  The  end  of  the  copper  is,  therefore,  termed  the  positive,  +  pole  or 
anode,  the  end  of  the  zinc  the  negative,— pole  or  kathode. 

Electric  Units. — Owing  to  the  difference  of  the  electric  potential  in  the 
cell,  the  electricity  leaves  the  cell  under  a  certain  degree  of  pressure,  termed 
the  "electro-motive  force."  As  it  passes  through  the  circuit  it  meets  with 
resistance,  the  amount  of  which  will  depend  on  the  nature  of  the  circuit 
material,  its  length,  and  the  area  of  its  cross-section.  In  accordance  with 
the  resistance  will  depend  the  quantity  of  electricity  that  a  given  electro- 
motive force  will  press  through  in  a  unit  of  time.  The  strength  of  the 
current  will,  therefore,  not  depend  entirely  on  the  electro-motive  force,  but 
rather  on  the  ratio  between  the  electro-motive  force  and  the  resistance. 


T  + 


Fig.  33S-— An 
Electric  Cell. 


Fig.  336. — Two  Simple  Electric  Cells  Joined 
IN  Series.     C.  Copper.     Z.  Zinc. 


For  the  measurement  of  electric  quantities,  a  system  of  units  has  been 
devised.  The  unit  of  electro-motive  force  is  the  volt;  the  unit  of  resistance 
is  the  ohm,  i.e.,  the  resistance  offered  by  a  column  of  mercury  106.3  cm.  long 
and  I  sq.  mm.  section  at  o°C.;  the  unit  of  quantity  is  the  coulomb;  the 
unit  of  time  is  one  second.  One  volt  is  the  electro-motive  force  which, 
when  steadily  apphed,  will  press  through  a  resistance  of  the  ohm,  one  coul- 
omb of  electricity  in  one  second  of  time  yielding  a  current  strength  of  one 
ampere. 

The  relation  may  be  expressed  in  the  following  formula.  Ohm's  law: 


C  (current  strength)  = 


Electro-motive  force  (E.  M.  F.) 
Resistance  (R) 


or  Ampers  = 


Volts 
Ohms 


In  practical  work  it  is  often  necessary  to  increase  the  strength  of  the 
current.  This  is  done  by  uniting  two  or  more  cells  in  series,  i.e.,  uniting  the 
copper  of  one  cell  to  the  zinc  of  a  second,  and  so  on  (Fig.  336).  If  the 
resistance  remains  the  same  the  total  voltage  and  current  are  those  of  one 
cell  multiplied  by  the  number  of  cells  united. 

The  cell  as  above  described  cannot  maintain  a  current  of  constant 
strength  for  any  length  of  time,  for  the  following  reasons: 

I.  The  sulphuric  acid  solution,  in  consequence  of  its  chemic  action, 
soon  becomes  nothing  more  than  a  saturated  solution  of  zinc  sulphate,  after 
which  its  chemic  activity  ceases.  The  current,  therefore,  soon  diminishes  in 
strength. 


PHYSIOLOGIC  APPARATUS 


743 


2.  The  accumulation  of  hydrogen  bubbles  on  the  surface  of  the  copper 
hinders  the  passage  of  the  electricity.  In  a  short  time  they  develop  a  current 
in  the  opposite  direction,  which  also  tends  to  weaken  the  original  current. 
This  action  is  termed  polarization  of  the  elements. 

Cells  of  this  character  are  not  suited  for  physiologic  work,  in  which 
constancy  in  the  strength  of  the  current  is  absolutely  necessary.  To  over- 
come these  disadvantages,  cells  have  been  devised  which  are  less  violent  in 
action,  which  prevent  polarization,  and  which  maintain  a  current  of  constant 
strength  for  a  long  period  of  time.  One  of  the  most  generally  used  for 
physiologic  purposes  is — 

The  Daniell  cell. — This  consists  of  a  porous  cup  containing  a  saturated 
solution  of  CuSO^,  copper  sulphate,  in  which  is  immersed  a  copper  plate  or 
rod.  This  combination  is  placed  in  a  glass  vessel  containing  a  solution  of 
H2SO4  (1:15).  In  this  solution  is  immersed  a  roll  of  sheet  zinc  (Fig.  337). 
Each  of  the  plates  is  provided  with 
a  binding  screw.  When  the  cell  is  in 
action  the  sulphuric  acid  attacks  the 
zinc,  forming  zinc  sulphate,  and  liber- 
ates hydrogen;  the  cup  being  porous, 
the  hydrogen  passes  into  the  copper 
sulphate  solution,  where  it  combines 
with  the  sulphuric  acid  radicle,  and 
liberates  metalHc  copper.  Polariza- 
tion of  the  copper  is  thus  prevented. 
The  metallic  copper  is  deposited  on 
the  copper  plate,  which  is  thus  kept 
bright.  The  copper  sulphate  solution 
is  kept  at  the  point  of  saturation  by 
packing  around  the  copper  cylinder  a 
quantity  of  the  crystals  of  the  salt. 
The  sulphuric  acid  passes  back  into 
the  porous  cup,  to  take  the  place  of  that  used.  This  cell  is  remarkably 
constant  for  these  reasons,  and  well  adapted  for  physiologic  as  well  as  other 
purposes  where  a  current  of  uniform  strength  is  necessary. 

The  projecting  ends  of  the  copper  and  zinc  plates  are  termed  respectively 
the  positive  pole  or  anode,  and  the  negative  pole  or  kathode.  The  electro- 
motive force  of  a  Daniell  cell  is  practically  i  volt;  but  when  the  two  poles 
are  connected  by  a  wire  of  i  ohm  resistance,  the  current  strength  will  be  less 
than  I  ampere,  possibly  only  0.7,  owing  to  the  resistance  offered  to  the  flow  of 
electricity  by  the  fluids  between  the  zinc  and  the  copper.  In  all  measurements, 
the  internal  resistance  of  the  cell  must  be  taken  into  consideration. 

The  Dry  Cell. — The  com_mercial  dry  cell  is  a  convenient  source  of 
electricity  for  general  laboratory  work.  It  consists  of  a  cup  of  zinc,  the  inner 
surface  of  which  is  covered  over  with  a  thick  layer  of  a  paste  of  plaster  of 
Paris,  saturated  with  ammonium  chlorid.  In  the  center  of  the  cup  there  is 
a  rod  of  carbon.  Surrounding  this  rod  and  occupying  the  space  between  it 
and  the  plaster-of-Paris  paste,  is  a  mixture  of  manganese  dioxid  and  charcoal. 
The  upper  surface  of  the  cell  is  sealed  to  prevent  evaporation.  The  electric- 
ity is  generated  at  the  surface  of  the  zinc  cup  by  the  chemic  action  of  the 
chlorin  which  arises  from  the  dissociation  of  the  ammonium  chlorid.     When 


Fig.  337. — D.\NiELL  Cell. 


744 


TEXT-BOOK  OF  PHYSIOLOGY 


the  plates  are  united  by  a  conjunctive  wire  the  current  within  the  cell  flows 
from  the  zinc  (the  positive  element)  to  the  carbon  (the  negative  element), 
and  without  the  cell  from  the  carbon  (the  positive  pole)  to  the  zinc  (the 
negative  pole). 

Leads. — By  means  of  insulated  wires  attached  to  the  poles  of  a  cell,  the 
electricity  may  be  conducted  from  the  cell  and  used  for  exciting  or  stimulat- 
ing purpose.  As  the  wires  thus  become  practically  prolongations  of  the 
plates  their  ends  become  the  corresponding  poles.  In  experimental  work 
the  ends  of  the  wires  are  provided  with  special  devices,  termed — 

Non-polarizable  Electrodes. — The  necessity  for  the  employment  of 
such  electrodes  arises  from  the  fact  that  when  the  ends  of  the  wires  from  a 
cell  are  placed  in  direct  contact  with  the  tissues  chemic  changes  are  produced 
in  a  short  time,  which  lead  to  their  polarization.     As  a  result,  a  current 

opposite  in  direction  to  that  of  the  cell  is 
developed,  which  tends  to  weaken  or  neu- 
tralize it.  This  polarization  current  vitiates 
the  result  of  many  experiments  made  with 
highly  irritable  tissue  such  as  nerve-tissue. 
Whether  for  stimulating  purposes  or  for  the 
purpose  of  detecting  the  existence  of  electric 
currents  in  living  tissues,  it  is  essential  that 
the  electrodes  used  shall  be  non-polarizable. 
The  earliest  electrodes  of  this  character  were 
made  by  du  Bois-Reymond  and  were  based 
on  the  fact  discovered  by  Regnault  that  a 
strip  of  chemically  pure  zinc  or  amalga- 
mated zinc  (Matteucci)  immersed  in  a  satu- 
rated solution  of  zinc  sulphate  would  not 
polarize.  One  form  made  by  du  Bois-Rey- 
mond is  shown  in  Fig.  338.  It  consists  of 
a  flattened  glass  tube  attached  to  a  universal 
joint  and  supported  by  an  insulated  brass 
stand.  The  lower  end  of  the  tube  is 
closed  with  kaolin  or  China  clay  made  into 
a  paste  with  a  0.6  per  cent,  solution  of  sodium  chlorid.  It  can  be  moulded 
into  any  desired  shape.  The  interior  of  the  tube  is  partially  filled  with  a 
saturated  solution  of  sulphate  of  zinc  in  which  is  immersed  the  strip  of  amal- 
gamated zinc.  To  the  upper  end  of  the  zinc  the  conducting  wire  is  attached. 
The  V.  Fleischl  brush  electrode  is  similar  to  the  preceding  except  that 
the  end  of  the  tube  is  closed  by  the  brush  of  a  camel's-hair  pencil. 

The  d'Arsonval  electrode  consists  of  a  glass  tube  containing  a  silver  rod 
coated  with  fused  silver  chlorid.  The  interior  of  the  tube  is  filled  with 
normal  salt  solution  0.6  per  cent,  and  the  end  closed  with  a  thread  or 
plug  of  asbestos  which  is  made  to  project  beyond  the  tube  for  a  short 
distance. 

Any  one  of  these  three  electrodes  is  suitable  for  physiologic  experimen- 
tation, as  their  free  ends  neither  corrode  the  tissues  nor  develop  electric 
currents. 

Keys. — Muscle  and  nerve-tissues  are  conductors  of  electricity.  When, 
therefore,  the  terminals  (the  non-polarizable  electrodes)  of  the  wires  of  a  cell 


Fig.  338. — NoN-POLARiz.^BLE  Elec- 
trodes. I.  Du  Bois-Reymond's.  2. 
Von  Fleischl's.     3.  d'Arsonval's. 


PHYSIOLOGIC  APPARATUS 


745. 


are  placed  in  contact  with  either  a  muscle  or  a  nerve  a  circuit  is  made  through 
which  a  current  of  electricity  flows;  when  one  or  both  are  removed,  the 
circuit  is  broken  and  the  current  ceases.  In  practical  work  it  is  often  neces- 
sary to  keep  the  electrodes  in  contact  with  the  tissues  for  a  variable  length 
of  time.  The  circuit,  however,  may  be  alternately  made  and  broken  at  will 
by  interposing  along  the  return  wire  a  mechanic  contrivance  known  as  a  key, 
of  which  there  are  many  forms. 

The  du  Bois-Reymond  Friction  Key. — This  consists  of  a  plate  of 
vulcanite  attached  to  a  screw  clamp  by  which  it  can  be  fastened  to  the  edge 
of  a  table  (Fig.  339).  The  surface  of  the  vulcanite 
plate  carries  two  rectangular  blocks  of  brass,  each 
of  which  has  two  holes  drilled  through  it,  for  the 
insertion  of  wires,  which  are  held  in  position  by 
small  screws.  A  movable  bridge  of  brass,  provided 
with  an  ebonite  handle,  serves  to  make  connection 
between  the  blocks.  There  are  two  ways  of  in- 
terposing this  key  in  the  circuit. 

1.  As  a  Simple  Key. — For  this  purpose  one  of  the 

wires,  usually  the  negative,  is  carried  from  the 
cell  to  one  block  and  then  continued  from  the 
second  block.  When  the  bridge  is  down,  the 
circuit  is  made  and  the  current  passes;  when 
it  is  up,  the  circuit  is  broken. 

2.  As  a  Short-circuiting  Key. — When  used  for  this 

purpose,  the  wires  "of  the  cell  are  carried  to 
the  inner  holes  of  each  block  and  then  con- 
tinued from  the  outer  holes  to  the  tissues  or 
to  some  form  of  apparatus  which  it  is  desired 
to  actuate.  When  the  key  is  closed,  i.e.,  when 
the  bridge  is  down,  the  current  on  reaching 
the  key,  will  divide,  one  portion  passing  across 

the  bridge  and  so  back  to  the  cell,  the  other     _  t.    ..      ^ 

^.        ^        .         ^     ^,        ^.  ^  Fig.  339.— Du  Bois-Rey- 

portion    passmg    to  the    tissue    or    apparatus,  ^j^ond  Friction  Key. 

The  amount  of  the  current  which  is  returned 

to  the  cell  through  the  short  circuit  will  be  proportional  to  the  resistance 

of  the  longer  circuit.     As  the  latter  is  usually  great  in  comparison  with 

the    former,  practically  all  the  current  is  short-circuited.     When  the 

bridge  is  lowered,  therefore,  the  current  is  short-circuited;  when  it  is 

raised,   the  current  flows  into  the  longer  circuit  through  the  tissue  or 

apparatus. 

The  Mercury  Key. — In  this  form  the  connection  is  established  by  means 

of  mercury.     It  consists  of  a  circular  block  in  the  center  of  which  there  is  a 

cup  containing  mercury  (Fig.  340).     At  opposite  points  there  are  binding 

posts,  one  of  which  is  provided  with  a  rigid  fixed  copper  rod  passing  into 

the   mercury;  the  other  is  provided  with  a  movable  bent  rod  which  may 

be  made  to  dip  into  or  be  withdrawn  from  the  mercury  by  the  ebonite 

handle. 

The  effect  of  a  constant  or  galvanic  current  on  a  muscle  or  nerve  will 
depend  to  some  extent  on  its  strength.  This  may  be  accurately  regulated 
by  means  of  an  apparatus  known  as — 


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The  Rheocord. — With  this  apparatus  an  electric  current  may  be  divided, 
one  portion  continuing  through  a  conductor  back  to  the  battery,  the  other 
portion  being  sent  off  through  the  nerve.  The  strengths  of  these  two  currents 
are  inversely  proportional  to  the  resistances  of  their  circuits.  A  simple  form 
of  rheocord  (Fig.  341)  consists  of  a  long  wire  arranged  for  convenience  in 
parallel  lines  on  a  small  wooden  base  and  connected  at  its  two  ends  with 
binding  posts  A  and  B.  The  resistance  of  this  wire,  1.6  ohms,  can  be 
increased  by  the  introduction  of  small  resistance  coils,  between  D  and  B, 
varying  from  5  to  20  ohms. 

The  two  binding  posts  A  and  B  are  connected  with  the  positive  and 
negative  poles  of  an  electric  cell  re- 
spectively.    A  simple  key  is  placed  in 
the  circuit. 

From  A,  a  wire  passes  to  one  of 
the  electrodes  on  which  the  muscle  or 
nerve  rests.  A  second  wire  passes  from 
the   second  electrode  to  a  clamp  S, 


Fig.  340. — A  Mercury  Key. 


Fig.  341. — Rheocord. 


by  way  of  the  binding  post  C,  which  can  be  fastened  to  the  long  wire  at  any 
given  point.  The  current,  on  reaching  A,  will  divide  into  two  branches,  one 
of  which  will  pass  along  the  wire  A,  B,  and  thence  back  to  the  cell;  the  other 
will  pass  through  the  nerve  and  back  to  S  and  thence  also  to  the  cell.  The 
amount  of  current  passing  through  the  nerve  circuit  will  be  inversely  pro- 
portional to  the  resistance  of  the  nerve  and  directly  proportional  to  the 
difference  of  potential  between  A  and  S.  If  S  is  close  to  A,  the  difference  of 
potential  is  slight.  If  S  is  removed  from  A  toward  B,  the  difference  of 
potential  is  increased  and  the  current  sent  through  the  nerve  circuit  is 
increased. 

In  many  experiments  it  is  necessary  to  reverse  the  direction  of  the  current, 
in  other  experiments  to  deflect  it,  without  changing  the  position  of  the  elec- 
trodes.    Both  these  results  may  be  accomplished  by  the  use  of — 

Pohl's  commutator.  This  is  a  round  block  of  wood  with  six  cups, 
each  of  which  is  in  connection  with  a  binding  post  (Fig.  342).  In  each  of  the 
two  cups  marked  i  and  2,  +  and  — ,  is  inserted  one  end  of  a  copper  wire 
bent  at  right  angles.  The  other  ends  of  the  wires  are  supported  and  in- 
sulated by  a  hard-rubber  handle.  To  the  top  of  each  wire  is  soldered  a 
semicircular  copper  wire.     This  arrangement  permits  of  a  rocking  move- 


PHYSIOLOGIC  APPARATUS 


747 


ment,  whereby  the  opposite  ends  of  the  semicircular  wires  can  be  made  to  dip 
into  cups  3  and  4,  and  into  cups  5  and  6  alternately.  Two  wires  crossed  in 
the  middle  of  the  block  serve  to  connect  opposite  pairs  of  cups.  When  in 
use,  the  cups  are  filled  with  clean  mercury.  The  method  of  using  the  com- 
mutator is  as  follows: 

1.  As  a  Current  Reverser. — The  positive  and  negative  poles  of  the  electric 
cell  are  connected  by  wires  with  binding  posts  i  and  2  respectively. 
A  key  is  interposed  in  the  circuit.  Wires  are  then  carried  from  binding 
posts  3  and  4  to  the  electrodes  in  connection  with  the  muscle  or  nerve. 
The  rocker  of  the  commutator  is  so  turned  that  the  ends  of  the  semicir- 
cular wires  dip  into  cups  3  and  4.  The  direction  of  the  current  will  be 
on  the  closure  of  the  circuit  from  i  to  3,  then  from  3  along  a  wire  to  and 
through  the  tissue  and  back  to  4,  and  thence  to  the  cell.  If  the  position  of 
the  rocker  be  now  reversed  so  that  the  opposite  ends  of  the  semicircular 


Fig.  342. — Pohl's  Commutator.     A.  Arranged  as  a  current  reverser;  B,  as  a  cur- 
rent deflector. 


wires  dip  into  cups  5  and  6,  the  direction  of  the  current  through  the  tissue 
will  be  reversed.     The  positive  current,  after  entering  binding  post  i, 
will  flow  to  5;  then  along  one  of  the  cross  wires  to  4;  then  along  a  wire 
to  and  through  the  tissue  and  back  to  3,  along  the  opposite  cross  wire 
to  6,  thence  to  2  and  so  back  to  the  cell. 
2.  As  a  Ctirrent  Deflector. — W^hen  it  is  desirable  to  deflect  the  current  to  two 
pairs  of  electrodes  differently  situated,  wires  are  carried  from  binding 
posts  3  and  4  to  one  pair,  and  from  5  and  6  to  the  other  pair.     The  cross 
wires  are  then  removed.     According  to  the  position  of  the  rocker  the 
current  will  be  deflected  to  one  or  the  other. 
The  Inductorium. — 'i'his  is  an  apparatus  designed  for  the  purpose  of 
obtaining  single  or  rapidly  succeeding  electric  currents  by  induction.     Its 
construction  is  based  on  facts  discovered  by  Faraday,  some  of  which  are  the 
following : 

If  two  circuits,  a  primary  and  a  secondary,  are  placed  parallel  to  each 
other,  the  former  connected  with  a  galvanic  cell,  the  latter  with  a  galvan- 
ometer, it  is  found  that,  at  the  moment  the  primary  circuit  is  made,  and  at  the 
moment  it  is  broken,  a  current  is  induced  in  the  secondary  circuit,  as  shown 
by  a  momentary  deflection  of  the  galvanometer  needle.     During  the  con- 


748 


TEXT-BOOK  OF  PHYSIOLOGY 


tinuous  flow  of  the  current  through  the  primary  circuit  there  is  no  evidence 
of  a  current  in  the  secondary  circuit.  The  induced  current  is  but  of  momen- 
tary duration.  The  current  flowing  through  the  primary  circuit  is  termed 
the  inducing,  the  current  flowing  through  the  secondary  circuit  the  induced 
current. 

The  induced  current  is  opposite  in  direction  to  that  of  the  inducing  cur- 
rent when  the  circuit  is  made  or  closed;  it  is  the  same  direction,  however, 
when  the  circuit  is  broken  or  opened. 

If  the  circuits  are  arranged  in  the  form  of  coils,  it  is  found  that,  other 
things  being  equal,  the  strength  of  the  induced  currents  will  be  proportional 
to  the  number  of  turns  in  the  coils. 

s"  If  the  coils  are  placed 

at  varying  distances 
from  each  other,  the 
strength  of  the  induced 
current  varies,  increas- 
ing as  the  coils  are  ap- 
proximated, decreasing 
as  they  are  separated. 

Approximation  or 
separation  of  the  coils 
while  the  current  is  flow- 
ing through  the  primary 
circuit  develops  an  in- 
duced current,  which 
disappears,  however,  the 
moment  the  movement 
of  the  coil  ceases.  A 
sudden  increase  or  de- 
crease in  the  strength  of  the  inducing  current  also  develops  an  induced 
current. 

When  the  coils  are  approximated  or  the  primary  current  increased  in 
strength,  the  induced  current  is  opposite  in  direction  to  that  of  the  inducing 
current;  with  the  reverse  conditions,  the  induced  current  has  the  same 
direction. 

The  induced  currents  have  been  termed,  in  honor  of  their  discoverer, 
Faradic  currents. 

The  du  Bois-Reymond  inductorium,  based  on  the  foregoing  facts, 
consists  essentially  of  two  coils  of  insulted  copper  wire,  termed  primary  and 
secondary  (Fig.  343). 

The  primary  coil,  R',  consists  of  thick  copper  wire  wound  around  a 
wooden  spool  attached  to  a  vertical  support.  The  beginning  of  this  coil  is  at 
the  binding  post  S",  its  termination  either  at  binding  post  P"  or  S'".  In  the 
course  of  this  primary  wire  or  circuit,  there  are  placed  two  vertical  bars  of 
soft  iron,  B',  connected  at  their  bases  to  form  a  horseshoe  magnet,  around  the 
ends  of  which  the  wire  is  coiled.  The  object  of  this  device  will  be  explained 
later. 

Inside  the  primary  coil  there  is  placed  a  bundle  of  soft  iron  wires,  which, 
as  soon  as  the  circuit  is  made,  become  magnetized,  with  the  effect  of 
increasing  the  action  of  the  inducing  current. 


Fig.  343. — iNDUCTOiau.M  01  du  Eois-Ri-y.mond.     R',  Pri- 
mar\',  R"    secondary  spiral.     B.  Board  on  which  R"  moves. 

I.  Scale.     -| .  Wires    from    batterv'.     P',    P".  Pillars.     H. 

Neef's  hammer.  B'.  Electro-magnet.  S'.  Binding  screw 
touching  the  steel  spring  (H).  S"  and  S'".  Binding  screws  to 
which  to  attach  wires  where  Neef's  hammer  is  not  required. 


PHYSIOLOGIC  APPARATUS  749 

The  secondary  coil,  R",  consists  of  a  much  greater  number  of  turns  of 
a  finer  copper  wire,  the  ratio  being  about  40  to  i,  also  wound  around  a  spool, 
having  a  tunnel  suflSciently  large  to  enable  it  to  slide  over  the  primary.  By 
these  means  the  strength  of  the  induced  current  is  increased.  As  a  result 
of  the  construction  of  the  inductorium,  the  low  electro-motive  force  of  the 
cell  is  transformed  into  the  high  electro-motive  force  characteristic  of  the 
induce  d  current.  As  the  number  of  turns  of  wire  in  the  secondary  coil  is  to 
the  number  in  the  primary,  so  are  the  electro-motive  forces  in  the  secondary 
coil  to  those  in  the  primary  coil. 

The  secondary  coil  slides  along  a  track,  B,  which  permits  it  to  be  moved 
toward  or  away  from  the  primary.  The  distance  between  the  two  coils  can 
be  measured  and  the  strength  of  the  induced  current  again  reproduced, 
other  things  being  equal,  by  means  of  a  centimeter-millimeter  scale  pasted 
on  the  edge  of  B. 

The  ends  of  the  wire  of  the  secondary  coil  are  fastened  to  two  binding 
posts  to  which  conducting  wires  provided  with  hand  electrodes  can  be 
attacked. 

The  inductorium  may  be  used  for  obtaining  either  a  single  current  or  a 
series  of  rapidly  repeated  induced  currents. 

The  Single  Induced  Current. — On  account  of  its  high  electro-motive  force, 
its  penetrative  power,  and  short  duration,  the  single  induced  current  is  a 
most  convenient  and  suitable  form  of  stimulus  for  many  purposes.  In 
order  to  obtain  such  a  current,  the  positive  wire  of  the  cell  is  carried  to 
binding  post  S",  and  the  negative  wire  either  to  S'"  or  P".  A  key  is  placed  in 
the  primary  circuit.  The  course  of  the  current  will  then  be  on  the  closure  of 
the  circuit  from  the  cell  to  S",  thence  around  R'  to  S'",  and  so  back  to 
the  cell;  or  if  the  negative  wire  is  connected  with  P",  the  course  of  the  cur- 
rent on  leaving  R'  will  be  through  the  coils  surrounding  the  two  vertical  bars 
B',  thence  to  P",  and  so  back  to  the  cell.  If  the  secondary  coil  be  placed 
close  to  the  primary  and  the  wires  of  the  secondary  brought  into  contact 
with  a  muscle,  it  will  be  found  that  with  both  the  make  and  the  break  of 
the  primary  circuit  a  current  is  induced  in  the  secondary,  as  shown  by  a 
short  quick  pulsation  of  the  muscle;  but  during  the  time  of  closure  of  the  cir- 
cuit the  induced  current  is  wanting,  as  shown  by  the  quiescent  condition  of  the 
muscle.  It  will  be  apparent,  however,  from  the  energy  of  the  contraction 
that  the  break-induced  current  is  a  more  efficient  stimulus  than  the  make- 
induced  current.  That  this  is  the  case  is  made  evident  by  removing  the 
secondary  to  the  end  of  the  slide- way  and  then  gradually  bringing  it  toward  the 
primary  half  a  centimeter  at  a  time,  making  and  breaking  the  circuit  after 
each  movement  until  a  pulsation  of  the  muscle  occurs.  It  will  be  found  to 
occur  first  on  the  break  of  the  circuit.  As  the  secondary  approaches  the  pri- 
mary a  position  will  be  reached  when  a  pulsation  occurs  on  the  make  as 
well  as  on  the  break  of  the  circuit,  though  it  will  be  less  pronounced. 

The  explanation  offered  for  this  difference  in  the  strength  of  the  two  induced  currents  is  as 
follows:  With  the  make  of  the  circuit  and  the  passage  of  the  battery  current  through  the  primary 
coil  there  is  induced  in  the  neighboring  and  parallel  turns  of  the  wire  and  extra  current  opposite  in 
direction  to  the  primary  current.  This  extra  or  self-induced  current  antagonizes  and  prevents 
the  current  from  attaining  its  maximum  development  as  quickly  as  it  otherwise  would,  and  therefore 
its  efiSciency  as  an  inducer  of  a  current  in  the  secondary  is  diminished.  On  the  break  of  the  cir- 
cuit the  primary  current  disappears  quickly,  and  as  there  is  nothing  to  retard  its  disappearance  its 
efficiency  as  an  inducer  of  a  current  in  the  secondary  coil  is  not  diminished.  It  is  not  infrequently 
stated  that  the  disappearance  of  the  primary  current  induces  in  the  neighboring  coils  a  break-extra 


750  TEXT-BOOK  OF  PHYSIOLOGY 

current  corresponding  in  direction  which  assists  in  the  development  of  the  induced  current.  This 
is  not  the  case,  however,  as  no  break-extra  current  is  developed  in  the  inductorium  as  ordinarily 
used  when  actuated  by  a  battery  current  of  moderate  strength. 

As  it  is  not  so  much  the  intensity  of  the  current  as  it  is  rapid  variations  in  intensity  that  produce 
effects,  it  is  readily  apparent  why  the  induced  current  developed  at  the  break  of  the  primary  is 
more  effective  as  a  stimulus  than  the  induced  current  developed  at  the  make  of  the  primary  circuit. 
The  quantity  of  electricity  is,  however,  the  same  in  both  cases. 

If  the  secondary  be  pushed  further  along  the  slideway  until  it  largely 
covers  the  primary  coil,  a  position  will  be  reached  when  the  make-induced 
current  equals  in  its  efficiency  as  a  stimulus  the  break-induced  current; 
and  if  the  secondary  be  yet  further  advanced,  a  position  is  reached  when  the 
make-induced  current  becomes  more  powerful  and  efficient  than  the  break- 
induced  current,  as  shown  by  the  greater  contraction  of  the  muscle.  This 
result  is  explained  by  the  fact  that  the  make-extra  current  is  now  able  of 
itself  to  induce  a  current  in  the  secondary  coil,  on  account  of  its  proximity, 
which,  added  to  that  induced  by  the  battery  current,  produces  a  current, 
greater  than  that  induced  on  the  break  of  the  circuit.^ 

Rapidly  Repeated  Induced  Currents. — As  the  single-induced  current  is  of 
extremely  short  duration,  it  is  inefficient  as  a  stimulus  in  the  conduct  of 
many  experiments.  It  is  necessary,  therefore,  to  develop  it  with  a  frequency 
that  is  sufficient  to  give  rise  to  a  summation  of  effects.  The  duration  of  the 
stimulation  may  be  thus  considerably  prolonged.  This  is  accomplished 
by  introducing  in  the  primary  circuit  close  to  the  primary  coil  an  automatic 
interrupter,  usually  Neef's  modification  of  Wagner's  hammer  (Fig.  344). 
This  consists  of  a  vertical  post,  P',  to  the  top  of  which  is  fastened  a  metallic 
spring  carrying  at  its  opposite  end  a  steel  or  iron  hammer,  H,  which  hangs  over, 
but  does  not  touch,  the  two  vertical  bars  of  soft  iron  around  which  the  wire 
of  the  primary  coil  is  wound.  About  the  middle  of  the  spring  on  its  upper 
surface  there  is  a  small  plate  of  platinum  which  is  in  contact  with  an  adjust- 
able, platinum-tipped  screw,  S',  carried  by  a  plate  of  brass  in  connection  with 
binding  post  S". 

For  the  purpose  of  interrupting  the  primary  circuit  frequently  in  a  unit 
of  time,  and  thus  developing  induced  currents  in  quick  succession,  the 
apparatus  is  arranged  in  the  following  way:  The  positive  and  negative 
poles  of  the  electric  cell  are  connected  by  wires  with  binding  post  P'  and  P^', 
a  key  being  interposed  in  the  circuit.  If  the  screw  S'  is  in  contact  with 
the  spring,  the  current  on  the  closure  of  the  circuit  will  enter  P',  pass 
along  the  spring  to  S',  thence  into  and  through  the  primary  coil  R',  to 
the  coils  surrounding  the  vertical  bars  B',  then  to  P",  and  so  back  to 
the  cell. 

As  the  current  passes  around  the  vertical  bars,  they  are  magnetized. 
The  magnetization  draws  down  the  hammer,  and,  in  so  doing,  breaks  the 
circuit  at  the  tip  of  the  screw,  S'.  The  vertical  bars  are  at  once  demagnetized, 
and  the  hammer  is  restored  to  its  original  position  by  the  elasticity  of  the 
spring.  The  circuit  is  thus  reestablished,  the  current  flows  through  the 
coils,  the  bars  are  again  magnetized,  the  hammer  is  drawn  down,  to  be 
followed  by  a  second  break  of  the  circuit. 

The  number  of  times  the  circuit  is  thus  made  and  broken  per  second  will 
vary  with  the  length  of  the  spring. 

*  "On  certain  peculiarities  of  the  inductorium,"  Prof.  CoUn  C.  Stewart,  "Univ.  Pa.  Medical 
Bulletin,"  Feb.,  1904. 


PHYSIOLOGIC  APPARATUS 


751 


As  each  interruption  of  the  primary  circuit  develops  an  induced  current, 
it  follows  that  the  latter  must  succeed  each  other  with  a  frequency  corres- 
ponding with  the  frequency  of  the  former.  If  while  the  primary  circuit 
is  thus  being  interrupted  the  wires  of  the  secondary  coil  be  placed  in  contact 
with  a  muscle,  the  induced  current  will  give  rise  to  contractions  which  will 
succeed  each  other  so  rapidly  that  they  fuse  together,  producing  a  spasm 
or  tetanus  of  the  muscle.  For  this  reason 
these  currents  are  frequently  spoken  of  as 
tetanizing  currents,  and  the  procedure  as 
tetanization  or  Faradization.  These  cur- 
rents also  increase  in  strength  as  the 
secondary  approaches  the  primary. 


344. — Helmholtz's  Modifi- 
cation OF  Neef's  Hammer.  As 
long  as  c  is  not  in  contact  with  d, 
g  h  remains  magnetic;  thus  c  is  at- 
tracted to  d  and  a  secondary  circuit, 
a,  b,  c,  d,  e,  is  formed;  c  then  springs 
back  again,  and  thus  the  process 
goes  on.  A  new  wire  is  introduced 
to  connect  a  with  /.     K.  Batter}-. 


Helmholtz's  Modification  of  the  Inductorium. — 

With  a  view  of  equalizing  the  strengths  of  the  induced 
currents,  Helmholtz  suggested  a  device  the  adoption  of 
which  accomplishes  this  to  a  certain  extent.  It  con- 
sists (Fig.  362)  in  connecting  with  a  wire  binding  posts 
P'  and  S",  and  in  providing  binding  post  P"  with  an 
adjustable  screw  which  can  be  raised  until  the  spring 
comes  in  contact  with  it,  when  the  hammer  is  drawn 
down  by  the  electromagnet  B'.  This  latter  arrange- 
ment is  practically  a  short-circuiting  key  by  which  a 
portion  of  the  current  is  returned  to  the  cell  without 
ever  entering  the  primary  coil.  The  same  arrange- 
ment, though  differently  lettered,  is  shown  in  Fig.  363. 
By  the  use  of  the  entire  device  the  changes  in  the 
primary  coil  are  made  not  by  making  and  breaking  the 
primary  current,  but  by  alternately  long-  and  short- 
circuiting  the  current.  "When  the  short-circuiting  key 
is  opened,  the  full  volume  of  the  primary  current  flows 
through  the  primary  coil.  When  the  short-circuiting  key  is  closed,  most  of  the  current  fails  to 
enter  the  coil,  taking  the  easier  path  through  the  key.  Some  of  the  current,  however,  always  flows 
through  the  coil  and  is  never  diverted. .  The  cycle  of  changes  in  the  electric  condition  of  the 
primary  coil  is  thus  altered  for  two  reasons: 

"First,  we  no  longer  have  an  alternation  between  a  full  primary  current  and  none  at  all — rather 
an  alternation  between  a  full  primary  current  and  a  weaker  one.  The  difference  in  the  phases  is 
thus  lessened,  the  extent  of  the  change  on  making  and  breaking  is  lessened,  and  correspondmgly 
the  efficiency  of  the  make  and  break  currents  induced  in  the  secondary  coil  is  sUghtly  decreased. 

"Second,  on  making  the  primary  current,  as  in  the  ordinary  coil,  the  sudden  appearance  of  the 
primary  current  is  antagonized  by  the  opposing  make-extra  current,  with  the  result  that  the  make- 
induced  current  is  still  further  reduced;  while  on  breaking  the  current  the  break-extra  current  can 
now  flow  through  the  primary  coil  across  the  short-circuiting  key.  This  current,  traiUng  behind 
the  disappearing  primary  current  in  the  same  direction,  produces  the  same  effect  as  if  the  primary 
current  itself  were  to  disappear  slowly.  As  a  result  the  disappearance  of  the  primary  current  loses 
its  former  efficiency  as  an  inducer  of  secondary  currents,  and  the  break-induction  current  is  reduced 
to  about  the  efficiency  of  the  make. 

"This  so-called  'equalizing'  of  the  make-  and  break-induced  currents  is  never  perfect,  if  for  no 
other  reason,  because  the  make-extra  current  must  take  the  long  circuit  through  the  battery,  while 
the  break-extra  current  has  an  easier  path  through  the  short-circuiting  key,  and  is  thus  greater  than 
the  make-extra  current."     (C.  C.  Stewart.) 


THE  GRAPHIC  METHOD 


The  term  graphic  is  applied  to  a  method  by  which  curves  or  tracings  are 
obtained  which  represent  the  extent,  duration,  and  time  relations  of  the 
movements  accompanying  physiologic  processes.  If  these  movements 
can  be  translated  in  one  direction,  they  may  be  recorded  in  different 
ways: 
I.  By    attaching    the    moving    structure — e.g.,    heart,   muscle,   etc. — to  a 


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Fig.  345. — A  Receiving  Tambour. 


delicate  lever  the  free  extremity  of  which  is  provided  with  a  writing 
point. 
By  transmitting  the  movement  through  a  column  of  air  enclosed  in  a 
rubber  tube  the  two  ends  of  which  are  attached  to  a  metallic  capsule, 
covered  by  a  rubber  membrane,  termed  a  drum  or  tambour.     When 

the  membrane  of  the 
first  tambour  is  pressed 
or  driven  inward,  the 
air  is  forced  through 
the  rubber  tube  into 
the  second  tambour 
and  its  membrane  is 
pushed  outward.  As 
soon  as  the  primary 
pressure  is  removed, 
the  membranes  return 
to  their  former  con- 
dition. If  the  mem- 
brane of  the  first  tam- 
bour is  drawn  outward,  the  air  in  the  system  is  rarefied  and  the  mem- 
brane of  the  second  tambour  is  pressed  inward.  For  the  purpose  of 
registering  the  movement  transmitted  by  the  column  of  air,  the  second 
tambour  is  provided  with  a  light  lever  supported  by  a  vertical 
bearing  resting  on  a  small  metallic  disc.  The  membrane  of  the  first 
tambour  is  frequently  provided  with  a  button,  which  is  placed  over  the 
moving  structure.  The  inward  movement  of  the  membrane  of  the 
first  tambour  produces  an  outward  movement  of  the  membrane  of  the 
second  tambour,  indicated,  though  magnified,  by  the  rise  of  the  free 
end  of  the  lever.  The 
reverse  movement  of 
the  membrane  is  at- 
tended by  a  fall  of  the 
lever.  The  first  tam- 
bour is  termed  the  re- 
ceiving, the  second  the 
recording  tambour 
(Figs.  345>  346). 
By  enclosing  an  organ- 


FiG.  346. — A  Recording  Tambour. — {Marey.) 


-e.g.,  kidney,  spleen,  arm,  finger,  etc. — in  a  rigid 

glass  or  metal  vessel  which  at  one  point  is  in  communication  with  a 

recording  apparatus — e.g.,  (i)  a  piston  provided  with  a  lever  (page  467); 

or  (2)  a  tambour  and  lever  (page  357);  or  (3)  a  mercurial  manometer 

carrying  a  float  and  pen  (page  343).     The  space  between  the  part 

investigated  and  the  vessel  is  filled  with  fluid.     The  variations  in  volume 

of  the  organ  cause  a  displacement  of  the  fluid  and  give  rise  to  a  to-and- 

fro  movement  which  is  taken  up  and  reproduced  by  the  recording 

apparatus. 

The  writing  point  may  be  (i)  some  form  of  pen  carrying  ink  which 

records    the    movement    on    a    white    paper   surface,    or   (2)    a   piece   of 

metal,  glass,  or  paper  which  records   the   movement   on  smoked  paper 

or  glass. 


PHYSIOLOGIC  APPARATUS 


753 


The  Recording  Surface. — The  surface  which  receives  and  records  the 
movements  of  a  pen  or  lever  is  usually  that  of  a  cylinder  which  is  covered 
with  glazed  paper  and  coated  with  a  thin  layer  of  soot,  obtained  by  passing 
the  cylinder  through  the  flame  of  a  gas  burner.  The  axis  of  the  cylinder  is 
supported  by  a  metal  framework.  If  the  writing  point  of  the  lever  be  placed 
against  the  cylinder  and  a  movement  be  imparted  to  it,  a  portion  of  the  soot  is 
rubbed  off,  leaving  a  white  line  behind.  If  the  cylinder  be  stationary,  the 
rise  and  fall  of  the  lever  are  recorded  as  a  vertical  line.  Such  a  record  shows 
only  the  extent  of  a  movement. 
If  the  cylinder  is  traveling,  how- 
ever, at  a  uniform  rate,  the  rise  and 
fall  of  the  lever  are  recorded  in  the 
form  of  a  curve  the  width  of  the 
two  arms  of  which  will  depend 
partly  on  the  rapidity  of  the  move- 
ment of  the  lever  and  partly  on  the 
rate  of  movement  of  the  cylinder. 
The  cylinder  movement  is  initiated 
and  maintained  by  clock-work  or 
by  the  transmission  of  power  by 
belting  to  a  system  of  pulleys  in 
connection  with  its  axis.  As  the 
tracing  is  wave-like  in  form,  the 
cylinder  is  frequently  spoken  of  as 
a    kymograph    or    wave   recorder 

(Fig.  347)- 

From  the  record  thus  obtained 
it  is  possible  to  determine  not  only 
the  extent  but  also  the  duration,  the 
form,  and  the  rate  of  recurrence  of 
any  given  movement. 

The  Extent  of  a  Movement. — As 
the  lever  not  only  takes  up  and  re- 
produces a  movement,  but  at  the 
same  time  magnifies  it,  it  is  essential 
that  the  degree  of  magnification  be 
known,  in  order  to  determine  the  actual  extent  of  the  movement.  The 
magnification  of  the  lever  is  readily  determined  by  dividing  the  distance 
between  the  axis  of  the  lever  and  its  writing  point  by  the  distance  between 
the  axis  and  the  point  of  attachment  of  the  structure,  and  then  dividing 
the  height  of  the  tracing  by  this  quotient.  The  final  quotient  represents 
the  extent  of  the  movement. 

The  Time  Relations  of  a  Movement. — When  recorded  in  the  form  of  a 
curve,  the  duration  of  the  entire  movement,  or  of  any  one  portion  of  it,  can 
be  determined  by  means  of  a  time  marking  or  chronographic  apparatus,  con- 
sisting of  (i)  a  small  signal  magnet  provided  with  a  movable  armature,  to 
which  is  attached  a  writing  style;  (2)  an  automatic  interrupter;  and  (3)  an 
electric  cell. 

The  Signal  Magnet. — The  magnet  (Fig.  348)  is  actuated  by  the  electric 
current  made  and  broken  at  regular  and  known  intervals  by  an  automatically 
48 


Fig.    347. — Kymograph. 
zold,  Leipzig.) 


(Boruttau's,    Pet- 


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TEXT-BOOK  OF  PHYSIOLOGY 


Fig.  348. — Signal  Magnet. 


acting  interrupter  placed  in  the  circuit.  With  each  make  and  break  of  the  cir- 
cuit the  armature  and  style  move  alternately  downward  and  upward.  The 
excursion  of  the  style  can  be  readily  recorded  on  a  traveling  surface.  The 
character  and  number  of  the  interruptions  per  second  will  determine 
the  character  of  the  tracing.     If  they  occur  in  a  rhythmic  manner,  the  tracing 

will  be  sinusoidal  or  wave-like 
in  form.  If  the  time  of  inter- 
ruption is  of  short  duration  as 
compared  with  the  time  of 
closure  of  the  circuit,  the  trac- 
ing will  be  a  horizontal  line 
with  short  vertical  elevations  at 
regular  intervals. 
The  Automatic  Interrupter. — The  circuit  may  be  interrupted  by 
vibrating  reeds,  tuning-forks,  metronomes,  etc.  A  well-known  form  of 
vibrating  reed  is  shown  in  Fig.  349.  This  consists  of  a  metallic  frame 
carrying  a  coil  of  wire  in  the  center  of  which  there  is  a  core  of  soft  iron.  To 
the  vertical  part  of  the  frame  there  is  fastened  the  reed,  the  distal  end  of 
which  is  bent  to  dip  into  an  adjustable  mercury  cup.  When  in  circuit  the 
current  enters  the  coil,  then  flows  into  and  through  the  frame  and  the 
reed  to  the  mercury,  and  thence  back  to  the  cell.  On  the  closure  of  the  cir- 
cuit and  the  magnetization  of  the  iron  core  the  reed  is  withdrawn  from  the 
mercury,  the  circuit  broken,  and  the  core  demagnetized.  The  elasticity  of 
the  spring  returns  it  to  the  mer- 
cury, when  the  circuit  is  again  re- 
stored. The  reed  may  be  so  con- 
structed that  it  will  be  raised  and 
lowered  50,  100,  or  200  times  a 
second.  The  armature  of  the 
signal  magnet  undergoes  a  corre- 
sponding number  of  elevations 
and  depressions.  If  the  reed  vi- 
brates 100  times  in  a  second,  the 
distance  from  crest  to  crest  of  the 
wave  tracing  will  represent  y-ro  ^^  ^^^ 
a  second.     Interrupters  of  various 


349. — Page's  Vibrating  Reed. 
modification.) 


(Reichert's 


kinds  have  been  devised  which  make  and  break  the  circuit  from  i  to  250 
times  a  second. 

Moist  Chamber. — In  many  experiments,  it  is  necessary  to  keep  the  nerve 
or  muscle  preparation  in  a  uniformly  moist  atmosphere.  To  secure  this,  a 
moist  chamber  is  employed  (Fig.  350).  This  consists  of  a  hard-rubber 
platform,  supported  by  a  piece  of  brass,  which  slides  up  and  down  a  vertical 
rod,  and  which  can  be  clamped  at  any  height.  By  means  of  a  short  lever 
the  vertical  rod  can  be  turned,  carrying  the  platform  from  side  to  side. 
The  rod  is  secured  to  a  firm  iron  base. 

Six  double  binding  posts  for  the  attachment  of  wires  pass  through  the 
platform.  Near  the  side  of  the  upper  surface  of  the  platform  there  rises  a 
vertical  rod,  carrying  a  clamp  for  holding  the  femur  of  a  nerve-muscle  prepa- 
ration, as  well  as  a  horizontal  rod  for  supporting  at  least  three  pairs  of 
non-polarizable  electrodes.     A  groove  around  the  outer  edge  of  the  platform 


PHYSIOLOGIC  APPARATUS 


755 


receives  a  glass  shade,  which  covers  the  whole.  The  air  of  the  chamber  is 
kept  moist  by  placing  in  it  pieces  of  blotting-paper  saturated  with  water. 

From  the  under  surface  of  the  platform  there  descends  a  rod,  which,  by 
means  of  a  double  binding  screw,  supports  a  horizontal  rod,  modified  at  one 
end  to  carry  the  delicate  axis  of  a  light  stiff  recording  lever.  The  end  of  this 
lever  is  pointed,  to  enable  it  to  write  on  a  smoked  glass  or  paper.  Beneath 
the  axis  is  a  strip  of  brass,  carrying  a  screw,  which  gives  support  to  the  lever 
until  the  instant  the  contraction  of  the  muscle  begins.  This  screw,  the 
after-loading  screw,  also  enables  the  lever  to  be  placed  in  a  horizontal  position. 
The  portion  of  the  lever  near  the  axis  is  provided  with  a  double  hook,  the 
lower  portion  of  which  serves  for  the  attachment  of  the  weight  by  which  the 
muscle  is  counterpoised. 

In  some  experiments,  as  in  the  registration  of  a  muscle  contraction  under 
varying  conditions,  it  is  necessary  to  give  the  lever  mass  by  attaching  weights 


Fig.  350. — Moist  Chamber. 


directly  beneath  the  muscle.  This,  however,  introduces  certain  errors  in 
the  movements  of  the  lever,  which  somewhat  deform  what  would  otherwise 
be  the  normal  curve.  If  the  weight  be  attached,  not  opposite  to  the  muscle 
attachment,  but  close  to  the  axis  of  the  lever,  the  undesirable  acceleration 
of  the  lever  movement,  during  both  contraction  and  relaxation,  is  largely 
prevented.  The  lever  may  be  a  straw,  a  strip  of  celluloid  or  aluminium. 
It  should  be  as  light  as  possible.  The  writing  point  may  be  made  of  stiff 
paper,  a  piece  of  tinsel,  glass,  or  aluminium.  It  should  have  sufficient  elas- 
ticity to  keep  it  in  contact  with  the  cylinder  during  the  excursion  of  the 
lever.  The  writing  point  should  be  placed  as  nearly  parallel  as  possible 
to  the  surface  of  the  cylinder. 

Normal  Saline  Solution. — To  prevent  drying  and  a  loss  of  irritability 
the  tissue  under  investigation  should  be  kept  moist  with  the  normal  saline 
solution  (NaCl,  0.6  per  cent.).     This  solution  very  largely  prevents  either 


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TEXT-BOOK  OF  PHYSIOLOGY 


absorption  or  extraction  of  water  from  the  tissues  and  thus  retards  chemic 
changes  in  their  composition. 

Ringer's  solution,  largely  used  for  the  same  purpose,  is  made  by 
saturating  0.65  per  cent.  NaCl  solution  with  calcium  phosphate  and  then 
adding  2  c.c.  of  a  i  per  cent,  solution  of  potassium  chlorid  to  each  100  c.c. 
The  Galvanometer  and  Capillary  Electrometer. — In  the  detection 
and  investigation  of  the  electric  currents  of  muscles,  nerves,  and  other 
tissues,  the  physiologist  is  limited  to  the  galvanometer  and  capillary  elec- 
trometer. The  principle  of  the  galvanometer  is  based  on  the  fact  that  an 
electric  current  flowing  through  a  wire  parallel  in  direction  with  a  magnetic 
needle  will  tend  to  set  the  needle  at  right  angles  to  the  direction  of  the 
current.     The  essential  requisite  of  any  galvanometer  used  for  physiologic 

purposes  is  that  it  will  re- 
spond quickly  to  the  influ- 
ence of  extremely  weak 
currents.  This  is  realized 
by  the  use  of  small  light 
needles,  the  adoption  of 
the  astatic  system,  or  some 
similar  device  by  which 
the  directive  influence  of 
the  earth's  magnetism  is 
eliminated,  and  the  multi- 
plication of  the  number  of 
turns  of  the  wire  in  the  coils 
which  surround  the  needle. 
The  tangent  galvanom- 
eter, or  boussole,  as  con- 
structed by  Wiedemann, 
is  the  form  most  frequently 
employed  in  physiologic  investigations  (Fig.  351).  It  consists  primarily 
of  a  thick  copper  cylinder,  through  which  a  tunnel  has  been  bored. 
Within  this  tunnel  is  suspended  a  magnetized  ring,  just  large  enough 
to  swing  clear  of  the  sides  of  the  chamber.  The  object  of  making 
the  magnet  ring-shaped  is  to  increase  its  strength  in  proportion  to  its  size, 
and  to  get  rid  of  the  central  inactive  part.  Connected  with  and  passing 
upward  from  the  magnetized  ring  through  the  copper  cylinder  is  an  alumin- 
ium rod,  surmounted  by  a  circular  plane  mirror.  Above  the  mirror  rises  a 
glass  tube,  which  carries  on  top,  on  an  ebonite  support,  a  little  windlass,  cap- 
able of  being  centered  by  three  small  screws.  On  the  windlass  is  wound  a 
single  filament  of  silk,  which  passes  down  the  tube  and  is  attached  to  the 
mirror.  The  magnet  can,  by  this  contrivance,  be  raised  or  lowered  and 
centered  in  the  copper  chamber.  Deflections  of  the  mirror  from  currents 
of  air  are  prevented  by  inclosing  it  with  a  brass  cover  provided  with  a  glass 
window.  The  coils  are  placed  on  each  side  of  the  copper  chamber,  and 
supported  by  a  rod,  on  which  they  slide.  By  this  arrangement  they  can  be 
approximated  until  they  meet  and  completely  conceal  the  cylinder.  By 
varying  the  position  of  the  coils  the  influence  of  the  current  upon  the  needle 
can  be  increased  or  diminished.  An  advantage  which  this  galvanometer 
possesses  is  the  damping  of  the  oscillation  of  the  needle,  so  that  it  quickly 


Fig.  351. — Wiedemann's  Boussole. 


PHYSIOLOGIC  APPARATUS  757 

comes  to  rest  after  deflection.  This  is  accomplished  by  the  development  of 
induction  currents  in  the  copper  cylinder,  the  direction  of  which  is  opposite 
to  that  of  the  movement  of  the  needle.  The  instrument,  therefore,  is 
aperiodic — that  is  to  say,  when  the  needle  is  influenced  by  a  current  it 
moves  comparatively  slowly  until  the  maximum  deflection  is  reached,  when 
it  comes  to  rest  without  oscillations.  When  the  circuit  is  broken  the  needle 
swings  slowly  back  to  zero,  and  again  comes  to  rest  without  oscillations. 

Inasmuch  as  the  needle  is  not  astatic,  it  is  rend  ered  so  by  the  use  of  an 
accessory  magnet — the  so-called  Hauy's  bar.  This  magnet,  supported  by  a 
rod  directed  perpendicular  to  the  coils,  is  placed  in  the  magnetic  meridian, 
horizontal  to  the  needle,  with  its  north  pole  pointing  north.  By  sliding  the 
magnet  toward  the  needle  the  directive  influence  of  the  earth's  magnetism 
is  gradually  diminished,  and  when  it  is  reduced  to  a  minimum  the  needle 
acquires  its  highest  degree  of  instability.  By  means  of  a  pulley  an  angular 
movement  can  be  imparted  to  the  end  of  the  accessory  magnet  in  the  direc- 
tion of  the  magnetic  meridian,  which  serves  to  keep  the  needle  on  the  zero  of 
the  scale.  The  deflections  of  the  needle  are  observed  by  means  of  an  astro- 
nomic telescope,  above  which  is  placed  a  scale  divided  into  centimeters  and 
millimeters,  and  distant  from  the  galvanometer  about  six  or  eight  feet.  As 
the  numbers  on  the  scale  are  reversed,  they  will  be  seen  in  the  mirror  in  their 
natural  position,  and  with  the  deflection  of  the  needle  the  number  will  appear 
as  if  drawn  across  the  mirror.  The  extent  of  the  deflection  is  readily 
determined  when  the  needle  comes  to  rest. 

The  reflecting  galvanometer  of  Sir  William  Thompson  is  also  used  for 
the  same  purposes. 

The  Capillary  Electrometer. — Not  withstanding  the  extreme  sensi- 
tiveness of  the  modern  galvanometer,  it  has  been  found  desirable,  in  the  in- 
vestigation of  many  physiologic  processes,  to  possess  some  means  which 
will  respond  even  more  promptly  to  slight  variations  in  electro-motive 
force.  This  has  been  realized  in  the  construction  by  Lippmann  of  the  capil- 
lary electrometer.  The  principle  of  this  apparatus  rests  upon  the  fact  that 
the  capillary  constant  or  the  surface-tension  of  mercury  undergoes  a  change 
upon  the  passage  of  an  electric  current,  in  consequence  of  a  polarization  by  hy- 
drogen taking  place  at  its  surface.  If  a  capillary  glass  tube  be  filled  with  mer- 
cury and  its  lower  end  inserted  into  a  solution  of  sulphuric  acid,  and  the  former 
connected  with  the  positive  and  the  latter  with  the  negative  electrode,  it 
will  be  observed,  upon  the  passage  of  the  current,  that  a  definite  movement 
of  the  mercury  takes  place,  in  the  direction  of  the  negative  electrode,  in  conse- 
quence of  the  diminution  of  its  capillary  constant  or  the  tension  of  its  surface  in 
contact  with  the  acid.  As  a  reverse  movement  follows  a  cessation  of  the  current, 
a  series  of  oscillations  will  follow  a  rapid  making  and  breaking  of  the  current. 
If  the  direction  of  the  current  is  reversed,  the  capillary  constant  is  increased 
and  the  mercury  ascends  the  tube  toward  the  negative  pole.  From  facts 
such  as  these  Lippmann  constructed  the  capillary  electrometer,  a  con- 
venient modification  of  which  devised  by  M.  v.  Frey,  is  shown  in  Fig.  352. 
This  consists  of  a  glass  tube.  A,  forty  millimeters  in  length,  three  milHmeters 
in  diameter,  the  lower  end  of  which  is  drawn  out  to  a  fine  capillary  point. 
The  tube  is  filled  with  mercury  and  its  capillary  point  immersed  in  a  10  per 
cent,  solution  of  sulphuric  acid.  The  vessel  containing  the  acid  is  filled  to 
the  extent  of  several  millimeters  with  mercury  also.     The  mercury  in  the 


758 


TEXT-BOOK  OF  PHYSIOLOGY 


tube  is  put  in  connection  with  a  platinum  wire  {a),  and  the  acid  in  the  vessel 
with  a  second  wire  {b).  When  a  constant  current  passes  into  the  apparatus 
in  the  direction  from  b  to  a  the  mercury  is  pushed  up  the  tube,  and,  upon 
the  breaking  of  the  current,  it  may  or  may  not  return  to  the  zero-point.  For 
the  purpose  of  measuring  in  millimeters  of  mercury  the  pressure  necessary 
to  compensate  this  change  in  the  capillary  constant  produced  by  the  electro- 
motive force  of  polarization,  the  apparatus  is  provided  with  a  pressure- vessel, 
H,  and  a  manometer,  B.  This  electrometer  can  be  applied  to  any  microscope 
having  a  reversible  stage.  The  oscillations  of  the  mercury  can  then  be 
observed  with  the  microscope  provided  with  an  ocular  micrometer  (Fig.  353). 
The  special  advantage  of  the  electrometer  is,  that  it  will  respond  instantly 
to  any  variation  in  the  electro-motive  force  and  indicated  a  difference  of 


Fig.  352. — Von  Frey's  Capillary  Electrometer. 


Fig.  353.— Capillary 
Electrometer.  R. 
Mercury  in  tube;  capil- 
lary tube.  s.  Sulphuric 
ac'id.  q.  Hg.  B.  Ob- 
server.    M.  Microscope. 


potential,  according  to  Lippmann's  observation,  as  slight  as  the  i-q^yt  ^^  ^ 
Daniell.     These  rapid  oscillations  can  be  recorded  by  photographic  methods. 

In  using  either  the  galvanometer  or  the  electrometer  for  detecting  the 
existence  of  electric  currents  or  differences  of  potential  in  living  tissues,  it  is 
absolutely  essential  that  non-polarizable  electrodes  be  employed  in  con- 
nection with  it. 

The  String  Galvanometer. — The  string  galvanometer  (Fig.  354)  as  de- 
vised by  Einthoven  is  a  far  more  sensitive  apparatus  than  either  the  usual 
galvanometer  or  the  capillary  electrometer,  and  responds  to  feeble  electric 
currents  more  quickly  than  either.  It  is  based  on  the  principle  that  a  con- 
ductor placed  near  a  stationary  magnet  will  tend  to  change  its  position  when 
a  current  is  passed  through  it,  just  as  a  magnet,  e.g.,  a  delicate  magnetic 
needle,  placed  near  a  conductor  will  change  its  position  when  a  current  is 
passing  through  the  conductor.     For  the  purpose  of  developing  a  high  degree 


PHYSIOLOGIC  APPARATUS 


759 


of  sensitiveness,  two  strong  magnets  are  employed,  across  the  field  of  which 
runs  a  very  fine  conductor  usually  made  by  drawing  out  molten  quartz,  by 
means  of  a  bow  and  arrow,  to  a  thread  much  thinner  than  the  smallest  possi- 
ble metal  wire,  and  then  silvering  the  filament  to  enable  it  to  conduct  elec- 
tricity. When  the  filament  is  securely  fastened  at  its  extremities,  it  will  be 
found  that  it  will  deviate  to  the  right  or  the  left  according  to  the  direction  of 
the  current  which  enters  it. 

By  reason  of  the  small  size  of  the  filament  employed,  it  must  be  strongly 
illuminated,  and  the  shadow  of  the  thread  must  be  magnified  by  an  optical 
device  and  then  projected  across  a  narrow  slit  on  a  sensitive  surface  behind 
it.  As  the  shadow  of  the  thread  moves  from  side  to  side  at  the^same  time 
that  the  sensitive  surface  moves  in  a  direction  at  right  angles  to  it,  a  graphic 


Fig.  354. — The  Cambridge  Electro-cardiograph  or  String  Galvanometer. 


record  will  be  obtained  in  which  the  abscissa  will  indicate  time  and  the  ordi- 
nates,  the  displacement  of  the  thread  or  the  strength  of  the  current  passing 
through  it.  A  typical  normal  record  or  electro-cardiogram  (Fig.  133,  page 
292)  shows  three  upward  directed  waves  separated  by  two  smaller  downward 
directed  waves  and  these  are  commonly  designated,  following  Einthoven, 
by  the  letters  P,  Q,  R,  S,  T  in  order.  In  the  conduct  of  an  experiment  a 
patient's  extremities  are  connected  with  some  suitable  low-resistance  elec- 
trodes. Under  such  circumstances  a  part  of  the  heart's  action  currents  may 
be  led  off  to  the  filament  of  the  galvanometer.  If  the  circuit  is  completed 
from  the  right  arm  and  the  left  arm,  the  arrangement  is  called  lead  I ;  if  from 
the  right  and  left  leg  lead  II;  if  from  the  left  arm  and  the  left  leg,  lead  III. 
The  apparatus  is  customarily  arranged  so  that  the  current  produced  by 


76o  TEXT-BOOK  OF  PHYSIOLOGY 

electro-negative  auricles  and  electro-positive  ventricles  gives  an  ui)ward  loop 
to  the  tracing,  one  centimeter  of  height  being  made  to  correspond  to  an  elec- 
tro-motive force  of  one  millivolt  on  suitable  adjustment  of  the  quartz  filament. 
Various  forms  of  string  galvanometers,  based  on  the  principle  stated  in 
a  previous  paragraph,  have  been  devised.  The  apparatus  shown  in  Fig.  354 
is  known  as  the  Cambridge  Electro-cardiogra])h. 

DISSECTION  OF  THE  HIND-LEG  OF  THE  FROG 

Much  of  our  knowledge  of  the  physiologic  properties  of  muscles  and 
nerves  has  been  derived  from  the  study  of  the  muscles  and  nerves  of  the 
cold-blooded  animals,  especially  of  the  frog,  for  the  reason  that  in  these 
animals  the  tissues  retain  their  vitality  under  appropriate  conditions  for  a 
considerable  period  of  time  after  death  or  removal  from  the  body.  The 
muscles  generally  employed  for  experimental  purposes  are  the  gastrocnemius, 
the  sartorius,  the  semimembranosus,  the  gracilis,  and  the  hyoglossus.  The 
nerve  generally  employed  is  the  sciatic.  Both  muscle  and  nerve  may  be 
studied  independently  of  each  other,  or  they  may  be  studied  together,  as 
when  in  their  usual  physiologic  relation.  For  this  latter  purpose  the  gas- 
trocnemius muscle  and  sciatic  nerv^e  are  employed,  constituting  the  so-called 
"nerve-muscle  preparation." 

For  these,  and  many  other  reasons,  the  student  should  familiarize  him- 
self with  the  general  anatomy  of  the  frog,  and  especially  with  the  anatomy 
of  the  posterior  extremities. 

Preparation  of  the  Frog. — Destroy  the  frog  by  plunging  a  pin 
through  the  skin  and  soft  tissues  covering  the  space  between  the  occipital 
bone  and  the  first  vertebra  until  the  point  is  stopped  by  the  vertebra.  Turn 
the  pin  toward  the  head  and  push  it  into  the  brain  cavity;  move  it  from  side 
to  side  and  destroy  the  brain.  Pass  the  pin  into  the  spinal  canal  and  destroy 
the  spinal  cord.  With  a  stout  pair  of  scissors  cut  off  the  body  behind  the 
fore  limbs.  Remove  the  viscera  and  the  abdominal  walls.  Draw  the 
hind-legs  out  of  the  skin.  Place  the  legs  on  a  glass  plate,  back  uppermost, 
and  moisten  them  freely  with  normal  saline  solution. 

Observe  on  the  outer  side  of  the  dorsal  surface  of  the  thigh  the  follow- 
ing muscles  (Fig.  355,  356).  The  triceps  femoris  (tr),  made  up  of  the 
rectus  anticus  (ra),  the  vastus  externus  (ve),  and  the  vastus  internus  (vi), 
not  seen  from  behind;  on  the  inner  side,  the  semi-membranosus  (sm)  and 
the  rectus  internus  minor  or  gracilis  (ri").  Between  these  two  groups, 
note  the  biceps  femoris  (b).  Above  the  thigh  observe  the  gluteus  (gl),  the 
ileo-coccygeus  (ci),  and  the  pyriformis  (p). 

In  the  leg  observe  the  gastrocnemius  (g)  with  its  tendon  (the  tendo- 
Achillis),  the  tibialis  anticus  (ta),  and  the  peroneus  (pe). 

Turn  the  frog  on  its  back  and  note  the  muscles  on  the  ventral  surface 
of  the  thigh,  the  rectus  internus  major  (ri'),  and  minor  (ri"),  the  adductor 
magnus  (ad"),  the  sartorius  (s),  the  adductor  longus  (ad'),  and  the  vastus 
internus  (vi).  In  the  leg,  in  addition  to  those  already  seen  from  behind, 
note  the  tibialis  posticus  (tp)  and  the  extensor  cruris  (ec). 

Note  in  the  abdominal  cavity  the  three  large  spinal  nerves,  the  seventh, 
eighth,  and  ninth. 

Dissection  of  the  Sciatic  Nerve. — The  sciatic  nerve  is  composed 
of  the  seventh,  eighth,  and  ninth  spinal  nerves.     After  its  emergence  from 


PHYSIOLOGIC  APPARATUS 


761 


the  pelvic  cavity,  it  passes  down  the  thigh  between  the  semi-membranosus 
and  the  biceps  muscles,  in  company  with  the  femoral  blood-vessels.  Below 
the  knee  it  di\ddes  into  the  tibialis  and  peroneus  nerves;  the  former  sending 
branches  into  the  gastrocnemius.  In  its  course,  the  sciatic  sends  branches 
to  the  muscles  of  the  entire  leg. 

Carefully  separate  the  biceps  and  semimembranosus  by  tearing  the 
connective  tissue  uniting  them.  The  sciatic  nerve  and  femoral  blood- 
vessels come  into  view;  with  a  bent  glass  rod  gently  separate  the  nerve 
from  its  surroundings  from  the  knee  to  the  thigh.  Begin  at  the  knee.  In 
order  to  expose  the  nerve  at  the  pelvis,  it  will  be  necessary  to  divide  the 
pyriformis  and  the  ileo-coccygeus  muscles.  Care  must  here  be  exercised, 
so  as  not  to  injure  the  nerve  which  lies  immediately  beneath.  Lift  up  the 
urostyle  with  the   forceps  and   separate  it  from  the  last  vertebra.     With 


ec 


I- ta 


Fig 


355. — Leg  Muscles  of  the  Frog. 
\'extral  StiBirACE. — (Ecker.) 


Fig.  356. — Leg  Muscles  of  the  Frog. 
Dorsal  Surface. — (Ecker.) 


the  scissors  cut  off  the  vertebral  column  above  the  seventh  vertebra.  Place 
the  legs  on  the  dorsal  surface  and  then  divide  the  seventh,  eighth,  and  ninth 
vertebrae  lengthwise.  With  the  forceps  lift  up  one  lateral  half  of  the  vertebrae 
and  free  the  nerve  as  far  as  the  knee  by  dividing  connective  tissue  and  nerve 
branches.     Be  careful  not  to  injure  the  nerve  with  scissors  or  forceps. 

The  Nerve-muscle  Preparation. — Divide  the  tendo-Achillis  just  below 
its  fibro-cartilaginous  thickening  at  the  heel,  and  detach  the  gastroc- 
nemius up  to  the  knee.  Cut  through  the  tibio-fibular  bone  just  below 
the  knee-joint.  Cut  the  femur  transversely  near  its  middle  and  remove 
the  muscles  from  the  lower  end,  carefully  avoiding  injury  to  the  nerve. 
The  completed  preparation  consists  of  the  gastrocnemius  muscle,  the  sci- 
atic nerve,  with  half  of  the  seventh,  eighth,  and  ninth  vertebrae  and  the  lower 
half  of  the  femur. 


762  TEXT-BOOK  OF  PHYSIOLOGY 

THE  ANATOMY  OF  THE  FROG  HEART  AND  THE  VASCULAR 

APPARATUS 

The  heart  of  the  frog  can  be  readily  exposed  after  the  animal  has  been 
made  insensible  by  destruction  of  the  brain.  The  sternum  is  divided  longi- 
tudinally and  each  half  drawn  outward  by  gentle  traction  of  the  anterior 
extremities.     The  pericardium  is   then  divided  and  turned  aside. 

When  viewed  from  the  ventral  surface,  Fig.  375,  the  heart  shows  two 
auricles,  a  right  and  left,  a  single  ventricle  and  a  more  or  less  conical  vessel, 
the  conus  arteriosus,  which  arises  from  the  right  side  of  the  base  of  the  ven- 
tricle. When  viewed  from  the  dorsal  surface,  Fig.  376,  it  presents  a  tri- 
angular-shaped vessel,  the  sinus  venosus,  formed  by  the  union  of  the 
terminations  of  the  two  superior  and  inferior  vense  cavse.  A  dissection  of 
the  heart  shows  that  the  cavity  of  the  sinus  venosus  communicates  with 


Fig.  357.— VejItral  Surface  of  the  Frog  Heart.  (After  Howes.)  ra.  Right  auricle 
la.  Left  auricle,  v.  Ventricle,  ca.  Conus  arteriosus,  p' .  Pulmo-cutaneous  trunk,  s'.  Sys- 
temic aortic  trunk,     c' .  Carotid  trunk,     ac.  Left  anterior  caval  vein. 

the  cavity  of  the  right  auricle  by  means  of  a  transversely  oval  foramen,  in 
the  posterior  wall  of  the  auricle.  This  opening  is  provided  with  two 
valves,  a  ventral  and  dorsal,  the  free  edges  of  which  are  directed  toward 
the  cavity  of  the  auricle.  Two  pulmonary  veins,  a  right  and  left,  penetrate 
the  dorsal  wall  of  the  left  auricle. 

A  longitudinal  section.  Fig.  377,  of  the  heart  shows  that  the  auricles, 
though  separated  by  a  septum,  communicate  below  by  a  common  orifice 
with  the  cavity  of  the  single  ventricle.  This  orifice,  the  auriculo-ventricu- 
lar,  is  provided  with  two  valves  the  free  edges  of  which  are  directed  toward 
the  cavity  of  the  ventricle. 


PHYSIOLOGIC  APPARATUS  763 

The  conus  arteriosus  is  separated  from  the  ventricle  by  three  semilunar 
valves.  The  interior  of  the  conus  is  traversed  by  a  longitudinally  disposed 
membranous  valve  attached  to  its  dorsal  surface;  the  ventral  edge  is,  however, 
free.  The  upper  extremity  of  the  conus  passes  into  the  bulbus  aortcB,  from 
which  it  is  separated  by  a  semilunar  valve  and  the  free  extremity  of  the 
longitudinal  valve.  From  the  bulbus  aortae  arise  two  large  branches,  a 
right  and  a  left,  each  of  which  is  subdivided  by  two  longitudinal  partitions 
into  three  vessels,  the  carotid  trunk,  the  aortic  arch,  and  the  pulmo-cutaneous 
trunk.  (See  Fig.  377.)  The  carotid  and  aortic  trunks  communicate 
separately  with  the  cavity  of  the  bulbus,  while  the  pulmo-cutaneous  trunk 
communicates  with  the  conus  arteriosus  by  a  single  orifice,  just  below  the 
free  end  of  the  longitudinal  valve.     After  pursuing  a  short  course  these 


la.. 


pv. 


SV 


IC. 


Fig.  358. — Dorsal  Surface  of  the  Frog  Heart.  {A  per  Howes)  ra.  Right  auricle. 
la.  Left  auricle,  sv.  Sinus  venosus.  sv' .  Opening  of  sinus  venosus  into  right  auricle. 
pv.  Pulmonary  vein.  v.  Right  anterior  caval  vein,  p'  s'  c'.  The  pulmo-cutaneous,  aortic  and 
carotid  trunks  respectively. 

three  vessels  separate  from  one  another  to  distribute  blood  to  the  various 
organs  of  the  body.  The  two  aortic  trunks  wind  around  the  esophagus 
and  unite  posteriorly  to  form  the  dorsal  aorta;  the  pulmo-cutaneous  divides 
into  a  pulmonary  artery  which  is  distributed  to  the  lung  and  a  cutaneous 
branch  which  is  distributed  to  the  skin. 

The  course  of  the  blood  through  the  heart  cavities  is,  therefore,  as  follows: 
The  venous  blood  poured  by  the  venae  cavae  into  the  sinus  venosus  passes 
through  the  sino-auricular  foramen  into  the  right  auricle.  While  the 
right  auricle  is  being  filled  from  this  source,  the  left  auricle  is  being  filled 
by  blood  coming  through  the  pulmonary  veins.  "When  the  auricles  contract, 
which  they  do  simultaneously,  each  passes  its  blood  into  the  corresponding 
part  of  the  ventricle,  which  then  instantly  contracts  before  the  venous  and 
arterial  bloods  have  time  to  mix.     Since  the  conus  arteriosus  springs  from 


764 


TEXT-BOOK  OF  PHYSIOLOGY 


the  right  side  of  the  ventricle  it  will  at  first  receive  only  venous  blood,  which 
on  the  contraction  of  the  conus  might  pass  either  into  the  bulbus  aortae 
or  into  the  aperture  of  the  pulmo-cutaneous  trunks.  But  the  carotid  and 
systemic  trunks  are  connected  with  a  much  more  extensive  capillary  system 
than  the  pulmo-cutaneous  and  the  pressure  in  them  is  proportionally  great, 
so  that  it  is  easier  for  the  blood  to  enter  the  pulmo-cutaneous  trunks  than  to 
force  aside  the  valves  between  the  conus  and  the  bulbus.  A  fraction  of  a 
second  is,  however,  enough  to  get  up  the  pressure  in  the  pulmonary  and 
cutaneous  arteries,  and  in  the  meantime  the  pressure  in  the  arteries  of  the 
head,  trunk,  etc.,  is  constantly  diminishing,  owing  to  the  continual  flow  of 
blood  toward  the  capillaries.     Very  soon,  therefore,  the  blood  forces  the 


Fig.  359. — Frog  Heart  with  Ventral  Surface  Dissected  Away  to  Show  its  Struc- 
ture. {After  Parker  and  Haswell.)  ra.  Right  auricle,  la.  Left  auricle,  sa.  Septum  between 
auricles.  sao.  Sino-auricular  opening,  ca.  Conus  arteriosus,  sv.  Semilunar  valves,  av. 
Auriculo-ventricular  valves.  Iv.  Longitudinal  valve,  sv' .  Semilunar  valve,  p'  s'  c' .  Pulmo- 
cutaneous  aortic  and  carotid  trunks  respectively,    cc.  Columnse  carnese. 

valves  aside  and  makes  its  way  into  the  bulbus  aortae.  Here  again  the 
course  taken  is  that  of  least  resistance;  owing  to  the  presence  of  the  carotid 
gland,  the  passage  of  blood  into  the  carotid  trunks  is  less  free  than  into 
the  wide  elastic  systemic  trunks.  These  will,  therefore,  receive  the  next 
portion  of  blood  which,  the  venous  blood  having  been  mostly  driven  to  the 
lungs,  will  be  a  mixture  of  venous  and  arterial.  Finally  as  the  pressure 
rises  in  the  systemic  trunks  the  last  portion  of  blood  from  the  ventricle, 
which  coming  from  the  left  side  is  arterial,  will  pass  into  the  carotids  and  so 
supply  the  head"  (Parker  and  Haswell). 

The  muscle-fibers  composing  the  walls  of  the  heart  from  the  sinus  venosus 
to  the  conus  arteriosus  are  continuous,  though  at  the  sino-auricular,  the 
auriculo-ventricular,  and  the  ventriculo-conic  junctions  the  continuity  is  to 


PHYSIOLOGIC  APPARATUS  765 

some  extent  interrupted  by  bands  of  circularly  disposed  fibrous  tissue,  serving 
for  the  support  of  the  valves,  which  momentarily  interfere  with  the  ready 
passage  of  the  contraction  wave  from  one  division  of  the  heart  to  another. 
The  frog  heart  receives  its  nutritive  material  from  the  blood  flowing  through 
its  cavities.  During  the  diastole  the  blood,  under  the  influence  of  the  shght 
pressure  developed,  passes  from  the  interior  of  the  heart  into  a  system  of 
irregular  passageways  or  channels  which  penetrate  the  heart-wall  in  all 
directions,  and  thus  comes  into  direct  contact  with  the  heart-cells.  With 
the  beginning  of  the  systole  the  blood  is  forced  out  of  these  channels  into  the 
interior  of  the  ventricle,  bringing  with  it  the  products  of  tissue  metabolism. 
The  Heart -beat. — If  the  heart  while  beating  is  lifted  up  by  a  ligature 
attached  to  the  apex  it  will  be  observed  that  the  contraction  begins  in  the 
walls  of  the  sinus  venosus,  then  passes  to  the  auricles,  thence  to  the  ventricle 
and  finally  to  the  conus;  from  this  it  may  be  inferred  that  the  physiologic 
stimulus  acts  primarily  in  the  walls  of  the  sinus  from  which  its  effect,  viz. : 
the  excitation  process,  is  conducted  from  one  cavity  to  another  in  quick 
succession. 


INDEX 


Abducent  nerve,  626 
Aberration,  chromatic,  702 

spheric,  702 
Absorption,  210 

by  epitheUum  of  villi,  219 

of  foods,  218 
of  fat,  222 
of  proteins,  220 
of  sugar,  220 
of  water,  220 

of  lymph,  217 

spectra  of  blood,  248 
A-C  interval,  302 
Acapnia,  431 
Accommodation  of  the  eye,  694 

convergence  of  eyes  during,  698 

force  of,  697 

mechanism  of,  695 

range,  697 
Acoustic  or  auditory  area,  587 

nerve,  631 
Action  currents  of  muscles,  79 
of  ner\''es,  106 

reflex,  114 

of  medulla  oblongata,  564 
of  spinal  cord,  537,  541 
Adams-Stokes  syndrome,  304 
Adrenal  bodies,  500 
Agraphia,  594 
Albuminoids,  16 
Albumins,  15 
Alcohol,  effects  of,  126 
Alimentary  canal,  136 
Amino-acids,  13 
Amnion,  734 
Amylase,  152 
Amylogenesis,  487 
Amylopsin,  191 
Amy  loses,  7 
Anastalsis,  201 
Animal  body,  structure  of,  2 
Animal  heat,  438 
Ankle  clonus,  543 

jerk,  543 
Anti-dromic  vaso-dilatator  nerves,  380 
Aphasia,  594 

agraphic,  594 


Aphasia,  amnesic,  595 

aphemic,  594 
Apnea,  433 

chemica  or  vera,  434 
vagi  or  inhibitoria,  434 
Arterial  circulation,  328 

pressure,  343 
Arteries,  structure  and  properties  of,  328 
Articulate  speech,  593 
Articulation,  temporo-maxillary,  143 
Asphyxia,  435 

Association  centers  of  cerebrum,  596 
Astigmatism,  701 
As-Vs  interval,  302 
Auriculo-ventricular  node,  272 
Autonomic  nerve  system,  642 

anatomic    relation  of    central    nerve 
system   to    sympathetic  ganglia, 
647 
anatomic    relation    of,    to    peripheral 

structures,  644 
autonomic  or  sympathetic  ganglia,  644 
autonomic   nerves  from   mid-brain 
region,  647- 
bulbar  region,  647 
mid-spinal  cord,  648 
sacral,  649 
functions  of  autonomic  nerve  sys- 
tem, 651 
functions  of  the    mid-brain    auto- 
nomic nerves,  651 
bulbar,  652 
sacral  autonomic,  654 
thoracic  and  lumbar,  652 

Basal  ganglia,  557 
Bile,  195 

composition  of,  195 

mode  of  secretion,  197 

physiologic  action,  198 

pigments,  197 

salts,  195 
Bilirubin,  197 
Biliverdin,  197 
Bioplasm,  29 

physiologic  properties,  29 
Blind  spot,  704 


767 


768 


INDEX 


Blood,  230 

changes  in,  during  respiration,  412 
circulation  of,  264 
coagulation  of,  234 

chemistry  of,  261 

extr avascular,  262 

intravascular,  263 
constituents  of,  230 
corpuscles,  238,  252 
defibrinated,  235 
general  composition  of,  269 
physical  constitution,  230 
physical  properties  of,  231 
plates,  259 
pressure,  341 

arterial,  343 

capillary,  347 

causes  of,  340 

determination  of,  in  man,  355 

methods  of  estimation,  346 

auscultatory  method,  358 

resume  of  factors,  348 

variations  in,  350 

venous,  347 
quantity  of,  260 
reaction,  232 
serum,  236 
temperature,  233 
velocity  of,  in  arteries,  360 

of,  in  capillaries,  362 

of,  in  veins,  363 

viscosity  of,  233 
Bronchial  innervation,  390 
Burdach,  column  of,  533 

Calcium  salts  of  the  body,  20 
Calorimeter,  444 
Capillary  blood-vessels,  330 
functions  of,  331 

circulation,  371 

electrometer,  757 
Capsule,  internal,  560 

functions  of,  567 
Carbo-hemoglobin,  251 
Carbohydrates,  7 
Carbon  monoxid  hemoglobin,  251 
Cardiac  cycle,  275 

impulse,  274 

movement  of  the  blood  during,  277 

schematic  representation  of  the  events, 
288 
Cardiac  nerves,  313 
Cardio-accelerator  center,  322 

factors  which  determine  its  activity, 
323 


Cardio-inhibitor  center,  323 

factors  which  determine  its  activity, 

324 

Cardio-pulmonic  vessels,  267 
Carotid  pulse,  368 
Caseinogen,  16,  19 
Catalysis,  139 
Caudate  nucleus,  558 
Cells,  structure  of,  24 

chemic  composition,  25 

manifestations  of  life  by,  26 

reproduction  of,  28 
Central  organs  of  the  nerve  system,  523 
Cerebellar  tract,  533 
Cerebellum,  603 

functions  of,  605 

results  of  experimental  lesions,  605 
Cerebrum,  570 

convolutions  of,  572 

fissures  of,  570 

functions  of,  577 

localization  of  function  in,  578 

motor  area  of  the  chimpanzee  brain, 

584 
motor  area  of  the  human  brain,  590 
motor  area  of  the  monkey's  brain,  580 
sensor  areas  of  the  human  brain,  585 
sensor  aieas  of  the    monkey's  brain, 

581 
structure  of  the  gray  matter,  570 
structure  of  the  white  matter,  575 

Chemic  composition  of  the  body,  6 

Cheyne-Stokes  respiration,  435 

Chimpanzee  brain,  motor  area  of,  584 

Cholesterin,  196 

Chorda  tympani  nerve,  153,  630 

Chorion,  735 

Chromo-proteins,  17 

Chyle,  223 

Ciliary  ganglion,  646 
movement,  88 
muscle,  682 

function  of,  696 

Circulation  of  blood,  264 
forces  concerned,  375 
hydrodynamic  considerations,  332 
vascular  apparatus,  328 

Clark's  vesicular  column,  530 

Classification  of  food  principles,  122 

Coagulated  proteins,  18 

Cochlea,  718 

functions  of,  723 

Colostrum,  483 

Color  perception,  709 

Commutator,  747 


•     INDEX 


769 


Complemental  air,  407 
Conjugated  proteins,  17 
Connective  tissues,  33 

physical  and  physiologic  properties  of, 

37 
Coordination,  mechanism  of,  607 
Coronary  arteries,  295 

effects  of  ligation,  297 
vaso-motor  nerves  of,  296 
Corpora  quadrigemina,  556 
functions  of,  565 
striatum,  557 
functions  of,  566 
Corpus  luteum,  729 
Cranial  nerves,  610 
Crura  cerebri,  555 

functions  of,  565 
Crystalline  lens,  685 

Defecation,  207 

ner\^e  mechanism  of,  207 
Degeneration,  Wallerian,  99 
Deglutition,  159 

ner\^e  mechanism  of,  164 
Demarcation  current,  80 
Depressor  nerve,  325,  386 
Dextrin,  8 
Dextroses,  8 
Diabetes,  521 

Diapedesis  of  leucocytes,  257 
Diaphragm,  393-398 
Diastalsis,  201 
Diffusion,  225 
Digestion,  136 
Digestive  apparatus,  136 
Dilatator  pupillee  muscle,  681 
Direct  cerebellar  tract,  533 

pyramidal  tract,  532 
Ductless  glands,  480 
Ductus  arteriosus,  739 

venosus,  739 
Dyspnea,  434 

Electro-cardiogram,  292 
Electro-cardiograph,  759 
Electrodes,  non-polarizable,  744 
Electrometer  capillary,  757 
Electrotonic  alterations  in  excitability  of 
nerves,  108 
current,  107 
Electrotonus,  107 
Encephalo-spinal  fluid,  524 
Endocardium,  266 
Enterokinase,  194 
Enzymes,  138 


Epidermis,  492 

Epididymis,  693 

Epinephrin,  503 

Epithelial  tissues,  functions  of,  31,  32 

Erepsin,  192,  193 

Erlanger's  sphygmomanometer,  356 

Erythrocytes,  238 

Eupnea,  433 

Eustachian  tube,  713,  722 

Excretion,  453 

Expiratory  forces  and  muscles,  400 

Expired  air,  composition  of,  410 

Fyp,  cardinal  points  of,  688 

blind  spot,  704 

dioptric  apparatus  of,  686 

muscles  of,  709 

physiologic  anatomy  of,  679 

reduced,  690 

schematic,  689 

Facial  nerve,  626 

function  of,  628 
paralysis  of,  627 
Fallopian  tube,  728 
Fat,  II 

absorption  of,  222 

digestion  of,  192 

emulsification  of,  12 

saponification  of,  1 1 
Feces,  206 
Fecundation,  732 
Fehling's  solution,  8 
Ferments,  138 
Petal  circulation^  737 

membranes,  734 
Fibrillary  contraction,  297 
Fibrin,  19 
Fibrinogen,  237 
Fillet,  544,  553 
Filtration,  229 
Follicle,  Graafian,  726 
Food,  116 

animal,  131 

cereal,  133 

composition  of,  131 

disposition  of,  123 

heat  value  of,  126 

principles,  122 

quantities  required  daily,  121 

vegetable,  134 
Forced  expiration,  401 
Forces  aiding  the  movement  of  lymph  and 

chyle,  223 
Fovea,  685 
Frog  heart,  anatomy  of,  762 


770 


INDEX 


Galactose,  9 

Gall-bladder,  194 

Galvanic  current,  effect  of,  on  nerves,  107 

muscles,  59,  60,  81 
Galvanometer,  756,  758 
Ganglia,  peripheral,  646 
Gaseous  exchange  in  lungs,  420 

in  tissues,  420 
Gases  of  blood,  relation  of,  413 
tension  of,  417 

carbon  dioxid,  416 

nitrogen,  417 

oxygen,  415 
Gastric  digestion,  165 

fistulae,  169 

glands,  167 

juice,  170 

mode  of  secretion,  172 
physiologic  action  of,  1 74 
Globulins,  15 

Glossopharyngeal  nerve,  633 
Gluco-proteins,  17 
Glycogen,  8,  487 

in  muscles,  515 
Glycogenosis,  487 

Glycogenic  function  of  the  liver,  487 
Glycogenolysis,  487 
Glycolysis,  488 
Glycosuria,  518 

alimentary,  518 

adrenal,  518 

pancreatic,  518 

parathyroid,  521 

pharmacologic,  520 

phlorhizin,  520 

pituitary,    520 
Gmelin's  test  for  bile  pigments,  197 
GoU,  column  of,  533 
Gower's  antero-lateral  tract,  532 
Graafian  follicle,  726 
Graphic  method,  756 
Green  vegetables,  135 

Hairs,  474 

Hearing,  sense  of,  713 

Heart,  264 

action  of  sympathetic  nerve  on,  313, 

319 

of  vagus  nerve  on,  317,  320 
auriculo- ventricular  bundle,  271 
conduction  system  of,  302 
beat,  nature  of  the  stimulus,  305 

action  of  inorganic  salts,  306 

cardio-pulmonic  vessels,  267 

frequency  of,  290 


Heart  beat,  influence  of  psychic  states,  325 

theory  of,  308 

of  the  excised  heart,  297 
blood-supply,  292 
causes  of  the  variations  of,  290 
compensatory  pause,  310 
coronary  arteries,  295 

effects  of  ligation,  297 

time  of  filling,  295 

vaso-motor  fibers  for,  296 
course  of  blood  through,  267 
curve  of  the  systole  and  diastole,  277 
cycle  of  275 
extra-systole,  310 
idio-ventricular  rhythm,  303 
intra-auricular  pressure,  286 
intracardiac  nerve-cells,  313 
intraventricular  pressure,  282,  339 

curve,  276,  285 

distribution  of,  341 
mechanics  of,  273 
modifications  of  beat  due  to  the  action 

of  drugs,  326 
muscle-band  of  His,  271 
muscle-fibers  of,  269 
negative  pressure  of,  282 
nerve,  mechanism  of,  311 
orifices  and  valves,  268 
origin  and  distribution  of  the  sympa- 
thetic nerves  to,  311 
origin  and  distribution  of  the  vagus 

nerve  to,  314 
physiologic  anatomy  of,  264 
refractory  period,  310 
relative  function  of  auricles  and  ven- 
tricles, 280 
sounds,  279 

synchronism  of  the  two  sides,  281 
valves,  action  of,  278 
ventricular  systole,  285 

the  pre-sphygmic  period,  286 

the  post-sphygmic  period,  286 

the  sphygmic  period,  286 
work  done  by,  376 
Heart-muscle,  properties  of,  298 
automaticity,  305 
conductivity,  299 
electric  currents  of,  290 
electro-cardiograms,  292 
irritability,  298 
response  to  the  action  of  an  artificial 

stimulus,  309 
rhythmicity,  304 
tonicity,  305 
Heat  dissipation,  448 


INDEX 


771 


Heat,  mechanism  of,  447 

production,  441 

influence  of  the  nerve  system,  450 

relation  to  work,  437 

rigor,  65 
Helmholtz's  theory  of  color  perception,  711 
Hematin,  252 
Hemianopsia,  588,  616 
Hemoglobin,  244 

absorption  spectra  of,  249 

chemic  composition  of,  245 

quantity  of,  245 
Hemoglobinometer,  Gowers',  246 

Haldane's,  247 
Hemometer,  v.  Fleischl's,  248 
Hering's  theory  of  color  perception,  711 
Histons,  14 
Hormone,  174 
Horopter,  707 
Hypermetropia,  700 
Hyperpnea,  433 
Hypoglossal  nerve,  640 

Ileo-colic  sphincter,  203 

Incus,  715 

Indol,  205 

Induced  currents,  713,  749,  7^50 

Inductorium,  747 

Infra-proteins,  18 

Insalivation,  146 

nerve  mechanism  of,  153 
Inspiration,  397 

mechanic  m^ovements  of  thorax,  394 

muscles,  397 
Insula,  574 

Intercostal  muscles,  393 
Internal  capsule,  560 

functions  of,  567 

secretion,  490 
Intestinal  digestion,  185 

fermentation,  205 

juice,  193 

physiologic  action  of,  193 

movements,  200 

nerve  mechanism  of,  202 

rhythmic  segmentation,  200 
Intraauricular  pressure,  286 
Intracranial  circulation,  597 

mechanism  of,  598 
Intrapulmonic  pressure,  395,  402 
Intrathoracic  pressure,  395,  403 
Intravascular  coagulation,  263 
Intraventricular  pressure,  282 

distribution  of,  341 
Invertase,  194 


Iris,  681 

functions  of,  698 

nerve  mechanism  of,  619,  620 
Iron  of  the  body,  22 
Irritability  of  muscles,  57 

of  nerves,  loi 
Island  of  Langerhans,  188 

of  Reil,  574 
Isometric  myogram,  69 
Isotonic  myogram,  64 
Isthmus  of  encephalon,  553 

functions  of,  564 

Jacobson's  nerve,  633,  634 
Joints,  48 

classification  of,  48 

Katastalsis,  201 
Keith-Flack  node,  272 
Kidney,  458 

histology  of,  459 
Knee-jerk,  542 
Kymograph,  753 

Labyrinth  of  ear,  715 
Lacrimal  glands,  712 
Lactation,  481,  739 
Lacteals,  223 
Lactose,  10 
Language,  592 
Large  intestine,  203 

function  of,  204 

movements  of,  204 

nerve  mechanism  of,  205 
Larynx,  658 

nerve  mechanism  of,  666 

structure  of,  659 
Lateral  columns  of  the  spinal  cord,  532 
Law  of  contraction,  no 
Lecithin,  197 
Lemniscus,  544,  553 
Lens,  crystalline,  685 
Lenticular  nucleus,  558 
Leucocytosis,  257 
Leucopenia,  257 
Leukocj^tes,  255 

chemic  composition  of,  256 

functions  of,  259 

number  of,  256 

origin  of,  258 

physiologic  properties,  257 

varieties  of,  258 
Levers,  83 
Levulose,  9 
Limbic  lobe,  573 


772 


INDEX 


Liver  ,  194,  483 

conjugation  of  the  products  of  protein 
putrefaction,  489 

elaboration  of  bile  by,  486 

formation  of  urea  in,  488 

functions  of,  485 

influence  of  the  nerve  system  on,  516 

production  of  glycogen  and  sugar,  487 
Localization  of  functions  in  cerebrum,  578 
Lungs,  structure  of  the,  389 
Lymph,  213 

absorption  of,  217 

composition  of,  215 

functions  of,  217 

movement  of,  223 

physical  properties,  214 

production  of,  215 
Lymph-capillaries,  211 
Lymph-glands,  212 
Lymph-nodes,  212 
Lymph- vessels,  212 
Lymphocytes,  213,  258 

Macula  lutea,  684 
Malleus,  715 
Maltose,  10 
Mammary  glands,  479 
Mastication,  141 

muscles  of,  143 

nerve  mechanism  of,  144 
Meats,  composition  of,  131 
Medulla  oblongata,  551 

reflex  activities  of,  564 
Meibomian  glands,  712 
Membrana  tympani,  714 

function  of,  720 
Menstruation,  729 
Metabolism,  118,  509 

during  starvation,  128,  510 

of  carbohydrates,  515 

of  fats,  513 

of  proteins,  512 

on  a  mixed  diet,  511 

on  protein  diet,  130 

on  fat  and  carbohydrate  diet,  131 
Methemoglobin,  252 
Migration  of  leukocytes,  257,  373 
Milk,  480 

composition  of,  132,  480 

mechanism  of  secretion,  481 
Moist  chamber,  755 
Motor  area  of  chimpanzee  brain,  584 
of  human  brain,  585 
of  monkey  brain,  580 

oculi  nerve,  617 


Mouth  digestion,  141 
Movements  of  the  eyeball,  708 
of  the  intestines,  200,  204 
of  the  lower  jaw,  143 
of  the  lungs,  402 
of  the  stomach,  179 
Muscle,  action  currents  of,  81 
contraction,  63 

chemic  phenomena  of,  76 
electric  phenomena  of,  79 
graphic  representation  of,  63 
modifying  influences  of,  65 
phenomena   following    stimulation, 

61 
physical  phenomena  of,  61 
rigor  mortis,  76 
summation  eff'ects,  71 
tetanus,  72,  74 
thermic  phenomena  of,  78 
Muscles,  electric  currents  from,  79 

electric  currents,  negative  variation  of, 

81 
energy,  source  of,  77 
fatigue,  68 

groups,  special  action  of,  83 
sense,  674 
sound,  76 
spindle,  674 
stimuli,  59 
tissue,  51 

chemic  composition  of,  54 
elasticity,  56,  62 
histology  of,  52,  85 
irritability,  57 
physical  properties  of,  55 
physiologic  properties  of,  55 
tonicity,  57 
Myenteric  plexus,  169,  202 

reflex,  202 
Myopia,  700 
Myosinogen,  15,  55 
Myxedema,  490 

Nerve,  abducent,  626 
acoustic,  531 
facial,  626 

glosso-pharyngeal,  635 
hypoglossal,  640 
oculo-motor,  617 
olfactory,  612 
optic,  613 

pneumogastric,  vagus,  634 
spinal  accessory,  639 
trigeminal,  623 
trochlear,  622 


INDEX 


773 


Nerve  impulse,  102 
Nerve  mechanism  of  heart,  311 
of  respiration,  424 
of  vascular  apparatus,  376 
Nerve-muscle  preparation,  104,  761 
Nerve  system,  functions  of,  524 
Nerve  tissue,  90 

histology  of,  90 
Nerves,  autonomic  system  of,  642 

chemic  composition  and  metabolism  of, 

.94 

classification  of,  99 

degeneration  of,  98 

development  of,  97 

effects  of  galvanic  current  on,  107 

electric  currents  of,  104 

electric  currents  of,  negative  variation 
of,  105 

electric  excitation  of,  107 

electric  phenomena  of,  104 
action  currents,  106 
diphasic  action  currents,  107 

peripheral  endings  of,  96 

physiologic  properties  of,  loi 

pilo-motor,  100 

polar  stimulation  of,  no 

relation  of,  to  central  nerve  system,  95 

stimuli  of,  1 01 
Neuron,  90 

Nicotin,  actions  of,  158,  326 
Nucleo-proteins,  17 
Nucleus  caudatus,  558 

cuneatus,  533 

gracilis,  533 

lenticularis,  558 
Nutritive  supply  of  the  embryo,  736 

Oculo-motor  nerve,  617 
Ohm's  law,  742 
Olein,  II 

Olfactory  nerve,  612 
Oncometer,  467 
Ophthalmic  ganglion,  646 
Opsonins,  259 
Optic  constants,  686 

nerve,  613 

thalamus,  558 
functions  of,  566 
Optogram,  706 
Organ  of  Corti,  718 
Osazones,  10 
Osmometer,  227 
Osmosis,  226 
Osmotic  pressure,  226 
Ossicles  of  ear,  714 


Otic  ganglion,  647 
Ovary,  726 
Ovulation,  728 
Ovum,  727 
Oxygen  in  blood,  415 

in  tissues,  418 

quantity  absorbed  daily,  410,  422 
Oxyhemoglobin,  250 

Pacinian  corpuscle,  671 
Palmitin,  1 1 
Pancreas,  185 
Pancreatic  juice,  188 

mode  of  secretion,  189 

physiologic  action  of,  190 
Parathyroids,  494 

effects  of  removal,  494 
Partial  pressure  of  gases,  414 
Parturition,  738 
Pepsin,  171 
Peptones,  175 

Peripheral  organs  of  the  nerve  system,  523 
Peristalsis,  158,  201 
Peristaltic  rush,  201 
Perspiration,  471 
Petrosal  nerves,  629 
Pettenkofer-Voit     respiration     apparatus, 

421 
Pexin,  171 
Phagocytosis,  259 
Phlorhizin  glycosuria,  520 
Phonation,  658 

mechanism  of,  664 
Phospho-proteins,  16 
Physiology  of  the  cell,  24 

of  movement,  39 
Pilleus  ventriculi,  201 
Pilo-motor  nerves,  100 
Pituitary  body,  495 

effects  of  total  removal,  496 
of  anterior  lobe  removal,  497 
of  injection  of  extracts,  499 
of  posterior  lobe  removal,  498 
Placenta,  736 

Plasma  of  blood,  composition  of,  235 
Pleura,  394 
Pneumatogram,  406 
Pneumogastric  nerve,  634 
Pneumograph,  406 
Polar  stimulation,  no 

of  human  nerves,  in 
Pons  varolii,  553 

functions  of,  565 
Postures,  84 
Presbyopia,  699 


774 


INDEX 


Prosecretin,  190 
Protamins,  14 
Proteins,  12 

chemic  composition,  12 

color  reactions,  19 

physical  properties,  13 

precipitation  tests,  20 

structure  of,  13 
Ptyalin,  152 
Pulmonic  blood-vessels,  374,  391 

vascular  apparatus,  374 

ventilation,  394 
Pulse,  364 

carotid,  368 

frequency,  260,  367 

radial  366 

venous,  369 

volume,  290 

wave,  velocity  of,  366 
Punctum  proximum,  697 

remotum,  697 
Pyramidal  tracts  of  spinal  cord,  532 

Reaction  of  degeneration,  114 
Red  corpuscles,  238 

chemic  composition  of,  244 

effects  of  reagents,  243 

function  of,  255 

life  history  of,  252 

number  of,  240    , 

of  vertebrated  animals,  254 

preservation  of,  242 
Reduced  hemoglobin,  250 
Reflex  action,  114,  536 

laws  of,  539 
Refractory  period  of  the  heart,  310 
Regnault's  and  Reisset's  respiration    ap- 
paratus, 422 
Relation  of  gases  in  the  blood,  413 
Renal  duct,  462 
Rennin,  171 
Reproduction,  726 

Reproductive  organs  of  the  female,   726 
Reproductive  organs  of  the  male,  730 
Reserve  air,  407 
Residual  air,  398,  407 
Residual  heat  of  the  body,  440 
Respiration,  388 

changes  in  composition  of  air   during, 
409 

changes  in  composition  of  blood,  412 

changes  in  tissues,  418 

chemistry  of,  409 

effects  of  a  change  of  pressure  of  the 
blood  gases  on,  413 


Respiration,  expiratory  forces  and  muscles, 
400 
first  inspiration,  432 
mechanism  of  gaseous  exchange,  420 
nerve  mechanism  of,  424 
number  per  minute,  405 
total  respiratory  exchange,  421 
volumes  of  air  breathed,  406 
Respiratory  apparatus,  388 
movements,  397 

effects  of,  on  arterial  pressure,  437 
effects   of,    on   the    flow  of    blood 
through  the  thoracic  vessels,  436 
of  upper  air  passages,  404 
Respiratory  nerve  mechanism,  424 
inspiratory  center,  425 

causes  of  rhythmic  activity  of,  425 
effect  of  a  change  of  pressure  of 

blood  gases  on,  431 
reflex  stimulation,  427 
relation  of  vagus  nerves,  427 
theories  of  mode  of  action,  430 
Respiratory  pressures,  395 
quotient,  411 
rhythm,  405 

Cheyne-Stokes,  435 
modification  of,  433 
sounds,  409 
types,  405 
Retina,  682 

functions  of,  704 
Retinal  image,  686 

size  of,  691 
Rheocord,  746 

Rhythmic  segmentation,   200 
Rigor  mortis,  76 
Rima  glottidis,  658 

respiratoria,  625,  663 
vocalis,  663 
Ringer-Locke  solution,  307 
Routes  for  the  absorbed  food,  223 
Rush  peristalsis,  201 

Saccharose,  9 

Saliva,  148 

physiologic  action  of,  150 

Salivary  glands,  146 

histologic  changes  in,  during  secre- 
tion, 150 
modification  of  by  drugs,  157 
nerve  mechanism  of,  153 

Sclero-proteins,  16 

Sebaceous  glands,  474 

Sebum,  474 

Secretin,  190 


INDEX 


775 


Secretion,  external,  476 

internal,  490 
Semen,  731 

Semicircular  canals,  716,  724 
Sensor  areas  of  human  brain,  585 

of  monkey  brain,  581 
Serum,  236 

Setchenow's  center,  541 
Sight,  sense  of,  679 
Sino-auricular  node,  272 
Skatol,  206 

Skeleton,  physiology  of,  47 
Skin,  472,  670 

nerve  endings  in,  670 
reflexes,  541 
Sleep,  600 
Smell,  sense  of,  677 
Sodium  glycocholate,  195 

taurocholate,  195 
Spectroscope,  249 
Speech,  658 
Spermatozoa,  731 
Spheno-palatine  ganglion,  646 
Sphygmograph,  367 
Sphygmomanometer,  355 
Spinal  accessory  ner^^e,  639 
Spinal  cord,  528 

encephalo-spinal  conduction,  547 

functions  of,  534 

nerve-cells,  classification  of,  530 

nerv'e-fibers  of,  531 
classification  of,  531 

redex  actions  of,  537,  541 

reflex  irritability  of,  539 

relation  of  spinal  ner\'es  to,  526 

segm.entation  of,  528 

spinal  nerve  roots,  functions  of,  527 

spino-encephalic  conduction,  544 

structure  of  gray  matter,  528 

structure  of  white  matter,  531 

tracts  of,  532 
Spinal  segments  as  conductor,  543 

as  independent  centers,  534 
Spirometer,  407 

Splanchnic  ner\'es,  184,  202,  387,  649,  654 
Spleen,  506 

functions  of,  507 
Stannius  ligatures,  300 
Stapes,  715 
Starch,  7 

digestion  of,  151,  190 
Starvation,  128 
Steapsin,  191 
Stearin,  11 
Stereognostic  area,  586 


Stomach,  165 

movements  of,  179 

nerve  mechanism  of,  183 
String  galvanometer,  759 
Suprarenal  capsules,  500 
Sweat-glands,  472 

Sweat,  influence  of  nerve  system  on  produc- 
tion of,  472 

Taste  buds,  676 

nerve  of,  675 

sense  of,  675 
Tawara's  node,  273 
Teeth,  141 
Tegmentum,  555 
Temperature  of  human  body,  438 

sense,  673 
Tendon  reflexes,  542 
Tension  of  gases  in  blood,  417 

tissues,  417 
Tensor  tympani  muscle,  715 

functions  of,  721 
Thermogenesis,  441 
Thermolysis,  448 
Testicles,  730 
Tetanus,  72 

experimental,  75 

pathologic,  75 

pharmacologic,  75 

physiologic,  74 
Thoracic  duct,  213 
Thorax,  392 

dynamic  condition  of,  397 

mechanic  movements  of,  394 

static  condition  of,  395 
Thyroid  gland,  490 

effects  of  removal,  492 
Tissue  spaces,  201 
Tongue,  676 
Total  carbon-dioxid  exhaled,  422 

oxygen  absorbed,  422 

respiratory  exchange,  421 
Touch,  sense  of,  670 
Trachea,  389 

Traube-Hering  waves,  387 
Trochlear  nerve,  622 
Trypsin,  191 
Turck,  column  of,  532 
Tympanum,  713 

Upper  air-passages,  respiratory  movements 

of,  404 
Urea,  455 

seat  of  formation,  455 
Ureter,  462 


776 


INDEX 


Uric  acid,  457 
Urination,  469 

nerve  mechanism  of,  470 
Urine,  453 

composition  of,  454 
mechanism  of  secretion,  462,  465 
influence  of  blood  composition,  469 
influence  of  nerve  system,  468 
of  blood-pressure,  466 
Uterus,  728 

Vagina,  728 
Vagus  nerve,  634 

bathmotropic,  322 

chronotropic,  322 

dromotropic,  322 

influence  on  heart,  317 

inotropic,  322 

seat  of  action,  321 
Valves  of  heart,  268-278 
Vas  deferens,  731 
Vascular  apparatus,  328 

application    to    vascular     apparatus, 

337 

hydrodynamic  considerations,  332 

stream  bed,  338 

nerve  mechanism  of,  376 
Vaso-motor  center,  382 

central  stimulation,  383 

nerves,  377 

reflex  stimulation,  387 

vaso-dilatator  nerves,  379 
Veins,  331 

structure  and  function,  331 
Velocity  of  blood,  360 
Venous  circulation,  331 

pulse,  369 
Vertebral  column,  47 
Vesiculae  seminales,  731 


Villi,  218 

functions  of,  219 
Visceral  muscle,  85 

functions  of,  88 

properties  of,  86 
Viscosity  of  blood,  233 
Vision,  679 

accommodation,  694 

astigmatism,  701 

binocular,  706 

color  perception,  709 

functions  of  retina,  704 

hypermetropia,  700 

myopia,  700 

presbyopia,  699 
Visual  angle,  692 
Vital  capacity  of  lungs,  407 
Vocal  bands,  661 

sounds,  664 
Voice  and  speech,  665 
Volume  pulse,  370 
Volumes  of  air  breathed,  407 

Wallerian  degeneration,  99 
Wernicke's  pupillary  reaction,  621 
White  blood-corpuscles,  255 

function  of,  259 

migration  of,  373 

origin  of,  258 

physiologic  properties,  257 

varieties  of,  258 
Wrisberg,  nerve  of,  629 

Yellow  spot,  682 

Zymogen,  152 

pepsinogen,  171 
ptyalogen,  152 
trypsinogen,  192 


k  M 


